View source for Dwarf planet

{{Short description|Small planetary-mass object}}
{{Distinguish|minor planet}}
{{Featured article}} 
{{Use mdy dates|date=September 2024}}
{{Multiple image
| header = Nine likeliest{{efn|The [[hydrostatic equilibrium]] criterion of a dwarf planet cannot be confirmed unless a spacecraft directly visits the object.}} dwarf planets<br/>and dates of discovery
| perrow = 2/3/4
| total_width = 315
| caption_align = center
| image1 = Ceres - RC3 - Haulani Crater (22381131691).jpg
| caption1 = [[Ceres (dwarf planet)|Ceres]] (1801)
| image2 = Pluto in True Color - High-Res.jpg
| caption2 = [[Pluto]] (1930)
| image3 = Quaoar-weywot hst.jpg
| caption3 = [[Quaoar]] (2002)
| image4 = Sedna PRC2004-14d.jpg
| caption4 = [[Sedna (dwarf planet)|Sedna]] (2003)
| image5 = Orcus-Vanth 10801.jpg
| caption5 = [[Orcus (dwarf planet)|Orcus]] (2004)<!--- Feb 2004 --->
| image6 = Haumea Hubble.png
| caption6 = [[Haumea]] (2004)<!--- Dec 2004 --->
| image7 = Eris and dysnomia2.jpg
| caption7 = [[Eris (dwarf planet)|Eris]] (2005)<!--- Jan 2005 --->
| image8 = Makemake and its moon.jpg
| caption8 = [[Makemake]] (2005)<!--- Mar 2005 --->
| image9 = 225088 Gonggong and Xiangliu by Hubble (clean).png
| caption9 = [[Gonggong (dwarf planet)|Gonggong]] (2007)
}}

A '''dwarf planet''' is a small [[planetary-mass object]] that is in direct orbit around the [[Sun]], [[mass]]ive enough to be [[hydrostatic equilibrium|gravitationally rounded]], but insufficient to achieve [[clearing the neighbourhood|orbital dominance]] like the eight classical [[planet]]s of the [[Solar System]]. The prototypical dwarf planet is [[Pluto]], which for decades was regarded as a planet before the "dwarf" concept was adopted in 2006.
Many planetary geologists consider dwarf planets and [[planetary-mass moon]]s to be planets,<ref name="Metzger">{{Cite journal |first1=Philip T. |last1=Metzger |author-link1=Philip T. Metzger |first2=W. M. |last2=Grundy |first3=Mark V. |last3=Sykes |first4=Alan |last4=Stern |author-link4=Alan Stern |first5=James F. |last5=Bell III |author-link5=James F. Bell III |first6=Charlene E. |last6=Detelich |first7=Kirby |last7=Runyon |first8=Michael |last8=Summers |display-authors=3 |date=March 1, 2022 |title=Moons Are Planets: Scientific Usefulness Versus Cultural Teleology in the Taxonomy of Planetary Science |url=https://doi.org/10.1016/j.icarus.2021.114768 |doi=10.1016/j.icarus.2021.114768 |s2cid=240071005 |journal=[[Icarus (journal)|Icarus]] |volume=374 |page=114768 |arxiv=2110.15285 |bibcode=2022Icar..37414768M |access-date=May 30, 2022}}</ref> but since 2006 the IAU and perhaps the majority of astronomers have excluded them from the roster of planets.

Dwarf planets are capable of being geologically active, an expectation that was borne out in 2015 by the ''[[Dawn (spacecraft)|Dawn]]'' mission to {{dp|Ceres}} and the ''[[New Horizons]]'' mission to Pluto. [[planetary geology|Planetary geologists]] are therefore particularly interested in them.

Astronomers are in general agreement that at least the [[List of possible dwarf planets#Likeliest dwarf planets|nine largest candidates]] are dwarf planets – in rough order of decreasing diameter, {{dp|Pluto}}, {{dp|Eris}}, {{dp|Haumea}}, {{dp|Makemake}}, {{dp|Gonggong}}, {{dp|Quaoar}}, {{dp|Sedna}}, {{dp|Ceres}}, and {{dp|Orcus}}. A considerable uncertainty remains over the tenth largest candidate {{dp|Salacia}}, which may thus be considered a borderline case. Of these ten, two have been visited by spacecraft (Pluto and Ceres) and seven others have at least one known moon (Eris, Haumea, Makemake, Gonggong, Quaoar, Orcus, and Salacia), which allows their masses and thus an estimate of their densities to be determined. Mass and density in turn can be fit into [[Computational geophysics#Geophysical models|geophysical models]] in an attempt to determine the nature of these worlds. Only one, Sedna, has neither been visited nor has any known moons, making an accurate estimate of mass difficult. Some astronomers include many smaller bodies as well,<ref>[https://www.hou.usra.edu/meetings/plutosystem2019/pdf/7016.pdf "Dwarf planets are planets, too: Planetary pedagogy after ''New Horizons''{{-"}}] {{Webarchive|url=https://web.archive.org/web/20210627154315/https://www.hou.usra.edu/meetings/plutosystem2019/pdf/7016.pdf |date=June 27, 2021 }}.</ref> but there is no consensus that these are likely to be dwarf planets.

== History of the concept ==
{{Main|Geophysical definition of planet|IAU definition of planet}}
[[File:Pluto and charon near true color.png|thumb|near true-colour image of [[Pluto]] and its moon [[Charon (moon)|Charon]]. Separation to scale]]
[[File:Vesta in natural color (cropped).jpg|thumb|[[4 Vesta]], an asteroid that was once a dwarf planet<ref>{{Cite web|url=https://solarsystem.nasa.gov/asteroids-comets-and-meteors/asteroids/4-vesta/in-depth|title=In Depth &#124; 4 Vesta|website=NASA Solar System Exploration|date=November 10, 2017 |access-date=February 29, 2020|archive-date=February 29, 2020|archive-url=https://web.archive.org/web/20200229050949/https://solarsystem.nasa.gov/asteroids-comets-and-meteors/asteroids/4-vesta/in-depth/|url-status=live}}</ref>]]
Starting in 1801, astronomers discovered [[Ceres (dwarf planet)|Ceres]] and other bodies between [[Mars]] and Jupiter that for decades were considered to be planets. Between then and around 1851, when the number of planets had reached 23, astronomers started using the word [[asteroid]] (from Greek, meaning 'star-like' or 'star-shaped') for the smaller bodies and began to distinguish them as ''minor planets'' rather than ''major planets''.<ref>{{cite journal |url=http://philsci-archive.pitt.edu/3418/ | first=Mauro | last=Murzi |title=Changes in a scientific concept: what is a planet? |date=2007 |access-date=April 6, 2013 |journal=Preprints in Philosophy of Science |type=Preprint |publisher=University of Pittsburgh |archive-date=June 11, 2019 |archive-url=https://web.archive.org/web/20190611023948/http://philsci-archive.pitt.edu/3418/ |url-status=live }}</ref>

With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine major planets, along with thousands of significantly smaller bodies ([[asteroid]]s and [[comet]]s). For almost 50&nbsp;years, [[Pluto]] was thought to be larger than [[Mercury (planet)|Mercury]],<ref>{{cite web |title=Pluto Revealed |url=http://www.discoveryofpluto.com/pluto06.html |first=Brad |last=Mager |access-date=January 26, 2008 |publisher=discoveryofpluto.com |archive-url=https://web.archive.org/web/20110722004610/http://www.discoveryofpluto.com/pluto06.html |archive-date=July 22, 2011 |url-status=dead }}</ref><ref>{{cite web |title=Is Pluto a planet? |url=http://curious.astro.cornell.edu/question.php?number=624 |date=September 14, 2007 | last1=Cuk | first1=Matija | last2=Masters | first2=Karen |publisher=Cornell University, Astronomy Department |access-date=January 26, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20071012225232/http://curious.astro.cornell.edu/question.php?number=624 |archive-date=October 12, 2007 }}</ref> but with the discovery in 1978 of Pluto's moon [[Charon (moon)|Charon]], it became possible to measure Pluto's mass accurately and to determine that it was much smaller than initial estimates.<ref>{{cite journal |title=Orbits and Photometry of Pluto's Satellites: Charon, S/2005 P1, and S/2005 P2 |journal=The Astronomical Journal |date=2006 |issue=1 |pages=290–298 |doi=10.1086/504422 |first=Marc W. |last=Buie | last2=Grundy | first2=William M. | last3=Young | first3=Eliot F. | last4=Young | first4=Leslie A. | last5=Stern | first5=S. Alan |volume=132 |bibcode=2006AJ....132..290B |arxiv=astro-ph/0512491|s2cid=119386667 }}</ref> It was roughly one-twentieth the mass of Mercury, which made Pluto by far the smallest planet. Although it was still more than ten times as massive as the largest object in the [[asteroid belt]], Ceres, it had only one-fifth the mass of Earth's [[Moon]].<ref>{{cite book |first=David |last=Jewitt | last2=Delsanti | first2=Audrey |title=The Solar System Beyond The Planets in Solar System Update : Topical and Timely Reviews in Solar System Sciences |publisher=Springer |doi=10.1007/3-540-37683-6 |isbn=978-3-540-37683-5 |date=2006 |url=http://www.ifa.hawaii.edu/faculty/jewitt/papers/2006/DJ06.pdf |access-date=February 10, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20060525051103/http://www.ifa.hawaii.edu/faculty/jewitt/papers/2006/DJ06.pdf |archive-date=May 25, 2006 }}</ref> Furthermore, having some unusual characteristics, such as large [[orbital eccentricity]] and a high [[Inclination|orbital inclination]], it became evident that it was a different kind of body from any of the other planets.<ref>{{cite book |title=Is Pluto a Planet? A Historical Journey through the Solar System |first=David A. |last=Weintraub |date=2006 |pages=[https://archive.org/details/isplutoplanethis00wein/page/1 1–272] |isbn=978-0-691-12348-6 |publisher=Princeton Univ. Press |location=Princeton, N.J. |url-access=registration |url=https://archive.org/details/isplutoplanethis00wein/page/1 }}</ref>

In the 1990s, astronomers began to find objects in the same region of space as Pluto (now known as the [[Kuiper belt]]), and some even farther away.<ref>{{cite web |title=Much Ado about Pluto |publisher=PlutoPetition.com |first=Tony |last=Phillips | last2=Phillips | first2=Amelia |date=September 4, 2006 |url=http://www.plutopetition.com/unplanet.php |access-date=January 26, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20080125225956/http://www.plutopetition.com/unplanet.php |archive-date=January 25, 2008 }}</ref> 
Many of these shared several of Pluto's key orbital characteristics, and Pluto started being seen as the largest member of a new class of objects, the [[plutino]]s. It became clear that either the larger of these bodies would also have to be classified as planets, or Pluto would have to be reclassified, much as Ceres had been reclassified after the discovery of additional asteroids.<ref>{{cite web |title=What is the definition of a planet? |date=2004 |url=http://web.gps.caltech.edu/~mbrown/sedna/index.html#planets |first=Michael E. |last=Brown |publisher=California Institute of Technology, Department of Geological Sciences |access-date=January 26, 2008 |archive-url=https://web.archive.org/web/20110719160217/http://web.gps.caltech.edu/~mbrown/sedna/index.html#planets |archive-date=July 19, 2011 |url-status=dead }}</ref>
This led some astronomers to stop referring to Pluto as a planet. Several terms, including ''subplanet'' and ''planetoid'', started to be used for the bodies now known as dwarf planets.<ref>{{cite web |last1=Eicher |first1=David J. |author1-link=David J. Eicher |title=Should Pluto Be Considered a Planet? |url=https://astronomy.com/magazine/greatest-mysteries/2007/07/32-should-pluto-be-considered-a-planet |website=[[Astronomy (magazine)|Astronomy]] |access-date=November 28, 2022 |language=en |date=July 21, 2007 |archive-date=November 28, 2022 |archive-url=https://web.archive.org/web/20221128115033/https://astronomy.com/magazine/greatest-mysteries/2007/07/32-should-pluto-be-considered-a-planet |url-status=live }}</ref><ref>{{cite news |url=https://hubblesite.org/contents/news-releases/2004/news-2004-14.html/ |date=April 14, 2004 |title=Hubble Observes Planetoid Sedna, Mystery Deepens |publisher=NASA's Hubble Space Telescope home site |access-date=January 26, 2008 |archive-url=https://web.archive.org/web/20210113110507/https://hubblesite.org/contents/news-releases/2004/news-2004-14.html |archive-date=January 13, 2021 |url-status=live}}</ref> 
Astronomers were also confident that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly if Pluto were to remain classified as a planet.<ref>{{cite news |title=War of the Worlds |work=[[The New York Times]] | last=Brown | first=Mike |date=August 16, 2006 |url=https://www.nytimes.com/2006/08/16/opinion/16brown.html |access-date=February 20, 2008 |archive-date=February 13, 2017 |archive-url=https://web.archive.org/web/20170213193022/http://www.nytimes.com/2006/08/16/opinion/16brown.html |url-status=live }}</ref>

[[Eris (dwarf planet)|Eris]] (then known as {{mp|2003 UB|313}}), a [[trans-Neptunian object]], was discovered in January 2005;<ref>{{Cite web |url=http://web.gps.caltech.edu/~mbrown/planetlila/ |title=California Institute of Technology, Retrieved 4-12-2015 |access-date=April 12, 2015 |archive-date=May 17, 2012 |archive-url=https://web.archive.org/web/20120517130215/http://web.gps.caltech.edu/~mbrown/planetlila/ |url-status=live }}</ref> it was thought to be slightly larger than Pluto, and some reports informally referred to it as the ''[[Planets beyond Neptune|tenth planet]]''.<ref>{{cite news |title=Astronomers Measure Mass of Largest Dwarf Planet |publisher=NASA's Hubble Space Telescope home site |url=http://hubblesite.org/newscenter/archive/releases/2007/24/full/ |date=June 14, 2007 |access-date=January 26, 2008 |archive-date=August 7, 2011 |archive-url=https://web.archive.org/web/20110807132150/http://hubblesite.org/newscenter/archive/releases/2007/24/full/ |url-status=live }}</ref> As a consequence, the issue became a matter of intense debate during the [[International Astronomical Union#The XXVIth General Assembly and the definition of a planet|IAU General Assembly]] in August 2006.<ref>{{cite web |title=What makes a planet? |url=http://www.gps.caltech.edu/~mbrown/whatsaplanet/ |first=Michael E. |last=Brown |publisher=California Institute of Technology, Department of Geological Sciences |access-date=January 26, 2008 |archive-date=May 16, 2012 |archive-url=https://web.archive.org/web/20120516023200/http://www.gps.caltech.edu/~mbrown/whatsaplanet/ |url-status=live }}</ref> The IAU's initial draft proposal included Charon, Eris, and Ceres in the list of planets. After many astronomers objected to this proposal, an alternative was drawn up by the Uruguayan astronomers [[Julio Ángel Fernández]] and [[Gonzalo Tancredi]]: They proposed an intermediate category for objects large enough to be round but that had not cleared their orbits of [[planetesimal]]s. Beside dropping Charon from the list, the new proposal also removed Pluto, Ceres, and Eris, because they have not cleared their orbits.<ref name="19aug">{{cite web |date=August 19, 2006 |first=Robert Roy |last=Britt |title=Details Emerge on Plan to Demote Pluto |publisher=Space.com |access-date=August 18, 2006 |url=http://www.space.com/scienceastronomy/060819_new_proposal.html |archive-date=June 28, 2011 |archive-url=https://web.archive.org/web/20110628232939/http://www.space.com/scienceastronomy/060819_new_proposal.html |url-status=live }}</ref>

Although concerns were raised about the classification of planets orbiting other stars,<ref name="Draft Resolution 5">{{cite web |title=The IAU draft definition of "planet" and "plutons" |url=http://www.iau.org/news/pressreleases/detail/iau0601/ |publisher=[[International Astronomical Union]] |date=August 16, 2006 |access-date=May 17, 2008 |archive-date=April 29, 2014 |archive-url=https://web.archive.org/web/20140429050716/http://www.iau.org/news/pressreleases/detail/iau0601/ |url-status=live }}</ref> the issue was not resolved; it was proposed instead to decide this only when dwarf-planet-size objects start to be observed.<ref name=19aug/>

In the immediate aftermath of the IAU definition of dwarf planet, some scientists expressed their disagreement with the IAU resolution.<ref name="Stern"/> Campaigns included car bumper stickers and T-shirts.<ref>{{cite news |url=https://www.usatoday.com/tech/science/space/2006-08-25-pluto-memorabilia_x.htm |title=Online merchants see green in Pluto news |first=Alicia |last=Chang |agency=Associated Press |publisher=USA Today |date=August 25, 2006 |access-date=January 25, 2008 |archive-date=May 11, 2008 |archive-url=https://web.archive.org/web/20080511172619/http://www.usatoday.com/tech/science/space/2006-08-25-pluto-memorabilia_x.htm |url-status=live }}</ref> [[Michael E. Brown|Mike Brown]] (the discoverer of Eris) agrees with the reduction of the number of planets to eight.<ref>{{cite web |title=The Eight Planets |first=Michael E. |last=Brown |url=http://web.gps.caltech.edu/~mbrown/eightplanets/ |publisher=California Institute of Technology, Department of Geological Sciences |access-date=January 26, 2008 |archive-url=https://web.archive.org/web/20110719162512/http://web.gps.caltech.edu/~mbrown/eightplanets/ |archive-date=July 19, 2011 |url-status=dead }}</ref>

NASA announced in 2006 that it would use the new guidelines established by the IAU.<ref>{{cite press release|title= {{Sic|hide=y|Hotly|-}}Debated Solar System Object Gets a Name|publisher=[[NASA]]|url= http://www.nasa.gov/vision/universe/solarsystem/erisf-20060914.html|date= September 14, 2006|access-date= January 26, 2008|archive-date= June 29, 2011|archive-url= https://web.archive.org/web/20110629155731/http://www.nasa.gov/vision/universe/solarsystem/erisf-20060914.html|url-status= dead}}</ref> [[Alan Stern]], the director of [[New Horizons|NASA's mission to Pluto]], rejects the current IAU definition of planet, both in terms of defining dwarf planets as something other than a type of planet, and in using orbital characteristics (rather than intrinsic characteristics) of objects to define them as dwarf planets.<ref>{{cite web |title=Unabashedly Onward to the Ninth Planet |first=Alan |last=Stern |date=September 6, 2006 |publisher=New Horizons Web Site |url=http://pluto.jhuapl.edu/overview/piPerspectives/piPerspective_09_06_2006.php |access-date=January 26, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20131207024954/http://pluto.jhuapl.edu/overview/piPerspectives/piPerspective_09_06_2006.php |archive-date=December 7, 2013 }}</ref> Thus, in 2011, he still referred to Pluto as a planet,<ref>{{cite web |url=http://www.space.com/12710-pluto-defender-alan-stern-dwarf-planet-interview.html |first=Mike |last=Wall |publisher=Space.com |title=Pluto's Planet Title Defender: Q & A With Planetary Scientist Alan Stern |date=August 24, 2011 |access-date=December 3, 2012 |archive-date=August 14, 2012 |archive-url=https://web.archive.org/web/20120814212232/http://www.space.com/12710-pluto-defender-alan-stern-dwarf-planet-interview.html |url-status=live }}</ref> and accepted other likely dwarf planets such as Ceres and Eris, as well as the [[satellite planet|larger moons]], as additional planets.<ref name="News.discovery.com"/> Several years before the IAU definition, he used orbital characteristics to separate "überplanets" (the dominant eight) from "unterplanets" (the dwarf planets), considering both types "planets".<ref name="Stern 2002"/>

== Name ==
[[File:Euler diagram of solar system bodies.svg|thumb|[[Euler diagram]] showing the IAU Executive Committee conception of the types of bodies in the Solar System (except the Sun)]]
Names for large subplanetary bodies include ''dwarf planet'', ''planetoid'' (more general term), ''[[Mesoplanet|meso-planet]]'' (narrowly used for sizes between Mercury and Ceres), ''quasi-planet'', and (in the transneptunian region) ''plutoid''. ''Dwarf planet'', however, was originally coined as a term for the smallest planets, not the largest sub-planets, and is still used that way by many planetary astronomers.

Alan Stern coined the term ''dwarf planet'' in 1990<ref name="AAAS"/><ref name="oralhistory"/> or 1991,<ref name="ChasingNH">{{Cite book |last1=Stern |first1=Alan |author-link1=Alan Stern |title=Chasing new horizons: inside the epic first mission to Pluto |title-link=Chasing New Horizons |last2=Grinspoon |first2=David |author-link2=David Grinspoon |date=2018 |publisher=[[Picador (imprint)|Picador]] |isbn=978-1-250-09896-2 |location=New York | pages=136 | quote=a term Alan coined in a 1991 research paper}}</ref>{{dubious|date=April 2025|Verifiability issues on Stern's coining of "dwarf planet"}} as an analogy to the term ''[[dwarf star]]'' for small stars.{{cn|date=April 2025}} Since the passing of the IAU definition, he and many of his colleagues continued to classify dwarf planets as a class of planets.<ref name="Metzger"/> The IAU decided that dwarf planets are not to be considered planets, but kept Stern's term for them. Other terms for the IAU definition of the largest subplanetary bodies that do not have such conflicting connotations or usage include ''quasi-planet''<ref>{{cite news |first=Tom |last=Service |title=Sounds of the solar system: probing Pluto's predicted score |newspaper=The Guardian |date=July 15, 2015 |url=https://www.theguardian.com/music/tomserviceblog/2015/jul/15/sounds-pluto-new-horizons-colin-matthews-holst |access-date=December 26, 2019 |archive-date=December 26, 2019 |archive-url=https://web.archive.org/web/20191226002852/https://www.theguardian.com/music/tomserviceblog/2015/jul/15/sounds-pluto-new-horizons-colin-matthews-holst |url-status=live }}</ref> and the older term ''planetoid'' ("having the form of a planet").<ref>{{cite book |editor=Karttunen |display-editors=etal |title=Fundamental Astronomy |publisher=Springer |date=2007 |edition=5 |title-link=Fundamental Astronomy (book)}}</ref> [[Michael E. Brown]] stated that ''planetoid'' is "a perfectly good word" that has been used for these bodies for years, and that the use of the term ''dwarf planet'' for a non-planet is "dumb", but that it was motivated by an attempt by the IAU division III plenary session to reinstate Pluto as a planet in a second resolution.<ref name=killed>{{cite book |first=Mike |last=Brown |title=How I Killed Pluto and Why It Had It Coming |page=[https://archive.org/details/howikilledplutow00mike/page/223 223] |publisher=Spiegel & Grau |date=2010 |title-link=How I Killed Pluto and Why It Had It Coming}}</ref> Indeed, the draft of Resolution 5A had called these median bodies ''planetoids,''<ref>{{cite web |title=Comments & discussions on Resolution 5: The definition of a planet – Planets Galore |first=Mark E. |last=Bailey |url=http://astro.cas.cz/nuncius/appendix.html |work=Dissertatio cum Nuncio Sidereo, Series Tertia – official newspaper of the IAU General Assembly 2006 |publisher=Astronomical Institute Prague |access-date=February 9, 2008 |archive-date=July 20, 2011 |archive-url=https://web.archive.org/web/20110720030946/http://astro.cas.cz/nuncius/appendix.html |url-status=live }}</ref><ref>{{cite web |title=Dos uruguayos, Julio Fernández y Gonzalo Tancredi en la historia de la astronomía:reducen la cantidad de planetas de 9 a 8 ...&Anotaciones de Tancredi |language=es |publisher=Science and Research Institute, Mercedes, Uruguay |url=http://www.ici.edu.uy/perfilASTRO.htm |access-date=February 11, 2008 |archive-url=https://web.archive.org/web/20071220063342/http://www.ici.edu.uy/perfilASTRO.htm |archive-date=December 20, 2007}}</ref> but the plenary session voted unanimously to change the name to ''dwarf planet.''<ref name="finalresolution">{{cite news |url=http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf |title=Definition of a Planet in the Solar System: Resolutions 5 and 6 |date=August 24, 2006 |work=IAU 2006 General Assembly |publisher=[[International Astronomical Union]] |access-date=January 26, 2008 |author=IAU |archive-date=June 20, 2009 |archive-url=https://web.archive.org/web/20090620102000/http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf |url-status=live }}</ref> The second resolution, 5B, defined ''dwarf planets'' as a subtype of ''planet'', as Stern had originally intended, distinguished from the other eight that were to be called "classical planets". Under this arrangement, the twelve planets of the rejected proposal were to be preserved in a distinction between eight ''classical planets'' and four ''dwarf planets''. Resolution 5B was defeated in the same session that 5A was passed.<ref name=killed/> Because of the semantic inconsistency of a ''dwarf planet'' not being a planet due to the failure of Resolution 5B, alternative terms such as ''nanoplanet'' and ''subplanet'' were discussed, but there was no consensus among the CSBN to change it.<ref name=D3-PSS-Cambridge/>

In most languages equivalent terms have been created by translating ''dwarf planet'' more-or-less literally: French ''{{lang|fr|planète [[wikt:nain#French|naine]]}}'', Spanish ''{{lang|es|planeta enano}}'', German ''{{lang|de|[[wikt:Zwerg|Zwergplanet]]}}'', Russian ''karlikovaya planeta'' ({{lang|ru|[[wikt:Карлик|карликовая]] планета}}), Arabic ''kaukab qazm'' ({{lang|ar|كوكب [[wikt:قزم|قزم]]}}), Chinese ''ǎixíngxīng'' ({{lang|cmn|[[wikt:矮|矮]]行星}}), Korean ''waesohangseong'' ({{lang|ko|왜소행성 / 矮小行星}}) or ''waehangseong'' ({{lang|ko|왜행성 / 矮行星}}), but in Japanese they are called ''junwakusei'' ({{lang|ja|準惑星}}), meaning "quasi-planets" or "peneplanets" ([[:wikt:pene-#English|pene-]] meaning "almost").

IAU Resolution 6a of 2006<ref name="Resolution Result">{{Cite web|date=August 24, 2006|url=https://www.iau.org/news/pressreleases/detail/iau0603/|title=International Astronomical Union 2006 General Assembly: Result of the IAU Resolution votes|website=[[IAU]]|access-date=August 10, 2021|archive-date=April 29, 2014|archive-url=https://web.archive.org/web/20140429212224/https://www.iau.org/news/pressreleases/detail/iau0603/|url-status=live}}</ref> recognizes Pluto as "the prototype of a new category of trans-Neptunian objects". The name and precise nature of this category were not specified but left for the IAU to establish at a later date; in the debate leading up to the resolution, the members of the category were variously referred to as ''plutons'' and ''plutonian objects'' but neither name was carried forward, perhaps due to objections from geologists that this would create confusion with their ''[[pluton]]''.<ref name="finalresolution" />

On June 11, 2008, the IAU Executive Committee announced a new term, ''plutoid'', and a definition: all trans-Neptunian dwarf planets are plutoids.<ref name=IAU2008>{{Cite web |date=June 11, 2008|url=https://www.iau.org/news/pressreleases/detail/iau0804/|title=Plutoid chosen as name for Solar System objects like Pluto|website=[[IAU]]|access-date=August 10, 2021|archive-date=November 23, 2020|archive-url=https://web.archive.org/web/20201123162642/https://www.iau.org/news/pressreleases/detail/iau0804/|url-status=live|location=Paris}}</ref> Other departments of the IAU have rejected the term: {{blockquote|...in part because of an email miscommunication, the [[International Astronomical Union|WG-PSN [Working Group for Planetary System Nomenclature]]] was not involved in choosing the word plutoid. ... In fact, a vote taken by the WG-PSN subsequent to the Executive Committee meeting has rejected the use of that specific term..."<ref name=D3-PSS-Cambridge>{{cite journal |last1=Bowell |first1=Edward L.G. |author-link1=Edward L. G. Bowell |last2=Meech |first2=Karen J. |author-link2=Karen Jean Meech |last3=Williams |first3=Iwan P. |author-link3=:fr:Iwan Williams |last4=Boss |first4=Alan P. |author-link4=Alan Boss |last5=Consolmagno |first5=Guy J. |author-link5=Guy Consolmagno |last6=Courtin |first6=Régis |last7=Fernández |first7=Julio A. |author-link7=Julio Ángel Fernández |last8=Gustafson |first8=Bo Å. S. |last9=Huebner |first9=Walter F. |author-link9=Walter F. Huebner |last10=Levasseur-Regourd |first10=Anny-Chantal |author-link10=Anny-Chantal Levasseur-Regourd |last11=Marov |first11=Mikhail Ya. |author-link11=Mikhail Marov |last12=Mayor |first12=Michel  |author-link12=Michel Mayor |last13=Schulz |first13=Rita M. |author-link13=:de:Rita Schulz |last14=Spurný |first14=Pavel |author-link14=:cs:Pavel Spurný |last15=Valsecchi |first15=Giovanni B. |last16=Watanabe |first16=Jun-ichi |author-link16=:ja:渡部潤一 |last17=Witt |first17=Adolf N. |display-authors=3 |date=2008-12-01 |title=Division III: Planetary Systems Sciences |journal=[[Proceedings of the International Astronomical Union]] |volume=4 |issue=T27A |pages=149–153 |publisher=[[Cambridge University Press]] |doi=10.1017/S1743921308025398 |doi-access=free}}</ref>}}

The category of 'plutoid' captured an earlier distinction between the 'terrestrial dwarf' Ceres and the 'ice dwarfs' of the outer Solar system,<ref>{{cite book |last=Carson |first=Mary Kay |date=2011 |title=Far-Out Guide to the Icy Dwarf Planets |url=https://archive.org/details/faroutguidetoicy0000cars/mode/2up |url-access=registration |publisher=[[Enslow Publishers]] |isbn=9780766031876 |oclc=441945398 |via=[[Internet Archive]]}}</ref> part of a conception of a threefold division of the Solar System into inner ''[[terrestrial planet]]s'', central ''[[giant planet]]s'', and outer ''ice dwarfs'', of which Pluto was the principal member.<ref>{{cite book |last=Lew |first=Kristi |date=2010 |title=Space! The Dwarf Planet Pluto |url=https://archive.org/details/dwarfplanetpluto0000lewk |url-access=registration |publisher=[[Marshall Cavendish Benchmark]] |place=New York |page=10 |isbn=9780761445531 |oclc=562529871 |via=[[Internet Archive]]}}</ref> 'Ice dwarf' also saw some use as an umbrella term for all trans-Neptunian [[minor planet]]s, or for the ice [[asteroid]]s of the outer Solar System; one attempted definition was that an ice dwarf "is larger than the [[comet nucleus|nucleus]] of a normal [[comet]] and icier than a typical asteroid."<ref name=darling>{{cite web
 |title=Ice dwarf |website=Encyclopedia of Astrobiology, Astronomy and Spaceflight |editor-last=Darling |editor-first=David |editor-link=David J. Darling |url=http://www.daviddarling.info/encyclopedia/I/icedwarf.html |access-date=June 22, 2008 |url-status =dead |archive-url=https://web.archive.org/web/20080706143614/http://www.daviddarling.info/encyclopedia/I/icedwarf.html |archive-date=July 6, 2008
}}</ref>
	
Since the ''[[Dawn (spacecraft)|Dawn]]'' mission, it has been recognized that Ceres is a geologically icy body that may have originated from the outer Solar System.<ref>{{Cite web|url = https://www.space.com/33934-dwarf-planet-ceres-ice-volcano-discoveries.html|title = Ice Volcanoes and More: Dwarf Planet Ceres Continues to Surprise|website = [[Space.com]]|date = September 2016|access-date = December 19, 2019|archive-date = October 12, 2019|archive-url = https://web.archive.org/web/20191012004925/https://www.space.com/33934-dwarf-planet-ceres-ice-volcano-discoveries.html|url-status = live}}</ref><ref>{{cite web|title=Dawn at Ceres: What Have we Learned?|display-authors=etal|first1=J. C.|last1=Castillo-Rogez|first2=C. A.|last2=Raymond|first3=C. T.|last3=Russell|url=http://sites.nationalacademies.org/cs/groups/ssbsite/documents/webpage/ssb_183286.pdf|publisher=Committee on Astrobiology and Planetary Science|date=September 12, 2017|access-date=October 12, 2019|archive-date=October 8, 2018|archive-url=https://web.archive.org/web/20181008123813/http://sites.nationalacademies.org/cs/groups/ssbsite/documents/webpage/ssb_183286.pdf|url-status=live}}</ref>
Ceres has since been called an ice dwarf as well.<ref>{{Cite book|chapter=Ceres: The First Known Ice Dwarf Planet |title=Ice Worlds of the Solar System: Their Tortured Landscapes and Biological Potential |first=Michael |last=Carroll|author-link=Michael Carroll (space artist) |date=October 23, 2019 |doi=10.1007/978-3-030-28120-5 |publisher=Springer Cham |isbn=978-3-030-28120-5}}</ref>

== Criteria ==
{| class="wikitable sortable floatright" style="text-align:center; width: 330px;"
|+ Planetary discriminants<ref name=soter/>
|-
! scope=col | Body
! scope=col style="font-weight: normal;" | {{nobr| '''{{sfrac|{{mvar|m}}|''M''<sub>🜨</sub> }}''' {{sup|[†]}} }}
! scope=col style="font-weight: normal;" | '''{{math|Λ}}'''&nbsp;{{sup|[‡]}}
! scope=col style="font-weight: normal;" | '''{{mvar|µ}}'''&nbsp;{{sup|[§]}}
! scope=col style="font-weight: normal;" | '''{{math|Π}}'''&nbsp;{{sup|[#]}}
|- style="background: #FFF;"
!scope=row align=left | [[Mercury (planet)|Mercury]]
| 0.055
| {{val|1.95e3}}
| {{val|9.1e4}}
| {{val|1.3e2}}
|- style="background: #FFF;"
!scope=row align=left | [[Venus]]
| 0.815
| {{val|1.66e5}}
| {{val|1.35e6}}
| {{val|9.5e2}}
|- style="background: #FFF;"
!scope=row align=left | [[Earth]]
| 1
| {{val|1.53e5}}
| {{val|1.7e6}}
| {{val|8.1e2}}
|- style="background: #FFF;"
!scope=row align=left | [[Mars]]
| 0.107
| {{val|9.42e2}}
| {{val|1.8e5}}
| {{val|5.4e1}}
|- style="background: #E8E0FF;"
!scope=row align=left | [[Ceres (dwarf planet)|Ceres]]
| 0.00016
| {{val|8.32e−4}}
| {{val|0.33}}
| {{val|4.0e−2}}
|- style="background: #FFF;"
!scope=row align=left | [[Jupiter]]
| 317.7
| {{val|1.30e9}}
| {{val|6.25e5}}
| {{val|4.0e4}}
|- style="background: #FFF;"
!scope=row align=left | [[Saturn]]
| 95.2
| {{val|4.68e7}}
| {{val|1.9e5}}
| {{val|6.1e3}}
|- style="background: #FFF;"
!scope=row align=left | [[Uranus]]
| 14.5
| {{val|3.85e5}}
| {{val|2.9e4}}
| {{val|4.2e2}}
|- style="background: #FFF;"
!scope=row align=left | [[Neptune]]
| 17.1
| {{val|2.73e5}}
| {{val|2.4e4}}
| {{val|3.0e2}}
|- style="background: #E8E0FF;"
!scope=row align=left | [[Pluto]]
| 0.0022
| {{val|2.95e−3}}
| {{val|0.077}}
| {{val|2.8e−2}}
<!--
|- style="background: #E8E0FF;"
!scope=row align=left | [[Haumea]]
| 0.00067
| {{val|2.68e–4}}
| {{val|0.02}}
| {{val|7.8e−3}}
|- style="background: #E8E0FF;"
!scope=row align=left | [[Makemake]]
| 0.00067
| {{val|2.22e–4}}
| {{val|0.02}}{{efn|Calculated using the estimate for the mass of the Kuiper belt found by Iorio (2007)<ref>{{cite journal |last=Iorio |first=Lorenzo |date=March 2007 |orig-date=February 5, 2007 |title=Dynamical determination of the mass of the Kuiper Belt from motions of the inner planets of the Solar system |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=375 |issue=4 |pages=1,311–1,314 |bibcode=2007MNRAS.375.1311I |arxiv=gr-qc/0609023 |doi=10.1111/j.1365-2966.2006.11384.x |doi-access=free |url=http://adsabs.harvard.edu/abs/2007MNRAS.tmp...24I}}</ref> of 0.033&nbsp;Earth masses.}}
| {{val|7.3e−3}}
-->
|- style="background: #E8E0FF;"
!scope=row align=left | [[Eris (dwarf planet)|Eris]]
| 0.0028
| {{val|2.13e−3}}
| {{val|0.10}}
| {{val|2.0e−2}}
|- style="background: #E8E0FF;"
!scope=row align=left | [[90377 Sedna|Sedna]]
| 0.0002
| {{val|3.64e−7}}
| {{val|0.07|p= < }}{{efn|Calculated using the minimum estimate from 15&nbsp;objects in its region with at least Sedna's mass, as estimated by Schwamb, Brown, & Rabinowitz (2009).<ref>{{cite journal |last1=Schwamb |first1=Megan E. |author-link1=Megan Schwamb |last2=Brown |first2=Michael E. |author-link2=Michael E. Brown |last3=Rabinowitz |first3=David L. |author-link3=David L. Rabinowitz |year=2009 |title=A search for distant solar system bodies in the region of Sedna |journal=[[The Astrophysical Journal]] |volume=694 |issue=1 |pages=L45–L48 |arxiv=0901.4173 |doi=10.1088/0004-637X/694/1/L45 |doi-access=free |bibcode=2009ApJ...694L..45S|s2cid=15072103 }}</ref>}}
| {{val|1.6e−4}}
|-
! colspan=5 style="font-size: 0.8em; font-weight: normal; text-align: center; padding: 4px;" |<!-- note -->
Planetary discriminants of the planets ({{background color|#FFF| white }}), and of the largest known dwarf planet ({{background color|#E8E0FF| light purple }}) in each orbital population ([[asteroid belt]], [[Kuiper belt]], [[scattered disc]], [[sednoid]]s). All other known objects in these populations have smaller discriminants than the one shown.
|-
! colspan=5 |<!-- legend -->
 {| style="font-size: 0.9em; font-weight: normal; text-align: left; padding: 4px"
 |-
 | valign=top |  [†]  || Mass in [[Earth mass|''M''<sub>🜨</sub>]], the unit of mass equal to that of Earth {{nobr| ( {{math|5.97 × {{10^|24}} kg}} ).}}
 |-
 | valign=top |  [‡]  || {{math| Λ }} is the capacity to [[Clearing the neighbourhood|clear the neighbourhood]] (greater than 1 for planets) by Stern & Levison (2002):<ref name="Stern 2002"/> {{nobr| {{math|Λ {{=}} ''k'' ''m''{{sup|2}} ''a''{{sup|{{sup|{{sfrac|− | 3 |2}}}} }} }} ,}} where {{nobr| {{math| ''k'' {{=}} 0.0043 }} }} for {{mvar| m }} in units of [[yottagram]]s ({{10^|18}}&nbsp;[[metric ton]]s) and {{mvar| a }} in [[astronomical unit]]s ({{sc|AU}}), where {{mvar| a }} is the body's semi-major axis.<ref name="Stern 2002"/>
 |-
 | valign=top |  [§]  || {{mvar| µ }} is Soter's planetary discriminant, which he finds greater than 100 for planets. {{nobr| {{math|  ''µ'' {{=}} {{sfrac|''m''| ''M'' − ''m'' }} }} ,}} where {{mvar| m }} is the mass of the body, and {{mvar| M }} is the aggregate mass of ''all'' the bodies that occupy its orbital zone.<ref name=soter/>
 |-
 | valign=top |  [#]  || {{math| Π  }} is the capacity to [[Clearing the neighbourhood|clear the neighbourhood]] (greater than 1 for planets) by Margot. {{nobr| {{math|  Π {{=}} ''k'' ''m'' ''a''{{sup| {{sup|{{sfrac|− | 9 |8}}}} }}  }},}}  where {{nobr| {{math|  ''k'' {{=}} 807 }} }} for units of [[Earth mass]]es and [[Astronomical unit|AU]].<ref name="Margot 2015">{{cite journal |first=Jean-Luc |last=Margot |author-link=Jean-Luc Margot |date=October 15, 2015 |title=A quantitative criterion for defining planets |journal=[[The Astronomical Journal]] |volume=150 |issue=6 |page=185 |doi=10.1088/0004-6256/150/6/185 |arxiv=1507.06300 |bibcode=2015AJ....150..185M |s2cid=51684830 }}</ref>
 |}
|}

The category ''dwarf planet'' arose from a conflict between dynamical and geophysical ideas of what a useful conception of a planet would be. In terms of the dynamics of the Solar System, the major distinction is between bodies that gravitationally dominate their neighbourhood (Mercury through Neptune) and those that do not (such as the asteroids and Kuiper belt objects). A celestial body may have a dynamic (planetary) geology at approximately the mass required for its mantle to become plastic under its own weight, which results in the body acquiring a round shape. Because this requires a much lower mass than gravitationally dominating the region of space near their orbit, there are a population of objects that are massive enough to have a world-like appearance and planetary geology, but not massive enough to clear their neighborhood. Examples are Ceres in the asteroid belt and Pluto in the Kuiper belt.<ref name=planetarysociety/>

Dynamicists usually prefer using gravitational dominance as the threshold for planethood, because from their perspective smaller bodies are better grouped with their neighbours, e.g. Ceres as simply a large asteroid and Pluto as a large Kuiper belt object.<ref>{{cite web |first=Mike |last=Brown |title=The eight planets |website=gps.caltech.edu |publisher=[[California Institute of Technology|Caltech]] |url=http://web.gps.caltech.edu/~mbrown/eightplanets/ |access-date=January 26, 2008 |archive-date=July 19, 2011 |archive-url=https://web.archive.org/web/20110719162512/http://web.gps.caltech.edu/~mbrown/eightplanets/ |url-status=live }}</ref><ref>{{cite web |first=David |last=Jewitt |title=Classification of Pluto |website=ess.ucla.edu |publisher=[[University of California, Los Angeles|UCLA]] |url=http://www2.ess.ucla.edu/~jewitt/kb/pluto_classification.html |access-date=August 19, 2021 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819074719/http://www2.ess.ucla.edu/~jewitt/kb/pluto_classification.html |url-status=live }}</ref> Geoscientists usually prefer roundness as the threshold, because from their perspective the internally driven geology of a body like Ceres makes it more similar to a classical planet like Mars, than to a small asteroid that lacks internally driven geology. This necessitated the creation of the category of ''dwarf planets'' to describe this intermediate class.<ref name=planetarysociety>{{cite web |first1=Emily |last1=Lakdawalla |author-link1=Emily Lakdawalla|display-authors=etal |date=April 21, 2020 |title=What is a planet? |website=planetary.org |publisher=[[The Planetary Society]] |url=https://www.planetary.org/worlds/what-is-a-planet |access-date=August 19, 2021 |archive-date=January 22, 2022 |archive-url=https://web.archive.org/web/20220122142140/https://www.planetary.org/worlds/what-is-a-planet |url-status=live }}</ref>

=== Orbital dominance ===
{{Main|Clearing the neighbourhood}}

Alan Stern and [[Harold F. Levison]] introduced a parameter {{math|Λ}} (upper case [[lambda]]) in 2000, expressing the likelihood of an encounter resulting in a given deflection of orbit.<ref name="Stern 2002">{{cite conference |last1=Stern |first1=S.A. |author1-link=Alan Stern |last2=Levison |first2=H.F. |author2-link=Harold F. Levison |date=August 7–18, 2000 |publication-date=2002 |title=Regarding the criteria for planethood and proposed planetary classification schemes |journal=Highlights of Astronomy |volume=12 |pages=205–213 |conference=XXIVth General Assembly of the IAU – 2000 |place=Manchester, UK |bibcode=2002HiA....12..205S |doi=10.1017/S1539299600013289 |doi-access=free |url=http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf |access-date=January 26, 2008 |archive-date=September 23, 2015 |archive-url=https://web.archive.org/web/20150923220319/http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf |url-status=live }}</ref> The value of this parameter in Stern's model is proportional to the square of the mass and inversely proportional to the period. This value can be used to estimate the capacity of a body to [[Clearing the neighbourhood|clear the neighbourhood]] of its orbit, where {{math|  Λ &gt; 1  }} will eventually clear it. A gap of five orders of magnitude in {{math|Λ}} was found between the smallest [[terrestrial planet]]s and the largest asteroids and Kuiper belt objects.<ref name=soter/>

Using this parameter, [[Steven Soter]] and other astronomers argued for a distinction between planets and dwarf planets based on the inability of the latter to "clear the neighbourhood around their orbits": planets are able to remove smaller bodies near their orbits by collision, capture, or gravitational disturbance (or establish orbital resonances that prevent collisions), whereas dwarf planets lack the mass to do so.<ref name="Stern 2002"/> Soter went on to propose a parameter he called the ''planetary discriminant'', designated with the symbol {{mvar|µ}} ([[mu (letter)|mu]]), that represents an experimental measure of the actual degree of cleanliness of the orbital zone (where {{mvar|µ}} is calculated by dividing the mass of the candidate body by the total mass of the other objects that share its orbital zone), where {{nobr| {{math|  ''µ'' > 100  }} }} is deemed to be cleared.<ref name=soter>{{cite journal |first=S. |last=Soter |author-link=Steven Soter |date=August 16, 2006 |title=What is a Planet? |journal=[[The Astronomical Journal]] |volume=132 |issue=6 |pages=2513–2519 |doi=10.1086/508861 |bibcode=2006AJ....132.2513S |s2cid=14676169 |arxiv=astro-ph/0608359 }}</ref>

[[Jean-Luc Margot]] refined Stern and Levison's concept to produce a similar parameter {{math|Π}} (upper case [[Pi (letter)|Pi]]).<ref name="Margot 2015"/> It is based on theory, avoiding the empirical data used by {{math|Λ&nbsp;.}} {{nobr| {{math|  Π &gt; 1  }} }} indicates a planet, and there is again a gap of several orders of magnitude between planets and dwarf planets.

There are several other schemes that try to differentiate between planets and dwarf planets,<ref name="Stern">{{cite news |first=Paul |last=Rincon |date=August 25, 2006 |title=Pluto vote 'hijacked' in revolt |agency=[[BBC News]] |publisher=[[British Broadcasting Corporation]] |url=http://news.bbc.co.uk/2/hi/science/nature/5283956.stm |access-date=January 26, 2008 |archive-date=July 23, 2011 |archive-url=https://web.archive.org/web/20110723033823/http://news.bbc.co.uk/2/hi/science/nature/5283956.stm |url-status=live }}</ref> but the 2006 definition uses this concept.<ref name=finalresolution/>

=== Hydrostatic equilibrium ===
{{Main|Hydrostatic equilibrium}}
{{Pie chart
|1= [ 
 {"value":16.6, "label": "Eris $d"},
 {"value":13.03, "label": "Pluto $d"},
 {"value":4.01, "label": "Haumea $d"},
 {"value":3.1, "label": "Makemake $d"},
 {"value":1.75, "label": "Gonggong $d"},
 {"value":1.59, "label": "Charon $d"},
 {"value":1.4, "label": "Quaoar $d"},
 {"value":0.94, "label": "Ceres $d"},
 {"value":0.61, "label": "Orcus $d"},
 {"value":0.492, "label": "Salacia $d"}
]
|legend=true
|autoscale=true
|radius=125
|caption=Comparative masses of the likeliest dwarf planets, with [[Charon (moon)|Charon]] for comparison. 
|footer=The unit of mass is {{e|21}} kg. [[Eris (dwarf planet)|Eris]] and Pluto dominate. Unmeasured [[90377 Sedna|Sedna]] is excluded, but is likely on the order of Ceres. The Moon in contrast is 73.5×{{10^|21}} kg, over four times more massive than Eris.
|border=no
}}

Enough internal pressure, caused by the body's gravitation, will turn a body [[Plasticity (physics)|plastic]], and enough plasticity will allow high elevations to sink and hollows to fill in, a process known as gravitational relaxation. Bodies smaller than a few kilometers are dominated by non-gravitational forces and tend to have an irregular shape and may be rubble piles. Larger objects, where gravity is significant but not dominant, are potato-shaped; the more massive the body, the higher its internal pressure, the more solid it is and the more rounded its shape, until the pressure is enough to overcome its [[compressive strength]] and it achieves [[hydrostatic equilibrium]]. Then, a body is as round as it is possible to be, given its rotation and tidal effects, and is an [[ellipsoid]] in shape. This is the defining limit of a dwarf planet.<ref>{{cite conference |first1=Charles H. |last1=Lineweaver |first2=Marc |last2=Norman |date=September 28–30, 2009 |publication-date=2010 |title=The potato radius: A lower minimum size for dwarf planets |editor-last1=Short |editor-first1=W. |editor-last2=Cairns |editor-first2=I. |conference=9th Australian Space Science Conference |book-title=Proceedings of 2009 Australian Space Science Conference |pages=67–78 |publisher=[[National Space Society#National Space Society of Australia|National Space Society of Australia]] |arxiv=1004.1091 |isbn=9780977574032 |url=https://www.mso.anu.edu.au/~charley/papers/Potato%20Radiusv8.pdf |access-date=August 11, 2023 |archive-date=March 10, 2023 |archive-url=https://web.archive.org/web/20230310103945/https://www.mso.anu.edu.au/~charley/papers/Potato%20Radiusv8.pdf |url-status=live }}</ref>

If an object is in hydrostatic equilibrium, a global layer of liquid on its surface would form a surface of the same shape as the body, apart from small-scale surface features such as craters and fissures. The body will have a spherical shape if it does not rotate and an ellipsoidal one if it does. The faster it rotates, the more [[Oblate spheroid|oblate]] or even [[scalene ellipsoid|scalene]] it becomes. If such a rotating body were heated until it melts, its shape would not change. The extreme example of a body that may be scalene due to rapid rotation is {{dp|Haumea}}, which is twice as long on its major axis as it is at the poles. If the body has a massive nearby companion, then tidal forces gradually slow its rotation until it is tidally locked; that is, it always presents the same face to its companion. Tidally locked bodies are also scalene, though sometimes only slightly so. Earth's [[Moon]] is tidally locked, as are all the rounded satellites of the gas giants. Pluto and Charon are tidally locked to each other, as are Eris and [[Dysnomia (moon)|Dysnomia]], and probably also Orcus and [[Vanth (moon)|Vanth]].

There are no specific size or mass limits of dwarf planets, as those are not defining features. There is no clear upper limit: an object very far out in the Solar System that is more massive than [[Mercury (planet)|Mercury]] might not have had time to clear its neighbourhood, and such a body would fit the definition of dwarf planet rather than planet. Indeed, Mike Brown set out to find such an object.<ref>{{cite AV media |people=[[Julia Sweeney]] (interviewer & host), [[Michael E. Brown|M.E. Brown]] (interviewed astronomer) |date=June 28, 2007 |title=Julia Sweeney and Michael E. Brown |series=Hammer Conversations |publisher=[[KCET]] |medium=podcast |quote=Actress and comedienne [[Julia Sweeney]] (''God Said Ha!'') discusses the discovery that dwarfed Pluto with [[Caltech]] astronomer [[Michael E. Brown]]. |url=http://www.pluggd.tv/audio/channels/kcet_podcast__hammer_conversations/episodes/2h10l |access-date=June 28, 2008 |archive-url=https://web.archive.org/web/20080626220548/http://www.pluggd.tv/audio/channels/kcet_podcast__hammer_conversations/episodes/2h10l <!--Added by H3llBot--> |archive-date=June 26, 2008}}</ref> The lower limit is determined by the requirements of achieving and retaining hydrostatic equilibrium, but the size or mass at which an object attains and retains equilibrium depends on its composition and thermal history, not simply its mass. An [[International Astronomical Union|IAU]] 2006 press release<ref name="Q&A"/> question-and-answer section estimated that objects with mass above {{val|0.5|e=21|u=kg}} and [[List of Solar System objects by size#Larger than 400 km|radius greater than 400&nbsp;km]] would "normally" be in hydrostatic equilibrium (''the shape ... would normally be determined by self-gravity''), but that ''all borderline cases would need to be determined by observation''.<ref name="Q&A">{{cite press release |title='Planet definition' questions & answers sheet |publisher=[[International Astronomical Union]] |date=August 24, 2006 |url=https://www.iau.org/static/archives/releases/doc/iau0601_q_answers.doc |access-date=October 16, 2021 |archive-date=May 7, 2021 |archive-url=https://web.archive.org/web/20210507125920/https://www.iau.org/static/archives/releases/doc/iau0601_q_answers.doc |url-status=live }}</ref> This is close to what as of 2019 is believed to be roughly the limit for objects beyond Neptune that are fully compact, solid bodies, with {{dp|Salacia}} {{nobr|( {{mvar|r}} {{=}} {{val|423|11|u=km}} ,}} {{nobr| {{mvar|m}} {{=}} {{val|0.492|0.007|e=21|u=kg}} )}} being a borderline case both for the 2006 Q&A expectations and in more recent evaluations, and with {{dp|Orcus}} being just above the expected limit.<ref name=Grundy2019/> No other body with a measured mass is close to the expected mass limit, though [[List of Solar System objects by size#From 200 to 399 km|several without a measured mass]] approach the expected size limit.
{{clear}}

== Population of dwarf planets ==
{{Main|List of possible dwarf planets}}
[[File:Trans-Neptunians_Size_Albedo_Color.svg|thumb|upright=1.5|Comparison of sizes, albedo, and colours of various large trans-Neptunian objects with sizes of >700 km. The dark-coloured arcs represent uncertainties of the object's size.]]

Though the definition of a dwarf planet is clear, evidence about whether a given trans-Neptunian object is large and malleable enough to be shaped by its own gravitational field is often inconclusive. There are also outstanding questions relating to the interpretation of the IAU criterion in certain instances. Consequently the number of currently conformed TNOs which meet the hydrostatic equilibrium criterion is uncertain.

The three objects under consideration during the debates leading up to the 2006 IAU acceptance of the category of dwarf planet – Ceres, Pluto and Eris – are generally accepted as dwarf planets, including by those astronomers who continue to classify dwarf planets as planets. Only one of them – Pluto – has been observed in enough detail to verify that its current shape fits what would be expected from hydrostatic equilibrium.<ref name="Nimmo2017">
{{cite journal |last1=Nimmo |first1=Francis |display-authors=etal |title=Mean radius and shape of Pluto and Charon from New Horizons images|journal=Icarus |date=2017 |volume=287  |pages=12–29 |doi=10.1016/j.icarus.2016.06.027|bibcode=2017Icar..287...12N |arxiv=1603.00821|s2cid=44935431 }}</ref> Ceres is close to equilibrium, but some gravitational anomalies remain unexplained.<ref name=EPSC>{{cite book|chapter-url=https://meetingorganizer.copernicus.org/EPSC2018/EPSC2018-645-1.pdf|display-authors=4 | last1=Raymond | first1=C. | last2=Castillo-Rogez | first2=J. C.| last3=Park | first3=R. S. | last4=Ermakov | first4=A.| last5=Bland | first5=M. T. | last6=Marchi | first6=S. | last7=Prettyman | first7=T. | last8=Ammannito | first8=E.| last9=De Sanctis | first9=M. C. | last10=Russell | first10=C. T.|date=September 2018|chapter=Dawn Data Reveal Ceres' Complex Crustal Evolution|title=European Planetary Science Congress|volume=12|access-date=July 19, 2020|archive-date=January 30, 2020|archive-url=https://web.archive.org/web/20200130111631/https://meetingorganizer.copernicus.org/EPSC2018/EPSC2018-645-1.pdf|url-status=live}}</ref>  Eris is generally assumed to be a dwarf planet because it is more massive than Pluto.

In order of discovery, these three bodies are:
# {{dp|Ceres}} – discovered January 1, 1801, and announced January 24, 45 years before [[Neptune]]. Considered a planet for half a century before reclassification as an asteroid. Considered a dwarf planet by the IAU since the adoption of Resolution 5A on August 24, 2006.
# {{dp|Pluto}} –  discovered February 18, 1930, and announced March 13. Considered a planet for 76 years. Explicitly reclassified as a dwarf planet by the IAU with Resolution 6A on August 24, 2006.<ref>'Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects'</ref> Five known moons.
# {{dp|Eris}} ({{mp|2003 UB|313}}) – discovered January 5, 2005, and announced July 29. Called the "[[Planets beyond Neptune|tenth planet]]" in media reports. Considered a dwarf planet by the IAU since the adoption of Resolution 5A on August 24, 2006, and named by the IAU dwarf-planet naming committee on September 13 of that year. One known moon.

The IAU only established guidelines for which committee would oversee the naming of likely dwarf planets: any unnamed trans-Neptunian object with an [[absolute magnitude#Solar System bodies (H)|absolute magnitude]] brighter than +1 (and hence a minimum diameter of 838 km at the maximum [[geometric albedo]] of 1)<ref name="bruton">{{cite web |title=Conversion of Absolute Magnitude to Diameter for Minor Planets |publisher=Department of Physics & Astronomy (Stephen F. Austin State University) | first=Dan | last=Bruton |url=http://www.physics.sfasu.edu/astro/asteroids/sizemagnitude.html |access-date=June 13, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20100323180835/http://www.physics.sfasu.edu/astro/asteroids/sizemagnitude.html |archive-date=March 23, 2010 }}</ref> was to be named by a joint committee consisting of the [[Minor Planet Center]] and the planetary working group of the IAU.<ref name=IAU2008/> At the time (and still as of 2023), the only bodies to meet this threshold were {{dp|Haumea}} and {{dp|Makemake}}. These bodies are generally assumed to be dwarf planets, although they have not yet been demonstrated to be in hydrostatic equilibrium, and there is some disagreement for 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 = January 14, 2022
| archive-date = November 7, 2020
| archive-url = https://web.archive.org/web/20201107052958/http://www.astrosurf.com/sogorb/occultations/nature24051.pdf
| url-status = live
}}</ref><ref name="Dunham2019">
{{cite journal
| title = Haumea's Shape, Composition, and Internal Structure
| last1 = Dunham | first1= E. T.
| last2 = Desch | first2= S. J.
| last3 = Probst | first3= L.
| date = April 2019
| journal = The Astrophysical Journal
| volume = 877
| issue = 1
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| doi = 10.3847/1538-4357/ab13b3
| bibcode = 2019ApJ...877...41D
| arxiv = 1904.00522
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| doi-access = free
}}
</ref>

{{ordered list|start=4
| {{dp|Haumea}} ({{mp|2003 EL|61}}) – discovered by Brown et al. December 28, 2004, and announced by Ortiz et al. on July 27, 2005. Named by the IAU dwarf planet naming committee on September 17, 2008. Two known moons and one known ring.
| {{dp|Makemake}} ({{mp|2005 FY|9}}) – discovered March 31, 2005, and announced July 29. Named by the IAU dwarf planet naming committee on July 11, 2008. One known moon. 
}}

These five bodies – the three under consideration in 2006 (Pluto, Ceres and Eris) plus the two named in 2008 (Haumea and Makemake) – are commonly presented as the dwarf planets of the Solar System, though the limiting factor (albedo) is not what defines an object as a dwarf planet.<ref name="WGPSN">{{cite web |date=July 11, 2008 <!--11:42:58 --> |title=Dwarf Planets and their Systems |publisher=Working Group for Planetary System Nomenclature (WGPSN) |url=http://planetarynames.wr.usgs.gov/append7.html#DwarfPlanets |access-date=September 12, 2019 |archive-date=July 14, 2007 |archive-url=https://web.archive.org/web/20070714225312/http://planetarynames.wr.usgs.gov/append7.html#DwarfPlanets |url-status=live }}</ref> 
 
The astronomical community commonly refers to other larger TNOs as dwarf planets as well.<ref name=Pinilla-Alonso>{{cite book |last1=Pinilla-Alonso |first1= Noemi |last2=Stansberry |first2= John A. |last3= Holler |first3= Bryan J. |date=November 22, 2019 |chapter=Surface properties of large TNOs: Expanding the study to longer wavelengths with the James Webb Space Telescope |arxiv=1905.12320 |title= The Transneptunian Solar System |editor= Dina Prialnik |editor2=Maria Antonietta Barucci |editor3=Leslie Young |publisher= Elsevier}}</ref> At least four additional bodies meet the preliminary criteria of Brown, of Tancredi et al., of Grundy et al., and of Emery et al. for identifying dwarf planets, and are generally called dwarf planets by astronomers as well:

{{ordered list|start=6
| {{dp|Quaoar}} ({{mp|2002 LM|60}}) – discovered June 5, 2002, and announced October 7 of that year.<!--per Govert Schilling, ''The Hunt for Planet X: New Worlds and the Fate of Pluto'', p. 289, autocorrected? to "Quasar"--> One known moon and two known rings. 
| {{dp|Sedna}} ({{mp|2003 VB|12}}) – discovered November 14, 2003, and announced March 15, 2004.<!--per Govert Schilling, ''The Hunt for Planet X: New Worlds and the Fate of Pluto'', p. 289-->
| {{dp|Orcus}} ({{mp|2004 DW}}) – discovered February 17, 2004, and announced two days later.<!--per Govert Schilling, ''The Hunt for Planet X: New Worlds and the Fate of Pluto'', p. 289--> One known moon. 
| {{dp|Gonggong}} ({{mp|2007 OR|10}}) – discovered July 17, 2007, and announced January 2009. One known moon. 
}}

For instance, JPL/NASA called [[225088 Gonggong|Gonggong]], a dwarf planet after observations in 2016,<ref name=OR10>{{cite web |url=https://www.jpl.nasa.gov/news/news.php?feature=6509 |title=2007 OR10: Largest Unnamed World in the Solar System |website=Jet Propulsion Laboratory |first=Preston |last=Dyches |date=May 11, 2016 |access-date=September 12, 2019 |archive-date=November 23, 2020 |archive-url=https://web.archive.org/web/20201123155324/https://www.jpl.nasa.gov/news/news.php?feature=6509 |url-status=live }}</ref> and Simon Porter of the Southwest Research Institute spoke of "the big eight [TNO] dwarf planets" in 2018, referring to Pluto, Eris, Haumea, Makemake, Gonggong, {{dp|Quaoar}}, {{dp|Sedna}} and {{dp|Orcus}}.<ref name="PorterTwitter">{{cite web
| first = Simon
| last = Porter
| url = https://twitter.com/AscendingNode/status/978592143626375169
| title = #TNO2018
| publisher = Twitter
| date = March 27, 2018
| access-date = March 27, 2018
| archive-date = October 2, 2018
| archive-url = https://web.archive.org/web/20181002133130/https://twitter.com/AscendingNode/status/978592143626375169
| url-status = live
}}</ref> The IAU itself has called Quaoar a dwarf planet in a 2022–2023 annual report.<ref>{{cite web |url=https://www.iau.org/static/science/scientific_bodies/divisions/f/division-f-annual-report-2022-2023.pdf |title=Report of Division F "Planetary Systems and Astrobiology": Annual Report 2022-23 |date=2022–2023 |website= |publisher=International Astronomical Union |access-date=December 8, 2023 |quote= |archive-date=December 8, 2023 |archive-url=https://web.archive.org/web/20231208112824/https://www.iau.org/static/science/scientific_bodies/divisions/f/division-f-annual-report-2022-2023.pdf |url-status=live }}</ref>

More bodies have been proposed, such as {{dp|Salacia}} and [[307261 Máni|Máni]] by Brown; {{dp|Varuna}} and {{dp|Ixion}} by Tancredi et al., and {{mpl|(532037) 2013 FY|27}} by [[Scott S. Sheppard|Sheppard]] et al.<ref>{{cite journal |last1=Sheppard |first1=Scott S. |last2=Fernandez |first2=Yanga R. |first3=Arielle |last3=Moullet |date=November 16, 2018 |title=The Albedos, Sizes, Colors, and Satellites of Dwarf Planets Compared with Newly Measured Dwarf Planet 2013 FY27 |journal=The Astronomical Journal |volume=156 |issue=6 |page=270 |doi=10.3847/1538-3881/aae92a |arxiv=1809.02184 |bibcode=2018AJ....156..270S |s2cid=119522310 |doi-access=free }}</ref> Most of the larger bodies have moons, which enables a determination of their mass and thus their density, which inform estimates of whether they could be dwarf planets. The largest TNOs that are not known to have moons are Sedna, Máni, [[55565 Aya|Aya]], and Ixion. In particular, Salacia has a known mass and diameter, putting it as a borderline case by the IAU's 2006 Q&A.

{{ordered list|start=10
| {{dp|Salacia}} ({{mp|2004 SB|60}}) –  discovered September 22, 2004. One known moon.
}}

At the time Makemake and Haumea were named, it was thought that [[trans-Neptunian object]]s (TNOs) with icy cores would require a diameter of only about 400 km (250 mi), or 3% the size of Earth{{snd}}the size of the moons [[Mimas (moon)|Mimas]], the smallest moon that is round, and [[Proteus (moon)|Proteus]], the largest that is not{{snd}}to relax into gravitational equilibrium.<ref name="Brown" /> Researchers thought that the number of such bodies could prove to be around 200 in the [[Kuiper belt]], with 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=January 26, 2008 |archive-date=July 19, 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 |last=Brown |first=Mike |author-link=Michael E. Brown |title=How many dwarf planets are there in the outer solar system? |url=http://www.gps.caltech.edu/~mbrown/dps.html |archive-url=https://web.archive.org/web/20111018154917/http://www.gps.caltech.edu/~mbrown/dps.html |archive-date=October 18, 2011 |url-status=live |access-date=November 15, 2013 |website=CalTech}}</ref><ref name=Stern2012>{{cite web |last=Stern |first=Alan  |author-link=Alan Stern |date=August 24, 2012 |title=The PI's Perspective. |url=https://pluto.jhuapl.edu/overview/piPerspective.php?page=piPerspective_08_24_2012 |url-status=dead |archive-url=https://web.archive.org/web/20141113225430/http://pluto.jhuapl.edu/overview/piPerspective.php?page=piPerspective_08_24_2012 |archive-date=November 13, 2014 |access-date=August 24, 2012}}</ref>
This was one of the reasons (keeping the roster of 'planets' to a reasonable number) that Pluto was reclassified in the first place. 
Research since then has cast doubt on the idea that bodies that small could have achieved or maintained equilibrium under the typical conditions of the Kuiper belt and beyond.

Individual astronomers have recognized a number of objects as dwarf planets or as likely to prove to be dwarf planets. In 2008, [[Gonzalo Tancredi|Tancredi]] et al. advised the IAU to officially accept Orcus, Sedna and Quaoar as dwarf planets (Gonggong was not yet known), though the IAU did not address the issue then and has not since. Tancredi also considered the five TNOs {{dp|Varuna}}, {{dp|Ixion}}, [[208996 Achlys|Achlys]], {{mpl-|90568|2004 GV|9}}, and [[55565 Aya|Aya]] to most likely be dwarf planets as well.<ref name=Tancredi2008>{{Cite journal |last1=Tancredi |first1=G. |last2=Favre |first2=S. A. |doi=10.1016/j.icarus.2007.12.020 |title=Which are the dwarfs in the Solar System? |journal=[[Icarus (journal)|Icarus]] |volume=195 |issue=2 |pages=851–862 |year=2008 |bibcode=2008Icar..195..851T}}</ref> 
Since 2011, Brown has maintained a list of hundreds of candidate objects, ranging from "nearly certain" to "possible" dwarf planets, based solely on estimated size.<ref name=free>{{cite web |last=Brown |first=Michael |author-link=Michael E. Brown |date=August 23, 2011 |title=Free the Dwarf Planets! |url=http://www.mikebrownsplanets.com/2011/08/free-dwarf-planets.html |website=Mike Brown's Planets |access-date=August 24, 2011 |archive-date=October 5, 2011 |archive-url=https://web.archive.org/web/20111005064654/http://www.mikebrownsplanets.com/2011/08/free-dwarf-planets.html |url-status=live }}</ref> 
As of September 13, 2019, Brown's list identifies ten [[trans-Neptunian object]]s with diameters then thought to be greater than 900 km (the four named by the IAU plus {{dp|Gonggong}}, {{dp|Quaoar}}, {{dp|Sedna}}, {{dp|Orcus}}, [[307261 Máni|Máni]], and {{dp|Salacia}}) as "near certain" to be dwarf planets, and another 16, with diameter greater than 600 km, as "highly likely".<ref name="BrownList"/> Notably, Gonggong may have a larger diameter ({{val|1230|50|u=km}}) than Pluto's round moon Charon (1212 km).

But in 2019 Grundy et al. proposed, based on their studies of [[229762 Gǃkúnǁʼhòmdímà|Gǃkúnǁʼhòmdímà]], that dark, low-density bodies smaller than about 900–1000 km in diameter, such as Salacia and {{dp|Varda}}, never fully collapsed into solid planetary bodies and retain internal porosity from their formation (in which case they could not be dwarf planets). They accept that brighter (albedo > ≈0.2)<ref>Of bodies smaller than 900 km in diameter, the only ones thought to have albedos much greater than this are fragments in the [[Haumea family|Haumea collisional family]] and possibly {{mpl|2005 QU|182}} (albedo between 0.2 and 0.5).</ref> or denser (> ≈1.4 g/cc) Orcus and Quaoar probably were fully solid:<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
 |title=The Mutual Orbit, Mass, and Density of Transneptunian Binary Gǃkúnǁʼhòmdímà ({{mp|(229762) 2007 UK|126}})
 |url=http://www2.lowell.edu/users/grundy/abstracts/2019.G-G.html
 |journal=Icarus
 |doi=10.1016/j.icarus.2018.12.037
 |year= 2019
 |volume=334
 |pages=30–38
 |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
 |url-status=live
 |archive-date=April 7, 2019}}</ref>

{{blockquote|Orcus and Charon probably melted and differentiated, considering their higher densities and spectra indicating surfaces made of relatively clean H<sub>2</sub>O ice.  But the lower albedos and densities of [[229762 Gǃkúnǁʼhòmdímà|Gǃkúnǁʼhòmdímà]], [[(55637) 2002 UX25|55637]], Varda, and Salacia suggest that they never did differentiate, or if they did, it was only in their deep interiors, not a complete melting and overturning that involved the surface.  Their surfaces could remain quite cold and uncompressed even as the interior becomes warm and collapses.  The liberation of volatiles could further help transport heat out of their interiors, limiting the extent of their internal collapse.  An object with a cold, relatively pristine surface and a partially collapsed interior should exhibit very distinctive surface geology, with abundant thrust faults indicative of the reduction in total surface area as the interior compresses and shrinks.<ref name=Grundy2019/>}}

Salacia was later found to have a somewhat higher density, comparable within uncertainties to that of Orcus, though still with a very dark surface. Despite this determination, Grundy et al. call it "dwarf-planet sized", while calling Orcus a dwarf planet.<ref name="Grundy-orbits">{{cite journal
 |title=Mutual Orbit Orientations of Transneptunian Binaries
 |url=http://www2.lowell.edu/users/grundy/abstracts/preprints/2019.TNB_orbits.pdf
 |first1=W. M.
 |last1=Grundy
 |first2=K. S.
 |last2=Noll
 |first3=H. G.
 |last3=Roe
 |first4=M. W.
 |last4=Buie
 |first5=S. B.
 |last5=Porter
 |first6=A. H.
 |last6=Parker
 |first7=D.
 |last7=Nesvorný
 |first8=S. D.
 |last8=Benecchi
 |first9=D. C.
 |last9=Stephens
 |first10=C. A.
 |last10=Trujillo
 |journal=Icarus
 |volume=334
 |pages=62–78
 |year=2019
 |doi=10.1016/j.icarus.2019.03.035
 |bibcode=2019Icar..334...62G
 |s2cid=133585837
 |issn=0019-1035
 |access-date=October 26, 2019
 |archive-url=https://web.archive.org/web/20200115091416/http://www2.lowell.edu/~grundy/abstracts/preprints/2019.TNB_orbits.pdf
 |archive-date=January 15, 2020
 |url-status=dead
 }}</ref> Later studies on Varda suggest that its density may also be high, though a low density could not be excluded.<ref name="Souami2020">{{cite journal
  |display-authors = etal
  |first1     = D. |last1 = Souami
  |first2     = F. |last2 = Braga-Ribas
  |first3     = B. |last3 = Sicardy
  |first4     = B. |last4 = Morgado
  |first5     = J. L. |last5 = Ortiz
  |first6     = J. |last6 = Desmars
  |date       = August 2020
  |title      = A multi-chord stellar occultation by the large trans-Neptunian object (174567) Varda
  |journal = Astronomy & Astrophysics |volume = 643 |pages = A125 |doi = 10.1051/0004-6361/202038526 |arxiv     = 2008.04818
  |bibcode = 2020A&A...643A.125S |s2cid = 221095753 }}</ref>

In 2023, Emery et al. wrote that [[near-infrared]] [[spectroscopy]] by the [[James Webb Space Telescope]] (JWST) in 2022 suggests that Sedna, Gonggong, and Quaoar underwent internal melting, differentiation, and chemical evolution, like the larger dwarf planets Pluto, Eris, Haumea, and Makemake, but unlike "all smaller KBOs". This is because light hydrocarbons are present on their surfaces (e.g. [[ethane]], [[acetylene]], and [[ethylene]]), which implies that methane is continuously being resupplied, and that methane would likely come from internal geochemistry. On the other hand, the surfaces of Sedna, Gonggong, and Quaoar have low abundances of CO and CO<sub>2</sub>, similar to Pluto, Eris, and Makemake, but in contrast to smaller bodies. This suggests that the threshold for dwarf planethood in the trans-Neptunian region is a diameter of ~900 km (thus including only Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Orcus, and Sedna), and that even Salacia may not be a dwarf planet.<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 |author-link=|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> A 2023 study of [[307261 Máni|Máni]] shows that it probably has an extremely large crater, whose depth takes up 5.7% of its diameter: this is proportionally larger than the Rheasilvia crater on Vesta, which is the reason Vesta is not usually considered a dwarf planet today.<ref name="Rommel2023">{{cite journal
  |display-authors = etal
  |first1 = F. L. |last1 = Rommel
  |first2 = F. |last2 = Braga-Ribas
  |first3 = J. L. |last3 = Ortiz
  |first4 = B. |last4 = Sicardy
  |first5 = P. |last5 = Santos-Sanz
  |first6 = J. |last6 = Desmars
  |title      = A large topographic feature on the surface of the trans-Neptunian object (307261) 2002 MS4 measured from stellar occultations
  |journal    = Astronomy & Astrophysics
  |date       = October 2023
  |volume     = 678
  |id         = A167
  |pages      = 25
  |doi-access = free
  |doi        = 10.1051/0004-6361/202346892
  |arxiv      = 2308.08062
  |bibcode    = 2023A&A...678A.167R
  |s2cid      = 260926329}}</ref>

In 2024, Kiss et al. found that Quaoar has an ellipsoidal shape incompatible with hydrostatic equilibrium for its current spin. They hypothesized that Quaoar originally had a rapid rotation and was in hydrostatic equilibrium, but that its shape became "frozen in" and did not change as it spun down due to tidal forces from its moon [[Weywot]].<ref name="Kiss2024">{{cite journal
 |display-authors = etal
 |first1 = C. |last1 = Kiss
 |first2 = T. G. |last2 = Müller
 |first3 = G. |last3 = Marton
 |first4 = R. |last4 = Szakáts
 |first5 = A. |last5 = Pál
 |first6 = L. |last6 = Molnár
 |title      = The visible and thermal light curve of the large Kuiper belt object (50000) Quaoar
 |journal    = Astronomy & Astrophysics
 |date       = March 2024
 |volume     = 684
 |issue      = 
 |pages = A50 |doi        = 10.1051/0004-6361/202348054
 |arxiv      = 2401.12679
 |bibcode    = 2024A&A...684A..50K}}</ref> If so, this would resemble the situation of Saturn's moon [[Iapetus (moon)|Iapetus]], which is too oblate for its current spin.<ref name="Science News">{{cite news | last=Cowen | first=R. | year=2007 | title=Idiosyncratic Iapetus | website=Science News | volume=172 | pages=104–106 | url=http://www.sciencenews.org/articles/20070818/bob8ref.asp }} {{Webarchive|url=https://web.archive.org/web/20071013165655/http://www.sciencenews.org/articles/20070818/bob8ref.asp |date=October 13, 2007 }}</ref><ref name="Thomas2010">{{cite journal| doi = 10.1016/j.icarus.2010.01.025| last = Thomas| first = 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 = September 25, 2015| archive-date = December 23, 2018| archive-url = https://web.archive.org/web/20181223003125/http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| url-status = dead}}</ref> Iapetus is generally still considered a [[planetary-mass moon]] nonetheless,<ref name=planetarysociety/> though not always.<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 |doi= 10.3847/1538-4357/834/1/17|arxiv=1603.08614 |s2cid=119114880 |doi-access=free |bibcode=2017ApJ...834...17C }}</ref>

===Most likely dwarf planets===
{{main|List of possible dwarf planets}}
[[File:Density vs albedo of dwarf planets.png|thumb|upright=1.5|Relative densities and albedos of the most likely dwarf planets]]

The trans-Neptunian objects in the following tables, except Salacia, are agreed by Brown, Tancredi et al., Grundy et al., and Emery et al. to be probable dwarf planets, or close to it. Salacia has been included as the largest TNO not generally agreed to be a dwarf planet; it is a borderline body by many criteria, and is therefore italicized. Charon, a moon of Pluto that was proposed as a dwarf planet by the IAU in 2006, is included for comparison. Those objects that have absolute magnitude greater than +1, and so meet the threshold of the joint planet&ndash;minor planet naming committee of the IAU, are highlighted, as is Ceres, which the IAU has assumed is a dwarf planet since they first debated the concept.

The masses of given dwarf planets are listed for their systems (if they have satellites) with exceptions for Pluto and Orcus.

{| class="wikitable sortable" style="text-align: center"
|+ Orbital attributes
|-
! scope=col | Name
! scope=col | Region of the<br /> Solar System
! scope=col | Semi-major <br /> axis ([[Astronomical unit|AU]])
! scope=col | [[Orbital period]] <br /> (years)
! scope=col | Mean orbital <br /> speed (km/s)
! scope=col | [[Inclination|Inclination <br /> to ecliptic]]
! scope=col | [[Orbital eccentricity|Orbital <br /> eccentricity]]
! scope=col | [[Clearing the neighbourhood|Planetary <br /> discriminant]]

|-style="background: #E5E5FF;"
! scope=row style="text-align: left" | [[Ceres (dwarf planet)|Ceres]]
| [[Asteroid belt]]
| 2.768
| 4.604
| 17.90
| 10.59°
| 0.079
| 0.3
|-
! scope=row style="text-align: left"| {{dp|Orcus}}
| [[Kuiper belt]] ([[Resonant trans-Neptunian object|resonant]] – [[plutino|2:3]])
| 39.40
| 247.3
| 4.75
| 20.58°
| 0.220
| 0.003
|-style="background: #E5E5FF;"
! scope=row style="text-align: left"| [[Pluto]]
| [[Kuiper belt]] ([[Resonant trans-Neptunian object|resonant]] – [[plutino|2:3]])
| 39.48
| 247.9
| 4.74
| 17.16°
| 0.249
| 0.08
|- align=center
! scope=row style="text-align: left"| ''{{dp|Salacia}}''
| [[Kuiper belt]] ([[Classical Kuiper belt object|cubewano]])
| 42.18
| 274.0
| 4.57
| 23.92°
| 0.106
| 0.003
|-style="background: #E5E5FF;"
! scope=row style="text-align: left" | {{dp|Haumea}}
| [[Kuiper belt]] ([[Resonant trans-Neptunian object#Haumea|resonant – 7:12]])
| 43.22
| 284.1
| 4.53
| 28.19°
| 0.191
| 0.02
|-
! scope=row style="text-align: left"| {{dp|Quaoar}}
| [[Kuiper belt]] ([[Classical Kuiper belt object|cubewano]])
| 43.69
| 288.8
| 4.51
| 7.99°
| 0.040
| 0.007
|-style="background: #E5E5FF;"
! scope=row style="text-align: left" | [[Makemake]]
| [[Kuiper belt]] ([[Classical Kuiper belt object|cubewano]])
| 45.56
| 307.5
| 4.41
| 28.98°
| 0.158
| 0.02
|-
! scope=row style="text-align: left"| [[225088 Gonggong|Gonggong]]
| [[Scattered disc]] ([[Resonant trans-Neptunian object#Other resonances|resonant – 3:10]])
| 67.49
| 554.4
| 3.63
| 30.74°
| 0.503
| 0.01<!--per Eris-->
|-style="background: #E5E5FF;"
! scope=row style="text-align: left" | [[Eris (dwarf planet)|Eris]]
| [[Scattered disc]]
| 67.86
| 559.1
| 3.62
| 44.04°
| 0.441
| 0.1
|-
! scope=row style="text-align: left"| {{dp|Sedna}}
| [[Detached object|Detached]]
| 506.8
|data-sort-value="11400"| ≈&nbsp;11,400
|data-sort-value="1.3"| ≈&nbsp;1.3
| 11.93°
| 0.855
|data-sort-value="0.06"| < 0.07
|}
{| class="wikitable sortable" style="text-align: center"
|+ Other attributes
|-
! scope=col width=54 |Name
! scope=col | Diameter <br /> relative to <br /> the [[Moon]]
! scope=col | Diameter <br /> (km)
! scope=col | Mass <br /> relative to <br /> the [[Moon]]
! scope=col | Mass <br /> ({{e|21}} kg)
! scope=col | [[Density]] <br /> (g/cm<sup>3</sup>)
! scope=col | [[Rotation period|Rotation <br /> period]] <br /> (hours)
! scope=col | [[Natural satellite|Moons]]
! scope=col | [[Geometric albedo|Albedo]]
! scope=col | [[absolute magnitude|H]]
|-style="background: #E5E5FF;"
! scope=row style="text-align: left" | [[Ceres (dwarf planet)|Ceres]]
| 27%
| {{val|939.4|0.2}}
| 1.3%
| {{val|0.93835|0.00001}}
| {{val|2.16}}
| {{val|9.1}}
| 0
| {{val|0.09}}
| 3.33
|-
! scope=row style="text-align: left" | {{dp|Orcus}}
| 26%
| {{val|910|50|40}}
| 0.8%
| {{val|0.55|0.01}}
| {{val|1.4|0.2}}
| {{val|13|4}}
| [[Vanth (moon)|1]]
| {{val|0.23|0.02|0.01}}
| 2.19
|-style="background: #E5E5FF;"
! scope=row style="text-align: left"| [[Pluto]]
| 68%
| {{val|2377|3}}
| 17.7%
| {{val|13.03|0.03}}
| {{val|1.85}}
|data-sort-value="{{val|153.3}}"| 6d 9.3h
| [[Moons of Pluto|5]]
|data-sort-value="{{val|0.575}}"| {{val|0.52}}
| −0.45
|-
! scope=row style="text-align: left" data-sort-value="Charon"| ([[Charon (moon)|Charon]])
| 35%
| {{val|1212|1}}
| 2.2%
| {{val|1.59|0.02}}
| {{val|1.70|0.02}}
|data-sort-value="{{val|153.3}}"| 6d 9.3h
| –
|data-sort-value="{{val|0.35}}"| {{val|0.38}}
| 1
|- align=center
! scope=row style="text-align: left"| ''{{dp|Salacia}}''
| 24%
| {{val|846|21}}
| 0.7%
| {{val|.49|0.01}}
| {{val|1.50|0.12}}
| {{val|6.1}}
| [[Actaea (moon)|1]]
| 0.04
| 4.27
|-style="background: #E5E5FF;"
! scope=row style="text-align: left"| {{dp|Haumea}}
|data-sort-value="45%"| ≈&nbsp;45%
|data-sort-value="{{val|1560}}"| ≈&nbsp;1560<ref name="Dunham2019" />
| 5.5%
| {{val|4.01|0.04}}
|data-sort-value="{{val|1.8}}"| ≈&nbsp;1.8
| {{val|3.9}}
| [[Moons of Haumea|2]]
|data-sort-value="{{val|0.66}}"| ≈&nbsp;0.66
| 0.23
|-
! scope=row style="text-align: left" | {{dp|Quaoar}}
| 32%
| {{val|1086|4}}
| 1.9%
| {{val|1.20|0.05}}
| {{val|1.7|0.1}}
| {{val|17.7}}
| [[Weywot (moon)|1]]
| {{val|0.11|0.01}}
| 2.42
|-style="background: #E5E5FF;"
! scope=row style="text-align: left"| [[Makemake]]
| 41%
| {{val|1430|38|22}}	
|data-sort-value="4.2%"| ≈&nbsp;4.2%
|data-sort-value="{{val|3.1}}"| ≈&nbsp;3.1
|data-sort-value="{{val|1.9}}"| ≈ {{val|1.9|0.2}}<ref>The range of two approximations</ref>
| {{val|22.8}}
| [[S/2015 (136472) 1|1]]
| {{val|0.81|0.03|0.05}}
| −0.20
|-
! scope=row style="text-align: left" | [[225088 Gonggong|Gonggong]]
| 35%
| {{val|1230|50}}
| 2.4%
| {{val|1.75|0.07}}
| {{val|1.74|0.16}}
| {{val|22.4|0.2}}?
| [[Xiangliu (moon)|1]]
| {{val|0.14|0.01}}
| 1.86
|-style="background: #E5E5FF;"
! scope=row style="text-align: left" | [[Eris (dwarf planet)|Eris]]
| 67%
| {{val|2326|12}}
| 22.4%
| {{val|16.47|0.09}}
| {{val|2.43|0.05}}
|data-sort-value="{{val|378.864}}"| 15d 18.9h
| [[Dysnomia (moon)|1]]
| {{val|0.96|0.04}}
| −1.21
|-
! scope=row style="text-align: left" | {{dp|Sedna}}
| 26% 
| {{val|906|314|258}}
|data-sort-value="1%"| ≈&nbsp;1%?
|data-sort-value="{{val|1}}"| ≈&nbsp;1?
| ?
| {{val|10|3}}
| 0?
| {{val|0.41|0.393|0.186}}
| 1.52
|}

=== Symbols ===
{{Main|Astronomical symbol#Symbols for trans-Neptunian objects}}
Ceres [[File:Ceres symbol (fixed width).svg|20px|⚳]]<ref name=bode-1801>{{cite book
| editor-last = Bode
| editor-first = J.E.
| date = 1801
| title = Berliner astronomisches Jahrbuch führ das Jahr 1804
| trans-title = The Berlin Astronomical Yearbook for 1804
| pages = 97–98
| url = https://books.google.com/books?id=6GElAQAAIAAJ&pg=RA2-PA98
| access-date = October 19, 2022
| archive-date = December 14, 2023
| archive-url = https://web.archive.org/web/20231214145307/https://books.google.com/books?id=6GElAQAAIAAJ&pg=RA2-PA98#v=onepage&q&f=false
| url-status = live
}}</ref> and Pluto [[File:Pluto monogram (fixed width).svg|20px|♇]]<ref name=popular-astronomy>{{cite magazine
| last = Slipher
| first = V. M.
| date = 1930
| title = The trans-Neptunian planet
| magazine = Popular Astronomy
| volume = 38
| page = 415
| url = http://articles.adsabs.harvard.edu/cgi-bin/nph-journal_query?volume=38&plate_select=NO&page=415&plate=&cover=&journal=PA...
| access-date = October 19, 2022
| archive-date = December 11, 2021
| archive-url = https://web.archive.org/web/20211211143952/http://articles.adsabs.harvard.edu/cgi-bin/nph-journal_query?volume=38&plate_select=NO&page=415&plate=&cover=&journal=PA...
| url-status = live
}}</ref> received planetary symbols, as they were considered to be planets when they were discovered. By the time the others were discovered, [[planetary symbol]]s had mostly fallen out of use among astronomers. [[Unicode]] includes symbols for Quaoar {{nobr|[[File:Quaoar symbol (fixed width).svg|20px|🝾]],}} Sedna {{nobr|[[File:Sedna symbol (fixed width).svg|20px|⯲]],}} Orcus {{nobr|[[File:Orcus symbol (fixed width).svg|20px|🝿]],}} Haumea {{nobr|[[File:Haumea symbol (fixed width).svg|20px|🝻]],}} Eris {{nobr|[[File:Eris symbol (fixed width).svg|20px|⯰]],}} Makemake {{nobr|[[File:Makemake symbol (fixed width).svg|20px|🝼]],}} and Gonggong {{nobr|[[File:Gonggong symbol (fixed width).svg|20px|🝽]],}} that are primarily used by astrologers: they were devised by Denis Moskowitz, a software engineer in Massachusetts.<ref name=anderson>{{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=May 4, 2022 |website=unicode.org |publisher=The Unicode Consortium |access-date=August 6, 2022 |archive-date=August 6, 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><ref name=DPU>{{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=October 26, 2021|website=unicode.org|access-date=October 19, 2022|archive-date=March 23, 2022|archive-url=https://web.archive.org/web/20220323174107/https://www.unicode.org/L2/L2021/21224-dwarf-planet-syms.pdf|url-status=live}}</ref><ref name="Unicode-1F700">{{cite web
| title = Alchemical Symbols
| date = 2022
| website = unicode.org
| publisher = The Unicode Consortium
| url = https://www.unicode.org/charts/PDF/U1F700.pdf
| access-date = October 19, 2022
| archive-date = April 2, 2020
| archive-url = https://web.archive.org/web/20200402042357/http://www.unicode.org/charts/PDF/U1F700.pdf
| url-status = live
}}</ref> NASA has used his Haumea, Eris, and Makemake symbols, as well as the traditional astrological symbol for Pluto [[File:Pluto symbol (large orb, fixed width).svg|20px|⯓]]<ref name=DP>{{cite web |url= https://www.jpl.nasa.gov/infographics/what-is-a-dwarf-planet |date= April 22, 2015 |website= Jet Propulsion Laboratory |publisher=[[NASA]] |title= What is a Dwarf Planet? |access-date= September 24, 2021 |archive-date= December 8, 2021 |archive-url= https://web.archive.org/web/20211208181916/https://www.jpl.nasa.gov/infographics/what-is-a-dwarf-planet |url-status= live }}</ref> when referring to it as a dwarf planet.<ref name=DPU/> Symbols for smaller objects are less established; a Unicode proposal notes the Moskowitz symbol [[File:Salacia symbol (fixed width).svg|20px]] for Salacia.<ref name=constellations>{{cite web |url=https://www.unicode.org/L2/L2024/24235-constellation-symbols.pdf |title=Preliminary presentation of constellation symbols |last=Miller |first=Kirk |date=18 October 2024 |website=unicode.org |publisher=The Unicode Consortium |access-date=22 October 2024 |quote=}}</ref> Moskowitz additionally devised a symbol for Charon [[File:Charon symbol (fixed width).svg|20px|⯕]].<ref name=moons>{{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>

== Exploration ==
[[File:PIA19562-Ceres-DwarfPlanet-Dawn-RC3-image19-20150506.jpg|thumb|The dwarf planet Ceres, as imaged by NASA's ''[[Dawn (spacecraft)|Dawn]]'' spacecraft]]

As of 2025, only two missions have targeted and explored dwarf planets up close. On March 6, 2015, the ''[[Dawn (spacecraft)|Dawn]]'' spacecraft entered orbit around [[Ceres (dwarf planet)|Ceres]], becoming the first spacecraft to visit a dwarf planet.<ref name="NASA-20150306">{{cite web |last1=Landau |first1=Elizabeth |last2=Brown |first2=Dwayne |title=NASA Spacecraft Becomes First to Orbit a Dwarf Planet |url=http://www.jpl.nasa.gov/news/news.php?feature=4503 |date=March 6, 2015 |work=[[NASA]] |access-date=March 6, 2015 |archive-date=March 7, 2015 |archive-url=https://web.archive.org/web/20150307062258/http://www.jpl.nasa.gov/news/news.php?feature=4503 |url-status=live }}</ref> On July 14, 2015, the ''[[New Horizons]]'' space probe flew by [[Pluto]] and its five moons.

Ceres displays such evidence of an active geology as salt deposits and [[cryovolcano]]s, while Pluto has water-ice mountains drifting in nitrogen-ice glaciers, as well as a significant atmosphere. 
Ceres evidently has brine percolating through its subsurface, while there is evidence that Pluto has an actual subsurface ocean.

''Dawn'' had previously orbited the asteroid Vesta. Saturn's moon [[Phoebe (moon)|Phoebe]] has been imaged by Cassini and before that by Voyager 2, which also encountered Neptune's moon [[Triton (moon)|Triton]]. All three bodies show evidence of once being dwarf planets, and their exploration helps clarify the evolution of dwarf planets.

''New Horizons'' has captured distant images of Triton, Quaoar, Haumea, Eris, and Makemake, as well as the smaller candidates Ixion, Máni, and {{mpl-|556416|2014 OE|394}}.<ref name="Verbiscer2022">{{cite journal
  |display-authors = etal
  |first1 = Anne J. |last1 = Verbiscer
  |first2 = Paul |last2 = Helfenstein
  |first3 = Simon B. |last3 = Porter
  |first4 = Susan D. |last4 = Benecchi
  |first5 = J. J. |last5 = Kavelaars
  |first6 = Tod R. |last6 = Lauer
  |title      = The Diverse Shapes of Dwarf Planet and Large KBO Phase Curves Observed from New Horizons
  |journal    = The Planetary Science Journal
  |date       = April 2022
  |volume     = 3
  |issue      = 4
  |id         = 95
  |pages      = 31
  |doi-access = free
  |doi        = 10.3847/PSJ/ac63a6
  |bibcode    = 2022PSJ.....3...95V}}</ref>
Quaoar has been proposed as a potential flyby target of the [[China National Space Administration]]'s two ''[[Shensuo (spacecraft)|Shensuo]]'' probes.<ref name="SpaceNews">{{cite news |last1=Jones |first1=Andrew |title=China to launch a pair of spacecraft towards the edge of the solar system |url=https://spacenews.com/china-to-launch-a-pair-of-spacecraft-towards-the-edge-of-the-solar-system/ |access-date=April 29, 2021 |work=SpaceNews |publisher=SpaceNews |date=April 16, 2021 |archive-date=September 29, 2021 |archive-url=https://web.archive.org/web/20210929080320/https://spacenews.com/china-to-launch-a-pair-of-spacecraft-towards-the-edge-of-the-solar-system/ |url-status=live }}</ref>

==Similar objects==
A number of bodies physically resemble dwarf planets. These include former dwarf planets, which may still have equilibrium shape or evidence of active geology; planetary-mass moons, which meet the physical but not the orbital definition for dwarf planet; and Charon in the Pluto–Charon system, which is arguably a binary dwarf planet. The categories may overlap: Triton, for example, is both a former dwarf planet and a planetary-mass moon.

=== Former dwarf planets ===
[[file:Triton.usgs23.png|thumb|A monochrome mosaic of Triton, from images by ''Voyager 2''. Triton is thought to be a captured dwarf planet.]]
[[4 Vesta|Vesta]], the next-most-massive body in the asteroid belt after Ceres, was once in hydrostatic equilibrium and is roughly spheroidal, deviating mainly due to massive impacts that formed the [[Rheasilvia]] and [[Veneneia]] craters after it solidified.<ref>{{cite journal |first=Peter C. |last=Thomas | last2=Binzelb | first2=Richard P. | last3=Gaffeyc | first3=Michael J. | last4=Zellnerd | first4=Benjamin H. | last5=Storrse | first5=Alex D. | last6=Wells | first6=Eddie |title=Vesta: Spin Pole, Size, and Shape from HST Images |journal=Icarus |volume=128 |pages=88–94 |date=1997 |issue=1 |doi=10.1006/icar.1997.5736 |bibcode=1997Icar..128...88T|doi-access=free }}</ref> 
Its dimensions are not consistent with it currently being in [[hydrostatic equilibrium]].<ref name="Asmar 2012">{{cite journal |last=Asmar |first=S. W. | last2=Konopliv | first2=A. S. | last3=Park | first3=R. S. | last4=Bills | first4=B. G. | last5=Gaskell | first5=R. | last6=Raymond | first6=C. A. | last7=Russell | first7=C. T. | last8=Smith | first8=D. E. | last9=Toplis | first9=M. J. | last10=Zuber | first10=M. T. |title=The Gravity Field of Vesta and Implications for Interior Structure |journal=43rd Lunar and Planetary Science Conference |issue=1659 |date=2012 |page=2600 |url=http://www.lpi.usra.edu/meetings/lpsc2012/pdf/2600.pdf |bibcode=2012LPI....43.2600A |access-date=July 15, 2015 |archive-date=October 20, 2013 |archive-url=https://web.archive.org/web/20131020181320/http://www.lpi.usra.edu/meetings/lpsc2012/pdf/2600.pdf |url-status=live }}</ref><ref name="Russel 2012">{{cite journal |last=Russel |first=C. T. |title=Dawn at Vesta: Testing the Protoplanetary Paradigm |journal=Science |date=2012 |volume=336 |issue=6082 |pages=684–686 |doi=10.1126/science.1219381 |url=http://www.phyast.pitt.edu/~wmwv/Classes/A1122/Literature/Science-2012-Russell-684-6.pdf |bibcode=2012Sci...336..684R |display-authors=etal |pmid=22582253 |s2cid=206540168 |access-date=July 15, 2015 |archive-date=July 15, 2015 |archive-url=https://web.archive.org/web/20150715181221/http://www.phyast.pitt.edu/~wmwv/Classes/A1122/Literature/Science-2012-Russell-684-6.pdf |url-status=live }}</ref>
[[Triton (moon)|Triton]] is more massive than Eris or Pluto, has an equilibrium shape, and is thought to be a captured dwarf planet (likely a member of a binary system), but no longer directly orbits the sun.<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=192–194 |year=2006 |last1=Agnor |first1=C. B. |last2=Hamilton |first2=D. P. |pmid=16688170 |bibcode=2006Natur.441..192A |s2cid=4420518 |access-date=August 29, 2015 |archive-date=October 14, 2016 |archive-url=https://web.archive.org/web/20161014134243/http://extranet.on.br/rodney/curso2010/aula9/tritoncapt_hamilton.pdf |url-status=dead }}</ref> 
[[Phoebe (moon)|Phoebe]] is a captured [[centaur (austronomy)|centaur]] that, like Vesta, is no longer in hydrostatic equilibrium, but is thought to have been so early in its history due to [[radiogenic heating]].<ref>{{cite news |last1=Cook |first1=Jia-Rui C. |last2=Brown |first2=Dwayne |date=April 26, 2012 |url=http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20120426/ |url-status=dead |title=Cassini Finds Saturn Moon Has Planet-Like Qualities |archive-url=https://web.archive.org/web/20150713125236/http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20120426/ |archive-date=July 13, 2015 |location=Pasadena, California|work=Jey Propoulsion Laboratory |publisher=[[NASA]]}}</ref>

=== Planetary-mass moons ===
{{Main|Planetary-mass moon}}

At least nineteen [[natural satellite|moons]] have equilibrium shapes from having collapsed into a solid body (or in a few cases into a nearly solid body) or even relaxed under self-gravity at some point, though some of the latter have since frozen solid and are no longer in equilibrium. Seven are more massive than either Eris or Pluto. These larger moons are not physically distinct from the dwarf planets, but do not fit the IAU definition because they do not directly orbit the Sun. (Indeed, Neptune's moon Triton is a captured dwarf planet, and Ceres formed in the same region of the Solar System as the moons of Jupiter and Saturn.) Alan Stern calls planetary-mass moons "[[satellite planet]]s", one of three categories of planet, together with dwarf planets and classical planets.<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'? |publisher=News.discovery.com |date=May 14, 2010 |access-date=November 4, 2011 |archive-date=May 5, 2012 |archive-url=https://web.archive.org/web/20120505221146/http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html |url-status=dead }}</ref> The term ''[[planemo]]'' ("planetary-mass object") also covers all three populations.<ref>{{cite journal |last1=Basri |first1=Gibor |last2=Brown |first2=Michael E. |date=2006 |title=Planetesimals to Brown Dwarfs: What is a Planet? |url=http://siba.unipv.it/fisica/articoli/A/Annual%20Review%20Earth%20Planetary%20Sciences_vol.34_2006_pp.193-216.pdf |journal=Annual Review of Earth and Planetary Sciences |volume=34 |pages=193–216 |doi=10.1146/annurev.earth.34.031405.125058 |arxiv=astro-ph/0608417 |bibcode=2006AREPS..34..193B |s2cid=119338327 |url-status=dead |archive-url=https://web.archive.org/web/20130731133553/http://siba.unipv.it/fisica/articoli/A/Annual%20Review%20Earth%20Planetary%20Sciences_vol.34_2006_pp.193-216.pdf |archive-date=July 31, 2013 }}</ref>

==== Charon ====
There has been some debate as to whether the Pluto–[[Charon (moon)|Charon]] system should be considered a [[binary planet|double dwarf planet]]. 
In a draft resolution for the [[IAU definition of planet]], both Pluto and Charon were considered planets in a binary system.<ref name="Draft Resolution 5"/>{{efn| The footnote in the original text reads: "For two or more objects comprising a multiple object system. ... A secondary object satisfying these conditions i.e. that of mass, shape is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are "satellites". Under this definition, Pluto's companion Charon is a planet, making Pluto–Charon a double planet."<ref name="Draft Resolution 5"/>}} The IAU currently says Charon is not considered a dwarf planet but rather a satellite of Pluto, though the idea that Charon might qualify as a dwarf planet may be considered at a later date.<ref>{{cite web |title=Pluto and the Solar System |website=iau.org |publisher=[[International Astronomical Union]] |url=http://www.iau.org/public/themes/pluto/ |access-date=July 10, 2013 |archive-date=April 17, 2020 |archive-url=https://web.archive.org/web/20200417193151/https://www.iau.org/public/themes/pluto/ |url-status=live }}</ref> Nonetheless, it is no longer clear that Charon is in hydrostatic equilibrium. Also, the location of the [[Center of mass#Barycenter in astrophysics and astronomy|barycentre]] depends not only on the relative masses of the bodies, but also on the distance between them; the barycentre of the Sun–Jupiter orbit, for example, lies outside the Sun, but they are not considered a binary object. Thus, a formal definition of what constitutes a binary (dwarf) planet must be established before Pluto and Charon are formally defined as binary dwarf planets.

== See also ==
{{portal|Solar System|Outer Space|Astronomy
}}
{{div col|colwidth=20em}}
* [[Centaur (minor planet)|Centaur]]
* [[Lists of astronomical objects]]
* [[List of former planets]]
* [[List of gravitationally rounded objects of the Solar System]]
* [[Lists of planets|List of planetary bodies]]
* [[List of possible dwarf planets]]
* [[Lists of small Solar System bodies]]
* [[Solar System belts]]
{{div col end}}
{{clear}}

== Notes ==
{{notelist}}

== References ==
{{reflist|refs=

<ref name="AAAS">{{cite web
  |title = From discovery to demotion: How a dwarf planet changed astronomy
  |url = https://www.aaas.org/taxonomy/term/8/discovery-demotion-how-dwarf-planet-changed-astronomy
  |first = Brad |last = Hooker
  |publisher = American Association for the Advancement of Science
  |date = September 24, 2013
  |accessdate = April 15, 2025
  |url-status = live
  |archive-url = https://web.archive.org/web/20240604163946/https://www.aaas.org/taxonomy/term/8/discovery-demotion-how-dwarf-planet-changed-astronomy
  |archive-date = June 4, 2024}}</ref>

<ref name="oralhistory">{{cite interview
  |title = NASA at 50 Oral History Project Edited Oral History Transcript
  |url = https://historycollection.jsc.nasa.gov/JSCHistoryPortal/history/oral_histories/NASA_HQ/NAF/SternA/SternA_4-15-08.htm
  |first = Alan |last = Stern
  |interviewer-first = Steven J. |interviewer-last = Dick
  |work = Johnson Space Center
  |publisher = NASA
  |date = April 15, 2008
  |accessdate = April 15, 2025
  |url-status = live
  |archive-url = https://web.archive.org/web/20250320054206/https://historycollection.jsc.nasa.gov/JSCHistoryPortal/history/oral_histories/NASA_HQ/NAF/SternA/SternA_4-15-08.htm
  |archive-date = March 20, 2025
  |quote = I coined the term dwarf planet in 1990 in the academic journal Icarus. It is a dwarf planet. But that's a kind of planet, just like dwarf human beings are still human beings, they're just smaller.}}</ref>

}}

== External links ==
{{Wiktionary}}
* [https://www.visualcapitalist.com/a-visual-introduction-to-the-dwarf-planets-in-our-solar-system/ A Visual Introduction to the Dwarf Planets in our Solar System] (Anshool Deshmukh, ''Visual Capitalist,'' October 8, 2021, graphics by Mark Belan)
* [[NPR]]: [https://www.npr.org/templates/story/story.php?storyId=5631291 Dwarf Planets May Finally Get Respect] (David Kestenbaum, ''[[Morning Edition]]'')
* [[BBC News]]: [http://news.bbc.co.uk/1/hi/sci/tech/4798205.stm Q&A New planets proposal], August 16, 2006
* ''[[Ottawa Citizen]]'': [https://web.archive.org/web/20070224221409/http://www.canada.com/ottawacitizen/news/story.html?id=a9591aed-f19f-4ac3-a324-1f8bb46d9379&p=2 The case against Pluto] (P. Surdas Mohit) August 24, 2006
* [[James L. Hilton]]: [https://web.archive.org/web/20070921162818/http://aa.usno.navy.mil/faq/docs/minorplanets.php When Did the Asteroids Become Minor Planets?]
* NASA: [https://web.archive.org/web/20100211161824/http://astronomy2009.nasa.gov/topics_sep.htm IYA 2009 Dwarf Planets]

{{Dwarf planets}}
{{Solar System}}
{{Portal bar|Astronomy|Stars|Spaceflight|Outer space|Solar System}}
{{Authority control}}

{{DEFAULTSORT:Dwarf Planet}}
[[Category:Dwarf planets| ]]
[[Category:1990s neologisms]]
[[Category:Minor planets]]
[[Category:Planetary-mass objects]]
[[Category:Types of planet]]
[[Category:Concepts in astronomy]]
[[Category:Solar System]]