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{{short description|Large, round non-stellar astronomical object}}
{{about|the astronomical object|other uses|Planet (disambiguation)}}
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[[File:Planet_collage_to_scale.jpg|thumb|upright=1.5|The eight planets of the [[Solar System]] with size to scale (up to down, left to right): [[Saturn]], [[Jupiter]], [[Uranus]], [[Neptune]] (outer planets), [[Earth]], [[Venus]], [[Mars]], and [[Mercury (planet)|Mercury]] (inner planets)]]
A '''planet''' is a large, [[Hydrostatic equilibrium|rounded]] [[Astronomical object|astronomical body]] that is generally required to be in [[orbit]] around a [[star]], [[stellar remnant]], or [[brown dwarf]], and is not one itself.<ref name="exodef"/> The [[Solar System]] has eight planets by the most restrictive definition of the term: the [[terrestrial planet]]s [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]], and the [[giant planet]]s [[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]]. The best available theory of planet formation is the [[nebular hypothesis]], which posits that an [[interstellar cloud]] collapses out of a [[nebula]] to create a young [[protostar]] orbited by a [[protoplanetary disk]]. Planets grow in this disk by the gradual accumulation of material driven by [[gravity]], a process called [[accretion (astrophysics)|accretion]].

The word ''planet'' comes from the Greek {{lang|grc|[[wikt:πλανήτης#Ancient Greek|πλανήται]]}} ({{Transliteration|grc|planḗtai}}) {{gloss|wanderers}}. In [[Classical antiquity|antiquity]], this word referred to the [[Sun]], [[Moon]], and five points of light visible to the naked eye that moved across the background of the stars—namely, Mercury, Venus, Mars, Jupiter, and Saturn. Planets have historically had religious associations: [[History of astronomy#Early History|multiple cultures]] identified celestial bodies with gods, and these connections with mythology and [[folklore]] persist in the schemes for naming newly discovered Solar System bodies. Earth itself was recognized as a planet when [[heliocentrism]] supplanted [[Geocentrism|geocentrism]] during the 16th and 17th centuries.

With the development of the [[telescope]], the meaning of ''planet'' broadened to include objects only visible with assistance: the [[natural satellite|moons]] of the planets beyond Earth; the [[ice giant]]s Uranus and Neptune; [[Ceres (dwarf planet)|Ceres]] and other bodies later recognized to be part of the [[asteroid belt]]; and [[Pluto (dwarf planet)|Pluto]], later found to be the largest member of the collection of icy bodies known as the [[Kuiper belt]]. The discovery of other large objects in the Kuiper belt, particularly [[Eris (dwarf planet)|Eris]], spurred debate about how exactly to define a planet. In 2006, the [[International Astronomical Union]] (IAU) adopted a definition of a planet in the Solar System, placing the four terrestrial planets and the four giant planets in the planet category; Ceres, Pluto, and Eris are in the category of [[dwarf planet]].<ref name="IAU">{{cite web |title=IAU 2006 General Assembly: Result of the IAU Resolution votes |url=http://www.iau.org/news/pressreleases/detail/iau0603/ |publisher=International Astronomical Union |date=2006 |access-date=30 December 2009 |archive-date=29 April 2014 |archive-url=https://web.archive.org/web/20140429212224/http://iau.org/news/pressreleases/detail/iau0603/ |url-status=live }}</ref><ref name="WSGESP">{{cite web|date=2001 |title=Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union |work=IAU |url=http://www.dtm.ciw.edu/boss/definition.html |access-date=23 August 2008 |archive-url=https://web.archive.org/web/20060916161707/http://www.dtm.ciw.edu/boss/definition.html |archive-date=16 September 2006 }}</ref><ref name=planetarysociety>{{cite web |first=Emily |last=Lakdawalla |author-link=Emily Lakdawalla |url=https://www.planetary.org/worlds/what-is-a-planet |title=What Is A Planet? |website=The Planetary Society |date=21 April 2020 |access-date=3 April 2022 |archive-date=22 January 2022 |archive-url=https://web.archive.org/web/20220122142140/https://www.planetary.org/worlds/what-is-a-planet |url-status=live }}</ref> Many [[Planetary science|planetary scientists]] have nonetheless continued to apply the term ''planet'' more broadly, including dwarf planets as well as rounded satellites like the Moon.<ref>{{Cite web|url=https://www.sciencenews.org/article/pluto-planet-vote-status-definition-demotion|title=The definition of planet is still a sore point – especially among Pluto fans|date=24 August 2021|first=Lisa|last=Grossman|website=[[Science News]]|access-date=10 July 2022|archive-date=10 July 2022|archive-url=https://web.archive.org/web/20220710033107/https://www.sciencenews.org/article/pluto-planet-vote-status-definition-demotion|url-status=live}}</ref>

Further advances in astronomy led to the discovery of over 5,900 planets outside the Solar System, termed [[exoplanet]]s. These often show unusual features that the Solar System planets do not show, such as [[hot Jupiter]]s—giant planets that orbit close to their parent stars, like [[51 Pegasi b]]—and extremely [[Orbital eccentricity|eccentric orbits]], such as [[HD 20782 b]]. The discovery of brown dwarfs and planets larger than Jupiter also spurred debate on the definition, regarding where exactly to draw the line between a planet and a star. Multiple exoplanets have been found to orbit in the [[circumstellar habitable zone|habitable zones]] of their stars (where liquid water can potentially exist on a [[planetary surface]]), but Earth remains the only planet known to support [[life]]. Such a habitable zone is complicated by many factors, such as the atmospheric composition (if there is an atmosphere) of the planet; Mars, for example, has the temperature during the day to form liquid water on its surface on the equator, if only the pressure were higher. 

== Formation ==
{{Main|Nebular hypothesis}}
{{multiple image
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| image1            = The_Mysterious_Case_of_the_Disappearing_Dust.jpg
| caption1          = A protoplanetary disk
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| caption2          = Protoplanets colliding during planet formation
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It is not known with certainty how planets are formed. The prevailing theory is that they coalesce during the collapse of a [[nebula]] into a thin disk of gas and dust. A [[protostar]] forms at the core, surrounded by a rotating [[protoplanetary disk]]. Through [[Accretion (astrophysics)|accretion]] (a process of sticky collision) dust particles in the disk steadily accumulate [[mass]] to form ever-larger bodies. Local concentrations of mass known as [[planetesimal]]s form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become increasingly dense until they collapse inward under gravity to form [[protoplanet]]s.<ref>{{cite journal | first=G. W. |last=Wetherill |title=Formation of the Terrestrial Planets |journal=Annual Review of Astronomy and Astrophysics |date=1980 |volume=18 | issue=1 |pages=77–113 |bibcode=1980ARA&A..18...77W |doi=10.1146/annurev.aa.18.090180.000453 |issn=0066-4146}}</ref> After a planet reaches a mass somewhat larger than Mars's mass, it begins to accumulate an extended [[atmosphere]],<ref name=dangelo_bodenheimer_2013>{{cite journal|last1=D'Angelo|first1=G.|last2=Bodenheimer|first2=P.|title=Three-dimensional Radiation-hydrodynamics Calculations of the Envelopes of Young Planets Embedded in Protoplanetary Disks|journal=The Astrophysical Journal|year=2013|volume=778|issue=1|pages=77 (29 pp.)|doi=10.1088/0004-637X/778/1/77|arxiv = 1310.2211 |bibcode = 2013ApJ...778...77D |s2cid=118522228}}</ref> greatly increasing the capture rate of the planetesimals by means of [[Drag (physics)|atmospheric drag]].<ref>{{cite journal | last1=Inaba | first1=S. | last2=Ikoma | first2=M. |title=Enhanced Collisional Growth of a Protoplanet that has an Atmosphere |journal=Astronomy and Astrophysics |date=2003 |volume=410 | issue=2 |pages=711–723 |bibcode=2003A&A...410..711I |doi = 10.1051/0004-6361:20031248|doi-access=free }}</ref><ref name=dangelo2014>{{cite journal|last1=D'Angelo|first1=G.|last2=Weidenschilling | first2=S. J. |last3=Lissauer | first3=J. J. |last4=Bodenheimer | first4=P. |title=Growth of Jupiter: Enhancement of core accretion by a voluminous low-mass envelope|journal=Icarus|date=2014|volume=241|pages=298–312|arxiv=1405.7305|doi=10.1016/j.icarus.2014.06.029|bibcode=2014Icar..241..298D |s2cid=118572605}}</ref> Depending on the accretion history of solids and gas, a [[giant planet]], an [[ice giant]], or a [[terrestrial planet]] may result.<ref name=lhdb2009>{{cite journal|last1=Lissauer|first1=J. J.|last2=Hubickyj | first2=O. |last3=D'Angelo | first3=G. |last4=Bodenheimer | first4=P. |title=Models of Jupiter's growth incorporating thermal and hydrodynamic constraints| journal=Icarus|year=2009|volume=199|issue=2| pages=338–350|arxiv=0810.5186|doi=10.1016/j.icarus.2008.10.004|bibcode=2009Icar..199..338L |s2cid=18964068}}</ref><ref name=ddl2011>{{cite book|last1=D'Angelo|first1=G.|last2=Durisen|first2=R. H.|last3=Lissauer|first3=J. J.|chapter=Giant Planet Formation|bibcode=2010exop.book..319D|title=Exoplanets|publisher=University of Arizona Press, Tucson, AZ|editor-first=S.|editor-last=Seager|pages=319–346|date=2011|chapter-url=http://www.uapress.arizona.edu/Books/bid2263.htm|arxiv=1006.5486|access-date=1 May 2016|archive-date=30 June 2015|archive-url=https://web.archive.org/web/20150630164645/http://www.uapress.arizona.edu/Books/bid2263.htm|url-status=live}}</ref><ref name=chambes2011>{{cite book|last=Chambers|first=J.|chapter=Terrestrial Planet Formation|bibcode=2010exop.book..297C|title=Exoplanets|publisher=University of Arizona Press|location=Tucson, AZ|editor-first=S.|editor-last=Seager|pages=297–317|date=2011|chapter-url=http://www.uapress.arizona.edu/Books/bid2263.htm|access-date=1 May 2016|archive-date=30 June 2015|archive-url=https://web.archive.org/web/20150630164645/http://www.uapress.arizona.edu/Books/bid2263.htm|url-status=live}}</ref> It is thought that the [[regular satellite]]s of Jupiter, Saturn, and Uranus formed in a similar way;<ref name="arxiv0812">{{cite book |author1=Canup, Robin M. |author1-link=Robin Canup |author2=Ward, William R. |title=Origin of Europa and the Galilean Satellites |publisher=[[University of Arizona Press]] |date=2008 |arxiv=0812.4995|bibcode = 2009euro.book...59C |page=59|isbn=978-0-8165-2844-8}}</ref><ref name=dangelo_podolak_2015>{{cite journal|last1=D'Angelo|first1=G.| last2=Podolak | first2=M.|title=Capture and Evolution of Planetesimals in Circumjovian Disks|journal=The Astrophysical Journal|date=2015|volume=806|issue=1|pages=29pp|doi=10.1088/0004-637X/806/2/203|arxiv = 1504.04364 |bibcode = 2015ApJ...806..203D |s2cid=119216797}}</ref> however, [[Triton (moon)|Triton]] was likely [[gravitational capture|captured]] by Neptune,<ref name="Agnor06">{{Cite journal| doi = 10.1038/nature04792| url = http://extranet.on.br/rodney/curso2010/aula9/tritoncapt_hamilton.pdf| title = Neptune's capture of its moon Triton in a binary–planet gravitational encounter| journal = Nature| volume = 441| issue = 7090| pages = 192–4| year = 2006| last1 = Agnor| first1 = C. B.| last2 = Hamilton| first2 = D. P.| pmid = 16688170| bibcode = 2006Natur.441..192A| s2cid = 4420518| access-date = 1 May 2022| archive-date = 14 October 2016| archive-url = https://web.archive.org/web/20161014134243/http://extranet.on.br/rodney/curso2010/aula9/tritoncapt_hamilton.pdf}}</ref> and Earth's Moon<ref name="taylor1998">{{cite web |url=http://www.psrd.hawaii.edu/Dec98/OriginEarthMoon.html |title=Origin of the Earth and Moon |last=Taylor |first=G. Jeffrey |date=31 December 1998 |work=Planetary Science Research Discoveries |publisher=Hawaiʻi Institute of Geophysics and Planetology |access-date=7 April 2010 |url-status=live |archive-url=https://web.archive.org/web/20100610011142/http://www.psrd.hawaii.edu/Dec98/OriginEarthMoon.html |archive-date=10 June 2010}}</ref> and Pluto's Charon might have formed in collisions.<ref name="Stern_2015">
{{cite journal |title=The Pluto system: Initial results from its exploration by New Horizons |first1=S.A. |last1=Stern |first2=F. |last2=Bagenal |first3=K. |last3=Ennico |first4=G.R. |last4=Gladstone |first5=W.M. |last5=Grundy |first6=W.B. |last6=McKinnon |first7=J.M. |last7=Moore |first8=C.B. |last8=Olkin |first9=J.R. |last9=Spencer |display-authors=4 |journal=Science |date=16 October 2015 |pmid=26472913 |article-number=aad1815 |volume=350 |issue=6258 |doi=10.1126/science.aad1815 |arxiv=1510.07704 |bibcode=2015Sci...350.1815S |s2cid=1220226 }}</ref>

When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by [[photoevaporation]], the [[solar wind]], [[Poynting–Robertson effect|Poynting–Robertson drag]] and other effects.<ref>{{cite thesis | last = Dutkevitch |first = Diane |date =1995 |url =http://www.astro.umass.edu/theses/dianne/thesis.html |archive-url =https://web.archive.org/web/20071125124958/http://www.astro.umass.edu/theses/dianne/thesis.html |archive-date=25 November 2007 |title =The Evolution of Dust in the Terrestrial Planet Region of Circumstellar Disks Around Young Stars |type=PhD thesis|publisher=University of Massachusetts Amherst |access-date = 23 August 2008 |bibcode=1995PhDT..........D}}</ref><ref>{{cite journal | last1=Matsuyama | first1=I. | last2=Johnstone | first2=D. | last3=Murray | first3=N. |title=Halting Planet Migration by Photoevaporation from the Central Source |journal=The Astrophysical Journal |date = 2005 |volume=585 |issue=2 |pages=L143–L146 |bibcode=2003ApJ...585L.143M |doi = 10.1086/374406|arxiv = astro-ph/0302042 | s2cid=16301955 }}</ref> Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a larger, combined protoplanet or release material for other protoplanets to absorb.<ref>{{cite journal | last1=Kenyon |first1=Scott J. | last2=Bromley | first2=Benjamin C. |journal=Astronomical Journal |volume=131 | issue=3 |pages=1837–1850 | date=2006 |doi=10.1086/499807 |title= Terrestrial Planet Formation. I. The Transition from Oligarchic Growth to Chaotic Growth | bibcode=2006AJ....131.1837K|arxiv = astro-ph/0503568 |s2cid=15261426 }}</ref> Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Protoplanets that have avoided collisions may become [[natural satellite]]s of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or [[small Solar System body|small bodies]].<ref>{{Cite journal |last1=Martin |first1=R. G. |last2=Livio |first2=M. |date=1 January 2013 |title=On the formation and evolution of asteroid belts and their potential significance for life |journal=Monthly Notices of the Royal Astronomical Society: Letters |language=en |volume=428 |issue=1 |pages=L11–L15 |doi=10.1093/mnrasl/sls003 |issn=1745-3925|doi-access=free |arxiv=1211.0023 }}</ref><ref>{{Cite journal |last=Peale |first=S. J. |date=September 1999 |title=Origin and Evolution of the Natural Satellites |url=https://www.annualreviews.org/doi/10.1146/annurev.astro.37.1.533 |journal=Annual Review of Astronomy and Astrophysics |language=en |volume=37 |issue=1 |pages=533–602 |doi=10.1146/annurev.astro.37.1.533 |bibcode=1999ARA&A..37..533P |issn=0066-4146 |access-date=13 May 2022 |archive-date=13 May 2022 |archive-url=https://web.archive.org/web/20220513181131/https://www.annualreviews.org/doi/10.1146/annurev.astro.37.1.533 }}</ref>

{{Multiple image
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| footer            = [[Supernova remnant]] ejecta producing planet-forming material
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The energetic impacts of the smaller planetesimals (as well as [[radioactive decay]]) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by density, with higher density materials sinking toward the [[planetary core|core]].<ref>{{cite journal | journal=Icarus |date=1987 |volume=69 | issue=2 |pages=239–248 |last1=Ida |first1=Shigeru | last2=Nakagawa | first2=Yoshitsugu | last3=Nakazawa | first3=Kiyoshi |title= The Earth's core formation due to the Rayleigh-Taylor instability |doi=10.1016/0019-1035(87)90103-5 |bibcode=1987Icar...69..239I}}</ref> Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the [[mantle (geology)|mantle]] and from the subsequent impact of [[comet]]s<ref>{{cite journal | last=Kasting |first=James F. |title=Earth's early atmosphere |journal=Science |date=1993 |volume=259 |bibcode=1993Sci...259..920K |doi=10.1126/science.11536547 |pmid=11536547 |issue=5097 | pages=920–926|s2cid=21134564 }}</ref> (smaller planets will lose any atmosphere they gain through various [[Atmospheric escape|escape mechanisms]]<ref>{{Cite web |last=Chuang |first=F. |date=6 June 2012 |title=FAQ – Atmosphere |url=https://www.psi.edu/epo/faq/atmosphere.html |access-date=13 May 2022 |website=Planetary Science Institute |language=en |archive-date=23 March 2022 |archive-url=https://web.archive.org/web/20220323224518/https://www.psi.edu/epo/faq/atmosphere.html |url-status=live }}</ref>).

With the discovery and observation of [[planetary system]]s around stars other than the Sun, it is becoming possible to elaborate, revise or even replace this account. The level of [[metallicity]]—an astronomical term describing the abundance of [[chemical element]]s with an [[atomic number]] greater than 2 ([[helium]])—appears to determine the likelihood that a star will have planets.<ref>{{cite journal |bibcode=2005ApJ...622.1102F |doi=10.1086/428383 |title=The Planet-Metallicity Correlation |journal=The Astrophysical Journal |volume=622 |issue=2 |page=1102 |year=2005 |last1=Fischer |first1=Debra A. |last2=Valenti |first2=Jeff|doi-access=free }}</ref><ref>{{Cite journal |arxiv=1310.7830 |last1=Wang |first1=Ji |title=Revealing a Universal Planet-Metallicity Correlation for Planets of Different Sizes Around Solar-Type Stars |journal=The Astronomical Journal |volume=149 |issue=1 |page=14 |last2=Fischer |first2=Debra A. |year=2013 |doi=10.1088/0004-6256/149/1/14 |bibcode=2015AJ....149...14W|s2cid=118415186 }}</ref> Hence, a metal-rich [[population I star]] is more likely to have a substantial planetary system than a metal-poor, [[population II star]].<ref>{{Cite book |last1=Harrison |first1=Edward Robert |url=https://books.google.com/books?id=kNxeHD2cbLYC |title=Cosmology: The Science of the Universe |date=2000 |publisher=Cambridge University Press |isbn=978-0-521-66148-5 |page=114 |language=en |access-date=13 May 2022 |archive-date=14 December 2023 |archive-url=https://web.archive.org/web/20231214142630/https://books.google.com/books?id=kNxeHD2cbLYC |url-status=live }}</ref>

== Planets in the Solar System ==

{{Main|Solar System}}
According to the [[IAU definition of planet|IAU definition]], there are eight planets in the Solar System, which are (in increasing distance from the Sun):<ref name="IAU"/> Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Jupiter is the largest, at 318 [[Earth mass]]es, whereas Mercury is the smallest, at 0.055 Earth masses.<ref name="JPL-physical-parameters">{{cite web |title=Planetary Physical Parameters |url=https://ssd.jpl.nasa.gov/planets/phys_par.html |url-status=live |archive-url=https://web.archive.org/web/20221004115344/https://ssd.jpl.nasa.gov/planets/phys_par.html |archive-date=4 October 2022 |access-date=11 July 2022 |website=Solar System Dynamics |publisher=Jet Propulsion Laboratory}}</ref>

The planets of the Solar System can be divided into categories based on their composition. [[Terrestrial planet|Terrestrials]] are similar to Earth, with bodies largely composed of [[Rock (geology)|rock]] and metal: Mercury, Venus, Earth, and Mars. Earth is the largest terrestrial planet.<ref name=Lewis59>{{cite book |first=John S. |last=Lewis |date=2004 |title=Physics and Chemistry of the Solar System |page=59 |edition=2nd |publisher=Academic Press |isbn=978-0-12-446744-6}}</ref> [[Giant planet]]s are significantly more massive than the terrestrials: Jupiter, Saturn, Uranus, and Neptune.<ref name=Lewis59/> They differ from the terrestrial planets in composition. The [[gas giant]]s, Jupiter and Saturn, are primarily composed of [[hydrogen]] and helium and are the most massive planets in the Solar System. Saturn is one third as massive as Jupiter, at 95 Earth masses.<ref name=M19>{{cite web |last1=Marley |first1=Mark |title=Not a Heart of Ice |date=2 April 2019 |url=https://www.planetary.org/articles/not-a-heart-of-ice |website=planetary.org |publisher=The Planetary Society |access-date=5 May 2022 |language=en |archive-date=12 August 2019 |archive-url=https://web.archive.org/web/20190812095448/http://www.planetary.org/blogs/guest-blogs/2019/not-a-heart-of-ice.html |url-status=live }}</ref> The [[ice giant]]s, Uranus and Neptune, are primarily composed of low-boiling-point materials such as water, [[methane]], and [[ammonia]], with thick atmospheres of hydrogen and helium. They have a significantly lower mass than the gas giants (only 14 and 17 Earth masses).<ref name=M19/>
[[File:Solar System true color (captions).jpg|center|thumb|600x600px|The Sun's, planets', dwarf planets' and moons' size to scale, labelled. Distance of objects is not to scale. The asteroid belt lies between the orbits of Mars and Jupiter, the Kuiper belt lies beyond Neptune's orbit.]]
Dwarf planets are gravitationally rounded, but have not cleared their orbits of other [[Small Solar System body|bodies]]. In increasing order of average distance from the Sun, the ones generally agreed among astronomers are {{dp|Ceres}}, {{dp|Orcus}}, {{dp|Pluto}}, {{dp|Haumea}}, {{dp|Quaoar}}, {{dp|Makemake}}, {{dp|Gonggong}}, {{dp|Eris}}, and {{dp|Sedna}}.<ref name=Grundy2019/><ref name=JWST/> Ceres is the largest object in the [[asteroid belt]], located between the orbits of Mars and Jupiter. The other eight all orbit beyond Neptune. Orcus, Pluto, Haumea, Quaoar, and Makemake orbit in the [[Kuiper belt]], which is a second belt of small Solar System bodies beyond the orbit of Neptune. Gonggong and Eris orbit in the [[scattered disc]], which is somewhat further out and, unlike the Kuiper belt, is unstable towards interactions with Neptune. Sedna is the largest known [[detached object]], a population that never comes close enough to the Sun to interact with any of the classical planets; the origins of their orbits are still being debated. All nine are similar to terrestrial planets in having a solid surface, but they are made of ice and rock rather than rock and metal. Moreover, all of them are smaller than Mercury, with Pluto being the largest known dwarf planet and Eris being the most massive.<ref name="Brown Schaller 2007">{{cite journal| doi = 10.1126/science.1139415| last1 = Brown| first1 = Michael E.| author-link = Michael E. Brown| last2 = Schaller| first2 = Emily L.| s2cid = 21468196| date = 15 June 2007| title = The Mass of Dwarf Planet Eris| journal = Science| volume = 316| issue = 5831| page = 1585| pmid = 17569855| bibcode = 2007Sci...316.1585B| url = http://hubblesite.org/pubinfo/pdf/2007/24/pdf.pdf| access-date = 27 September 2015| archive-url = https://web.archive.org/web/20160304053122/http://hubblesite.org/pubinfo/pdf/2007/24/pdf.pdf| archive-date = 4 March 2016}}</ref><ref>{{cite web |url=https://www.nasa.gov/feature/how-big-is-pluto-new-horizons-settles-decades-long-debate |title=How Big Is Pluto? New Horizons Settles Decades-Long Debate |website=NASA |date=7 August 2017 |access-date=5 May 2022 |archive-date=9 November 2019 |archive-url=https://web.archive.org/web/20191109182908/https://www.nasa.gov/feature/how-big-is-pluto-new-horizons-settles-decades-long-debate/ }}</ref>

There are at least nineteen [[planetary-mass moon]]s or satellite planets—moons large enough to take on ellipsoidal shapes:<ref name=planetarysociety/>
* One satellite of Earth: the [[Moon]]
* Four [[Moons of Jupiter|satellites of Jupiter]]: [[Io (moon)|Io]], [[Europa (moon)|Europa]], [[Ganymede (moon)|Ganymede]], and [[Callisto (moon)|Callisto]]
* Seven [[Moons of Saturn|satellites of Saturn]]: [[Mimas (moon)|Mimas]], [[Enceladus]], [[Tethys (moon)|Tethys]], [[Dione (moon)|Dione]], [[Rhea (moon)|Rhea]], [[Titan (moon)|Titan]], and [[Iapetus (moon)|Iapetus]]
* Five [[Moons of Uranus|satellites of Uranus]]: [[Miranda (moon)|Miranda]], [[Ariel (moon)|Ariel]], [[Umbriel]], [[Titania (moon)|Titania]], and [[Oberon (moon)|Oberon]]
* One [[Moons of Neptune|satellite of Neptune]]: [[Triton (moon)|Triton]]
* One [[Moons of Pluto|satellite of Pluto]]: [[Charon (moon)|Charon]]

The Moon, Io, and Europa have compositions similar to the terrestrial planets; the others are made of ice and rock like the dwarf planets, with [[Tethys (moon)|Tethys]] being made of almost pure ice. Europa is often considered an icy planet, though, because its surface ice layer makes it difficult to study its interior.<ref name=planetarysociety/><ref>{{cite book |first=John S. |last=Lewis |date=2004 |title=Physics and Chemistry of the Solar System |page=425 |edition=2nd |publisher=Academic Press |isbn=978-0-12-446744-6}}</ref> Ganymede and Titan are larger than Mercury by radius, and Callisto almost equals it, but all three are much less massive. Mimas is the smallest object generally agreed to be a [[Geophysical definition of planet|geophysical planet]], at about six&nbsp;millionths of Earth's mass, though there are many larger bodies that may not be geophysical planets (e.g. {{dp|Salacia}}).<ref name=Grundy2019/>

== Exoplanets ==
{{Main|Exoplanet}}
[[File:Exoplanet detections per year.png|alt=Exoplanet detections per year |thumb|upright=1.2|Exoplanet detections per year as of August 2023 (by [[NASA Exoplanet Archive]])<ref>{{cite web |title=Pre-generated Exoplanet Plots |url=https://exoplanetarchive.ipac.caltech.edu/exoplanetplots/ |website=exoplanetarchive.ipac.caltech.edu |publisher=[[NASA Exoplanet Archive]] |access-date=24 June 2022 |archive-date=30 April 2012 |archive-url=https://web.archive.org/web/20120430080407/https://exoplanetarchive.ipac.caltech.edu/exoplanetplots/ |url-status=live }}</ref>]]
An [[exoplanet]] is a planet outside the Solar System. {{Extrasolar planet counts|full}} Known exoplanets range in size from gas giants [[WASP-17b|about twice as large as Jupiter]] down to [[Kepler-37b|just over the size of the Moon]]. Analysis of [[gravitational microlensing]] data suggests a minimum average of 1.6 bound planets for every star in the [[Milky Way]].<ref name="nature.com">{{cite journal |display-authors=4 |last1=Cassan |first1=Arnaud | first2=D. | last2=Kubas | first3=J.-P. | last3=Beaulieu | first4=M. | last4=Dominik | first5=K. | last5=Horne | first6=J. | last6=Greenhill | first7=J. | last7=Wambsganss | first8=J. | last8=Menzies | first9=A. | last9=Williams | first10=U. G. | last10=Jørgensen | first11=A. | last11=Udalski | first12=D. P. | last12=Bennett | first13=M. D. | last13=Albrow | first14=V. | last14=Batista | first15=S. | last15=Brillant | first16=J. A. R. | last16=Caldwell | first17= A. | last17=Cole | first18=Ch. | last18=Coutures | first19=K. H. | last19=Cook | first20=S. | last20=Dieters | first21=D. | last21=Dominis Prester | first22=J. | last22=Donatowicz | first23=P. | last23=Fouqué | first24=K. | last24=Hill | first25=N. | last25=Kains |title=One or more bound planets per Milky Way star from microlensing observations |journal=Nature |date=12 January 2012 |volume=481 |pages=167–169 |doi=10.1038/nature10684 |issue=7380 |bibcode=2012Natur.481..167C |pmid=22237108 |arxiv=1202.0903|s2cid=2614136 }}</ref>

In early 1992, radio astronomers [[Aleksander Wolszczan]] and [[Dale Frail]] announced the discovery of two planets orbiting the [[pulsar]] [[PSR 1257+12]].<ref name="Wolszczan">{{Cite journal | last1 = Wolszczan | first1 = A. |bibcode=1992Natur.355..145W| last2 = Frail | first2 = D. A. | doi = 10.1038/355145a0 | title = A planetary system around the millisecond pulsar PSR1257 + 12 | journal = Nature | volume = 355 | issue = 6356 | pages = 145–147 | year = 1992 | s2cid = 4260368 }}</ref> This discovery was confirmed and is generally considered to be the first definitive detection of exoplanets. Researchers suspect they formed from a disk remnant left over from the [[supernova]] that produced the pulsar.<ref>{{Cite journal |title=Planets Around the Pulsar PSR B1257+12 |url=https://adsabs.harvard.edu/full/2008ASPC..398....3W |access-date=13 May 2022 |journal=Extreme Solar Systems |bibcode=2008ASPC..398....3W |last1=Wolszczan |first1=Alex |year=2008 |volume=398 |pages=3+ |archive-date=13 May 2022 |archive-url=https://web.archive.org/web/20220513200823/https://adsabs.harvard.edu/full/2008ASPC..398....3W |url-status=live }}</ref>

The first confirmed discovery of an exoplanet orbiting an ordinary [[main sequence|main-sequence]] star occurred on 6 October 1995, when [[Michel Mayor]] and [[Didier Queloz]] of the [[University of Geneva]] announced the detection of [[51 Pegasi b]], an exoplanet around [[51 Pegasi]].<ref name=whatworlds>{{cite web |url=http://www.cbc.ca/news/technology/what-worlds-are-out-there-a-field-guide-to-exoplanets-1.3734359 |title=What worlds are out there? |work=[[Canadian Broadcasting Corporation]] |language=en |date=25 August 2016 |access-date=5 June 2017 |archive-date=25 August 2016 |archive-url=https://web.archive.org/web/20160825161047/http://www.cbc.ca/news/technology/what-worlds-are-out-there-a-field-guide-to-exoplanets-1.3734359 |url-status=live }}</ref> From then until the [[Kepler space telescope]] mission, most of the known exoplanets were gas giants comparable in mass to Jupiter or larger as they were more easily detected. The [[List of exoplanets discovered using the Kepler space telescope|catalog of Kepler candidate planets]] consists mostly of planets the size of Neptune and smaller, down to smaller than Mercury.<ref>{{Cite web |last=Chen |first=Rick |date=23 October 2018 |title=Top Science Results from the Kepler Mission |url=http://www.nasa.gov/kepler/topscience |access-date=11 July 2022 |website=NASA |quote=The most common size of planet Kepler found doesn't exist in our solar system—a world between the size of Earth and Neptune—and we have much to learn about these planets. |archive-date=11 July 2022 |archive-url=https://web.archive.org/web/20220711224819/https://www.nasa.gov/kepler/topscience/ }}</ref><ref name="Barclay-2013">{{Cite journal |last1=Barclay |first1=Thomas |last2=Rowe |first2=Jason F. |last3=Lissauer |first3=Jack J. |last4=Huber |first4=Daniel |last5=Fressin |first5=François |last6=Howell |first6=Steve B. |last7=Bryson |first7=Stephen T. |last8=Chaplin |first8=William J. |last9=Désert |first9=Jean-Michel |last10=Lopez |first10=Eric D. |last11=Marcy |first11=Geoffrey W. |display-authors=4 |date=28 February 2013 |title=A sub-Mercury-sized exoplanet |url=http://www.nature.com/articles/nature11914 |journal=Nature |language=en |volume=494 |issue=7438 |pages=452–454 |doi=10.1038/nature11914 |pmid=23426260 |arxiv=1305.5587 |bibcode=2013Natur.494..452B |s2cid=205232792 |issn=0028-0836 |access-date=11 July 2022 |archive-date=19 October 2022 |archive-url=https://web.archive.org/web/20221019020640/https://www.nature.com/articles/nature11914 |url-status=live }}</ref>

In 2011, the [[Kepler space telescope]] team reported the discovery of the first Earth-sized exoplanets orbiting a [[Solar analog|Sun-like star]], [[Kepler-20e]] and [[Kepler-20f]].<ref name="NASA-20111220">{{cite web|last=Johnson|first=Michele|title=NASA Discovers First Earth-size Planets Beyond Our Solar System|url=http://www.nasa.gov/mission_pages/kepler/news/kepler-20-system.html|publisher=[[NASA]]|date=20 December 2011|access-date=20 December 2011|archive-date=16 May 2020|archive-url=https://web.archive.org/web/20200516230313/https://www.nasa.gov/mission_pages/kepler/news/kepler-20-system.html}}</ref><ref name="Nature-20111220">{{cite journal |last=Hand |first=Eric |title=Kepler discovers first Earth-sized exoplanets |doi=10.1038/nature.2011.9688 |date=20 December 2011 |journal=[[Nature (journal)|Nature]]|s2cid=122575277 }}</ref><ref name="NYT-20111220">{{cite news |last=Overbye |first=Dennis |title=Two Earth-Size Planets Are Discovered |url=https://www.nytimes.com/2011/12/21/science/space/nasas-kepler-spacecraft-discovers-2-earth-size-planets.html |date=20 December 2011 |newspaper=The New York Times |access-date=21 December 2011 |archive-date=20 December 2011 |archive-url=https://web.archive.org/web/20111220234625/http://www.nytimes.com/2011/12/21/science/space/nasas-kepler-spacecraft-discovers-2-earth-size-planets.html |url-status=live }}</ref> Since that time, more than 100 planets have been identified that are approximately the [[Earth radius|same size as Earth]], 20 of which orbit in the [[circumstellar habitable zone|habitable zone]] of their star—the range of orbits where a terrestrial planet could sustain liquid water on its surface, given enough atmospheric pressure.<ref name=kopparapu-2013>{{cite journal |title=A revised estimate of the occurrence rate of terrestrial planets in the habitable zones around kepler m-dwarfs |author=Kopparapu, Ravi Kumar |journal=The Astrophysical Journal Letters |date=2013 |volume=767 |issue=1 |doi=10.1088/2041-8205/767/1/L8 |arxiv=1303.2649 |page=L8|bibcode = 2013ApJ...767L...8K|s2cid=119103101 }}</ref><ref>{{cite web|first=Traci|last=Watson|url=https://www.usatoday.com/story/news/2016/05/10/kepler-finds-new-planets/84187098/|title=NASA discovery doubles the number of known planets|date=10 May 2016|work=[[USA Today]]|access-date=10 May 2016|archive-date=10 May 2016|archive-url=https://web.archive.org/web/20160510184735/http://www.usatoday.com/story/news/2016/05/10/kepler-finds-new-planets/84187098/|url-status=live}}</ref><ref>{{Cite web |url=http://phl.upr.edu/projects/habitable-exoplanets-catalog |title=The Habitable Exoplanets Catalog |website=Planetary Habitability Laboratory |publisher=University of Puerto Rico at Arecibo |access-date=12 July 2022 |archive-date=20 October 2011 |archive-url=https://web.archive.org/web/20111020180543/http://phl.upr.edu/projects/habitable-exoplanets-catalog |url-status=live }}</ref> One in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone, which suggests that the nearest would be expected to be within 12&nbsp;[[light-year]]s distance from Earth.{{efn|name=1in5earthsized|Here, "Earth-sized" means 1–2 Earth radii, and "habitable zone" means the region with 0.25 to 4 times Earth's stellar flux (corresponding to 0.5–2 AU for the Sun). Data for [[G-type star]]s like the Sun is not available. This statistic is an extrapolation from data on [[K-type star]]s.<ref name="ucb1in5">{{cite web|last=Sanders|first=R.|date=4 November 2013|title=Astronomers answer key question: How common are habitable planets?|url=http://newscenter.berkeley.edu/2013/11/04/astronomers-answer-key-question-how-common-are-habitable-planets/|work=newscenter.berkeley.edu|access-date=7 November 2013|archive-url=https://web.archive.org/web/20141107081158/http://newscenter.berkeley.edu/2013/11/04/astronomers-answer-key-question-how-common-are-habitable-planets/|archive-date=7 November 2014}}</ref><ref name="earthsunhz">{{cite journal|last1=Petigura |first1=E. A.|last2=Howard |first2=A. W.|last3=Marcy |first3=G. W.|date=2013|title=Prevalence of Earth-size planets orbiting Sun-like stars|journal=[[Proceedings of the National Academy of Sciences]]|volume= 110|issue= 48|pages=19273–19278|arxiv= 1311.6806|bibcode= 2013PNAS..11019273P|doi=10.1073/pnas.1319909110|pmid=24191033|pmc=3845182|doi-access=free}}</ref>}} The frequency of occurrence of such terrestrial planets is one of the variables in the [[Drake equation]], which estimates the number of [[Extraterrestrial life|intelligent, communicating civilizations]] that exist in the Milky Way.<ref>{{cite news | last=Drake |first=Frank |title=The Drake Equation Revisited |publisher=Astrobiology Magazine |date=29 September 2003 |url=http://www.astrobio.net/index.php?option=com_retrospection&task=detail&id=610 |archive-url=https://web.archive.org/web/20110628180502/http://www.astrobio.net/index.php?option=com_retrospection&task=detail&id=610 |archive-date=28 June 2011 |access-date=23 August 2008 |url-status=usurped}}</ref>

There are types of planets that do not exist in the Solar System: [[super-Earth]]s and [[mini-Neptune]]s, which have masses between that of Earth and Neptune. Objects less than about twice the mass of Earth are expected to be rocky like Earth; beyond that, they become a mixture of volatiles and gas like Neptune.<ref name="ChenKipping">{{cite journal |last1=Chen |first1=Jingjing |last2=Kipping |first2=David |date=2016 |title=Probabilistic Forecasting of the Masses and Radii of Other Worlds |journal=The Astrophysical Journal |volume=834 |issue=1 |page=17 |arxiv=1603.08614 |doi=10.3847/1538-4357/834/1/17 |s2cid=119114880 |doi-access=free |bibcode=2017ApJ...834...17C }}</ref> The planet [[Gliese 581c]], with a mass 5.5–10.4 times the mass of Earth,<ref>{{cite journal |doi=10.1051/0004-6361/200912172 |bibcode=2009A&A...507..487M |journal=[[Astronomy and Astrophysics]] |volume=507 |issue=1 |year=2009 |pages=487–494 |last1=Mayor |first1=Michel |last2=Bonfils |first2=Xavier |last3=Forveille |first3=Thierry |last4=Delfosse |first4=Xavier |last5=Udry |first5=Stéphane |last6=Bertaux |first6=Jean-Loup |last7=Beust |first7=Hervé |last8=Bouchy |first8=François |last9=Lovis |first9=Christophe |last10=Pepe |first10=Francesco |last11=Perrier |first11=Christian |last12=Queloz |first12=Didier |last13=Santos |first13=Nuno C. |title=The HARPS search for southern extra-solar planets, XVIII. An Earth-mass planet in the GJ 581 planetary system |arxiv=0906.2780 |display-authors=3 |url=http://obswww.unige.ch/~udry/Gl581_preprint.pdf |s2cid=2983930 |archive-url=https://web.archive.org/web/20090521052641/http://obswww.unige.ch/~udry/Gl581_preprint.pdf |archive-date=21 May 2009 }}</ref> attracted attention upon its discovery for potentially being in the habitable zone,<ref name="BBC1">{{cite news |url=https://news.bbc.co.uk/2/hi/science/nature/6589157.stm |title=New 'super-Earth' found in space |access-date=25 April 2007 |date=25 April 2007 |work=BBC News |archive-date=10 November 2012 |archive-url=https://web.archive.org/web/20121110172300/http://news.bbc.co.uk/2/hi/science/nature/6589157.stm |url-status=live }}</ref> though later studies concluded that it is actually too close to its star to be habitable.<ref name="blo07">{{cite journal|bibcode=2007A&A...476.1365V|author=von Bloh|display-authors=etal|date=2007|title=The Habitability of Super-Earths in Gliese 581 |journal=[[Astronomy and Astrophysics]]|volume=476|issue=3|pages=1365–1371|doi=10.1051/0004-6361:20077939 |arxiv = 0705.3758 |s2cid=14475537}}</ref> Planets more massive than Jupiter are also known, extending seamlessly into the realm of brown dwarfs.<ref name=Hatzes/>

Exoplanets have been found that are much closer to their parent star than any planet in the Solar System is to the Sun. Mercury, the closest planet to the Sun at 0.4&nbsp;[[astronomical unit|AU]], takes 88&nbsp;days for an orbit, but [[ultra-short period planet]]s can orbit in less than a day. The [[Kepler-11]] system has five of its planets in shorter orbits than Mercury's, all of them much more massive than Mercury. There are [[hot Jupiter]]s, such as 51 Pegasi b,<ref name=whatworlds/> that orbit very close to their star and may evaporate to become [[chthonian planet]]s, which are the leftover cores. There are also exoplanets that are much farther from their star. Neptune is 30&nbsp;AU from the Sun and takes 165&nbsp;years to orbit, but there are exoplanets that are thousands of AU from their star and take more than a million&nbsp;years to orbit (e.g. [[COCONUTS-2b]]).<ref>{{Cite journal |last1=Zhang |first1=Zhoujian |last2=Liu |first2=Michael C. |last3=Claytor |first3=Zachary R. |last4=Best |first4=William M. J. |last5=Dupuy |first5=Trent J. |last6=Siverd |first6=Robert J. | display-authors=4 |date=1 August 2021 |title=The Second Discovery from the COCONUTS Program: A Cold Wide-orbit Exoplanet around a Young Field M Dwarf at 10.9 pc |journal=The Astrophysical Journal Letters |volume=916 |issue=2 |page=L11 |doi=10.3847/2041-8213/ac1123 |arxiv=2107.02805 |bibcode=2021ApJ...916L..11Z |hdl=20.500.11820/4f26e8e5-5d42-4259-bc20-fcb093d664b6 |s2cid=236464073 |issn=2041-8205 |doi-access=free }}</ref>

{{clear}}

== Attributes ==<!-- This section is linked from [[Earth radius]] -->
Although each planet has unique physical characteristics, a number of broad commonalities do exist among them. Some of these characteristics, such as [[ring system|rings]] or natural satellites, have only as yet been observed in planets in the Solar System, whereas others are commonly observed in exoplanets.<ref>{{Cite web |title=Extrasolar Planets |url=https://lasp.colorado.edu/outerplanets/exoplanets.php |access-date=13 May 2022 |website=lasp.colorado.edu |archive-date=5 April 2019 |archive-url=https://web.archive.org/web/20190405075609/https://lasp.colorado.edu/outerplanets/exoplanets.php |url-status=live }}</ref>

=== Dynamic characteristics ===

==== Orbit ====
{{Main|Orbit|orbital elements}}
{{see also|Kepler's laws of planetary motion|Exoplanetology#Orbital parameters}}
[[File:TheKuiperBelt Orbits Pluto Ecliptic.svg|thumb|right|upright=1.35|The orbit of the planet Neptune compared to that of [[Pluto]]. Note the elongation of Pluto's orbit in relation to Neptune's ([[orbital eccentricity|eccentricity]]), as well as its large angle to the ecliptic ([[inclination]]).]]

In the Solar System, all the planets orbit the Sun in the same direction as the [[Solar rotation|Sun rotates]]: [[counter-clockwise]] as seen from above the Sun's north pole. At least one exoplanet, [[WASP-17b]], has been found to orbit in the opposite direction to its star's rotation.<ref>{{cite journal | first1 = D. R. |last1=Anderson | title = WASP-17b: an ultra-low density planet in a probable retrograde orbit | arxiv = 0908.1553 | date = 2009 | last2 = Hellier | first2 = C. | last3 = Gillon | first3 = M. | last4 = Triaud | first4 = A. H. M. J. | last5 = Smalley | first5 = B. | last6 = Hebb | first6 = L. | last7 = Collier Cameron | first7 = A. | last8 = Maxted | first8 = P. F. L. | last9 = Queloz | first9 = D.| last10 =  West | first10 = R. G. | last11 =  Bentley | first11 = S. J. | last12 =  Enoch | first12 = B. | last13 =  Horne | first13 = K. | last14 =  Lister | first14 = T. A. | last15 =  Mayor | first15 = M. | last16 =  Parley | first16 = N. R. | last17 =  Pepe | first17 = F. | last18 =  Pollacco | first18 = D. | last19 =  Ségransan | first19 = D. | last20 =  Udry | first20 = S. | last21 =  Wilson | first21 = D. M. | display-authors= 4| doi=10.1088/0004-637X/709/1/159 | volume=709 | issue = 1 | journal=The Astrophysical Journal | pages=159–167 | bibcode=2010ApJ...709..159A| s2cid = 53628741 }}</ref> The period of one revolution of a planet's orbit is known as its [[sidereal period]] or ''year''.<ref name="young">{{cite book | first=Charles Augustus |last=Young |date=1902 |title=Manual of Astronomy: A Text Book | url=https://archive.org/details/manualastronomy05youngoog |publisher=Ginn & company |pages=[https://archive.org/details/manualastronomy05youngoog/page/n342 324]–327}}</ref> A planet's year depends on its distance from its star; the farther a planet is from its star, the longer the distance it must travel and the slower its speed, since it is less affected by its star's [[gravity]].{{citation needed|date=April 2025}}

No planet's orbit is perfectly circular, and hence the distance of each from the host star varies over the course of its year. The closest approach to its star is called its [[periastron]], or [[perihelion]] in the Solar System, whereas its farthest separation from the star is called its [[apastron]] ([[aphelion]]). As a planet approaches periastron, its speed increases as it trades [[gravitational energy|gravitational potential energy]] for [[kinetic energy]], just as a falling object on Earth accelerates as it falls. As the planet nears apastron, its speed decreases, just as an object thrown upwards on Earth slows down as it reaches the apex of its [[trajectory]].<ref>{{cite book | last1=Dvorak |first1=R. | last2=Kurths |first2= J. | last3=Freistetter |first3= F. |date=2005 |title=Chaos And Stability in Planetary Systems |publisher=Springer |location=New York |isbn=978-3-540-28208-2 |page=90}}</ref>

Each planet's orbit is delineated by a set of elements:
* The ''[[Orbital eccentricity|eccentricity]]'' of an orbit describes the elongation of a planet's elliptical (oval) orbit. Planets with low eccentricities have more circular orbits, whereas planets with high eccentricities have more elliptical orbits. The planets and large moons in the Solar System have relatively low eccentricities, and thus nearly circular orbits.<ref name="young"/> The comets and many Kuiper belt objects, as well as several exoplanets, have very high eccentricities, and thus exceedingly elliptical orbits.<ref>{{cite journal |title=Eccentricity evolution of giant planet orbits due to circumstellar disk torques |last1=Moorhead |first1=Althea V. |last2=Adams |first2= Fred C. |journal=Icarus |date=2008 |volume=193 |issue=2 |pages=475–484 |doi=10.1016/j.icarus.2007.07.009 |arxiv=0708.0335 |bibcode=2008Icar..193..475M|s2cid=16457143 }}</ref><ref>{{cite web |title=Planets – Kuiper Belt Objects |work=The Astrophysics Spectator |date=15 December 2004 | url=http://www.astrophysicsspectator.com/topics/planets/KuiperBelt.html |access-date=23 August 2008 | archive-url=https://web.archive.org/web/20210323161115/https://www.astrophysicsspectator.com/topics/planets/KuiperBelt.html |archive-date=23 March 2021}}</ref>
* The ''[[semi-major axis]]'' gives the size of the orbit. It is the distance from the midpoint to the longest diameter of its elliptical orbit. This distance is not the same as its apastron, because no planet's orbit has its star at its exact centre.<ref name="young" />
* The ''[[inclination]]'' of a planet tells how far above or below an established reference plane its orbit is tilted. In the Solar System, the reference plane is the plane of Earth's orbit, called the [[ecliptic]]. For exoplanets, the plane, known as the ''sky plane'' or ''plane of the sky'', is the plane perpendicular to the observer's line of sight from Earth.<ref>{{cite book |chapter-url=http://astrowww.phys.uvic.ca/~tatum/celmechs.html |title=Celestial Mechanics |date=2007 |chapter=17. Visual binary stars |first=J. B. |last=Tatum |access-date=2 February 2008 |publisher=Personal web page |archive-date=6 July 2007 |archive-url=https://web.archive.org/web/20070706031613/http://astrowww.phys.uvic.ca/~tatum/celmechs.html |url-status=live }}</ref> The orbits of the eight major planets of the Solar System all lie very close to the ecliptic; however, some smaller objects like Pallas, Pluto, and Eris orbit at far more extreme angles to it, as do comets.<ref>{{cite journal |title=A Correlation between Inclination and Color in the Classical Kuiper Belt | last1=Trujillo |first1=Chadwick A. | last2=Brown | first2=Michael E. |journal=Astrophysical Journal |date=2002 |bibcode=2002ApJ...566L.125T | volume=566 |issue=2 | page=L125 |doi=10.1086/339437|arxiv = astro-ph/0201040 | s2cid=11519263 }}</ref> The large moons are generally not very inclined to their parent planets' [[equator]]s, but Earth's Moon, Saturn's Iapetus, and Neptune's Triton are exceptions. Triton is unique among the large moons in that it orbits [[Retrograde and prograde motion|retrograde]], i.e. in the direction opposite to its parent planet's rotation.<ref>{{cite journal|last1=Peter Goldreich|title=History of the Lunar Orbit|journal=[[Reviews of Geophysics]]|volume=4|issue=4|pages=411–439|date=Nov 1966|doi=10.1029/RG004i004p00411|bibcode=1966RvGSP...4..411G}}</ref>
* The points at which a planet crosses above and below its reference plane are called its [[ascending node|ascending]] and [[descending node]]s.<ref name="young" /> The [[longitude of the ascending node]] is the angle between the reference plane's 0 longitude and the planet's ascending node. The [[argument of periapsis]] (or perihelion in the Solar System) is the angle between a planet's ascending node and its closest approach to its star.<ref name="young" />

==== Axial tilt ====
{{Main|Axial tilt}}
[[File:AxialTiltObliquity.png|thumb|Earth's [[axial tilt]] is about 23.4°. It oscillates between 22.1° and 24.5° on a 41,000-year cycle and is currently decreasing.]]

Planets have varying degrees of axial tilt; they spin at an angle to the [[reference plane|plane]] of their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when the [[Northern Hemisphere]] points away from its star, the [[Southern Hemisphere]] points towards it, and vice versa. Each planet therefore has [[season]]s, resulting in changes to the [[climate]] over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as its [[solstice]]. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice with its day being the longest, the other has its winter solstice when its day is shortest. The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of the planet. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either continually in sunlight or continually in darkness around the time of [[Climate of Uranus|its solstices]].<ref name="Weather">{{cite web | last=Harvey |first=Samantha |date=1 May 2006 |url=http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=725 |archive-url=https://web.archive.org/web/20060831201346/http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=725 |archive-date=31 August 2006 |title=Weather, Weather, Everywhere? |publisher=NASA |access-date=23 August 2008}}</ref> In the Solar System, Mercury, Venus, Ceres, and Jupiter have very small tilts; Pallas, Uranus, and Pluto have extreme ones; and Earth, Mars, Vesta, Saturn, and Neptune have moderate ones.<ref name="factsheets">[https://web.archive.org/web/20160304052405/http://nssdc.gsfc.nasa.gov/planetary/planetfact.html Planetary Fact Sheets], NASA</ref><ref name="Schorghofer2016">{{Cite journal |last1=Schorghofer |first1=N. |last2=Mazarico |first2=E. |last3=Platz |first3=T. |last4=Preusker |first4=F. |last5=Schröder |first5=S. E. |last6=Raymond |first6=C. A. |last7=Russell |first7=C. T. |date=6 July 2016 |title=The permanently shadowed regions of dwarf planet Ceres |journal=Geophysical Research Letters |volume=43 |issue=13 |pages=6783–6789 |bibcode=2016GeoRL..43.6783S |doi=10.1002/2016GL069368 |doi-access=free}}</ref><ref name=Carry2009>{{Cite journal|title=Physical properties of (2) Pallas|author=Carry, B.|date=2009|doi=10.1016/j.icarus.2009.08.007 |arxiv=0912.3626|display-authors=etal|bibcode = 2010Icar..205..460C|volume=205|issue=2|journal=Icarus|pages=460–472|s2cid=119194526}}</ref><ref name="Thomas1997b">{{cite journal | title=Vesta: Spin Pole, Size, and Shape from HST Images | date=1997 | author=Thomas, P. C. | bibcode=1997Icar..128...88T | display-authors=etal | journal=Icarus | volume=128 | issue=1 | pages=88–94 | doi=10.1006/icar.1997.5736| doi-access=free }}</ref> Among exoplanets, axial tilts are not known for certain, though most hot Jupiters are believed to have a negligible axial tilt as a result of their proximity to their stars.<ref>{{cite journal |title=Obliquity Tides on Hot Jupiters | last1=Winn | first1=Joshua N. | last2=Holman | first2=Matthew J. |journal=The Astrophysical Journal |date=2005 | doi=10.1086/432834 | volume=628 |issue=2 |page=L159 |bibcode=2005ApJ...628L.159W|arxiv = astro-ph/0506468 |s2cid=7051928 }}</ref> Similarly, the axial tilts of the planetary-mass moons are near zero,<ref>{{cite book |title=Explanatory Supplement to the Astronomical Almanac |editor-first=P. Kenneth |editor-last=Seidelmann |publisher=University Science Books |date=1992 |page=384 }}</ref> with Earth's Moon at 6.687° as the biggest exception;<ref name="Lang2011">{{cite book |last=Lang |first=Kenneth R. |url=https://books.google.com/books?id=S4xDhVCxAQIC&pg=PA184 |title=The Cambridge Guide to the Solar System |publisher=Cambridge University Press |year=2011 |isbn=978-1-139-49417-5 |edition=2nd |archive-url=https://web.archive.org/web/20160101071141/https://books.google.com/books?id=S4xDhVCxAQIC&pg=PA184 |archive-date=1 January 2016}}</ref> additionally, Callisto's axial tilt varies between 0 and about 2 degrees on timescales of thousands of years.<ref name=galileantilt>{{cite journal |first=Bruce G. |last=Bills |title=Free and forced obliquities of the Galilean satellites of Jupiter |date=2005 |volume=175 |issue=1 |pages=233–247 |doi=10.1016/j.icarus.2004.10.028 |bibcode=2005Icar..175..233B |journal=Icarus |url=https://zenodo.org/record/1259023 |access-date=6 April 2023 |archive-date=27 July 2020 |archive-url=https://web.archive.org/web/20200727063125/https://zenodo.org/record/1259023 |url-status=live }}</ref>

==== Rotation ====
{{See also|Exoplanetology#Rotation and axial tilt}}
The planets rotate around invisible axes through their centres. A planet's [[rotation period]] is known as a [[day|stellar day]]. Most of the planets in the Solar System rotate in the same direction as they orbit the Sun, which is counter-clockwise as seen from above the Sun's [[Poles of astronomical bodies#Geographic poles|north pole]]. The exceptions are Venus<ref>{{cite journal |title=Rotation of Venus: Period Estimated from Radar Measurements | last1=Goldstein | first1=R. M. | last2=Carpenter | first2=R. L. |date=1963 |journal =Science |volume=139 |doi=10.1126/science.139.3558.910 |pmid=17743054 |issue=3558 |bibcode=1963Sci...139..910G |pages=910–911|s2cid=21133097 }}</ref> and Uranus,<ref name="Belton-1984">{{cite conference |title=Rotational properties of Uranus and Neptune |first1=M. J. S. |last1=Belton | last2=Terrile | first2=R. J. |date=1984 |conference=Voyager "Uranus-Neptune" Workshop Pasadena 6–8 February 1984 |pages=327–347|bibcode=1984NASCP2330..327B |editor=Bergstralh, J. T.}}</ref> which rotate clockwise, though Uranus's extreme axial tilt means there are differing conventions on which of its poles is "north", and therefore whether it is rotating clockwise or anti-clockwise.<ref>{{cite book |title=The Outer Worlds; Uranus, Neptune, Pluto, and Beyond |pages=195–206 |date=2006 |first=Michael P. |last=Borgia |publisher=Springer New York}}</ref> Regardless of which convention is used, Uranus has a [[retrograde rotation]] relative to its orbit.<ref name="Belton-1984" />

{{solar_system_bodies_rotation_animation.svg|upright}}

The rotation of a planet can be induced by several factors during formation. A net [[angular momentum]] can be induced by the individual angular momentum contributions of accreted objects. The accretion of gas by the giant planets contributes to the angular momentum. Finally, during the last stages of planet building, a [[stochastic process]] of protoplanetary accretion can randomly alter the spin axis of the planet.<ref name="araa31">{{cite journal | title=Planet formation |last=Lissauer |first=Jack J. |journal=Annual Review of Astronomy and Astrophysics |volume=31 |pages=129–174 |date=September 1993 |doi=10.1146/annurev.aa.31.090193.001021 |bibcode=1993ARA&A..31..129L}}</ref> There is great variation in the length of day between the planets, with Venus taking 243&nbsp;[[Julian day|days]] to rotate, and the giant planets only a few hours.<ref name="planetcompare">{{cite web |title=Planet Compare |url=https://solarsystem.nasa.gov/planet-compare/ |url-status=live |archive-url=https://web.archive.org/web/20180309204400/https://solarsystem.nasa.gov/planet-compare/ |archive-date=9 March 2018 |access-date=12 July 2022 |website=Solar System Exploration |publisher=NASA}}</ref> The rotational periods of exoplanets are not known, but for [[hot Jupiter]]s, their proximity to their stars means that they are [[Tidal locking|tidally locked]] (that is, their orbits are in sync with their rotations). This means, they always show one face to their stars, with one side in perpetual day, the other in perpetual night.<ref>{{cite journal |title=Magnetically-Driven Planetary Radio Emissions and Application to Extrasolar Planets | last1=Zarka |first1=Philippe | last2=Treumann | first2=Rudolf A. | last3=Ryabov | first3=Boris P. | last4=Ryabov | first4=Vladimir B. |date=2001 |journal=Astrophysics and Space Science |volume=277 |issue=1/2 |pages=293–300 |doi = 10.1023/A:1012221527425|bibcode = 2001Ap&SS.277..293Z | s2cid=16842429 }}</ref> Mercury and Venus, the closest planets to the Sun, similarly exhibit very slow rotation: Mercury is tidally locked into a 3:2 spin–orbit resonance (rotating three times for every two revolutions around the Sun),<ref>{{cite journal |last1=Liu |first1=Han-Shou |last2=O'Keefe |first2=John A. |title=Theory of Rotation for the Planet Mercury |journal=Science |year=1965 |volume=150 |issue=3704 |page=1717 |doi=10.1126/science.150.3704.1717 |pmid=17768871 |bibcode=1965Sci...150.1717L|s2cid=45608770 }}</ref> and Venus's rotation may be in equilibrium between [[tidal force]]s slowing it down and [[atmospheric tide]]s created by solar heating speeding it up.<ref>{{cite journal |last1=Correia |first1=Alexandre C. M. |last2=Laskar |first2=Jacques |last3=De Surgy |first3=Olivier Néron |title=Long-Term Evolution of the Spin of Venus, Part I: Theory |journal=Icarus |volume=163 |issue=1 |pages=1–23 |date=May 2003 |url=http://www.imcce.fr/Equipes/ASD/preprints/prep.2002/venus1.2002.pdf |doi=10.1016/S0019-1035(03)00042-3 |bibcode=2003Icar..163....1C |access-date=9 September 2006 |archive-date=27 September 2019 |archive-url=https://web.archive.org/web/20190927122047/https://www.imcce.fr/Equipes/ASD/preprints/prep.2002/venus1.2002.pdf |url-status=live }}</ref><ref>{{cite journal |last1=Laskar |first1=Jacques |last2=De Surgy |first2=Olivier Néron |title=Long-Term Evolution of the Spin of Venus, Part II: Numerical Simulations |journal=Icarus |volume=163 |issue=1 |pages=24–45 |url=http://www.imcce.fr/Equipes/ASD/preprints/prep.2002/venus2.2002.pdf |doi=10.1016/S0019-1035(03)00043-5 |bibcode=2003Icar..163...24C |year=2003 |access-date=9 September 2006 |archive-date=2 May 2019 |archive-url=https://web.archive.org/web/20190502225637/https://www.imcce.fr/Equipes/ASD/preprints/prep.2002/venus2.2002.pdf |url-status=live }}</ref>

All the large moons are tidally locked to their parent planets;<ref>{{cite book|last1=Schutz|first1=Bernard|title=Gravity from the Ground Up|publisher=Cambridge University Press|isbn=978-0-521-45506-0|page=43|url=https://books.google.com/books?id=P_T0xxhDcsIC&pg=PA43|access-date=24 April 2017|date=2003|archive-date=6 August 2023|archive-url=https://web.archive.org/web/20230806164032/https://books.google.com/books?id=P_T0xxhDcsIC&pg=PA43|url-status=live}}</ref> Pluto and Charon are tidally locked to each other,<ref name="Young1997">{{cite web
| title = The Once and Future Pluto
| first = Leslie A.
| last = Young
| work = Southwest Research Institute, Boulder, Colorado
| url = http://www.boulder.swri.edu/~layoung/projects/talks03/IfA-jan03v1.ppt
| date = 1997
| access-date = 26 March 2007
| archive-date = 30 March 2004
| archive-url = https://web.archive.org/web/20040330212503/http://www.boulder.swri.edu/~layoung/projects/talks03/IfA-jan03v1.ppt
| url-status = live
}}</ref> as are Eris and Dysnomia,<ref name="Szakats2022">{{cite journal
  |display-authors = etal
  |first1 = R. |last1 = Szakáts
  |first2 = Cs. |last2 = Kiss
  |first3 = J. L. |last3 = Ortiz
  |first4 = N. |last4 = Morales
  |first5 = A. |last5 = Pál
  |first6 = T. G. |last6 = Müller
  |title      = Tidally locked rotation of the dwarf planet (136199) Eris discovered via long-term ground-based and space photometry
  |journal = Astronomy & Astrophysics |year = 2023 |volume = 669 |page = L3 |doi = 10.1051/0004-6361/202245234 |arxiv      = 2211.07987
|bibcode = 2023A&A...669L...3S |s2cid = 253522934 }}</ref> and probably {{dp|Orcus}} and its moon [[Vanth (moon)|Vanth]].<ref name="Brown2023"/> The other dwarf planets with known rotation periods rotate faster than Earth; Haumea rotates so fast that it has been distorted into a [[triaxial ellipsoid]].<ref name="Rabinowitz2005">
{{cite journal
| author = Rabinowitz, D. L.
| date = 2006
| title = Photometric Observations Constraining the Size, Shape, and Albedo of 2003&nbsp;EL<sub>61</sub>, a Rapidly Rotating, Pluto-Sized Object in the Kuiper Belt
| journal = [[Astrophysical Journal]]
| volume = 639
| issue = 2
| pages = 1238–1251
| doi = 10.1086/499575
| bibcode = 2006ApJ...639.1238R
| arxiv = astro-ph/0509401
| last2 = Barkume | first2 = Kristina
| last3 = Brown | first3 = Michael E.
| last4 = Roe | first4 = Henry
| last5 = Schwartz | first5 = Michael
| last6 = Tourtellotte | first6 = Suzanne
| last7 = Trujillo | first7 = Chad
| s2cid = 11484750
}}
</ref> The exoplanet [[Tau Boötis b]] and its parent star [[Tau Boötis]] appear to be mutually tidally locked.<ref>{{cite journal | title=Life on a tidally-locked planet | last=Singal | first=Ashok K. | journal=Planex Newsletter | volume=4 | issue=2 | page=8 | date=May 2014 | bibcode=2014arXiv1405.1025S | arxiv=1405.1025 }}</ref><ref>{{cite journal | title=MOST detects variability on tau Bootis possibly induced by its planetary companion | url=http://www.aanda.org/articles/aa/full/2008/17/aa8952-07/aa8952-07.html | last1=Walker | first1=G. A. H. | last2=Croll | first2=B. | last3=Matthews | first3=J. M. | last4=Kuschnig | first4=R. | last5=Huber | first5=D. | last6=Weiss | first6=W. W. | last7=Shkolnik | first7=E. | last8=Rucinski | first8=S. M. | last9=Guenther | first9=D. B. | display-authors=1 | year=2008 | journal=Astronomy and Astrophysics | volume=482 | issue=2 | pages=691–697 | doi=10.1051/0004-6361:20078952 | arxiv=0802.2732 | bibcode=2008A&A...482..691W | s2cid=56317105 | access-date=6 August 2022 | archive-date=25 February 2021 | archive-url=https://web.archive.org/web/20210225212508/https://www.aanda.org/articles/aa/full/2008/17/aa8952-07/aa8952-07.html | url-status=live }}</ref>

==== Orbital clearing ====
{{Main|Clearing the neighbourhood}}
The defining dynamic characteristic of a planet, according to the IAU definition, is that it has ''cleared its neighborhood''. A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all the [[planetesimal]]s in its orbit. In effect, it orbits its star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects. As described above, this characteristic was mandated as part of the [[International Astronomical Union|IAU]]'s official [[2006 definition of planet|definition of a planet]] in August 2006.<ref name="IAU" /> Although to date this criterion only applies to the Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their [[circumstellar disc]]s.<ref>{{cite journal |title=The Total Number of Giant Planets in Debris Disks with Central Clearings |date=26 November 2007 | last1=Faber | first1=Peter | last2=Quillen | first2=Alice C. |journal=Monthly Notices of the Royal Astronomical Society |volume=382 |number=4 |pages=1823–1828 |doi=10.1111/j.1365-2966.2007.12490.x |doi-access=free |arxiv=0706.1684 |bibcode=2007MNRAS.382.1823F |s2cid=16610947 }}</ref>

=== Physical characteristics ===

==== Size and shape ====
{{see also|Earth#Size and shape|Astronomical object#Shape|Planetary coordinate system}}
Gravity causes planets to be pulled into a roughly spherical shape, so a planet's size can be expressed roughly by an average radius (for example, [[Earth radius (unit)|Earth radius]] or [[Jupiter radius]]). However, planets are not perfectly spherical; for example, the [[Earth's rotation]] causes it to be slightly flattened at the poles with a [[equatorial bulge|bulge around the equator]].<ref name=milbert_smith96>{{cite web |last1=Milbert |first1=D. G. |last2=Smith |first2=D. A |url=http://www.ngs.noaa.gov/PUBS_LIB/gislis96.html |title=Converting GPS Height into NAVD88 Elevation with the GEOID96 Geoid Height Model |publisher=National Geodetic Survey, NOAA |access-date=7 March 2007 |archive-date=20 August 2011 |archive-url=https://web.archive.org/web/20110820090214/http://www.ngs.noaa.gov/PUBS_LIB/gislis96.html |url-status=live }}</ref> Therefore, a better approximation of Earth's shape is an [[oblate spheroid]], whose equatorial diameter is {{convert|43|km|mi|sp=us}} larger than the [[Geographical pole|pole]]-to-pole diameter.<ref name="ngdc2006">{{cite web |last1=Sandwell |first1=D. T. |last2=Smith |first2=Walter H. F. |author-link2=Walter H. F. Smith|date=7 July 2006 |url=http://www.ngdc.noaa.gov/mgg/bathymetry/predicted/explore.HTML |title=Exploring the Ocean Basins with Satellite Altimeter Data |publisher=NOAA/NGDC |access-date=21 April 2007|archive-url=https://archive.today/20140715142212/http://www.ngdc.noaa.gov/mgg/bathymetry/predicted/explore.HTML|archive-date=15 July 2014}}</ref> Generally, a planet's shape may be described by giving polar and equatorial radii of a [[spheroid]] or specifying a [[reference ellipsoid]]. From such a specification, the planet's flattening, surface area, and volume can be calculated; its [[normal gravity]] can be computed knowing its size, shape, rotation rate, and mass.<ref>{{Citation |last=Wieczorek |first=M. A. |title=10.05 – Gravity and Topography of the Terrestrial Planets |date=2015 |url=https://www.sciencedirect.com/science/article/pii/B978044453802400169X |work=Treatise on Geophysics |edition=2nd |pages=153–193 |editor-last=Schubert |editor-first=Gerald |place=Oxford |publisher=Elsevier |language=en |isbn=978-0-444-53803-1 |access-date=13 May 2022 |archive-date=13 May 2022 |archive-url=https://web.archive.org/web/20220513203935/https://www.sciencedirect.com/science/article/pii/B978044453802400169X |url-status=live }}</ref>

==== Mass ====
{{Main|Planetary mass}}
A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over the [[electromagnetic force]]s binding its physical structure, leading to a state of [[hydrostatic equilibrium]]. This effectively means that all planets are spherical or spheroidal. Up to a certain mass, an object can be irregular in shape, but beyond that point, which varies depending on the chemical makeup of the object, gravity begins to pull an object towards its own centre of mass until the object collapses into a sphere.<ref>{{cite web |title=The Dwarf Planets |url=http://www.gps.caltech.edu/~mbrown/dwarfplanets/ |author-link=Michael E. Brown |last=Brown |first=Michael E. |work=California Institute of Technology |date=2006 |access-date=1 February 2008 |archive-date=16 January 2011 |archive-url=https://web.archive.org/web/20110116181239/http://www.gps.caltech.edu/~mbrown/dwarfplanets/ |url-status=live }}</ref>

Mass is the prime attribute by which planets are distinguished from stars. No objects between the masses of the Sun and Jupiter exist in the Solar System, but there are exoplanets of this size. The lower [[stellar mass]] limit is estimated to be around 75 to 80 times that of Jupiter ({{Jupiter mass|link=yes}}). Some authors advocate that this be used as the upper limit for planethood, on the grounds that the internal physics of objects does not change between approximately one Saturn mass (beginning of significant self-compression) and the onset of hydrogen burning and becoming a [[red dwarf]] star.<ref name=ChenKipping/> Beyond roughly 13 {{Jupiter mass}} (at least for objects with solar-type [[isotopic abundance]]), an object achieves conditions suitable for [[nuclear fusion]] of [[deuterium]]: this has sometimes been advocated as a boundary,<ref name=exoworkdef/> even though deuterium burning does not last very long and most brown dwarfs have long since finished burning their deuterium.<ref name=Hatzes/> This is not universally agreed upon: the [[Extrasolar Planets Encyclopaedia|exoplanets Encyclopaedia]] includes objects up to 60 {{Jupiter mass}},<ref name=corot/> and the [[Exoplanet Data Explorer]] up to 24 {{Jupiter mass}}.<ref name=eod/>

The smallest known exoplanet with an accurately known mass is [[PSR B1257+12A]], one of the first exoplanets discovered, which was found in 1992 in orbit around a [[pulsar]]. Its mass is roughly half that of the planet Mercury.<ref name="konacki2003">{{cite journal | author=Konacki, M. | author2=Wolszczan, A. | title=Masses and Orbital Inclinations of Planets in the PSR B1257+12 System | journal=The Astrophysical Journal | volume=591 | issue=2 | pages=L147–L150 | date=2003 | doi=10.1086/377093 | bibcode=2003ApJ...591L.147K|arxiv = astro-ph/0305536 | s2cid=18649212 }}</ref> Even smaller is [[WD 1145+017 b]], orbiting a white dwarf; its mass is roughly that of the dwarf planet Haumea, and it is typically termed a minor planet.<ref>{{cite book |last=Veras |first=Dimitri |chapter=Planetary Systems Around White Dwarfs |date=2021 |url=https://oxfordre.com/planetaryscience/view/10.1093/acrefore/9780190647926.001.0001/acrefore-9780190647926-e-238 |title=Oxford Research Encyclopedia of Planetary Science |publisher=Oxford University Press |language=en |arxiv=2106.06550 |doi=10.1093/acrefore/9780190647926.013.238 |isbn=978-0-19-064792-6 |access-date=12 July 2022 |archive-date=6 June 2022 |archive-url=https://web.archive.org/web/20220606003104/https://oxfordre.com/planetaryscience/view/10.1093/acrefore/9780190647926.001.0001/acrefore-9780190647926-e-238 |url-status=live }}</ref> The smallest known planet orbiting a main-sequence star other than the Sun is [[Kepler-37b]], with a mass (and radius) that is probably slightly higher than that of the Moon.<ref name="Barclay-2013" /> The smallest object in the Solar System generally agreed to be a geophysical planet is Saturn's moon Mimas, with a radius about 3.1% of Earth's and a mass about 0.00063% of Earth's.<ref name="Jacobson2022">{{cite journal |last1=Jacobson |first1=Robert. A. |title=The Orbits of the Main Saturnian Satellites, the Saturnian System Gravity Field, and the Orientation of Saturn's Pole* |journal=The Astronomical Journal |date=1 November 2022 |volume=164 |issue=5 |page=199 |doi=10.3847/1538-3881/ac90c9|bibcode=2022AJ....164..199J |s2cid=252992162 |doi-access=free }}</ref> Saturn's smaller moon [[Phoebe (moon)|Phoebe]], currently an irregular body of 1.7% Earth's radius<ref name=Thomas2010>{{cite journal| doi = 10.1016/j.icarus.2010.01.025| last1 = Thomas| first1 = P. C.| date = July 2010| title = Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission| journal = Icarus| volume = 208| issue = 1| pages = 395–401| url = http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| bibcode = 2010Icar..208..395T| access-date = 7 May 2023| archive-date = 23 December 2018| archive-url = https://web.archive.org/web/20181223003125/http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf}}</ref> and 0.00014% Earth's mass,<ref name="Jacobson2022"/> is thought to have attained hydrostatic equilibrium and differentiation early in its history before being battered out of shape by impacts.<ref name=planetlike>{{cite web |date=26 April 2012 |author=Jia-Rui C. Cook and Dwayne Brown |title=Cassini Finds Saturn Moon Has Planet-Like Qualities |url=http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20120426/ |publisher=JPL/NASA |archive-url=https://web.archive.org/web/20120427192715/http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20120426/ | archive-date=27 April 2012 }}</ref> Some asteroids may be fragments of [[protoplanet]]s that began to accrete and differentiate, but suffered catastrophic collisions, leaving only a metallic or rocky core today,<ref name=Gaffey1984>{{cite journal
  |last=Gaffey |first=Michael
  |title=Rotational spectral variations of asteroid (8) Flora: Implications for the nature of the S-type asteroids and for the parent bodies of the ordinary chondrites
  |journal=Icarus
  |volume=60
  |issue=1 |pages=83–114 |date=1984
  |doi=10.1016/0019-1035(84)90140-4
  |bibcode=1984Icar...60...83G}}</ref><ref name=Hardersen-Gaffey-Abell-2005>
{{cite journal
 |first1=Paul S.     |last1=Hardersen
 |first2=Michael J.  |last2=Gaffey
 |first3=Paul A.     |last3=Abell
 |name-list-style=amp
 |year=2005
 |title=Near-IR spectral evidence for the presence of iron-poor orthopyroxenes on the surfaces of six M-type asteroid
 |journal=Icarus
 |volume=175 |issue=1 |page=141
 |bibcode=2005Icar..175..141H  |doi=10.1016/j.icarus.2004.10.017
}}
</ref><ref name=Asphaug-Reufer-2014>
{{cite journal
 |first1=E. |last1=Asphaug
 |first2=A. |last2=Reufer
 |year=2014
 |title=Mercury and other iron-rich planetary bodies as relics of inefficient accretion
 |journal=Nature Geoscience
 |volume=7  |issue=8  |pages=564–568
 |bibcode=2014NatGe...7..564A   |doi=10.1038/NGEO2189
}}</ref> or a reaccumulation of the resulting debris.<ref name=Yang2020/>

==== Internal differentiation ====
{{Main|Planetary differentiation}}
[[File:Jupiter interior.png|upright|thumb|Illustration of the interior of Jupiter, with a rocky core overlaid by a deep layer of metallic hydrogen]]

Every planet began its existence in an entirely fluid state; in early formation, the denser, heavier materials sank to the centre, leaving the lighter materials near the surface. Each therefore has a [[Planetary differentiation|differentiated]] interior consisting of a dense [[planetary core]] surrounded by a [[Mantle (geology)|mantle]] that either is or was a [[fluid]]. The terrestrial planets' mantles are sealed within hard [[Crust (geology)|crusts]],<ref name="terrestrial">{{cite web |title=Planetary Interiors |work=Department of Physics, University of Oregon |url=http://abyss.uoregon.edu/~js/ast121/lectures/lec16.html |access-date=23 August 2008 |archive-date=8 August 2012 |archive-url=https://web.archive.org/web/20120808155809/http://abyss.uoregon.edu/~js/ast121/lectures/lec16.html }}</ref> but in the giant planets the mantle simply blends into the upper cloud layers. The terrestrial planets have cores of elements such as [[iron]] and [[nickel]] and mantles of [[silicate]]s. Jupiter and Saturn are believed to have cores of rock and metal surrounded by mantles of [[metallic hydrogen]].<ref>{{cite book | first=Linda T. |last=Elkins-Tanton |date=2006 |title=Jupiter and Saturn |publisher=Chelsea House |location=New York |isbn=978-0-8160-5196-0}}</ref> Uranus and Neptune, which are smaller, have rocky cores surrounded by mantles of water, [[ammonia]], [[methane]], and other [[Volatile (astrogeology)|ices]].<ref>{{cite journal| doi = 10.1016/0032-0633(95)00061-5| last1 = Podolak| first1 = M.| last2 = Weizman| first2 = A.| last3 = Marley| first3 = M.| date=December 1995 | title = Comparative models of Uranus and Neptune| journal = Planetary and Space Science| volume = 43| issue = 12| pages = 1517–1522| bibcode = 1995P&SS...43.1517P| ref = {{sfnRef|Podolak Weizman et al.|1995}}}}</ref> The fluid action within these planets' cores creates a [[geodynamo]] that generates a [[magnetic field]].<ref name="terrestrial" /> Similar differentiation processes are believed to have occurred on some of the large moons and dwarf planets,<ref name=Grundy2019/> though the process may not always have been completed: Ceres, Callisto, and Titan appear to be incompletely differentiated.<ref name="Neumann2015">{{Cite journal |last1=Neumann |first1=W. |last2=Breuer |first2=D. |last3=Spohn |first3=T. |date=2 December 2015 |title=Modelling the internal structure of Ceres: Coupling of accretion with compaction by creep and implications for the water-rock differentiation |url=http://www.aanda.org/articles/aa/pdf/2015/12/aa27083-15.pdf |url-status=live |journal=Astronomy & Astrophysics |volume=584 |page=A117 |bibcode=2015A&A...584A.117N |doi=10.1051/0004-6361/201527083 |archive-url=https://web.archive.org/web/20160822053141/http://www.aanda.org/articles/aa/pdf/2015/12/aa27083-15.pdf |archive-date=22 August 2016 |access-date=10 July 2016 |doi-access=free}}</ref><ref name=Monteux2014>{{cite journal |last1=Monteux |first1=J. |last2=Tobie |first2=G. |last3=Choblet |first3=G. |last4=Le Feuvre |first4=M. |title=Can large icy moons accrete undifferentiated? |journal=Icarus |year=2014 |volume=237 |pages=377–387 |doi=10.1016/j.icarus.2014.04.041 |bibcode=2014Icar..237..377M |s2cid=46172826 |url=https://hal.uca.fr/hal-01636068/file/Monteux-Icarus-V3-1-Final-2014.pdf |access-date=6 August 2022 |archive-date=9 October 2022 |archive-url=https://ghostarchive.org/archive/20221009/https://hal.uca.fr/hal-01636068/file/Monteux-Icarus-V3-1-Final-2014.pdf |url-status=live }}</ref> The asteroid Vesta, though not a dwarf planet because it was battered by impacts out of roundness, has a differentiated interior<ref name=Vestainterior>{{cite web|title=A look into Vesta's interior|url=https://www.mpg.de/877913/Vesta_asteroid|work=Max-Planck-Gesellschaft|date=6 January 2011|access-date=7 May 2023|archive-date=5 March 2023|archive-url=https://web.archive.org/web/20230305200352/https://www.mpg.de/877913/Vesta_asteroid|url-status=live}}</ref> similar to that of Venus, Earth, and Mars.<ref name=Asphaug-Reufer-2014/>

==== Atmosphere ====
{{Main|Atmosphere|extraterrestrial atmospheres}}
{{see also|Extraterrestrial skies}}
[[File:Top of Atmosphere.jpg|thumb|left|Earth's atmosphere]]
All of the Solar System planets [[Atmosphere of Mercury|except Mercury]]<ref>{{cite journal |last1=Zurbuchen |first1=Thomas H. |last2=Raines |first2=Jim M. |last3=Gloeckler |first3=George |last4=Krimigis |first4=Stamatios M. |last5=Slavin |first5=James A. |last6=Koehn |first6=Patrick L. |last7=Killen |first7=Rosemary M. |last8=Sprague |first8=Ann L. |last9=McNutt Jr. |first9=Ralph L. |last10=Solomon |first10=Sean C. |display-authors=4 |name-list-style=vanc |year=2008 |title=MESSENGER Observations of the Composition of Mercury's Ionized Exosphere and Plasma Environment |journal=Science |volume=321 |issue=5885 |pages=90–92 |bibcode=2008Sci...321...90Z |doi=10.1126/science.1159314 |pmid=18599777 |s2cid=206513512}}</ref> have substantial [[atmosphere]]s because their gravity is strong enough to keep gases close to the surface. Saturn's largest moon [[Titan (moon)|Titan]] also has a substantial atmosphere thicker than that of Earth;<ref>{{cite book|title=Titan: Exploring an Earthlike World|author1=Coustenis, Athéna|author2=Taylor, F. W.|name-list-style=amp|publisher=World Scientific|year=2008|page=130|url=https://books.google.com/books?id=j3O47dxrDAQC&q=Titan|access-date=25 March 2010|isbn=978-981-270-501-3|archive-date=14 December 2023|archive-url=https://web.archive.org/web/20231214142630/https://books.google.com/books?id=j3O47dxrDAQC&q=Titan#v=snippet&q=Titan&f=false|url-status=live}}</ref> Neptune's largest moon [[Triton (moon)|Triton]]<ref name="Solar System">{{cite web |url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Nep_Triton |title=Neptune: Moons: Triton |work=Solar System Exploration |access-date=31 December 2007 |archive-url=https://web.archive.org/web/20080110095537/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Nep_Triton |archive-date=10 January 2008 }}</ref> and the dwarf planet [[Pluto]] have more tenuous atmospheres.<ref name=Lellouch_2015>{{cite journal
 |author1=Lellouch, E. |author2=de Bergh, C. |author3=Sicardy, B. |author4=Forget, F. |author5=Vangvichith, M. |author6=Käufl, H.-U. |title  =Exploring the spatial, temporal, and vertical distribution of methane in Pluto's atmosphere
 |journal=Icarus
 |date   =January 2015
 |doi    =10.1016/j.icarus.2014.03.027
 |bibcode=2015Icar..246..268L
 |arxiv  =1403.3208
 |volume=246 |pages=268–278
|s2cid=119194193 }}</ref> The larger giant planets are massive enough to keep large amounts of the light gases hydrogen and helium, whereas the smaller planets lose these gases into [[Interplanetary medium|space]].<ref>{{Cite journal | last1 = Sheppard | first1 = S. S. | last2 = Jewitt | first2 = D. | last3 = Kleyna | first3 = J. | title = An Ultradeep Survey for Irregular Satellites of Uranus: Limits to Completeness | doi = 10.1086/426329 | journal = The Astronomical Journal | volume = 129 | issue = 1 | pages = 518–525 | year = 2005 |arxiv = astro-ph/0410059 |bibcode = 2005AJ....129..518S | s2cid = 18688556 }}</ref> Analysis of exoplanets suggests that the threshold for being able to hold on to these light gases occurs at about {{val|2.0|0.7|0.6}} ''M''<sub>🜨</sub>, so that Earth and Venus are near the maximum size for rocky planets.<ref name=ChenKipping/>

The composition of Earth's atmosphere is different from the other planets because the various life processes that have transpired on the planet have introduced free molecular [[oxygen]].<ref name="zeilik">{{cite book | last1=Zeilik |first1=Michael A. |author2=Gregory, Stephan A. |title=Introductory Astronomy & Astrophysics |edition=4th |date=1998 |publisher=Saunders College Publishing |isbn=978-0-03-006228-5 |page=67}}</ref> The atmospheres of Mars and Venus are both dominated by [[carbon dioxide]], but differ drastically in density: the average surface pressure of [[Atmosphere of Mars|Mars's atmosphere]] is less than 1% that of Earth's (too low to allow liquid water to exist),<ref>{{Citation|last=Haberle|first=R. M.|title=Solar System/Sun, Atmospheres, Evolution of Atmospheres {{!}} Planetary Atmospheres: Mars|date=2015|encyclopedia=Encyclopedia of Atmospheric Sciences |edition=2nd|pages=168–177|editor-last=North|editor-first=Gerald R.|publisher=Academic Press|doi=10.1016/b978-0-12-382225-3.00312-1|isbn=978-0-12-382225-3|editor2-last=Pyle|editor2-first=John|editor3-last=Zhang|editor3-first=Fuqing}}</ref> while the average surface pressure of [[Atmosphere of Venus|Venus's atmosphere]] is about 92 times that of Earth's.<ref name=Basilevsky2003>{{cite journal |last=Basilevsky|first=Alexandr T.|author2=Head, James W.|title=The surface of Venus|journal=Rep. Prog. Phys.|date=2003|volume=66|issue=10|pages=1699–1734|doi=10.1088/0034-4885/66/10/R04 |bibcode= 2003RPPh...66.1699B|s2cid=250815558 }}</ref> It is likely that Venus's atmosphere was the result of a [[runaway greenhouse effect]] in its history, which today makes it the hottest planet by surface temperature, hotter even than Mercury.<ref>{{cite journal |author=S. I. Rasoonl|author2=C. de Bergh|name-list-style=amp |title=The Runaway Greenhouse Effect and the Accumulation of CO<sub>2</sub> in the Atmosphere of Venus |journal=Nature |volume=226 |pages=1037–1039 |date=1970 |pmid=16057644 |issue=5250 |doi=10.1038/2261037a0 |bibcode=1970Natur.226.1037R|s2cid=4201521}}</ref> Despite hostile surface conditions, temperature, and pressure at about 50–55&nbsp;km altitude in Venus's atmosphere are close to Earthlike conditions (the only place in the Solar System beyond Earth where this is so), and this region has been suggested as a plausible base for future [[Space exploration#Human spaceflight and habitation|human exploration]].<ref name="Badescu">{{cite book |author=Badescu, Viorel |editor=Zacny, Kris |title=Inner Solar System: Prospective Energy and Material Resources |url=https://www.springer.com/us/book/9783319195681 |location=Heidelberg |publisher=Springer-Verlag GmbH |page=492 |date=2015 |isbn=978-3-319-19568-1 |access-date=4 May 2023 |archive-date=21 August 2018 |archive-url=https://web.archive.org/web/20180821093729/https://www.springer.com/us/book/9783319195681 |url-status=live }}.<!--Based on ''Technica Molodezhi TM - 9 1971''--></ref> Titan has the only dense [[nitrogen]]-rich planetary atmosphere in the Solar System other than Earth's. Just as Earth's conditions are close to the [[triple point]] of water, allowing it to exist in all three states on the planet's surface, so Titan's are to the triple point of [[methane]].<ref>{{Cite journal|last=Horst|first=Sarah|date=2017|title=Titan's Atmosphere and Climate|journal=J. Geophys. Res. Planets|volume=122|issue=3|pages=432–482|doi=10.1002/2016JE005240|arxiv=1702.08611|bibcode=2017JGRE..122..432H|s2cid=119482985}}</ref>

Planetary atmospheres are affected by the varying [[insolation]] or internal energy, leading to the formation of dynamic [[weather system]]s such as [[hurricane]]s (on Earth), planet-wide [[dust storm]]s (on Mars), a greater-than-Earth-sized [[Anticyclonic storm|anticyclone]] on Jupiter (called the [[Great Red Spot]]), and [[Great Dark Spot|holes in the atmosphere]] (on Neptune).<ref name="Weather" /> Weather patterns detected on exoplanets include a hot region on [[HD 189733 b]] twice the size of the Great Red Spot,<ref name="knutson">{{cite journal | last1=Knutson |first1=Heather A. | last2=Charbonneau | first2=David | last3=Allen | first3=Lori E. |author3-link=Lori Allen (astronomer)| last4=Fortney | first4=Jonathan J. |title=A map of the day-night contrast of the extrasolar planet HD 189733 b |journal=Nature |date=2007 |volume=447 |doi=10.1038/nature05782 |pmid=17495920 |issue=7141 |bibcode=2007Natur.447..183K | pages=183–186|arxiv = 0705.0993|s2cid=4402268}}
* {{cite press release |date=9 May 2007 |title=First Map of an Extrasolar Planet |website=Center for Astrophysics |url=https://pweb.cfa.harvard.edu/news/first-map-extrasolar-planet |access-date=10 July 2022 |archive-date=5 December 2022 |archive-url=https://web.archive.org/web/20221205045421/https://pweb.cfa.harvard.edu/news/first-map-extrasolar-planet |url-status=live }}</ref> as well as [[cloud]]s on the hot Jupiter [[Kepler-7b]],<ref name="ArXiv-20130930">{{cite journal |last1=Demory |first1= Brice-Olivier |first2= Julien |last2= de Wit |first3= Nikole |last3= Lewis |first4= Jonathan |last4= Fortney |first5= Andras |last5= Zsom |first6= Sara |last6= Seager |display-authors=4 |year=2013 |title=Inference of Inhomogeneous Clouds in an Exoplanet Atmosphere |journal=The Astrophysical Journal Letters |volume=776 |issue=2 |page=L25 |arxiv=1309.7894 |bibcode=2013ApJ...776L..25D |doi=10.1088/2041-8205/776/2/L25 |s2cid=701011}}</ref> the super-Earth [[Gliese 1214 b]], and others.<ref name="NAT-20140101a">{{cite journal |last=Moses |first=Julianne |date=1 January 2014 |title=Extrasolar planets: Cloudy with a chance of dustballs |journal=[[Nature (journal)|Nature]] |volume=505 |issue=7481 |pages=31–32 |bibcode=2014Natur.505...31M |doi=10.1038/505031a |pmid=24380949|s2cid=4408861 }}</ref><ref>{{Cite journal |last1=Benneke |first1=Björn |last2=Wong |first2=Ian |last3=Piaulet |first3=Caroline |last4=Knutson |first4=Heather A. |last5=Lothringer |first5=Joshua |last6=Morley |first6=Caroline V. |last7=Crossfield |first7=Ian J. M. |last8=Gao |first8=Peter |last9=Greene |first9=Thomas P. |last10=Dressing |first10=Courtney |last11=Dragomir |first11=Diana |display-authors= 4 |date=10 December 2019 |title=Water Vapor and Clouds on the Habitable-zone Sub-Neptune Exoplanet K2-18b |journal=The Astrophysical Journal Letters |volume=887 |issue=1 |pages=L14 |doi=10.3847/2041-8213/ab59dc |arxiv=1909.04642 |bibcode=2019ApJ...887L..14B |s2cid=209324670 |issn=2041-8205 |doi-access=free }}</ref>

Hot Jupiters, due to their extreme proximities to their host stars, have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets.<ref>{{cite journal |journal=Nature |last1=Ballester |first1=Gilda E. |last2=Sing |first2=David K. |last3=Herbert |first3=Floyd |title=The signature of hot hydrogen in the atmosphere of the extrasolar planet HD 209458b |volume=445 |date=2007 |doi=10.1038/nature05525 |pmid=17268463 |issue=7127 |bibcode=2007Natur.445..511B |pages=511–514 |hdl=10871/16060 |s2cid=4391861 |url=https://ore.exeter.ac.uk/repository/bitstream/10871/16060/2/HD209458.nature.rev105.pdf |hdl-access=free |access-date=24 September 2019 |archive-date=28 July 2020 |archive-url=https://web.archive.org/web/20200728035216/https://repository/bitstream/handle/10871/16060/HD209458.nature.rev105.pdf;jsessionid=35C3149FC9764FBF9D4ADEA8F1DA25E4?sequence=2 |url-status=live }}
* {{cite press release |first1=Donna |last1=Weaver |first2=Ray |last2=Villard |url=https://hubblesite.org/contents/news-releases/2007/news-2007-07.html |title=Hubble Probes Layer-cake Structure of Alien World's Atmosphere |publisher=Space Telescope Science Institute |date=31 January 2007 |access-date=23 October 2011 |archive-date=9 July 2016 |archive-url=https://web.archive.org/web/20160709121743/http://hubblesite.org/newscenter/archive/releases/2007/07/full/ |url-status=live }}</ref><ref>{{Cite journal |last1=Villarreal D'Angelo |first1=Carolina |last2=Esquivel |first2=Alejandro |last3=Schneiter |first3=Matías |last4=Sgró |first4=Mario Agustín |date=21 September 2018 |title=Magnetized winds and their influence in the escaping upper atmosphere of HD 209458b |url=https://academic.oup.com/mnras/article/479/3/3115/5035846 |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=479 |issue=3 |pages=3115–3125 |doi=10.1093/mnras/sty1544 |doi-access=free |issn=0035-8711 |hdl=11336/86936 |hdl-access=free |access-date=10 July 2022 |archive-date=10 July 2022 |archive-url=https://web.archive.org/web/20220710233411/https://academic.oup.com/mnras/article/479/3/3115/5035846 |url-status=live }}</ref> These planets may have vast differences in temperature between their day and night sides that produce supersonic winds,<ref>{{cite journal | last1=Harrington |first1=Jason | last2=Hansen | first2=Brad M. | last3=Luszcz | first3=Statia H. | last4=Seager | first4=Sara |title=The phase-dependent infrared brightness of the extrasolar planet Andromeda b |journal=Science |volume=314 |date=2006 |doi=10.1126/science.1133904 |pmid=17038587 |issue=5799 |bibcode=2006Sci...314..623H | pages=623–626|arxiv = astro-ph/0610491 |s2cid=20549014}}
* {{cite press release |date=12 October 2006 |title=NASA's Spitzer Sees Day and Night on Exotic World |website=NASA |url=http://www.nasa.gov/vision/universe/starsgalaxies/spitzer-20061012.html |access-date=16 August 2007 |archive-date=13 July 2017 |archive-url=https://web.archive.org/web/20170713035307/https://www.nasa.gov/vision/universe/starsgalaxies/spitzer-20061012.html }}</ref> although multiple factors are involved and the details of the atmospheric dynamics that affect the day-night temperature difference are complex.<ref>{{Cite journal |last1=Showman |first1=Adam P. |last2=Tan |first2=Xianyu |last3=Parmentier |first3=Vivien |date=December 2020 |title=Atmospheric Dynamics of Hot Giant Planets and Brown Dwarfs |url=http://link.springer.com/10.1007/s11214-020-00758-8 |journal=Space Science Reviews |language=en |volume=216 |issue=8 |page=139 |doi=10.1007/s11214-020-00758-8 |arxiv=2007.15363 |bibcode=2020SSRv..216..139S |s2cid=220870881 |issn=0038-6308 |access-date=10 July 2022 |archive-date=14 December 2023 |archive-url=https://web.archive.org/web/20231214142633/https://link.springer.com/article/10.1007/s11214-020-00758-8 |url-status=live }}</ref><ref>{{Cite journal |last1=Fortney |first1=Jonathan J. |last2=Dawson |first2=Rebekah I. |last3=Komacek |first3=Thaddeus D. |date=March 2021 |title=Hot Jupiters: Origins, Structure, Atmospheres |url=https://onlinelibrary.wiley.com/doi/10.1029/2020JE006629 |journal=Journal of Geophysical Research: Planets |language=en |volume=126 |issue=3 |doi=10.1029/2020JE006629 |arxiv=2102.05064 |bibcode=2021JGRE..12606629F |s2cid=231861632 |issn=2169-9097 |access-date=10 July 2022 |archive-date=14 December 2023 |archive-url=https://web.archive.org/web/20231214142634/https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020JE006629 |url-status=live }}</ref>

==== Magnetosphere ====
{{Main|Magnetosphere}}
[[File:Structure_of_the_magnetosphere_LanguageSwitch.svg|lang=en|thumb|[[Earth's magnetic field|Earth's magnetosphere]] (diagram)]]

One important characteristic of the planets is their intrinsic [[magnetic moment]]s, which in turn give rise to magnetospheres. The presence of a magnetic field indicates that the planet is still geologically alive. In other words, magnetized planets have flows of [[electrical conductivity|electrically conducting]] material in their interiors, which generate their magnetic fields. These fields significantly change the interaction of the planet and solar wind. A magnetized planet creates a cavity in the solar wind around itself called the magnetosphere, which the wind cannot penetrate. The magnetosphere can be much larger than the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the [[ionosphere]] with the solar wind, which cannot effectively protect the planet.<ref name="Kivelson2007" />

Of the eight planets in the Solar System, only Venus and Mars lack such a magnetic field.<ref name="Kivelson2007" /> Of the magnetized planets, the magnetic field of Mercury is the weakest and is barely able to deflect the [[solar wind]]. Jupiter's moon [[Ganymede (moon)|Ganymede]] has a magnetic field several times stronger, and Jupiter's is the strongest in the Solar System (so intense in fact that it poses a serious health risk to future crewed missions to all its moons inward of Callisto<ref>{{Cite journal |last1=De Angelis |first1=G. |last2=Clowdsley |first2=M. S. |last3=Nealy |first3=J. E. |last4=Tripathi |first4=R. K. |last5=Wilson |first5=J. W. |display-authors=4 |date=January 2004 |title=Radiation analysis for manned missions to the Jupiter system |url=https://linkinghub.elsevier.com/retrieve/pii/S0273117704003205 |journal=Advances in Space Research |language=en |volume=34 |issue=6 |pages=1395–1403 |doi=10.1016/j.asr.2003.09.061 |pmid=15881781 |bibcode=2004AdSpR..34.1395D |access-date=13 July 2022 |archive-date=25 April 2022 |archive-url=https://web.archive.org/web/20220425152824/https://linkinghub.elsevier.com/retrieve/pii/S0273117704003205 |url-status=live }}</ref>). The magnetic fields of the other giant planets, measured at their surfaces, are roughly similar in strength to that of Earth, but their magnetic moments are significantly larger. The magnetic fields of Uranus and Neptune are strongly tilted relative to the planets' rotational [[Axis of rotation|axes]] and displaced from the planets' centres.<ref name="Kivelson2007">{{cite book |last1=Kivelson |first1=Margaret Galland | last2=Bagenal | first2=Fran |chapter=Planetary Magnetospheres |title=Encyclopedia of the Solar System |date=2007 |publisher=Academic Press |editor=Lucy-Ann McFadden |editor2=Paul Weissman |editor3=Torrence Johnson |isbn=978-0-12-088589-3 |page=[https://archive.org/details/encyclopediaofso0000unse_u6d1/page/519 519] |chapter-url=https://archive.org/details/encyclopediaofso0000unse_u6d1/page/519 }}</ref>

In 2003, a team of astronomers in Hawaii observing the star [[HD 179949]] detected a bright spot on its surface, apparently created by the magnetosphere of an orbiting hot Jupiter.<ref>{{cite web |last=Gefter |first=Amanda |date=17 January 2004 |title=Magnetic planet |url=https://astronomy.com/news-observing/news/2004/01/magnetic%20planet |access-date=29 January 2008 |work=Astronomy |archive-date=1 June 2019 |archive-url=https://web.archive.org/web/20190601224551/http://www.astronomy.com/news-observing/news/2004/01/magnetic%20planet }}</ref><ref>{{Cite journal |last1=Shkolnik |first1=E. |last2=Walker |first2=G. A. H. |last3=Bohlender |first3=D. A. |date=10 November 2003 |title=Evidence for Planet-induced Chromospheric Activity on HD 179949 |url=https://iopscience.iop.org/article/10.1086/378583 |journal=The Astrophysical Journal |language=en |volume=597 |issue=2 |pages=1092–1096 |doi=10.1086/378583 |bibcode=2003ApJ...597.1092S |s2cid=15829056 |issn=0004-637X |access-date=10 July 2022 |archive-date=10 July 2022 |archive-url=https://web.archive.org/web/20220710171419/https://iopscience.iop.org/article/10.1086/378583 |url-status=live |arxiv=astro-ph/0303557 }}</ref>

===Secondary characteristics===
{{Main|Natural satellite|ring system}}
[[File:Voyager 2 - Saturn Rings - 3085 7800 2.png|thumb|upright|The [[rings of Saturn]]]]

Several planets or dwarf planets in the Solar System (such as Neptune and Pluto) have orbital periods that are in [[Orbital resonance|resonance]] with each other or with smaller bodies. This is common in satellite systems (e.g. the resonance between Io, [[Europa (moon)|Europa]], and Ganymede around Jupiter, or between Enceladus and Dione around Saturn). All except Mercury and Venus have [[natural satellite]]s, often called "moons". Earth has one, Mars has two, and the giant planets have numerous moons in complex planetary-type systems. Except for Ceres and Sedna, all the consensus dwarf planets are known to have at least one moon as well. Many moons of the giant planets have features similar to those on the terrestrial planets and dwarf planets, and some have been studied as possible abodes of life (especially Europa and Enceladus).<ref name="Grasset2000">{{cite journal | last1=Grasset |first1=O. | last2=Sotin | first2=C. | last3=Deschamps | first3=F. |title = On the internal structure and dynamic of Titan |date = 2000 |journal = Planetary and Space Science |volume = 48 | issue= 7–8 | pages = 617–636 |doi=10.1016/S0032-0633(00)00039-8 | bibcode=2000P&SS...48..617G}}</ref><ref name="Fortes2000">{{cite journal | journal = Icarus |volume= 146 |issue = 2 |pages = 444–452 |date= 2000 |doi = 10.1006/icar.2000.6400 |title = Exobiological implications of a possible ammonia-water ocean inside Titan | last=Fortes | first=A. D. |bibcode=2000Icar..146..444F}}</ref><ref>{{cite news |first=Nicola |last=Jones |date=11 December 2001 |work=New Scientist Print Edition |url=https://www.newscientist.com/article.ns?id=dn1647 |title=Bacterial explanation for Europa's rosy glow |access-date=23 August 2008 |archive-date=10 April 2008 |archive-url=https://web.archive.org/web/20080410053352/http://www.newscientist.com/article.ns?id=dn1647 }}</ref><ref name="Taubner et al. 2018">{{Cite journal| doi = 10.1038/s41467-018-02876-y| issn = 2041-1723| volume = 9| issue = 1| page = 748| last1 = Taubner| first1 = Ruth-Sophie| last2 = Pappenreiter| first2 = Patricia| last3 = Zwicker| first3 = Jennifer| last4 = Smrzka| first4 = Daniel| last5 = Pruckner| first5 = Christian| last6 = Kolar| first6 = Philipp| last7 = Bernacchi| first7 = Sébastien| last8 = Seifert| first8 = Arne H.| last9 = Krajete| first9 = Alexander| last10 = Bach| first10 = Wolfgang| last11 = Peckmann| first11 = Jörn| last12 = Paulik| first12 = Christian| last13 = Firneis| first13 = Maria G.| last14 = Schleper| first14 = Christa| last15 = Rittmann| first15 = Simon K.-M. R.| title = Biological methane production under putative Enceladus-like conditions| journal = Nature Communications| date = 27 February 2018| pmid = 29487311| pmc = 5829080| bibcode = 2018NatCo...9..748T}}</ref><ref name="Affholder et al. 2021">{{cite journal |author=Affholder, Antonin |display-authors=et al. |title=Bayesian analysis of Enceladus's plume data to assess methanogenesis |url=https://www.nature.com/articles/s41550-021-01372-6 |date=7 June 2021 |journal=[[Nature Astronomy]] |volume=5 |issue=8 |pages=805–814 |doi=10.1038/s41550-021-01372-6 |bibcode=2021NatAs...5..805A |s2cid=236220377 |access-date=7 July 2021 |archive-date=7 July 2021 |archive-url=https://web.archive.org/web/20210707121118/https://www.nature.com/articles/s41550-021-01372-6 |url-status=live }}</ref>

The four giant planets are orbited by [[planetary ring]]s of varying size and complexity. The rings are composed primarily of dust or particulate matter, but can host tiny '[[Rings of Saturn#Propeller moonlets|moonlets]]' whose gravity shapes and maintains their structure. Although the origins of planetary rings are not precisely known, they are believed to be the result of natural satellites that fell below their parent planets' [[Roche limit]]s and were torn apart by [[tidal force]]s.<ref>{{cite journal | last1=Molnar | first1=L. A. | last2=Dunn | first2=D. E. |title=On the Formation of Planetary Rings |journal=Bulletin of the American Astronomical Society |date=1996 |volume=28 |pages=77–115 |bibcode=1996DPS....28.1815M }}</ref><ref>{{cite book | first=Encrenaz |last=Thérèse |author-link=Thérèse Encrenaz|date=2004 |title=The Solar System |edition=3rd |pages=388–390 |publisher=Springer |isbn=978-3-540-00241-3}}</ref> The dwarf planets Haumea<ref name="Ortiz2017">{{cite journal
| display-authors = etal
| last1 = Ortiz
| first1 = J. L.
| last2 = Santos-Sanz
| first2 = P.
| last3 = Sicardy
| first3 = B.
| last4 = Benedetti-Rossi
| first4 = G.
| last5 = Bérard
| first5 = D.
| last6 = Morales
| first6 = N.
| title = The size, shape, density and ring of the dwarf planet Haumea from a stellar occultation
| url = http://www.astrosurf.com/sogorb/occultations/nature24051.pdf
| journal = Nature
| volume = 550
| issue = 7675
| year = 2017
| pages = 219–223
| doi = 10.1038/nature24051
| pmid = 29022593
| bibcode = 2017Natur.550..219O
| arxiv = 2006.03113
| hdl = 10045/70230
| s2cid = 205260767
| hdl-access = free
| access-date = 6 October 2022
| archive-date = 7 November 2020
| archive-url = https://web.archive.org/web/20201107052958/http://www.astrosurf.com/sogorb/occultations/nature24051.pdf
| url-status = live
}}</ref> and Quaoar also have rings.<ref name="Morgado2023">{{Cite Q|Q116754015|display-authors=1}}</ref>

No secondary characteristics have been observed around exoplanets. The [[sub-brown dwarf]] [[Cha 110913−773444]], which has been described as a [[rogue planet]], is believed to be orbited by a tiny [[protoplanetary disc]],<ref name="Luhman">{{cite journal | journal=Astrophysical Journal |last1=Luhman |first1=K. L. | last2=Adame | first2=Lucía | last3=D'Alessio | first3=Paola | last4=Calvet | first4=Nuria|author4-link=Nuria Calvet |title= Discovery of a Planetary-Mass Brown Dwarf with a Circumstellar Disk |volume=635 | issue=1 |page=L93 |doi=10.1086/498868 |date= 2005 |bibcode=2005ApJ...635L..93L|arxiv = astro-ph/0511807 |s2cid=11685964}}
* {{cite press release |author=Whitney Clavin |date=29 November 2005 |title=A Planet With Planets? Spitzer Finds Cosmic Oddball |website=NASA |url=http://www.nasa.gov/vision/universe/starsgalaxies/spitzerf-20051129.html |access-date=10 September 2007 |archive-date=11 October 2012 |archive-url=https://web.archive.org/web/20121011011111/http://www.nasa.gov/vision/universe/starsgalaxies/spitzerf-20051129.html }}</ref> and the sub-brown dwarf [[OTS 44]] was shown to be surrounded by a substantial protoplanetary disk of at least 10 Earth masses.<ref name=joergens2013_AA558>{{cite journal|last1=Joergens|first1=V.|display-authors=4|last2=Bonnefoy|first2=M.|last3=Liu|first3=Y.|last4=Bayo|first4=A.|last5=Wolf|first5=S.|last6=Chauvin|first6=G.|last7=Rojo|first7=P.|title=OTS 44: Disk and accretion at the planetary border|journal=Astronomy & Astrophysics|volume=558|number=7|date=2013|doi=10.1051/0004-6361/201322432|bibcode=2013A&A...558L...7J|arxiv = 1310.1936|page=L7|s2cid=118456052}}</ref>

== History and etymology ==
{{Further|History of astronomy|Timeline of Solar System astronomy}}The idea of planets has evolved over the history of astronomy, from the divine lights of antiquity to the earthly objects of the scientific age. The concept has expanded to include worlds not only in the Solar System, but in multitudes of other extrasolar systems. The consensus as to what counts as a planet, as opposed to other objects, has changed several times. It previously encompassed [[asteroid]]s, [[natural satellite|moons]], and [[dwarf planet]]s like [[Pluto]],<ref>{{Cite web |title=What is a Planet? {{!}} Planets |url=https://solarsystem.nasa.gov/planets/in-depth |access-date=2 May 2022 |website=NASA Solar System Exploration |archive-date=26 April 2022 |archive-url=https://web.archive.org/web/20220426123245/https://solarsystem.nasa.gov/planets/in-depth/ |url-status=live }}</ref><ref name="asteroids"/><ref name=metzger22/> and there continues to be some disagreement today.<ref name=metzger22/>

=== Ancient civilizations and classical planets ===
[[File:Apparent retrograde motion of Mars in 2003.gif|thumb|The motion of 'lights' moving across the sky is the basis of the classical definition of planets: wandering stars.]]
The five [[classical planet]]s of the [[Solar System]], being visible to the naked eye, have been known since ancient times and have had a significant impact on [[mythology]], [[religious cosmology]], and ancient [[astronomy]]. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the "[[fixed stars]]", which maintained a constant relative position in the sky.<ref>{{cite web|title=Ancient Greek Astronomy and Cosmology|publisher=[[The Library of Congress]]|url=https://www.loc.gov/collections/finding-our-place-in-the-cosmos-with-carl-sagan/articles-and-essays/modeling-the-cosmos/ancient-greek-astronomy-and-cosmology|access-date=19 May 2016|archive-date=1 May 2015|archive-url=https://web.archive.org/web/20150501051354/https://www.loc.gov/collections/finding-our-place-in-the-cosmos-with-carl-sagan/articles-and-essays/modeling-the-cosmos/ancient-greek-astronomy-and-cosmology|url-status=live}}</ref> Ancient Greeks called these lights {{lang|grc|[[wikt:πλάνης|πλάνητες]] [[wikt:ἀστήρ|ἀστέρες]]}} ({{Transliteration|grc|planētes asteres}}) {{gloss|wandering stars}} or simply {{lang|grc|[[wikt:πλανήτης#Ancient Greek|πλανῆται]]}} ({{Transliteration|grc|planētai}}) {{gloss|wanderers}}<ref>{{LSJ|pla/nhs|πλάνης}}, {{LSJ|planh/ths|πλανήτης|ref}} Retrieved on 11 July 2022.</ref> from which today's word "planet" was derived.<ref>{{cite web |url=http://www.merriam-webster.com/dictionary/planet |title=Definition of planet |publisher=Merriam-Webster OnLine |access-date=23 July 2007 |archive-date=1 June 2012 |archive-url=https://web.archive.org/web/20120601063256/http://www.merriam-webster.com/dictionary/planet |url-status=live }}</ref><ref>{{cite encyclopedia|url=http://dictionary.reference.com/browse/planet|title=''Planet'' Etymology|dictionary=[[dictionary.com]]|access-date=29 June 2015|archive-date=2 July 2015|archive-url=https://web.archive.org/web/20150702151908/http://dictionary.reference.com/browse/planet|url-status=live}}</ref><ref name="oed" /> In [[ancient Greece]], [[History of China#Ancient China|China]], [[Babylon]], and indeed all pre-modern civilizations,<ref>{{cite journal |first=Otto E. |last=Neugebauer |date=1945 |title=The History of Ancient Astronomy Problems and Methods |journal=Journal of Near Eastern Studies |volume=4 |issue=1 |pages=1–38 |doi=10.1086/370729|s2cid=162347339 }}</ref><ref>{{cite book |first=Colin |last=Ronan |title=Astronomy in China, Korea and Japan |editor=Walker, Christopher |chapter=Astronomy Before the Telescope |publisher=British Museum Press |pages=264–265 |date=1996|bibcode=1996abt..conf..245R }}</ref> it was almost universally believed that Earth was the [[Geocentric|center of the Universe]] and that all the "planets" circled Earth. The reasons for this perception were that stars and planets appeared to revolve around Earth each day<ref>{{cite book |first=Thomas S. |last=Kuhn |title=The Copernican Revolution |url=https://archive.org/details/copernicanrevolu0008kuhn |url-access=registration |pages=[https://archive.org/details/copernicanrevolu0008kuhn/page/5 5–20] |publisher=Harvard University Press |date=1957 |isbn=978-0-674-17103-9}}</ref> and the apparently [[common sense|common-sense]] perceptions that Earth was solid and stable and that it was not moving but at rest.<ref name="TMU">{{Cite book |last1=Frautschi |first1=Steven C. |title=The Mechanical Universe: Mechanics and Heat |title-link=The Mechanical Universe |last2=Olenick |first2=Richard P. |last3=Apostol |first3=Tom M. |last4=Goodstein |first4=David L. |date=2007 |publisher=Cambridge University Press |isbn=978-0-521-71590-4 |edition=Advanced |location=Cambridge [Cambridgeshire] |oclc=227002144 |author-link=Steven Frautschi |author-link3=Tom M. Apostol |author-link4=David L. Goodstein |page=58}}</ref>

==== Babylon ====
{{Main|Babylonian astronomy}}

The first civilization known to have a functional theory of the planets were the [[Babylonia]]ns, who lived in [[Mesopotamia]] in the first and second millennia BC. The oldest surviving planetary astronomical text is the Babylonian [[Venus tablet of Ammisaduqa]], a 7th-century BC copy of a list of observations of the motions of the planet Venus, that probably dates as early as the second millennium BC.<ref name="practice" /> The [[MUL.APIN]] is a pair of [[cuneiform]] tablets dating from the 7th century BC that lays out the motions of the Sun, Moon, and planets over the course of the year.<ref>{{cite book | first=Francesca | last=Rochberg | chapter=Astronomy and Calendars in Ancient Mesopotamia | title=Civilizations of the Ancient Near East | volume=III | editor=Jack Sasson | date=2000 | page=1930 }}</ref> Late Babylonian astronomy is the origin of Western astronomy and indeed all Western efforts in the [[exact science]]s.<ref name="Aaboe, Asger">{{ Citation | last = Aaboe | first = Asger | author-link = Asger Aaboe | editor-last = Boardman | editor-first = John | editor-link = John Boardman (art historian) | editor2-last = Edwards | editor2-first = I. E. S. | editor2-link = I. E. S. Edwards | editor3-last = Hammond | editor3-first = N. G. L. | editor3-link =  N. G. L. Hammond | editor4-last = Sollberger | editor4-first = E. | editor5-last = Walker | editor5-first = C. B. F | date = 1991 | title = The Assyrian and Babylonian Empires and other States of the Near East, from the Eighth to the Sixth Centuries B.C. | chapter = The culture of Babylonia: Babylonian mathematics, astrology, and astronomy | series = The Cambridge Ancient History | volume = 3 | issue = 2 | publisher = Cambridge University Press | location = Cambridge | pages = 276–292 | isbn = 978-0-521-22717-9 }}</ref> The ''[[Enuma anu enlil]]'', written during the [[Neo-Assyrian]] period in the 7th century BC,<ref>{{cite book | volume=8 |series=State Archives of Assyria |title=Astrological reports to Assyrian kings |editor=Hermann Hunger |date=1992 |publisher=Helsinki University Press |isbn=978-951-570-130-5}}</ref> comprises a list of [[omen]]s and their relationships with various celestial phenomena including the motions of the planets.<ref>{{cite journal |title=Babylonian Planetary Omens. Part One. Enuma Anu Enlil, Tablet 63: The Venus Tablet of Ammisaduqa. |first1=W. G. |last1=Lambert |date=1987 |journal=Journal of the American Oriental Society |doi=10.2307/602955 |volume=107 |issue=1 |last2=Reiner |first2=Erica |jstor=602955 |pages=93–96}}</ref><ref name="ancientmes">{{cite journal |url=http://www.folklore.ee/Folklore/vol16/planets.pdf |last1=Kasak |first1=Enn |last2=Veede |first2=Raul |title=Understanding Planets in Ancient Mesopotamia |journal=Electronic Journal of Folklore |access-date=6 February 2008 |volume=16 |date=2001 |pages=7–35 |editor=Mare Kõiva |editor2=Andres Kuperjanov |doi=10.7592/fejf2001.16.planets |citeseerx=10.1.1.570.6778 |archive-date=4 February 2019 |archive-url=https://web.archive.org/web/20190204141254/http://www.folklore.ee/folklore/vol16/planets.pdf |url-status=live }}</ref> The [[inferior planet]]s [[Venus]] and [[Mercury (planet)|Mercury]] and the superior planets [[Mars]], [[Jupiter]], and [[Saturn]] were all identified by [[Babylonian astronomy|Babylonian astronomers]]. These would remain the only known planets until the invention of the [[telescope]] in early modern times.<ref>{{cite journal |title=Babylonian Observational Astronomy |first=A. |last=Sachs |journal=[[Philosophical Transactions of the Royal Society]] |volume=276 |issue=1257 |date=2 May 1974 |pages=43–50 [45 & 48–49] |jstor=74273 |doi=10.1098/rsta.1974.0008 |bibcode=1974RSPTA.276...43S|s2cid=121539390 }}</ref>

==== Greco-Roman astronomy ====
{{See also|Ancient Greek astronomy}}

The [[Ancient Greece|ancient Greeks]] initially did not attach as much significance to the planets as the Babylonians. In the 6th and 5th centuries BC, the [[Pythagoreans]] appear to have developed [[Pythagorean astronomical system|their own independent planetary theory]], which consisted of the Earth, Sun, Moon, and planets revolving around a "Central Fire" at the center of the Universe. [[Pythagoras]] or [[Parmenides]] is said to have been the first to identify the evening star ([[Hesperos]]) and morning star ([[Phosphoros]]) as one and the same ([[Aphrodite]], Greek corresponding to Latin [[Venus]]),<ref name="burnet">{{cite book | first=John |last=Burnet |title= Greek philosophy: Thales to Plato |date=1950 |publisher=Macmillan and Co. |pages=7–11 |url=https://books.google.com/books?id=7yUAmmqHHEgC&pg=PR4 |access-date=7 February 2008 |isbn=978-1-4067-6601-1}}</ref> though this had long been known in Mesopotamia.<ref name=Cooley>{{cite journal |last=Cooley |first=Jeffrey L. |title=Inana and Šukaletuda: A Sumerian Astral Myth |url=https://www.academia.edu/1247599 |journal=KASKAL |volume=5 |pages=161–172 |year=2008 |issn=1971-8608 |quote=The Greeks, for example, originally identified the morning and evening stars with two separate deities, Phosphoros and Hesporos respectively. In Mesopotamia, it seems that this was recognized prehistorically. Assuming its authenticity, a cylinder seal from the Erlenmeyer collection attests to this knowledge in southern Iraq as early as the Late Uruk / Jemdet Nasr Period, as do the archaic texts of the period. [...] Whether or not one accepts the seal as authentic, the fact that there is no epithetical distinction between the morning and evening appearances of Venus in any later Mesopotamian literature attests to a very, very early recognition of the phenomenon. |access-date=26 November 2022 |archive-date=24 December 2019 |archive-url=https://web.archive.org/web/20191224105634/https://www.academia.edu/1247599 |url-status=live }}</ref><ref>{{Cite journal |last=Kurtik |first=G. E. |date=June 1999 |title=The identification of Inanna with the planet Venus: A criterion for the time determination of the recognition of constellations in ancient Mesopotamia |url=http://www.tandfonline.com/doi/abs/10.1080/10556799908244112 |journal=Astronomical & Astrophysical Transactions |language=en |volume=17 |issue=6 |pages=501–513 |doi=10.1080/10556799908244112 |bibcode=1999A&AT...17..501K |issn=1055-6796 |access-date=13 July 2022 |archive-date=16 June 2022 |archive-url=https://web.archive.org/web/20220616151834/https://www.tandfonline.com/doi/abs/10.1080/10556799908244112 |url-status=live }}</ref> In the 3rd century BC, [[Aristarchus of Samos]] proposed a [[Heliocentrism|heliocentric]] system, according to which Earth and the planets revolved around the Sun. The geocentric system remained dominant until the [[Scientific Revolution]].<ref name="TMU"/>

By the 1st century BC, during the [[Hellenistic period]], the Greeks had begun to develop their own mathematical schemes for predicting the positions of the planets. These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness and account for most of the astronomical movements observed from Earth with the naked eye. These theories would reach their fullest expression in the ''[[Almagest]]'' written by [[Ptolemy]] in the 2nd century CE. So complete was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries.<ref name="practice" /><ref name="almagest" /> To the Greeks and Romans, there were seven known planets, each presumed to be [[Geocentrism|circling Earth]] according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order and using modern names): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.<ref name="oed">{{cite encyclopedia | url= http://dictionary.oed.com/cgi/entry/50180718?query_type=word&queryword=planet | dictionary= Oxford English Dictionary | title= planet, n | access-date= 7 February 2008 | date= 2007 | archive-date= 3 July 2012 | archive-url= https://web.archive.org/web/20120703072456/http://oed.com/public/redirect/welcome-to-the-new-oed-online | url-status= live }} ''Note: select the Etymology tab ''</ref><ref name="almagest">{{cite journal |first=Bernard R. |last=Goldstein |title=Saving the phenomena: the background to Ptolemy's planetary theory | journal=Journal for the History of Astronomy |volume=28 |issue=1 |date=1997 |pages=1–12 |bibcode=1997JHA....28....1G|doi=10.1177/002182869702800101 |s2cid=118875902 }}</ref><ref>{{cite book |title=Ptolemy's Almagest |author1= Ptolemy |author-link=Ptolemy |author2=Toomer, G. J. |author2-link=G. J. Toomer |publisher=Princeton University Press |date=1998 |isbn=978-0-691-00260-6}}</ref>

=== Medieval astronomy ===
{{Main|Astronomy in the medieval Islamic world|Indian astronomy}}[[File:1660 illustration of Claudius Ptolemy's geocentric model of the Universe.jpg|thumb|upright=1.5|1660 illustration of Claudius Ptolemy's geocentric model]]

After the [[fall of the Western Roman Empire]], astronomy developed further in India and the medieval Islamic world. In 499 CE, the Indian astronomer [[Aryabhata]] propounded a planetary model that explicitly incorporated [[Earth's rotation]] about its axis, which he explains as the cause of what appears to be an apparent westward motion of the stars. He also theorized that the orbits of planets were [[Ellipse|elliptical]].<ref>{{cite web | first1=J. J. | last1=O'Connor | first2=E. F. | last2=Robertson | url=https://mathshistory.st-andrews.ac.uk/Biographies/Aryabhata_I/ | title=Aryabhata the Elder | website=[[MacTutor History of Mathematics archive]] | access-date=10 July 2022 | archive-date=1 February 2022 | archive-url=https://web.archive.org/web/20220201090937/https://mathshistory.st-andrews.ac.uk/Biographies/Aryabhata_I/ | url-status=live }}</ref> Aryabhata's followers were particularly strong in [[South India]], where his principles of the diurnal rotation of Earth, among others, were followed and a number of secondary works were based on them.<ref>{{cite book | author-link=K. V. Sarma | last=Sarma | first=K. V. | year=1997 |chapter= Astronomy in India| editor-link=Helaine Selin | editor-last=Selin | editor-first=Helaine |title=Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures | publisher=Kluwer Academic Publishers | isbn=0-7923-4066-3 | page=116 }}</ref>

The astronomy of the [[Islamic Golden Age]] mostly took place in the [[Middle East]], [[Central Asia]], [[Al-Andalus]], and [[North Africa]], and later in the [[Far East]] and India. These astronomers, like the polymath [[Ibn al-Haytham]], generally accepted geocentrism, although they did dispute Ptolemy's system of epicycles and sought alternatives. The 10th-century astronomer [[Abu Sa'id al-Sijzi]] accepted that the Earth rotates around its axis.<ref name=Scientia>{{Cite journal| volume = 108| issue = 67| page = 762| first = Alessandro| last = Bausani| title = Cosmology and Religion in Islam| journal = Scientia/Rivista di Scienza| date = 1973}}</ref> In the 11th century, the [[transit of Venus]] was observed by [[Avicenna]].<ref>{{cite encyclopedia |title=Ibn Sīnā: Abū ʿAlī al-Ḥusayn ibn ʿAbdallāh ibn Sīnā |first=Sally P. |last= Ragep |editor=Thomas Hockey |encyclopedia=The Biographical Encyclopedia of Astronomers |publisher=[[Springer Science+Business Media]] |date=2007 |pages=570–572 |doi=10.1888/0333750888/3736 |bibcode=2000eaa..bookE3736. |isbn=978-0-333-75088-9|chapter=Ibn Sina, Abu Ali [known as Avicenna] (980?1037) }}</ref> His contemporary [[Al-Biruni]] devised a method of determining the Earth's radius using [[trigonometry]] that, unlike the older method of [[Eratosthenes]], only required observations at a single mountain.<ref name="The Lost Art of Finding Our Way">{{Cite book|url={{google books |plainurl=y |id=9QAuAAAAQBAJ |page=216}}|title= The Lost Art of Finding Our Way |year=2013|publisher=Harvard University Press|isbn=978-0-674-07282-4 |first=John Edward |last=Huth|pages=216–217}}</ref>

=== Scientific Revolution and discovery of outer planets ===
{{See also|Heliocentrism}}
[[File:The_Solar_System,_with_the_orbits_of_5_remarkable_comets._LOC_2013593161.jpg|thumb|True-scale Solar System poster made by [[Emanuel Bowen]] in 1747. At that time, Uranus, Neptune, and the asteroid belts had all not yet been discovered.]]
With the advent of the [[Scientific Revolution]] and the [[heliocentric model]] of [[Copernicus]], [[Galileo]], and [[Kepler]], use of the term "planet" changed from something that moved around the sky relative to the [[fixed star]] to a body that orbited the Sun, directly (a primary planet) or indirectly (a secondary or satellite planet). Thus the Earth was added to the roster of planets,<ref name="galileo_project" /> and the Sun was removed. The Copernican count of primary planets stood until 1781, when [[William Herschel]] discovered [[Uranus]].<ref name="Dreyer">{{cite book|first=J. L. E. | last=Dreyer |year=1912|title=The Scientific Papers of Sir William Herschel|publisher=Royal Society and Royal Astronomical Society|volume=1|page=100|author-link=J. L. E. Dreyer|url=https://archive.org/details/scientificpapers032804mbp/page/100/mode/2up}}</ref>

When four satellites of Jupiter (the [[Galilean moons]]) and five of Saturn were discovered in the 17th century, they joined Earth's Moon in the category of "satellite planets" or "secondary planets" orbiting the primary planets, though in the following decades they would come to be called simply "satellites" for short. Scientists generally considered planetary satellites to also be planets until about the 1920s, although this usage was not common among non-scientists.<ref name=metzger22>{{cite journal |last1=Metzger |first1=Philip T. |author-link1=Philip T. Metzger |last2=Grundy |first2=W. M. |first3=Mark V. |last3=Sykes |first4=Alan |last4=Stern |first5=James F. |last5=Bell III |first6=Charlene E. |last6=Detelich |first7=Kirby |last7=Runyon |first8=Michael |last8=Summers |date=2022 |title=Moons are planets: Scientific usefulness versus cultural teleology in the taxonomy of planetary science |url=https://www.sciencedirect.com/science/article/pii/S0019103521004206 |journal=Icarus |volume=374 |issue= |article-number=114768 |doi=10.1016/j.icarus.2021.114768 |arxiv=2110.15285 |bibcode=2022Icar..37414768M |s2cid=240071005 |access-date=8 August 2022 |archive-date=11 September 2022 |archive-url=https://web.archive.org/web/20220911060134/https://www.sciencedirect.com/science/article/pii/S0019103521004206 |url-status=live }}</ref>

In the first decade of the 19th century, four new 'planets' were discovered: [[Ceres (dwarf planet)|Ceres]] (in 1801), [[2 Pallas|Pallas]] (in 1802), [[3 Juno|Juno]] (in 1804), and [[4 Vesta|Vesta]] (in 1807). It soon became apparent that they were rather different from previously known planets: they shared the same general region of space, between Mars and Jupiter (the [[asteroid belt]]), with sometimes overlapping orbits. This was an area where only one planet had been expected, and they were much smaller than all other planets; indeed, it was suspected that they might be shards of a larger planet that had broken up. Herschel called them ''[[asteroid]]s'' (from the Greek for "starlike") because even in the largest telescopes they resembled stars, without a resolvable disk.<ref name="asteroids" /><ref>{{cite OED|asteroid}}</ref>

The situation was stable for four decades, but in the 1840s several additional asteroids were discovered ([[5 Astraea|Astraea]] in 1845; [[6 Hebe|Hebe]], [[7 Iris|Iris]], and [[8 Flora|Flora]] in 1847; [[9 Metis|Metis]] in 1848; and [[10 Hygiea|Hygiea]] in 1849). New "planets" were discovered every year; as a result, astronomers began tabulating the asteroids ([[minor planet]]s) separately from the major planets and assigning them numbers instead of abstract [[planetary symbol]]s,<ref name="asteroids">{{cite web | last =Hilton |first =James L. |date = 17 September 2001 |url =http://aa.usno.navy.mil/faq/docs/minorplanets.php |title =When Did the Asteroids Become Minor Planets? |publisher =U.S. Naval Observatory |access-date = 8 April 2007 |archive-url = https://web.archive.org/web/20070921162818/http://aa.usno.navy.mil/faq/docs/minorplanets.php |archive-date = 21 September 2007}}</ref> although they continued to be considered as small planets.<ref name=metzger19/>

[[Discovery of Neptune|Neptune was discovered in 1846]], its position having been predicted thanks to its gravitational influence upon Uranus. Because the orbit of Mercury appeared to be affected in a similar way, it was believed in the late 19th century that there might be [[Vulcan (hypothetical planet)|another planet even closer to the Sun]]. However, the discrepancy between Mercury's orbit and the predictions of Newtonian gravity was instead explained by an improved theory of gravity, Einstein's [[general relativity]].<ref>{{cite book | first1=Richard P. | last1=Baum  | first2=William | last2=Sheehan | title=In Search of Planet Vulcan: The Ghost in Newton's Clockwork | publisher=Basic Books | year=2003 | page=264 | isbn=978-0-7382-0889-3 }}</ref><ref>{{cite journal | last1 = Park | first1 = Ryan S. |  last2 = Folkner  | first2= William M. |last3 = Konopliv |first3 = Alexander S. | last4 = Williams | first4 = James G. |last5 = Smith |first5 = David E. |last6 =  Zuber | first6 = Maria T.| display-authors = 4 | year = 2017 | title = Precession of Mercury's Perihelion from Ranging to the MESSENGER Spacecraft | journal = The Astronomical Journal | volume = 153 | issue = 3| page = 121 | doi=10.3847/1538-3881/aa5be2| bibcode = 2017AJ....153..121P | hdl = 1721.1/109312 | s2cid = 125439949 | hdl-access = free | doi-access = free }}</ref>

[[Pluto]] was discovered in 1930. After initial observations led to the belief that it was larger than Earth,<ref>{{cite book |title=Planet Quest: The Epic Discovery of Alien Solar Systems |first=Ken |last=Croswell |author-link=Ken Croswell |publisher=The Free Press |date=1997 |page=57 |isbn=978-0-684-83252-4 }}</ref> the object was immediately accepted as the ninth major planet. Further monitoring found the body was actually much smaller: in 1936, [[Raymond Arthur Lyttleton|Ray Lyttleton]] suggested that Pluto may be an escaped satellite of [[Neptune]],<ref>{{cite journal | last=Lyttleton |first=Raymond A. |date=1936 |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=97 |issue=2 |pages=108–115 |title= On the possible results of an encounter of Pluto with the Neptunian system |bibcode=1936MNRAS..97..108L |doi=10.1093/mnras/97.2.108|doi-access=free }}</ref> and [[Fred Lawrence Whipple|Fred Whipple]] suggested in 1964 that Pluto may be a comet.<ref>{{cite journal | journal=Proceedings of the National Academy of Sciences of the United States of America |volume=52 |pages=565–594 |last=Whipple |first=Fred |date=1964 |bibcode=1964PNAS...52..565W |title= The History of the Solar System |doi= 10.1073/pnas.52.2.565 | pmid=16591209 | issue=2 | pmc=300311|doi-access=free }}</ref> The discovery of its large moon [[Charon (moon)|Charon]] in 1978 showed that Pluto was only 0.2% the mass of Earth.<ref name="ChristyHarrington1978">{{cite journal| first1 = James W.| last1 = Christy| first2 = Robert Sutton| last2 = Harrington| author-link2 = Robert Sutton Harrington| title = The Satellite of Pluto| journal = Astronomical Journal| date = 1978| volume = 83| issue = 8| pages = 1005–1008| bibcode = 1978AJ.....83.1005C| doi = 10.1086/112284| s2cid = 120501620}}</ref> As this was still substantially more massive than any known asteroid, and because no other [[trans-Neptunian objects]] had been discovered at that time, Pluto kept its planetary status, only officially losing it in 2006.<ref>{{cite journal | journal=Scientific American |date=1996 |pages=46–52 |last1=Luu |first1=Jane X. |last2=Jewitt |first2=David C. |title=The Kuiper Belt |volume=274 |issue=5 |doi=10.1038/scientificamerican0596-46|bibcode = 1996SciAm.274e..46L }}</ref><ref name="Pluto loses status as a planet-2006">{{cite news |date=24 August 2006 |title=Pluto loses status as a planet |url=https://news.bbc.co.uk/2/hi/5282440.stm |url-status=live |archive-url=https://web.archive.org/web/20120530155226/http://news.bbc.co.uk/2/hi/5282440.stm |archive-date=30 May 2012 |access-date=23 August 2008 |department=[[BBC News]] |publisher=[[British Broadcasting Corporation]]}}</ref>

In the 1950s, [[Gerard Kuiper]] published papers on the origin of the asteroids. He recognized that asteroids were typically not spherical, as had previously been thought, and that the [[asteroid family|asteroid families]] were remnants of collisions. Thus he differentiated between the largest asteroids as "true planets" versus the smaller ones as collisional fragments. From the 1960s onwards, the term "minor planet" was mostly displaced by the term "asteroid", and references to the asteroids as planets in the literature became scarce, except for the geologically evolved largest three: Ceres, and less often Pallas and Vesta.<ref name=metzger19>{{cite journal |last1=Metzger |first1=Philip T. |author-link1=Philip T. Metzger |last2=Sykes |first2=Mark V. |last3=Stern |first3=Alan |last4=Runyon |first4=Kirby |date=2019 |title=The Reclassification of Asteroids from Planets to Non-Planets |journal=Icarus |volume=319 |pages=21–32 |doi=10.1016/j.icarus.2018.08.026|arxiv=1805.04115 |bibcode=2019Icar..319...21M |s2cid=119206487 }}</ref>

The beginning of Solar System exploration by space probes in the 1960s spurred a renewed interest in planetary science. A split in definitions regarding satellites occurred around then: planetary scientists began to reconsider the large moons as also being planets, but astronomers who were not planetary scientists generally did not.<ref name=metzger22/> (This is not exactly the same as the definition used in the previous century, which classed ''all'' satellites as secondary planets, even non-round ones like Saturn's [[Hyperion (moon)|Hyperion]] or Mars's [[Phobos (moon)|Phobos]] and [[Deimos (moon)|Deimos]].)<ref>{{cite book |last=Hind |first=John Russell |author-link=John Russell Hind |date=1863 |title=An introduction to astronomy, to which is added an astronomical vocabulary |url=https://books.google.com/books?id=d9aWKHamz8sC&dq=%22hyperion%22+%22secondary+planet%22&pg=PA205 |location=London |publisher=Henry G. Bohn |page=204 |isbn= |access-date=25 October 2023 |archive-date=30 October 2023 |archive-url=https://web.archive.org/web/20231030064739/https://books.google.com/books?id=d9aWKHamz8sC&dq=%22hyperion%22+%22secondary+planet%22&pg=PA205 |url-status=live }}</ref><ref>{{cite book |author=<!--Staff writer(s); no by-line.--> |editor-first1=Robert |editor-last1=Hunter |editor-first2=John A. |editor-last2=Williams |editor-first3=S. J. |editor-last3=Heritage |date=1897 |title=The American Encyclopædic Dictionary |url=https://books.google.com/books?id=P_VOAAAAYAAJ&dq=%22phobos%22+%22secondary+planet%22&pg=PA3553 |location=Chicago and New York |publisher=R. S. Peale and J. A. Hill |volume=8 |pages=3553–3554 |isbn= |access-date=25 October 2023 |archive-date=30 October 2023 |archive-url=https://web.archive.org/web/20231030063236/https://books.google.com/books?id=P_VOAAAAYAAJ&dq=%22phobos%22+%22secondary+planet%22&pg=PA3553 |url-status=live }}</ref> All the eight major planets and their planetary-mass moons have since been explored by spacecraft, as have many asteroids and the dwarf planets Ceres and Pluto; however, so far the only planetary-mass body beyond Earth that has been explored by humans is the Moon.{{efn|See [[Timeline of Solar System exploration]].}}

=== Defining the term ''planet'' ===
{{Further|Definition of planet}}
{{Anchor|Definition and similar concepts}}A growing number of astronomers argued for Pluto to be declassified as a planet, because many similar objects approaching its size had been found in the same region of the Solar System (the [[Kuiper belt]]) during the 1990s and early 2000s. Pluto was found to be just one "small" body in a population of thousands.<ref name="plutoplanet">{{cite journal |last1=Basri |first1=Gibor |last2=Brown |first2=Michael E. |date=2006 |title=Planetesimals to Brown Dwarfs: What is a Planet? |url=http://www.gps.caltech.edu/~mbrown/papers/ps/basribrown.pdf |url-status=live |journal=Annual Review of Earth and Planetary Sciences |volume=34 |pages=193–216 |arxiv=astro-ph/0608417 |bibcode=2006AREPS..34..193B |doi=10.1146/annurev.earth.34.031405.125058 |s2cid=119338327 |archive-url=https://web.archive.org/web/20080704213644/http://www.gps.caltech.edu/~mbrown/papers/ps/basribrown.pdf |archive-date=4 July 2008 |access-date=4 August 2008}}</ref> They often referred to the demotion of the asteroids as a precedent, although that had been done based on their geophysical differences from planets rather than their being in a belt.<ref name="metzger22" /> Some of the larger [[trans-Neptunian object]]s, such as [[50000 Quaoar|Quaoar]], [[90377 Sedna|Sedna]], [[Eris (dwarf planet)|Eris]], and [[Haumea]],<ref name="planeta">{{cite web|title=Estados Unidos "conquista" Haumea|work=[[ABC.es|ABC]]|date=20 September 2008|url=http://www.abc.es/20080920/nacional-sociedad/estados-unidos-conquista-haumea-20080920.html|access-date=18 September 2008|language=es|archive-date=6 October 2017|archive-url=https://web.archive.org/web/20171006013056/http://www.abc.es/20080920/nacional-sociedad/estados-unidos-conquista-haumea-20080920.html|url-status=live}}</ref> were heralded in the popular press as the [[tenth planet]].

The announcement of Eris in 2005, an object 27% more massive than Pluto, created the impetus for an official definition of a planet,<ref name="plutoplanet" /> as considering Pluto a planet would logically have demanded that Eris be considered a planet as well. Since different procedures were in place for naming planets versus non-planets, this created an urgent situation because under the rules Eris could not be named without defining what a planet was.<ref name="metzger22" /> At the time, it was also thought that the size required for a trans-Neptunian object to become round was about the same as that required for the moons of the giant planets (about 400&nbsp;km diameter), a figure that would have suggested about 200 round objects in the Kuiper belt and thousands more beyond.<ref name="Brown">{{cite web |author-link=Michael E. Brown |title=The Dwarf Planets |first=Michael E. |last=Brown |publisher=California Institute of Technology, Department of Geological Sciences |url=http://web.gps.caltech.edu/~mbrown/dwarfplanets/ |access-date=26 January 2008 |archive-date=19 July 2011 |archive-url=https://web.archive.org/web/20110719164447/http://web.gps.caltech.edu/~mbrown/dwarfplanets/ |url-status=live }}</ref><ref name="BrownList">{{cite web |last1=Brown |first1=Mike |author1-link=Michael E. Brown |title=How Many Dwarf Planets Are There in the Outer Solar System? |url=http://web.gps.caltech.edu/~mbrown/dps.html |publisher=California Institute of Technology |access-date=11 August 2022 |archive-url=https://web.archive.org/web/20220719141419/http://web.gps.caltech.edu/~mbrown/dps.html |archive-date=19 July 2022 |date=23 February 2021}}</ref> Many astronomers argued that the public would not accept a definition creating a large number of planets.<ref name="metzger22" />

{{Quote box
| title = The [[International Astronomical Union]]'s<br />definition of a planet in the [[Solar System]]
| quote = {{ordered list|
Object is in [[orbit]] around the Sun|
Object has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a [[hydrostatic equilibrium]] (nearly round) shape|
Object has [[cleared the neighbourhood]] around its orbit}}

Source: {{cite news|url=http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf|title=IAU 2006 General Assembly: Resolutions 5 and 6|date=24 August 2006|publisher=IAU|access-date=23 June 2009}}
| width = 375px
}}To acknowledge the problem, the [[International Astronomical Union]] (IAU) set about creating the [[IAU definition of planet|definition of planet]] and produced one in August 2006. Under this definition, the Solar System is considered to have eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune). Bodies that fulfill the first two conditions but not the third are classified as [[dwarf planet]]s, provided they are not [[natural satellite]]s of other planets. Originally an IAU committee had proposed a definition that would have included a larger number of planets as it did not include (c) as a criterion.<ref>{{cite news |last=Rincon |first=Paul |date=16 August 2006 |title=Planets plan boosts tally 12 |url=https://news.bbc.co.uk/2/hi/science/nature/4795755.stm |url-status=live |archive-url=https://web.archive.org/web/20070302051348/http://news.bbc.co.uk/1/hi/sci/tech/4795755.stm |archive-date=2 March 2007 |access-date=23 August 2008 |department=[[BBC News]] |publisher=[[British Broadcasting Corporation]]}}</ref> After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.<ref name="Pluto loses status as a planet-2006" /><ref>{{cite journal |last=Green |first=D. W. E. |date=13 September 2006 |title=(134340) Pluto, (136199) Eris, and (136199) Eris I (Dysnomia) |url=http://www.cbat.eps.harvard.edu/iauc/08700/08747.html |journal=IAU Circular |publisher=Central Bureau for Astronomical Telegrams, International Astronomical Union |issue=8747 |page=1 |bibcode=2006IAUC.8747....1G |id=Circular No. 8747 |archive-url=https://web.archive.org/web/20080624225029/http://www.cfa.harvard.edu/iau/special/08747.pdf |archive-date=24 June 2008 |access-date=5 July 2011}}</ref>

==== Criticisms and alternatives to IAU definition ====
{{See also|List of gravitationally rounded objects of the Solar System}}
[[File:25 solar system objects smaller than Earth.jpg|thumb|upright=1.6|The planetary-mass moons to scale, compared with Mercury, Venus, Earth, Mars, and Pluto. Sub-planetary [[Proteus (moon)|Proteus]] and [[Nereid (moon)|Nereid]] (about the same size as Mimas) have been included for comparison. Unimaged [[Dysnomia (moon)|Dysnomia]] (intermediate in size between Tethys and Enceladus) is not shown; it is in any case probably not a solid body.<ref name="Brown2023">{{cite journal |last1=Brown |first1=Michael E. |last2=Butler |first2=Bryan |date=October 2023 |title=Masses and densities of dwarf planet satellites measured with ALMA |journal=The Planetary Science Journal |volume=4 |issue=10 |page=6 |arxiv=2307.04848 |bibcode=2023PSJ.....4..193B |doi=10.3847/PSJ/ace52a |s2cid= |id=193 |doi-access=free}}</ref>]]

The IAU definition has not been universally used or accepted. In [[planetary geology]], celestial objects are [[geophysical definition of planet|defined as planets by geophysical characteristics]]. A celestial body may acquire a dynamic (planetary) geology at approximately the mass required for its mantle to become plastic under its own weight. This leads to a state of [[hydrostatic equilibrium]] where the body acquires a stable, round shape, which is adopted as the hallmark of planethood by geophysical definitions. For example:<ref name="Stern_Levison_2002">{{citation
 | last1=Stern | first1=S. Alan
 | last2=Levison | first2=Harold F.
 | editor1-first=H. | editor1-last=Rickman
 | title=Regarding the criteria for planethood and proposed planetary classification schemes
 | journal=Highlights of Astronomy | volume=12
 | pages=205–213 | date=2002
 | publisher=Astronomical Society of the Pacific
 | location=San Francisco
 | bibcode=2002HiA....12..205S
 | isbn=978-1-58381-086-6
 | doi=10.1017/S1539299600013289
 | doi-access=free
 }} See p. 208.</ref>

{{blockquote|a substellar-mass body that has never undergone nuclear fusion and has enough gravitation to be round due to hydrostatic equilibrium, regardless of its orbital parameters.<ref name=Astronomy>{{cite web
 |url          = http://www.astronomy.com/magazine/2018/05/an-organically-grown-planet-definition
 |title        = An organically grown planet definition — Should we really define a word by voting?
 |last1        = Runyon
 |first1       = Kirby D.
 |last2        = Stern
 |first2       = S. Alan
 |date         = 17 May 2018
 |website      = [[Astronomy (magazine)|Astronomy]]
 |access-date  = 12 October 2019
 |archive-date = 10 October 2019
 |archive-url  = https://web.archive.org/web/20191010153028/http://astronomy.com/magazine/2018/05/an-organically-grown-planet-definition
 |url-status   = live
}}</ref>}}

In the Solar System, this mass is generally less than the mass required for a body to clear its orbit; thus, some objects that are considered "planets" under geophysical definitions are not considered as such under the IAU definition, such as Ceres and Pluto.<ref name=planetarysociety/> (In practice, the requirement for hydrostatic equilibrium is universally relaxed to a requirement for rounding and compaction under self-gravity; Mercury is not actually in hydrostatic equilibrium,<ref name="Mercury">Sean Solomon, Larry Nittler & Brian Anderson, eds. (2018) ''Mercury: The View after MESSENGER''. Cambridge Planetary Science series no. 21, Cambridge University Press, pp. 72–73.</ref> but is universally included as a planet regardless.)<ref>{{Cite tweet |user=plutokiller |last=Brown |first=Mike |number=1624127764969459713 |title=The real answer here is to not get too hung up on definitions, which I admit is hard when the IAU tries to make them sound official and clear, but, really, we all understand the intent of the hydrostatic equilibrium point, and the intent is clearly to include Merucry & the moon}}</ref> Proponents of such definitions often argue that location should not matter and that planethood should be defined by the intrinsic properties of an object.<ref name=planetarysociety/> Dwarf planets had been proposed as a category of small planet (as opposed to [[Minor planet|planetoid]]s as sub-planetary objects) and planetary geologists continue to treat them as planets despite the IAU definition.<ref name=Grundy2019>{{cite journal
 |first1=W.M. |last1=Grundy
 |first2=K.S. |last2=Noll
 |first3=M.W. |last3=Buie
 |first4=S.D. |last4=Benecchi
 |first5=D. |last5=Ragozzine
 |first6=H.G. |last6=Roe
 | display-authors=4
 |title=The Mutual Orbit, Mass, and Density of Transneptunian Binary Gǃkúnǁʼhòmdímà ({{mp|(229762) 2007 UK|126}})
 |url=http://www2.lowell.edu/~grundy/abstracts/2019.G-G.html
 |journal=Icarus
 |doi=10.1016/j.icarus.2018.12.037
 |date=December 2018
 |volume=334
 |page=30
 |bibcode=2019Icar..334...30G
 |s2cid=126574999
 |archive-url=https://web.archive.org/web/20190407045339/http://www2.lowell.edu/~grundy/abstracts/preprints/2019.G-G.pdf
 |archive-date=7 April 2019}}</ref>

The number of dwarf planets even among known objects is not certain. In 2019, Grundy et al. argued based on the low densities of some mid-sized trans-Neptunian objects that the limiting size required for a trans-Neptunian object to reach equilibrium was in fact much larger than it is for the icy moons of the giant planets, being about 900–1000&nbsp;km diameter.<ref name=Grundy2019/> There is general consensus on Ceres in the asteroid belt<ref>{{Cite journal |last1=Raymond |first1=C. A. |last2=Ermakov |first2=A. I. |last3=Castillo-Rogez |first3=J. C. |last4=Marchi |first4=S. |last5=Johnson |first5=B. C. |last6=Hesse |first6=M. A. |last7=Scully |first7=J. E. C. |last8=Buczkowski |first8=D. L. |last9=Sizemore |first9=H. G. |last10=Schenk |first10=P. M. |last11=Nathues |first11=A. |display-authors=4 |date=August 2020 |title=Impact-driven mobilization of deep crustal brines on dwarf planet Ceres |url=https://www.nature.com/articles/s41550-020-1168-2 |journal=Nature Astronomy |language=en |volume=4 |issue=8 |pages=741–747 |bibcode=2020NatAs...4..741R |doi=10.1038/s41550-020-1168-2 |s2cid=211137608 |issn=2397-3366 |access-date=27 June 2022 |archive-date=21 June 2022 |archive-url=https://web.archive.org/web/20220621225448/https://www.nature.com/articles/s41550-020-1168-2 |url-status=live }}</ref> and on the eight trans-Neptunians that probably cross this threshold—{{dp|Orcus}}, {{dp|Pluto}}, {{dp|Haumea}}, {{dp|Quaoar}}, {{dp|Makemake}}, {{dp|Gonggong}}, {{dp|Eris}}, and {{dp|Sedna}}.<ref>{{Cite journal |last1=Barr |first1=Amy C. |last2=Schwamb |first2=Megan E. |date=1 August 2016 |title=Interpreting the densities of the Kuiper belt's dwarf planets |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=460 |issue=2 |pages=1542–1548 |doi=10.1093/mnras/stw1052 |issn=0035-8711|doi-access=free |arxiv=1603.06224 }}</ref><ref name=JWST>{{cite journal|last1=Emery|first1=J. P. |first2=I. |last2=Wong |first3=R. |last3=Brunetto |first4=J. C. |last4=Cook |first5=N. |last5=Pinilla-Alonso |first6=J. A. |last6=Stansberry |first7=B. J. |last7=Holler |first8=W. M. |last8=Grundy |first9=S. |last9=Protopapa |first10=A. C. |last10=Souza-Feliciano |first11=E. |last11=Fernández-Valenzuela |first12=J. I. |last12=Lunine |first13=D. C. |last13=Hines |date=2024|title=A Tale of 3 Dwarf Planets: Ices and Organics on Sedna, Gonggong, and Quaoar from JWST Spectroscopy|journal=Icarus |volume=414 |doi=10.1016/j.icarus.2024.116017 |arxiv=2309.15230|bibcode=2024Icar..41416017E }}</ref>

Planetary geologists may include the nineteen known [[planetary-mass moon]]s as "satellite planets", including Earth's Moon and Pluto's [[Charon (moon)|Charon]], like the early modern astronomers.<ref name=planetarysociety/><ref name="News.discovery.com">{{cite web |url=http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html |title=Should Large Moons Be Called 'Satellite Planets'?|last1=Villard|first1=Ray|website=Discovery News|publisher=[[Discovery, Inc.]] |date=14 May 2010 |access-date=4 November 2011 |archive-date=5 May 2012 |archive-url=https://web.archive.org/web/20120505221146/http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html }}</ref> Some go even further and include as planets relatively large, geologically evolved bodies that are nonetheless not very round today, such as Pallas and Vesta;<ref name="planetarysociety" /> rounded bodies that were completely disrupted by impacts and re-accreted like Hygiea;<ref>{{Cite web|url=https://www.space.com/asteroid-hygiea-may-be-smallest-dwarf-planet.html|title=Asteroid Hygiea May be the Smallest Dwarf Planet in the Solar System|website=[[Space.com]]|publisher=[[Purch Group]]|last1=Urrutia|first1=Doris Elin|date=28 October 2019|access-date=28 August 2022|archive-date=5 November 2019|archive-url=https://web.archive.org/web/20191105002904/https://www.space.com/asteroid-hygiea-may-be-smallest-dwarf-planet.html|url-status=live}}</ref><ref>{{Cite news|url=https://www.sciencenews.org/article/hygiea-may-be-solar-system-smallest-dwarf-planet|title=The solar system may have a new smallest dwarf planet: Hygiea|website=[[Science News]]|publisher=[[Society for Science]]|date=28 October 2019|access-date=28 August 2022|archive-date=31 August 2022|archive-url=https://web.archive.org/web/20220831083918/https://www.sciencenews.org/article/hygiea-may-be-solar-system-smallest-dwarf-planet|url-status=live}}</ref><ref name=Yang2020>{{citation|arxiv=2007.08059|title=Binary asteroid (31) Euphrosyne: Ice-rich and nearly spherical|year=2020|doi=10.1051/0004-6361/202038372|last1=Yang|first1=B.|last2=Hanuš|first2=J.|last3=Carry|first3=B.|last4=Vernazza|first4=P.|last5=Brož|first5=M.|last6=Vachier|first6=F.|last7=Rambaux|first7=N.|last8=Marsset|first8=M.|last9=Chrenko|first9=O.|last10=Ševeček|first10=P.|last11=Viikinkoski|first11=M.|last12=Jehin|first12=E.|last13=Ferrais|first13=M.|last14=Podlewska-Gaca|first14=E.|last15=Drouard|first15=A.|last16=Marchis|first16=F.|last17=Birlan|first17=M.|last18=Benkhaldoun|first18=Z.|last19=Berthier|first19=J.|last20=Bartczak|first20=P.|last21=Dumas|first21=C.|last22=Dudziński|first22=G.|last23=Ďurech|first23=J.|last24=Castillo-Rogez|first24=J.|last25=Cipriani|first25=F.|last26=Colas|first26=F.|last27=Fetick|first27=R.|last28=Fusco|first28=T.|last29=Grice|first29=J.|last30=Jorda|first30=L.|journal=Astronomy & Astrophysics|volume=641|page=A80|bibcode=2020A&A...641A..80Y|s2cid=220546126|display-authors=29}}</ref> or even everything at least the diameter of Saturn's moon [[Mimas (moon)|Mimas]], the smallest planetary-mass moon. (This may even include objects that are not round but happen to be larger than Mimas, like Neptune's moon [[Proteus (moon)|Proteus]].)<ref name=planetarysociety/>

Astronomer [[Jean-Luc Margot]] proposed a mathematical criterion that determines whether an object can clear its orbit during the lifetime of its host star, based on the mass of the planet, its semimajor axis, and the mass of its host star.<ref>{{cite news |last=Netburn |first=Deborah |date=13 November 2015 |title=Why we need a new definition of the word 'planet' |url=http://www.latimes.com/science/sciencenow/la-sci-sn-new-planet-definition-margot-20151113-htmlstory.html |url-status=live |archive-url=https://web.archive.org/web/20210603084521/https://www.latimes.com/science/sciencenow/la-sci-sn-new-planet-definition-margot-20151113-htmlstory.html |archive-date=3 June 2021 |access-date=24 July 2016 |newspaper=[[Los Angeles Times]]}}</ref> The formula produces a value called {{mvar|π}} that is greater than 1 for planets.{{efn|name=not-confuse-π|Margot's parameter<ref name=Margot/> is not to be confused with the [[Pi|famous mathematical constant]] {{nowrap|{{mvar|π}}≈3.14159265 ... .}} }} The eight known planets and all known exoplanets have {{mvar|π}} values above 100, while Ceres, Pluto, and Eris have {{mvar|π}} values of 0.1, or less. Objects with {{mvar|π}} values of 1 or more are expected to be approximately spherical, so that objects that fulfill the orbital-zone clearance requirement around Sun-like stars will also fulfill the roundness requirement<ref name="Margot">
{{cite journal |last=Margot |first=Jean-Luc |author-link=Jean-Luc Margot |year=2015 |title=A quantitative criterion for defining planets |journal=[[The Astronomical Journal]] |volume=150 |issue=6 |page=185 |arxiv=1507.06300 |bibcode=2015AJ....150..185M |doi=10.1088/0004-6256/150/6/185 |s2cid=51684830}}
</ref> – though this may not be the case around very low-mass stars.<ref name="Margot 2024"/> In 2024, Margot and collaborators proposed a revised version of the criterion with a uniform clearing timescale of 10&nbsp;billion years (the approximate main-sequence lifetime of the Sun) or 13.8&nbsp;billion years (the [[age of the universe]]) to accommodate planets orbiting brown dwarfs.<ref name="Margot 2024">{{cite journal |last1=Margot |first1=Jean-Luc |last2=Gladman |first2=Brett |last3=Yang |first3=Tony |title=Quantitative Criteria for Defining Planets |journal=The Planetary Science Journal |date=1 July 2024 |volume=5 |issue=7 |page=159 |doi=10.3847/PSJ/ad55f3|doi-access=free |arxiv=2407.07590 |bibcode=2024PSJ.....5..159M }}</ref>

=== Exoplanets ===
{{Further|Exoplanet#History of detection|Brown dwarf}}
Even before the discovery of [[exoplanet]]s, there were particular disagreements over whether an object should be considered a planet if it was part of a distinct population such as a [[asteroid belt|belt]], or if it was large enough to generate energy by the [[thermonuclear fusion]] of [[deuterium]].<ref name="plutoplanet" /> Complicating the matter even further, bodies too small to generate energy by fusing deuterium can form by [[nebula|gas-cloud]] collapse just like stars and brown dwarfs, even down to the mass of Jupiter:<ref>{{citation |last1=Boss |first1=Alan P. |title=Nomenclature: Brown Dwarfs, Gas Giant Planets, and ? |journal=Brown Dwarfs |volume=211 |page=529 |year=2003 |bibcode=2003IAUS..211..529B |last2=Basri |first2=Gibor |last3=Kumar |first3=Shiv S. |last4=Liebert |first4=James |last5=Martín |first5=Eduardo L. |last6=Reipurth |first6=Bo |last7=Zinnecker |first7=Hans}}</ref> there was thus disagreement about whether how a body formed should be taken into account.<ref name="plutoplanet" />

In 1992, astronomers [[Aleksander Wolszczan]] and [[Dale Frail]] announced the discovery of planets around a [[pulsar]], [[PSR B1257+12]].<ref name="Wolszczan" /> This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on 6 October 1995, [[Michel Mayor]] and [[Didier Queloz]] of the [[Geneva Observatory]] announced the first definitive detection of an exoplanet orbiting an ordinary [[main sequence|main-sequence]] star ([[51 Pegasi]]).<ref name="Mayor">{{cite journal |last1=Mayor |first1=Michel |author2=Queloz, Didier |date=1995 |title=A Jupiter-mass companion to a solar-type star |journal=Nature |volume=378 |issue=6356 |pages=355–359 |bibcode=1995Natur.378..355M |doi=10.1038/378355a0 |s2cid=4339201}}</ref>

The discovery of exoplanets led to another ambiguity in defining a planet: the point at which a planet becomes a star. Many known exoplanets are many times the mass of Jupiter, approaching that of stellar objects known as [[brown dwarf]]s. Brown dwarfs are generally considered stars due to their theoretical ability to fuse [[deuterium]], a heavier isotope of [[hydrogen]]. Although objects more massive than 75 times that of Jupiter fuse simple hydrogen, objects of 13 Jupiter masses can fuse deuterium. Deuterium is quite rare, constituting less than 0.0026% of the hydrogen in the galaxy, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.<ref>{{cite journal |last=Basri |first=Gibor |date=2000 |title=Observations of Brown Dwarfs |journal=Annual Review of Astronomy and Astrophysics |volume=38 |issue=1 |pages=485–519 |bibcode=2000ARA&A..38..485B |doi=10.1146/annurev.astro.38.1.485}}</ref>

==== IAU working definition of exoplanets ====
The 2006 IAU definition presents some challenges for exoplanets because the language is specific to the Solar System and the criteria of roundness and orbital zone clearance are not presently observable for exoplanets.<ref name="exodef">{{Cite journal |last1=Lecavelier des Etangs |first1=A. |last2=Lissauer |first2=Jack J. |date=1 June 2022 |title=The IAU working definition of an exoplanet |url=https://www.sciencedirect.com/science/article/pii/S138764732200001X |journal=New Astronomy Reviews |language=en |volume=94 |article-number=101641 |doi=10.1016/j.newar.2022.101641 |arxiv=2203.09520 |bibcode=2022NewAR..9401641L |s2cid=247065421 |issn=1387-6473 |access-date=13 May 2022 |archive-date=13 May 2022 |archive-url=https://web.archive.org/web/20220513110848/https://www.sciencedirect.com/science/article/pii/S138764732200001X |url-status=live }}</ref> In 2018, this definition was reassessed and updated as knowledge of exoplanets increased.<ref name="iauexo">{{cite journal |last1=Lecavelier des Etangs |first1=A. |last2=Lissauer |first2=Jack J. |date=2022 |title=The IAU working definition of an exoplanet |journal=New Astronomy Reviews |volume=94 |issue= |article-number=101641 |arxiv=2203.09520 |bibcode=2022NewAR..9401641L |doi=10.1016/j.newar.2022.101641 |s2cid=247065421}}</ref> The current official working definition of an exoplanet is as follows:<ref name="exoworkdef" />

{{blockquote|
# Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars, brown dwarfs, or stellar remnants and that have a mass ratio with the central object below the [[Lagrange point#Stability|L4/L5 instability]] (M/M<sub>central</sub> < 2/(25+{{math|{{radical|621}}}}) are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.
# Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.
# Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).<ref name=exoworkdef/>
}}

The IAU noted that this definition could be expected to evolve as knowledge improves.<ref name="exoworkdef">{{cite web |title=Official Working Definition of an Exoplanet |url=https://www.iau.org/science/scientific_bodies/commissions/F2/info/documents/ |access-date=29 November 2020 |work=IAU position statement |archive-date=3 July 2022 |archive-url=https://web.archive.org/web/20220703184850/https://www.iau.org/science/scientific_bodies/commissions/F2/info/documents/ |url-status=live }}</ref> A 2022 review article discussing the history and rationale of this definition suggested that the words "in young star clusters" should be deleted in clause 3, as such objects have now been found elsewhere, and that the term "sub-brown dwarfs" should be replaced by the more current "free-floating planetary mass objects". The term "[[Planetary-mass object|planetary mass object]]" has also been used to refer to ambiguous situations concerning exoplanets, such as objects with mass typical for a planet that are free-floating or orbit a brown dwarf instead of a star.<ref name="iauexo" /> Free-floating objects of planetary mass have sometimes been called planets anyway, specifically [[rogue planet]]s.<ref name="eso2120">{{cite web | title = ESO telescopes help uncover largest group of rogue planets yet | url = https://www.eso.org/public/news/eso2120/ | publisher = [[European Southern Observatory]] | date = 22 December 2021 | access-date = 22 December 2021}}</ref>

The limit of 13 Jupiter masses is not universally accepted. Objects below this mass limit can sometimes burn deuterium, and the amount of deuterium that is burned depends on an object's composition.<ref name="bodenheimer2013">{{cite journal |title=Deuterium Burning in Massive Giant Planets and Low-mass Brown Dwarfs Formed by Core-nucleated Accretion |journal=The Astrophysical Journal |date=2013 |volume=770 |issue=2 |page=120 |doi=10.1088/0004-637X/770/2/120 |arxiv=1305.0980 |bibcode=2013ApJ...770..120B |last1=Bodenheimer |first1=Peter |last2=D'Angelo |first2=Gennaro |last3=Lissauer |first3=Jack J. |last4=Fortney |first4=Jonathan J. |last5=Saumon |first5=Didier |s2cid=118553341 }}</ref><ref>{{Cite journal | doi = 10.1088/0004-637X/727/1/57| title = The Deuterium-Burning Mass Limit for Brown Dwarfs and Giant Planets| journal = The Astrophysical Journal| volume = 727| issue = 1| page = 57| year = 2011| last1 = Spiegel | first1 = D. S. |last2=Burrows |first2=Adam | last3 = Milsom | first3 = J. A. | bibcode = 2011ApJ...727...57S|arxiv = 1008.5150 | s2cid = 118513110}}</ref> Furthermore, deuterium is quite scarce, so the stage of deuterium burning does not actually last very long; unlike hydrogen burning in a star, deuterium burning does not significantly affect the future evolution of an object.<ref name="Hatzes" /> The relationship between mass and radius (or density) show no special feature at this limit, according to which brown dwarfs have the same physics and internal structure as lighter Jovian planets, and would more naturally be considered planets.<ref name="Hatzes">{{cite journal |arxiv=1506.05097 |last1=Hatzes |first1=Artie P. |author-link1=Artie P. Hatzes |last2=Rauer |first2=Heike |author-link2=Heike Rauer |title=A Definition for Giant Planets Based on the Mass-Density Relationship |year=2015 |doi=10.1088/2041-8205/810/2/L25 |volume=810 |issue=2 |journal=The Astrophysical Journal |page=L25 |bibcode=2015ApJ...810L..25H |s2cid= 119111221 }}</ref><ref name="ChenKipping" />

Thus, many catalogues of exoplanets include objects heavier than 13 Jupiter masses, sometimes going up to 60 Jupiter masses.<ref>{{cite journal |last1=Schneider |first1=Jean |last2=Dedieu |first2=Cyril |last3=Le Sidaner |first3=Pierre |last4=Savalle |first4=Renaud |last5=Zolotukhin |first5=Ivan |title=Defining and cataloging exoplanets: The exoplanet.eu database |date=2011 |volume=532 |issue=79 |journal=[[Astronomy & Astrophysics]] |arxiv=1106.0586 |doi=10.1051/0004-6361/201116713 |pages=A79 |bibcode=2011A&A...532A..79S |s2cid=55994657 }}</ref><ref name="corot">{{cite book |last=Schneider |first=Jean |arxiv=1604.00917 |chapter=Exoplanets versus brown dwarfs: the CoRoT view and the future |title=The CoRoT Legacy Book |date=July 2016 |page=157 |doi=10.1051/978-2-7598-1876-1.c038 |isbn=978-2-7598-1876-1|s2cid=118434022 }}</ref><ref name="eod">{{cite journal |arxiv=1012.5676 |title=The Exoplanet Orbit Database |date=2010 |bibcode=2011PASP..123..412W |doi=10.1086/659427 |volume=123 |issue=902 |journal=[[Publications of the Astronomical Society of the Pacific]] |pages=412–422 |last1=Wright |first1=Jason T. |last2=Fakhouri |first2=Onsi |last3=Marcy |first3=Geoffrey W. |author-link3=Geoffrey Marcy |last4=Han |first4=Eunkyu |last5=Feng |first5=Y. Katherina |last6=Johnson |first6=John Asher |author-link6=John Johnson (astronomer) |last7=Howard |first7=Andrew W. |last8=Fischer |first8=Debra A. |author-link8=Debra Fischer |last9=Valenti |first9=Jeff A. |last10=Anderson |first10=Jay |last11=Piskunov |first11=Nikolai |s2cid=51769219 }}</ref><ref>[http://exoplanetarchive.ipac.caltech.edu/docs/exoplanet_criteria.html Exoplanet Criteria for Inclusion in the Archive] {{Webarchive|url=https://web.archive.org/web/20150127102447/http://exoplanetarchive.ipac.caltech.edu/docs/exoplanet_criteria.html |date=27 January 2015 }}, NASA Exoplanet Archive</ref> (The limit for hydrogen burning and becoming a [[red dwarf]] star is about 80 Jupiter masses.)<ref name="Hatzes" /> The situation of main-sequence stars has been used to argue for such an inclusive definition of "planet" as well, as they also differ greatly along the two orders of magnitude that they cover, in their structure, atmospheres, temperature, spectral features, and probably formation mechanisms; yet they are all considered as one class, being all hydrostatic-equilibrium objects undergoing nuclear burning.<ref name="Hatzes" />

== Mythology and naming ==
{{See also|Names of the days of the week|classical planet}}

=== Classical planets ===
The names for the planets of the [[Solar System]] (other than [[Earth]]) in the [[English language]] are derived from naming practices developed consecutively by the [[Babylonians]], [[Ancient Greece|Greeks]], and [[Roman people|Romans]] of [[Classical antiquity|antiquity]]. The practice of grafting the names of gods onto the planets was almost certainly borrowed from the Babylonians by the ancient Greeks, and thereafter from the Greeks by the Romans. The Babylonians named Venus after the [[Sumer]]ian goddess of love with the [[Akkadian language|Akkadian]] name [[Ishtar]]; Mars after their god of war, [[Nergal]]; Mercury after their god of wisdom [[Nabu]]; Jupiter after their chief god, [[Marduk]]; and Saturn after their god of farming, [[Ninurta]].<ref>{{Cite journal |last=Huxley |first=Margaret |date=2000 |title=The Gates and Guardians in Sennacherib's Addition to the Temple of Assur |journal=Iraq |volume=62 |pages=109–137 |doi=10.2307/4200484 |jstor=4200484 |s2cid=191393468 |issn=0021-0889}}</ref> There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.<ref name=practice/> Given the differences in mythology, the correspondence was not perfect. For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. Unlike Ares, Nergal was also a god of pestilence and ruler of the underworld.<ref>{{cite book|last=Wiggermann|first=Frans A. M.|chapter=Nergal A. Philological|title=Reallexikon der Assyriologie|chapter-url=http://publikationen.badw.de/en/rla/index#8358|year=1998|publisher=Bavarian Academy of Sciences and Humanities|access-date=12 July 2022|archive-date=6 June 2021|archive-url=https://web.archive.org/web/20210606083853/http://publikationen.badw.de/en/rla/index#8358|url-status=live}}</ref><ref>{{Cite book |last=Koch |first=Ulla Susanne |url=https://books.google.com/books?id=8QiwAqGlmAQC |title=Mesopotamian Astrology: An Introduction to Babylonian and Assyrian Celestial Divination |date=1995 |publisher=Museum Tusculanum Press |isbn=978-87-7289-287-0 |pages=128–129 |language=en}}</ref><ref>{{Cite journal |last=Cecilia |first=Ludovica |date=6 November 2019 |title=A Late Composition Dedicated to Nergal |url=https://www.degruyter.com/document/doi/10.1515/aofo-2019-0014/html |journal=Altorientalische Forschungen |volume=46 |issue=2 |pages=204–213 |doi=10.1515/aofo-2019-0014 |hdl=1871.1/f23ff882-1539-4906-bc08-049906f8d505 |s2cid=208269607 |issn=2196-6761 |hdl-access=free |access-date=12 July 2022 |archive-date=22 March 2022 |archive-url=https://web.archive.org/web/20220322014922/https://www.degruyter.com/document/doi/10.1515/aofo-2019-0014/html |url-status=live }}</ref>

In ancient Greece, the two great luminaries, the Sun and the Moon, were called ''[[Helios]]'' and ''[[Selene]]'', two ancient [[Titan (mythology)|Titanic]] deities; the slowest planet, Saturn, was called ''[[Phaenon|Phainon]]'', the shiner; followed by ''[[Phaethon]]'', Jupiter, "bright"; the red planet, Mars was known as ''[[Pyroeis]]'', the "fiery"; the brightest, Venus, was known as ''[[Phosphorus (morning star)|Phosphoros]]'', the light bringer; and the fleeting final planet, Mercury, was called ''[[Stilbon (mythology)|Stilbon]]'', the gleamer. The Greeks assigned each planet to one among their pantheon of gods, the [[Twelve Olympians|Olympians]] and the earlier Titans:<ref name=practice/>
* Helios and Selene were the names of both planets and gods, both of them Titans (later supplanted by Olympians [[Apollo]] and [[Artemis]]);
* Phainon was sacred to [[Cronus]], the Titan who fathered the Olympians, associated with the harvest;
* Phaethon was sacred to [[Zeus]], Cronus's son who deposed him as king;
* Pyroeis was given to [[Ares]], son of Zeus and god of war;
* Phosphoros was ruled by [[Aphrodite]], the goddess of love; and
* Stilbon with its speedy motion, was ruled over by [[Hermes]], messenger of the gods and god of learning and wit.<ref name="practice">
{{cite book
 |first=James |last=Evans
 |year=1998
 |title=The History and Practice of Ancient Astronomy
 |pages=296–297
 |publisher=Oxford University Press
 |isbn=978-0-19-509539-5
 |url=https://books.google.com/books?id=nS51_7qbEWsC&pg=PA17
 |access-date=4 February 2008
}}
</ref>

[[File:Olympians.jpg|thumb|upright|The Greek gods of [[Mount Olympus|Olympus]], after whom the [[Solar System]]'s Roman names of the planets are derived]]

Although modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the [[Roman Empire]] and, later, the [[Catholic Church]], use the Roman (Latin) names rather than the Greek ones. The Romans inherited [[Proto-Indo-European mythology]] as the Greeks did and shared with them a [[Roman mythology|common pantheon]] under different names, but the Romans lacked the rich narrative traditions that Greek poetic culture had given [[Greek mythology|their gods]]. During the later period of the [[Roman Republic]], Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable.<ref>Rengel,&nbsp;Marian;&nbsp;Daly,&nbsp;Kathleen&nbsp;N.&nbsp;(2009).&nbsp;''[https://books.google.com/books?id=c7cNB-JaZA8C&q=%22Hellenization%20refers%22 Greek and Roman Mythology, A to Z] {{Webarchive|url=https://web.archive.org/web/20221229103416/https://www.google.com/books/edition/Greek_and_Roman_Mythology_A_to_Z/c7cNB-JaZA8C?hl=en&gbpv=1&bsq=%22Hellenization%20refers%22 |date=29 December 2022 }}''.&nbsp;United States:&nbsp;Facts On File, Incorporated. p. 66.</ref> When the Romans studied Greek astronomy, they gave the planets their own gods' names: ''[[Mercury (mythology)|Mercurius]]'' (for Hermes), ''[[Venus (mythology)|Venus]]'' (Aphrodite), ''[[Mars (mythology)|Mars]]'' (Ares), ''[[Jupiter (mythology)|Iuppiter]]'' (Zeus), and ''[[Saturn (mythology)|Saturnus]]'' (Cronus). However, there was not much agreement on which god a particular planet was associated with; according to [[Pliny the Elder]], while Phainon and Phaethon's associations with Saturn and Jupiter respectively were widely agreed upon, Pyroeis was also associated with the demi-god [[Hercules]], Stilbon was also associated with [[Apollo]], god of music, healing, and prophecy; Phosphoros was also associated with prominent goddesses [[Juno (mythology)|Juno]] and [[Isis]].<ref name="DRGA Planetae">[https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0063%3Aalphabetic+letter%3DP%3Aentry+group%3D4%3Aentry%3Dplanetae-cn "Planetae"], in ''Dictionary of Greek and Roman Antiquities'', pp. 922, 923.</ref> Some Romans, following a belief possibly originating in [[Mesopotamia]] but developed in [[Hellenistic Egypt]], believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from the farthest to the closest planet).<ref name="zerubavel">
{{cite book
 | first=Eviatar |last=Zerubavel
 |year=1989
 |title=The Seven Day Circle: The history and meaning of the week
 |page=14
 |publisher=University of Chicago Press
 |isbn=978-0-226-98165-9
 |url=https://books.google.com/books?id=aGahKeojIUoC&pg=PA14
 |access-date=7 February 2008 }}
</ref> Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter, and Venus. Because each day was named by the god that started it, this became the order of the [[Week-day names|days of the week]] in the [[Roman calendar]].<ref name="weekdays">
{{cite journal
 |last1=Falk    |first1=Michael
 |last2=Koresko |first2=Christopher
 |year=2004
 |title=Astronomical names for the days of the week
 |journal=[[Journal of the Royal Astronomical Society of Canada]]
 |volume=93 |pages=122–133
 |bibcode=1999JRASC..93..122F |doi=10.1016/j.newast.2003.07.002
 |arxiv=astro-ph/0307398 |s2cid=118954190
}}
</ref> In English, ''Saturday'', ''Sunday'', and ''Monday'' are straightforward translations of these Roman names. The other days were renamed after ''[[Týr|Tīw]]'' (Tuesday), ''[[Woden|Wōden]]'' (Wednesday), ''[[Thor|Þunor]]'' (Thursday), and ''[[Frige|Frīġ]]'' (Friday), the [[Anglo-Saxon gods]] considered similar or equivalent to Mars, Mercury, Jupiter, and Venus, respectively.<ref>{{Cite web |last=Ross |first=Margaret Clunies |title=Explainer: the gods behind the days of the week |url=http://theconversation.com/explainer-the-gods-behind-the-days-of-the-week-87170 |access-date=13 May 2022 |website=The Conversation |date=January 2018 |language=en |archive-date=13 May 2022 |archive-url=https://web.archive.org/web/20220513181134/https://theconversation.com/explainer-the-gods-behind-the-days-of-the-week-87170 |url-status=live }}</ref>

Earth's name in English is not derived from Greco-Roman mythology. Because it was only generally accepted as a planet in the 17th&nbsp;century,<ref name="galileo_project">{{cite web
 |last=Van Helden
 |first=Al
 |date=1995
 |title=Copernican System
 |department=The Galileo Project
 |publisher=[[Rice University]]
 |url=http://galileo.rice.edu/sci/theories/copernican_system.html
 |access-date=28 January 2008
|archive-date=19 July 2012
|archive-url=https://web.archive.org/web/20120719043059/http://galileo.rice.edu/sci/theories/copernican_system.html
 |url-status=live
 }}</ref> there is no tradition of naming it after a god. (The same is true, in English at least, of the Sun and the Moon, though they are no longer generally considered planets.) The name originates from the [[Old English]] word ''eorþe'', which was the word for "ground" and "dirt" as well as the world itself.<ref>{{cite encyclopedia
 |title=earth
 |dictionary=Oxford English Dictionary
 |url=https://www.oed.com/viewdictionaryentry/Entry/59023
 |access-date=7 May 2021
|archive-date=10 May 2021
|archive-url=https://web.archive.org/web/20210510142438/https://www.oed.com/viewdictionaryentry/Entry/59023
 |url-status=live
 }}</ref> As with its equivalents in the other [[Germanic languages]], it derives ultimately from the [[Proto-Germanic]] word ''erþō'', as can be seen in the English ''earth'', the German ''Erde'', the Dutch ''aarde'', and the Scandinavian ''jord''. Many of the [[Romance languages]] retain the old Roman word ''[[Terra (mythology)|terra]]'' (or some variation of it) that was used with the meaning of "dry land" as opposed to "sea".<ref>{{cite web
 |last=Harper
 |first=Douglas
 |date=September 2001
 |title=Etymology of "terrain"
 |website=Online Etymology Dictionary
 |url=http://www.etymonline.com/index.php?term=terrain
 |access-date=30 January 2008
|archive-date=23 August 2012
|archive-url=https://web.archive.org/web/20120823192155/http://www.etymonline.com/index.php?term=terrain
 |url-status=live
 }}</ref> The non-Romance languages use their own native words. The Greeks retain their original name, ''[[Gaia (mythology)|Γή]]'' ''(Ge)''.<ref>{{cite book |last=Kambas |first=Michael |date=2004 |title=Greek-English, English-Greek Dictionary |location= |publisher=Hippocrene Books |page=259 |isbn=978-0-7818-1002-9}}</ref>

Non-European cultures use other planetary-naming systems. [[India]] uses a system based on the [[Navagraha]], which incorporates the seven traditional planets and the ascending and descending [[lunar node]]s ''[[Rahu]]'' and [[Ketu (mythology)|''Ketu'']]. The planets are ''[[Surya]]'' 'Sun', ''[[Chandra]]'' 'Moon', ''[[Budha]]'' for Mercury, ''[[Shukra]]'' ('bright') for Venus, ''[[Mangala]]'' (the god of war) for Mars, ''[[Bṛhaspati|{{IAST|Bṛhaspati}}]]'' (councilor of the gods) for Jupiter, and ''[[Shani]]'' (symbolic of time) for Saturn.<ref>{{cite thesis |last=Markel |first=Stephen Allen |date=1989 |title=The Origin and Early Development of the Nine Planetary Deities (Navagraha) |type=PhD |publisher=University of Michigan |url=https://www.proquest.com/openview/b2b11d92cbfe7471459b34ddaa2fa18f/1?pq-origsite=gscholar&cbl=18750&diss=y |access-date=11 August 2022 |archive-date=13 May 2022 |archive-url=https://web.archive.org/web/20220513182636/https://www.proquest.com/openview/b2b11d92cbfe7471459b34ddaa2fa18f/1?pq-origsite=gscholar&cbl=18750&diss=y |url-status=live }}</ref>

The native [[Persian language|Persian]] names of most of the planets are based on identifications of the Mesopotamian gods with Iranian gods, analogous to the Greek and Latin names. Mercury is ''Tir'' (Persian: {{lang|fa|تیر}}) for the western Iranian god Tīriya (patron of scribes), analogous to Nabu; Venus is ''Nāhid'' ({{lang|fa|ناهید}}) for [[Anahita]]; Mars is ''Bahrām'' ({{lang|fa|بهرام}}) for [[Verethragna]]; and Jupiter is ''Hormoz'' ({{lang|fa|هرمز}}) for [[Ahura Mazda]]. The Persian name for Saturn, ''Keyvān'' ({{lang|fa|کیوان}}), is a borrowing from [[Akkadian language|Akkadian]] ''kajamānu'', meaning "the permanent, steady".<ref>{{cite encyclopedia |url=https://iranicaonline.org/articles/planets |date=20 September 2016 |first=Antonio |last=Panaino |title=Planets |encyclopedia=Encyclopædia Iranica |access-date=24 February 2023 |archive-date=24 February 2023 |archive-url=https://web.archive.org/web/20230224004240/https://iranicaonline.org/articles/planets |url-status=live }}</ref>

China and the countries of eastern Asia historically subject to [[Sinosphere|Chinese cultural influence]] (such as [[Japan]], [[Korea]], and [[Vietnam]]) use a naming system based on the [[Wuxing (Chinese philosophy)|five Chinese elements]]: [[Water (wuxing)|water]] (Mercury [[wikt:水|水]]星 "water star"), [[Metal (wuxing)|metal]] or gold (Venus [[wikt:金|金]]星 "gold star"), [[Fire (wuxing)|fire]] (Mars [[wikt:火|火]]星 "fire star"), [[Wood (wuxing)|wood]] (Jupiter [[wikt:木|木]]星 "wood star"), and [[Earth (wuxing)|earth]] or soil (Saturn [[wikt:土|土]]星 "soil star").<ref name="weekdays"/>

In traditional [[Hebrew astronomy]], the seven traditional planets have (for the most part) descriptive names—the Sun is חמה ''Ḥammah'' or "the hot one", the Moon is לבנה ''Levanah'' or "the white one", Venus is כוכב נוגה ''Kokhav Nogah'' or "the bright planet", Mercury is כוכב ''Kokhav'' or "the planet" (given its lack of distinguishing features), Mars is מאדים ''Ma'adim'' or "the red one", and Saturn is שבתאי ''Shabbatai'' or "the resting one" (in reference to its slow movement compared to the other visible planets).<ref name=Hebrew>
{{cite journal
 |last1=Stieglitz |first1=Robert
 |date=Apr 1981
 |title=The Hebrew names of the seven planets
 |journal=Journal of Near Eastern Studies
 |volume=40 |issue=2 |pages=135–137
 |doi=10.1086/372867 |jstor=545038 |s2cid=162579411
}}
</ref> The odd one out is Jupiter, called צדק ''Tzedeq'' or "justice".<ref name=Hebrew/> These names, first attested in the [[Babylonian Talmud]], are not the original Hebrew names of the planets. In 377 [[Epiphanius of Salamis]] recorded another set of names that seem to have pagan or [[Canaanite religion|Canaanite]] associations: those names, since replaced for religious reasons, were probably the historical Semitic names, and may have much earlier roots going back to Babylonian astronomy.<ref name="Hebrew"/> The etymologies for the Arabic names of the planets are less well understood. Mostly agreed among scholars are Venus (Arabic: {{lang|ar|الزهرة}}, ''az-Zuhara'', "the bright one"<ref>{{cite journal
 |first1=F.J.
 |last1=Ragep
 |first2=W.
 |last2=Hartner
 |date=24 April 2012
|title=Zuhara
 |journal=Encyclopaedia of Islam
 |edition=2nd
 |url=https://referenceworks.brillonline.com/entries/encyclopaedia-of-islam-2/zuhara-SIM_8195
 |via=referenceworks.brillonline.com
 |access-date=16 January 2019
|archive-date=9 July 2021
|archive-url=https://web.archive.org/web/20210709220502/https://referenceworks.brillonline.com/entries/encyclopaedia-of-islam-2/zuhara-SIM_8195
 |url-status=live
 }}</ref>), Earth ({{lang|ar|الأرض}}, ''al-ʾArḍ'', from the same root as [[wikt:ארץ|eretz]]), and Saturn ({{lang|ar|زُحَل}}, ''Zuḥal'', "withdrawer"<ref>
{{cite book
 |last1=Meyers    |first1=Carol L.
 |last2=O'Connor  |first2=M.
 |last3=O'Connor  |first3=Michael Patrick
 |date= 1983
|title=The Word of the Lord Shall Go Forth: Essays in honor of David Noel Freedman in celebration of his sixtieth birthday
 |publisher=Eisenbrauns
 |isbn=978-0-931464-19-5
 |url=https://books.google.com/books?id=leQtcmpcQ-EC&q=zuhal+saturn&pg=PA53
 |via=Google Books
}}
</ref>). Multiple suggested etymologies exist for Mercury ({{lang|ar|عُطَارِد}}, ''ʿUṭārid''), Mars ({{lang|ar|اَلْمِرِّيخ}}, ''al-Mirrīkh''), and Jupiter ({{lang|ar|المشتري}}, ''al-Muštarī''), but there is no agreement among scholars.<ref>{{cite book |last=Eilers |first=Wilhelm |date=1976 |title=Sinn und Herkunft der Planetennamen |url=https://www.zobodat.at/pdf/Sitz-Ber-Akad-Muenchen-phil-hist-Kl_1975_0001-0138.pdf |location=Munich |publisher=[[Bavarian Academy of Sciences and Humanities]] |access-date=28 August 2022 |archive-date=10 October 2022 |archive-url=https://web.archive.org/web/20221010105840/https://www.zobodat.at/pdf/Sitz-Ber-Akad-Muenchen-phil-hist-Kl_1975_0001-0138.pdf |url-status=live }}</ref><ref>
{{cite conference
 |first=Hannes D. |last=Galter
 |publication-date=31 July 1993
|title=Die Rolle der Astronomie in den Kulturen Mesopotamiens
 |trans-title=The role of astronomy in the cultures of the Mesopotamians
 |book-title=Beiträge Zum 3. Grazer Morgenländischen Symposion ( 23–27 September 1991)
 |conference=3.&nbsp;Grazer Morgenländischen Symposion [Third Graz Oriental Symposium]
 |date= 23–27 September 1991
 |place=Graz, Austria
 |publisher=GrazKult
 |isbn=978-3-85375-009-4
 |url=https://books.google.com/books?id=dMgfAQAAIAAJ&q=mustari+jupiter
 |via=Google Books
}}</ref><ref>
{{cite web
 |author=al-Masūdī
 |date=1841
|title=El-Masūdī's Historical Encyclopaedia, entitled "Meadows of Gold and Mines of Gems."
 |publisher=Oriental Translation Fund of Great Britain and Ireland
 |url=https://books.google.com/books?id=XaVmAAAAMAAJ&q=utarid+mercury+penman&pg=PA204
 |via=Google Books
}}
</ref><ref>
{{cite web
 |first=Mas'ûdi |last=Ali-Abu'l-Hassan
 |date=1841
|title=Historical Encyclopaedia: Entitled "Meadows of gold and mines of gems"
 |publisher=Printed for the Oriental Translation Fund of Great Britain and Ireland
 |url=https://books.google.com/books?id=SqJEAQAAIAAJ&q=Mirrikh+mars&pg=PA204
 |via=Google Books
}}</ref>

=== Modern discoveries ===
When subsequent planets were discovered in the 18th and 19th centuries, Uranus was named for a [[Uranus (mythology)|Greek deity]] and Neptune for a [[Neptune (mythology)|Roman one]] (the counterpart of [[Poseidon]]). The asteroids were initially named from mythology as well—[[Ceres (mythology)|Ceres]], [[Juno (mythology)|Juno]], and [[Vesta (mythology)|Vesta]] are major Roman goddesses, and Pallas is an epithet of the major Greek goddess [[Athena (mythology)|Athena]]—but as more and more were discovered, they first started being named after more minor goddesses, and the mythological restriction was dropped starting from the twentieth asteroid [[20 Massalia|Massalia]] in 1852.<ref>{{cite book|last=Schmadel|first=Lutz|title=Dictionary of Minor Planet Names|date=2012|edition=6th|publisher=Springer|page=15|url=https://books.google.com/books?id=aeAg1X7afOoC&pg=PA15|isbn=978-3-642-29718-2}}</ref> Pluto (named after the [[Pluto (mythology)|Greek god of the underworld]]) was given a classical name, as it was considered a major planet when it was discovered.{{citation needed|date=April 2025}}

The names of Uranus ([[wikt:天|天]]王星 "sky king star"), Neptune ([[wikt:海|海]]王星 "sea king star"), and Pluto ([[wikt:冥|冥]]王星 "underworld king star") in Chinese, Korean, and Japanese are [[calque]]s based on the roles of those gods in Roman and Greek mythology.<ref name=bianyulin/><ref>{{cite web |title=Planetary linguistics |publisher=nineplanets.org |url=http://nineplanets.org/days.html |access-date=8 April 2010 |archive-url=https://web.archive.org/web/20100407154905/http://www.nineplanets.org/days.html |archive-date=7 April 2010 |url-status=live }}</ref>{{efn|In Korean, these names are more often written in [[Hangul]] rather than Chinese characters, e.g. 명왕성 for Pluto. In Vietnamese, [[calque]]s are more common than directly reading these names as [[Sino-Vietnamese vocabulary|Sino-Vietnamese]], e.g. ''sao Thuỷ'' rather than ''Thuỷ tinh'' for Mercury. Pluto is not ''sao Minh Vương'' but ''sao Diêm Vương'' "[[Yama]] star".<ref>{{Cite web |url=https://dictionary.cambridge.org/dictionary/english-vietnamese/ |title=Cambridge English-Vietnamese Dictionary |access-date=21 September 2022 |archive-date=7 October 2022 |archive-url=https://web.archive.org/web/20221007123000/https://dictionary.cambridge.org/dictionary/english-vietnamese/ |url-status=live }}</ref>}} In the 19th century, [[Alexander Wylie (missionary)|Alexander Wylie]] and [[Li Shanlan]] calqued the names of the first 117 asteroids into Chinese, and many of their names are still used today, e.g. Ceres ([[wikt:穀|穀]]神星 "grain goddess star"), Pallas ([[wikt:智|智]]神星 "wisdom goddess star"), Juno ([[wikt:婚|婚]]神星 "marriage goddess star"), Vesta ([[wikt:灶|灶]]神星 "hearth goddess star"), and Hygiea ([[wikt:健|健]]神星 "health goddess star").<ref name=lijing>{{cite journal |author1=李竞 [Li Jing] |date=2018 |title=小行星世界中的古典音乐 |url=http://www.term.org.cn/CN/10.3969/j.issn.1673-8578.2018.03.015 |language=zh-cn |journal=中国科技术语 [China Terminology] |volume=20 |issue=3 |pages=66–75 |doi=10.3969/j.issn.1673-8578.2018.03.015 |access-date=5 May 2023 |archive-date=5 May 2023 |archive-url=https://web.archive.org/web/20230505191254/http://www.term.org.cn/CN/10.3969/j.issn.1673-8578.2018.03.015 |url-status=live }}</ref> Such translations were extended to some later minor planets, including some of the dwarf planets discovered in the 21st century, e.g. Haumea ([[wikt:妊|妊]]神星 "pregnancy goddess star"), Makemake ([[wikt:鳥|鳥]]神星 "bird goddess star"), and Eris ([[wikt:鬩|鬩]]神星 "quarrel goddess star"). However, except for the better-known asteroids and dwarf planets, many of them are rare outside Chinese astronomical dictionaries.<ref name=bianyulin>{{cite journal |author1=卞毓麟 [Bian Yulin] |title="阋神星"的来龙去脉 |url=https://nadc.china-vo.org/astrodict/s/2016/committeeArticles/Eris_history_BianYuLin_2007.pdf |journal=中国科技术语 [China Terminology] |volume=9 |issue=4 |date=2007 |pages=59–61 |doi=10.3969/j.issn.1673-8578.2007.04.020 |access-date=21 September 2022 |language=zh-cn |url-access=limited |archive-date=21 September 2022 |archive-url=https://web.archive.org/web/20220921143511/https://nadc.china-vo.org/astrodict/s/2016/committeeArticles/Eris_history_BianYuLin_2007.pdf |url-status=live }}</ref>

Hebrew names were chosen for Uranus (אורון ''Oron'', "small light") and Neptune (רהב ''Rahab'', a Biblical sea monster) in 2009;<ref>{{cite news |last=Ettinger |first=Yair |date=31 December 2009 |title=Uranus and Neptune Get Hebrew Names at Last |url=https://www.haaretz.com/2009-12-31/ty-article/uranus-and-neptune-get-hebrew-names-at-last/0000017f-ec5c-ddba-a37f-ee7ecaf10000 |work=[[Haaretz]] |location= |access-date=5 October 2022 |archive-date=5 October 2022 |archive-url=https://web.archive.org/web/20221005082216/https://www.haaretz.com/2009-12-31/ty-article/uranus-and-neptune-get-hebrew-names-at-last/0000017f-ec5c-ddba-a37f-ee7ecaf10000 |url-status=live }}</ref> prior to that the names "Uranus" and "Neptune" had simply been borrowed.<ref>{{cite journal |last1=Zucker |first1=Shay |date=2011 |title=Hebrew names of the planets |journal=Proceedings of the International Astronomical Union |volume=260 |issue= |pages=301–305 |doi=10.1017/S1743921311002432 |bibcode=2011IAUS..260..301Z |s2cid=162671357 |doi-access=free }}</ref>

After more objects were discovered beyond Neptune, naming conventions depending on their orbits were put in place: those in the 2:3 resonance with Neptune (the [[plutino]]s) are given names from underworld myths, while others are given names from creation myths. Most of the trans-Neptunian objects are named after gods and goddesses from other cultures (e.g. [[50000 Quaoar|Quaoar]] is named after a [[Tongva]] god). There are a few exceptions which continue the Roman and Greek scheme, notably including Eris as it had initially been considered a tenth planet.<ref name="WGSBN-Naming-Guidelines">{{cite web
  |title        = Minor Planet Naming Guidelines (Rules and Guidelines for naming non-cometary small Solar-System bodies) – v1.0
  |work        = Working Group Small Body Nomenclature
  |type        = PDF
  |date        = 20 December 2021
|url        = https://www.wgsbn-iau.org/documentation/NamesAndCitations.pdf
  |access-date        = 1 May 2022
|archive-date        = 20 March 2023
|archive-url        = https://web.archive.org/web/20230320000458/https://www.wgsbn-iau.org/documentation/NamesAndCitations.pdf
  |url-status        = live
  }}</ref><ref name="WGSBN-home">{{cite web
  |title      = IAU: WG Small Body Nomenclature (WGSBN)
  |work      = Working Group Small Body Nomenclature
  |url      = https://www.wgsbn-iau.org/
  |access-date      = 9 February 2022
|archive-date      = 8 February 2022
|archive-url      = https://web.archive.org/web/20220208031049/https://wgsbn-iau.org/
  |url-status      = live
  }}</ref>

The moons (including the planetary-mass ones) are generally given names with some association with their parent planet. The planetary-mass moons of Jupiter are named after four of Zeus's lovers (or other sexual partners); those of Saturn are named after Cronus's brothers and sisters, the Titans; those of Uranus are named after characters from [[William Shakespeare|Shakespeare]] and [[Alexander Pope|Pope]] (originally specifically from fairy mythology,<ref name="Lassell 1852">
{{cite journal
| last = Lassell
| first = W.
| date = 1852
| title = Beobachtungen der Uranus-Satelliten
| journal = Astronomische Nachrichten
| volume = 34
| page = 325
| bibcode = 1852AN.....34..325.
}}
</ref> but that ended with the naming of [[Miranda (moon)|Miranda]]). Neptune's planetary-mass moon Triton is named after [[Triton (mythology)|the god's son]]; Pluto's planetary-mass moon Charon is named after the [[Charon|ferryman of the dead]], who carries the souls of the newly deceased to the underworld (Pluto's domain).<ref>{{cite web |url=https://planetarynames.wr.usgs.gov/Page/Planets |title=Gazetteer of Planetary Nomenclature |website=IAU |access-date=27 June 2022 |archive-date=21 August 2014 |archive-url=https://web.archive.org/web/20140821014052/http://planetarynames.wr.usgs.gov/Page/Planets |url-status=live }}</ref>

=== Exoplanets ===
Exoplanets are commonly named after their parent star and their order of discovery within its planetary system, such as [[Proxima Centauri b]]. (The lettering starts at b, with a considered to represent the parent star.){{citation needed|date=April 2025}}

===Symbols===
{{main|Planetary symbol}}
{| class="wikitable skin-invert" style="margin:1em auto 1em auto; float:right; margin:10px"

|+ Most common planetary symbols
|- style="font-size:smaller; text-align:center;"
| Sun <br /> [[File:Sun symbol (fixed width).svg|14px|☉]] || Mercury <br /> [[File:Mercury symbol (fixed width).svg|14px|☿]] || Venus <br /> [[File:Venus symbol (fixed width).svg|14px|♀]] || Earth <br /> [[File:Earth symbol (fixed width).svg|14px|🜨]] || Moon <br /> [[File:Moon symbol decrescent (fixed width).svg|14px|☾]] || Mars <br /> [[File:Mars symbol (fixed width).svg|14px|♂]] || Jupiter <br /> [[File:Jupiter symbol (fixed width).svg|14px|♃]] || Saturn <br /> [[File:Saturn symbol (fixed width).svg|14px|♄]] || Uranus <br /> [[File:Uranus symbol (fixed width).svg|14px|⛢]] or [[File:Uranus monogram (fixed width).svg|14px|♅]] || Neptune <br /> [[File:Neptune symbol (fixed width).svg|14px|♆]]
|}
The written symbols for Mercury, Venus, Jupiter, Saturn, and possibly Mars have been traced to forms found in late Greek papyrus texts.<ref name=jones-1999>{{cite book
| last = Jones  | first = Alexander
| date = 1999
| title = Astronomical Papyri from Oxyrhynchus
| pages = 62–63
| publisher = American Philosophical Society
| isbn = 978-0-87169-233-7
}}
</ref> The symbols for Jupiter and Saturn are identified as [[monogram]]s of the corresponding Greek names, and the symbol for Mercury is a stylized [[caduceus]].<ref name=jones-1999/>

According to [[Annie Scott Dill Maunder]], antecedents of the planetary symbols were used in art to represent the gods associated with the classical planets. ''Bianchini's [[planisphere]]'', discovered by Francesco Bianchini in the 18th century but produced in the 2nd century,<ref name=imss>
{{cite web
| title = Bianchini's planisphere
| publisher = Istituto e Museo di Storia della Scienza [Institute and Museum of the History of Science]
| location = Florence, Italy
| url = https://brunelleschi.imss.fi.it/galileopalazzostrozzi/object/BianchinisPlanisphere.html
| access-date = 20 August 2018 | url-status=live
| archive-url=https://web.archive.org/web/20180227062732/https://brunelleschi.imss.fi.it/galileopalazzostrozzi/object/BianchinisPlanisphere.html
| archive-date=27 February 2018 }}
</ref> shows Greek personifications of planetary gods charged with early versions of the planetary symbols. Mercury has a [[caduceus]]; Venus has, attached to her necklace, a cord connected to another necklace; Mars, a spear; Jupiter, a staff; Saturn, a scythe; the [[Sun]], a [[circlet]] with rays radiating from it; and the Moon, a headdress with a crescent attached.<ref name=maunder>
{{cite magazine
| last = Maunder  | first = A.S.D.
| date = 1934
| title = The origin of the symbols of the planets
| magazine = The Observatory
| volume = 57 | pages = 238–247
| bibcode = 1934Obs....57..238M
}}
</ref> The modern shapes with the cross-marks first appeared around the 16th century. According to Maunder, the addition of crosses appears to be "an attempt to give a savour of Christianity to the symbols of the old pagan gods."<ref name=maunder/> Earth itself was not considered a classical planet; its symbol descends from a pre-heliocentric symbol for the [[four corners of the world]].<ref name=high-school-astronomy>
{{cite book
 | last = Mattison | first = Hiram
 | date = 1872
 | title = High-School Astronomy
 | pages = 32–36
 | publisher = Sheldon & Co.
 | url = https://books.google.com/books?id=XksAAAAAYAAJ&pg=PA32
}}
</ref>

When further planets were discovered orbiting the Sun, symbols were invented for them. The most common astronomical symbol for Uranus, ⛢,<ref name=iancu/> was invented by [[Johann Gottfried Köhler]], and was intended to represent the newly discovered metal [[platinum]].<ref name=bode-uranus>{{cite book
| title = Von dem neu entdeckten Planeten
| publisher = Beim Verfaszer
| url = https://archive.org/details/bub_gb_ZqA5AAAAcAAJ
| last = Bode  | first = J.E.
| date = 1784
| pages = [https://archive.org/details/bub_gb_ZqA5AAAAcAAJ/page/n98 95]–96
| bibcode = 1784vdne.book.....B
}}
</ref><ref name=gould-uranus>
{{cite book
| title = Report on the history of the discovery of Neptune
| url = https://archive.org/details/bub_gb_uyANAQAAIAAJ
| last = Gould  | first = B.A.
| publisher = Smithsonian Institution
| date = 1850
| pages = 5, 22
}}</ref> An alternative symbol, ♅, was invented by [[Jérôme Lalande]], and represents a globe with a H on top, for Uranus's discoverer Herschel.<ref name=Francisca>{{Cite journal |title=The meaning of the symbol H+o for the planet Uranus |author=Francisca Herschel|date=August 1917|journal=The Observatory |volume=40 |page=306 |bibcode=1917Obs....40..306H}}</ref> Today, ⛢ is mostly used by astronomers and ♅ by [[astrology|astrologers]], though it is possible to find each symbol in the other context.<ref name=iancu>{{cite web|url=https://www.unicode.org/L2/L2009/09300-uranus.pdf|title=Proposal to Encode the Astronomical Symbol for Uranus|last=Iancu|first=Laurentiu|date=14 August 2009|website=unicode.org|access-date=12 September 2022|archive-date=2 October 2022|archive-url=https://web.archive.org/web/20221002155531/http://www.unicode.org/L2/L2009/09300-uranus.pdf|url-status=live}}</ref> The first few asteroids were considered to be planets when they were discovered, and were likewise given abstract symbols, e.g. Ceres's sickle (⚳), Pallas's spear (⚴), Juno's sceptre (⚵), and Vesta's hearth (⚶). However, as their number rose further and further, this practice stopped in favour of numbering them instead. (Massalia, the first asteroid not named from mythology, is also the first asteroid that was not assigned a symbol by its discoverer.) The symbols for the first four asteroids, Ceres through Vesta, remained in use for longer than the others,<ref name=asteroids/> and even in the modern day [[NASA]] has used the Ceres symbol—Ceres being the only asteroid that is also a dwarf planet.<ref name=miller/> Neptune's symbol (♆) represents [[Neptune's trident|the god's trident]].<ref name=gould-uranus/> The astronomical symbol for Pluto is a P-L monogram (♇),<ref name="JPL/NASA Pluto's Symbol">
{{cite web
 |title=NASA's Solar System Exploration: Multimedia: Gallery: Pluto's Symbol
 |url=http://sse.jpl.nasa.gov/multimedia/display.cfm?IM_ID=263
 |publisher=NASA
 |access-date=29 November 2011
|archive-url=https://web.archive.org/web/20061001015053/http://sse.jpl.nasa.gov/multimedia/display.cfm?IM_ID=263
 |archive-date=1 October 2006
}}</ref> though it has become less common since the IAU definition reclassified Pluto.<ref name=miller/> Since Pluto's reclassification, NASA has used the traditional astrological symbol of Pluto (⯓), a planetary orb over Pluto's [[bident]].<ref name=miller/>

{| class="wikitable skin-invert" style="margin:1em auto 1em auto; float:right; margin:10px"
|- style="background:#ccf; font-size:smaller;"
|+ Some rarer planetary symbols in Unicode
|- style="font-size:smaller; text-align:center;"
| Earth <br /> [[File:Globus cruciger (fixed width).svg|14px|♁]] || Vesta <br /> [[File:Vesta symbol (fixed width).svg|14px|⚶]] || Juno <br /> [[File:Juno symbol (fixed width).svg|14px|⚵]] || Ceres <br /> [[File:Ceres symbol (fixed width).svg|14px|⚳]] || Pallas <br /> [[File:Pallas symbol (fixed width).svg|14px|⚴]] || Hygiea <br /> [[File:Hygiea symbol (fixed width).svg|14px|⯚]] || Orcus <br /> [[File:Orcus symbol (fixed width).svg|14px|🝿]] || Pluto <br /> [[File:Pluto monogram (fixed width).svg|14px|♇]] or [[File:Pluto symbol (large orb, fixed width).svg|14px|⯓]] || Charon <br /> [[File:Charon symbol (fixed width).svg|14px|⯕]] || Haumea <br /> [[File:Haumea symbol (fixed width).svg|14px|🝻]] || Quaoar <br /> [[File:Quaoar symbol (fixed width).svg|14px|🝾]] || Makemake <br /> [[File:Makemake symbol (fixed width).svg|14px|🝼]] || Gonggong <br /> [[File:Gonggong symbol (fixed width).svg|14px|🝽]] || Eris <br /> [[File:Eris symbol (fixed width).svg|14px|⯰]] || Sedna <br /> [[File:Sedna symbol (fixed width).svg|14px|⯲]]
|}
The IAU discourages the use of planetary symbols in modern journal articles in favour of one-letter or (to disambiguate Mercury and Mars) two-letter abbreviations for the major planets. The symbols for the Sun and Earth are nonetheless common, as [[solar mass]], [[Earth mass]], and similar units are common in astronomy.<ref name=iau-style-manual >{{cite book
| title = The IAU Style Manual
| url = http://www.iau.org/static/publications/stylemanual1989.pdf
| date = 1989
| page = 27
| access-date = 8 August 2022
| archive-date = 26 July 2011
| archive-url = https://web.archive.org/web/20110726170213/http://www.iau.org/static/publications/stylemanual1989.pdf
| url-status = live
}}</ref> Other planetary symbols today are mostly encountered in astrology. Astrologers have resurrected the old astronomical symbols for the first few asteroids and continue to invent symbols for other objects.<ref name=miller>{{cite web|url=https://www.unicode.org/L2/L2021/21224-dwarf-planet-syms.pdf|title=Unicode request for dwarf-planet symbols|last=Miller|first=Kirk|date=26 October 2021|website=unicode.org|access-date=8 August 2022|archive-date=23 March 2022|archive-url=https://web.archive.org/web/20220323174107/https://www.unicode.org/L2/L2021/21224-dwarf-planet-syms.pdf|url-status=live}}</ref> This includes relatively standard astrological symbols for the dwarf planets discovered in the 21st century, which were not given symbols by astronomers because planetary symbols had mostly fallen out of use in astronomy by the time they were discovered. Many astrological symbols are included in [[Unicode]], and a few of these new inventions (the symbols of Haumea, Makemake, and Eris) have since been used by NASA in astronomy.<ref name=miller/> The Eris symbol is a traditional one from [[Discordianism]], a religion worshipping the goddess Eris. The other dwarf-planet symbols are mostly initialisms (except Haumea) in the native scripts of the cultures they come from; they also represent something associated with the corresponding deity or culture, e.g. Makemake's face or Gonggong's snake-tail.<ref name=miller/><ref>{{cite web |url=http://blog.unicode.org/2022/05/out-of-this-world-new-astronomy-symbols.html |title=Out of this World: New Astronomy Symbols Approved for the Unicode Standard |last=Anderson |first=Deborah |date=4 May 2022 |website=unicode.org |publisher=The Unicode Consortium |access-date=6 August 2022 |archive-date=6 August 2022 |archive-url=https://web.archive.org/web/20220806075352/http://blog.unicode.org/2022/05/out-of-this-world-new-astronomy-symbols.html |url-status=live }}</ref> Moskowitz also devised symbols for the planetary-mass moons; most of them are initialisms combined with a feature of their parent planet's symbol. The exception is Charon, which combines the high orb of Pluto's bident symbol with a crescent, suggesting both Charon as a moon and the mythological Charon's boat crossing the river [[Styx]].<ref>{{cite web |url=https://www.unicode.org/L2/L2025/25079-phobos-and-deimos.pdf |title=Phobos and Deimos symbols |last1=Bala |first1=Gavin Jared |last2=Miller |first2=Kirk |date=7 March 2025 |website=unicode.org |publisher=The Unicode Consortium |access-date=14 March 2025 |quote=}}</ref>

== See also ==
{{div col|colwidth=20em}}
* {{annotated link|Double planet}}
* [[List of landings on extraterrestrial bodies]]
* [[Lists of planets]] – A list of lists of planets sorted by diverse attributes
* {{annotated link|Mesoplanet}}
* {{annotated link|Planetary habitability}}
* {{annotated link|Planetary mnemonic}}
* {{annotated link|Theoretical planetology}}
{{div col end}}

== Notes ==
{{reflist|group=lower-alpha|30em}}{{Clear}}

== References ==
{{reflist|30em}}

== External links ==
{{Commons category|Planets}}
{{Wikiquote}}
{{Wiktionary|planet}}
{{Wikisource|1911 Encyclopædia Britannica/Planet|Planet}}
* [http://photojournal.jpl.nasa.gov/ Photojournal NASA]
* [http://www.psrd.hawaii.edu/ ''Planetary Science Research Discoveries''] (educational site with illustrated articles)

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