View source for Planets beyond Neptune

{{Short description|Hypothetical planets further than Neptune}}
{{about|hypothetical planets in the [[Solar System]]|known objects beyond Neptune|Trans-Neptunian object|planets beyond the Solar System|Extrasolar planet}}
{{redirect|Venkatesh Ketakar|the sociologist|Shridhar Venkatesh Ketkar}}
{{redirect-multi|2|Ninth planet|Tenth planet}}

[[File:Percival Lowell observing Venus from the Lowell Observatory in 1914.jpg|thumb|upright=1.25|right|[[Percival Lowell]], originator of the Planet X hypothesis]]
Following the discovery of the planet [[Neptune]] in 1846, there was considerable speculation that another [[planet]] might exist beyond its [[orbit]].<ref>{{Cite book |last=Meyersohn |first=Ryanne Maxine |url=https://books.google.com/books?id=m0HOEAAAQBAJ |title=Estudio Comprensivo del Origen de la Humanidad |date=2023-05-28 |publisher=(n.p.) |editor-last=Figueroa Nieves |editor-first=Rosa Amelia |language=en |trans-title=Comprehensive Study of the Origin of Humanity |access-date=2024-10-19}}</ref> The search began in the mid-19th century and continued at the start of the 20th with [[Percival Lowell]]'s quest for Planet X. Lowell proposed the [[Planet Nine|Planet&nbsp;X hypothesis]] to explain [[Discovery of Neptune#Irregularities in Uranus' orbit|apparent discrepancies]] in the [[orbit]]s of the giant planets, particularly [[Uranus]] and Neptune,<ref name=bower/> speculating that the gravity of a large unseen ninth planet could have [[gravitational perturbation|perturbed]] Uranus enough to account for the irregularities.<ref>Tombaugh (1946), p.&nbsp;73.</ref>

[[Clyde Tombaugh]]'s discovery of [[Pluto]] in 1930 appeared to validate Lowell's hypothesis, and Pluto was officially named the ninth planet. In 1978, Pluto was conclusively determined to be too small for its gravity to affect the giant planets, resulting in a brief search for a tenth planet. The search was largely abandoned in the early 1990s, when a study of measurements made by the ''[[Voyager 2]]'' spacecraft found that the irregularities observed in Uranus's orbit were due to a slight overestimation of Neptune's mass.<ref name="standage">{{Cite book |last=Standage |first=Tom |url=https://archive.org/details/neptunefilestory00stan/page/188 |title=The Neptune file: a story of astronomical rivalry and the pioneers of planet hunting |date=2000 |publisher=Walker |isbn=978-0-8027-1363-6 |location=New York |page=[https://archive.org/details/neptunefilestory00stan/page/188 188] |url-access=registration}}</ref> After 1992, the discovery of numerous small icy objects with similar or even wider orbits than Pluto led to a debate over whether Pluto should remain a planet, or whether it and its neighbours should, like the [[asteroid]]s, be given [[definition of planet|their own separate classification]]. Although a number of the larger members of this group were initially described as planets, in 2006 the [[International Astronomical Union]] (IAU) reclassified Pluto and its largest neighbours as [[dwarf planet]]s, leaving Neptune the farthest known planet in the [[Solar System]].<ref name=IAU>{{cite news | url=http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf| title=IAU 2006 General Assembly: Resolutions 5 and 6|publisher=International Astronomical Union|date=2006-08-24}}</ref>

While the astronomical community widely agrees that Planet&nbsp;X, as originally envisioned, does not exist, the concept of an as-yet-unobserved planet has been revived by a number of astronomers to explain other anomalies observed in the outer Solar System.<ref name="varuna">{{Cite journal |last1=Tegler |first1=S. C. |last2=Romanishin |first2=W. |name-list-style=amp |date=May 2001 |title=Almost Planet X |journal=Nature |language=en |volume=411 |issue=6836 |pages=423–424 |doi=10.1038/35078164 |issn=0028-0836 |pmid=11373654 |s2cid=5135498 |doi-access=free|bibcode=2001Natur.411..423T }}</ref> As of March 2014, observations with the [[Wide-field Infrared Survey Explorer|WISE telescope]] have ruled out the possibility of a [[Saturn]]-sized object (95 Earth masses) out to 10,000 [[astronomical unit|AU]], and a [[Jupiter]]-sized (≈318 Earth masses) or larger object out to 26,000 AU.<ref name="Luhman2014">{{cite journal | last=Luhman |first=K. L. | title=A Search for a Distant Companion to the Sun with the Wide-field Infrared Survey Explorer | journal=[[The Astrophysical Journal]] | volume=781 |issue=1 |pages= 4|date=2014 | doi=10.1088/0004-637X/781/1/4 | bibcode= 2014ApJ...781....4L|s2cid=122930471 }}</ref>

In 2014, based on similarities of the orbits of a group of recently discovered [[extreme trans-Neptunian object]]s, astronomers hypothesized the existence of a super-Earth or ice giant planet, 2 to 15 times the mass of the Earth and beyond 200 AU with possibly a highly inclined orbit at some 1,500 AU.<ref name="nature letter">{{Cite journal| doi = 10.1038/nature13156| title = A Sedna-like body with a perihelion of 80 astronomical units| url = http://home.dtm.ciw.edu/users/sheppard/pub/TrujilloSheppard2014.pdf| archive-url = https://web.archive.org/web/20141216183818/http://home.dtm.ciw.edu/users/sheppard/pub/TrujilloSheppard2014.pdf| archive-date = 2014-12-16| url-status = dead| journal = Nature| volume = 507| issue = 7493| pages = 471–474| date = 2014| last1 = Trujillo| first1 = C. A.| last2 = Sheppard| first2 = S. S.| bibcode = 2014Natur.507..471T| pmid = 24670765| s2cid = 4393431| access-date = 2016-01-25}}</ref> In 2016, further work showed this unknown distant planet is likely to be on an inclined, eccentric orbit that goes no closer than about 200 AU and no farther than about 1,200 AU from the Sun. The orbit is predicted to be anti-aligned to the clustered extreme trans-Neptunian objects.<ref name="AJ121-2-22"/> Because Pluto is no longer considered a planet by the IAU, this new hypothetical object has become known as [[Planet Nine]].<ref>{{cite magazine|url=http://www.newyorker.com/tech/elements/discovering-planet-nine|title=Discovering Planet Nine|magazine=The New Yorker|first=Alan|last=Burdick|date=20 January 2016|access-date=20 January 2016}}</ref>

{{TOC limit|3}}

== Early speculation ==
{{see also|Discovery of Neptune}}
[[File:Jacques Babinet.jpg|thumb|right|[[Jacques Babinet]], an early proponent of a trans-Neptunian planet]]

In the 1840s, the French mathematician [[Urbain Le Verrier]] used [[Classical mechanics|Newtonian mechanics]] to analyse perturbations in the orbit of Uranus, and hypothesised that they were caused by the gravitational pull of a yet-undiscovered planet. Le Verrier predicted the position of this new planet and sent his calculations to German astronomer [[Johann Gottfried Galle]]. On 23 September 1846, the night following his receipt of the letter, Galle and his student [[Heinrich Louis d'Arrest|Heinrich d'Arrest]] discovered Neptune, exactly where Le Verrier had predicted.<ref>Croswell (1997), p. 43</ref> There remained some slight discrepancies in the [[giant planet]]s' orbits. These were taken to indicate the existence of yet another planet orbiting beyond Neptune.

Even before Neptune's discovery, some speculated that one planet alone was not enough to explain the discrepancy. On 17 November 1834, the British amateur astronomer the Reverend [[Thomas John Hussey]] reported a conversation he had had with French astronomer [[Alexis Bouvard]] to [[George Biddell Airy]], the British Astronomer Royal. Hussey reported that when he suggested to Bouvard that the unusual motion of Uranus might be due to the gravitational influence of an undiscovered planet, Bouvard replied that the idea had occurred to him, and that he had corresponded with [[Peter Andreas Hansen]], director of the [[Gotha Observatory|Seeberg Observatory]] in [[Gotha]], about the subject. Hansen's opinion was that a single body could not adequately explain the motion of Uranus, and postulated that two planets lay beyond Uranus.<ref name="grosser">{{Cite journal |last=Grosser |first=Morton |date=June 1964 |title=The Search for a Planet beyond Neptune |journal=Isis |language=en |volume=55 |issue=2 |pages=163–183 |doi=10.1086/349825 |issn=0021-1753 |jstor=228182 |s2cid=144255699}}</ref>

In 1848, [[Jacques Babinet]] raised an objection to Le Verrier's calculations, claiming that Neptune's observed mass was smaller and its orbit larger than Le Verrier had initially predicted. He postulated, based largely on simple subtraction from Le Verrier's calculations, that another planet of roughly 12 Earth masses, which he named "Hyperion", must exist beyond Neptune.<ref name=grosser/> Le Verrier denounced Babinet's hypothesis, saying, "[There is] absolutely nothing by which one could determine the position of another planet, barring hypotheses in which imagination played too large a part."<ref name=grosser/>

In 1850 [[James Ferguson (American astronomer)|James Ferguson]], Assistant Astronomer at the [[United States Naval Observatory]], noted that he had "lost" a star he had observed, GR1719k, which Lt. Matthew Maury, the superintendent of the Observatory, claimed was evidence that it must be a new planet. Subsequent searches failed to recover the "planet" in a different position, and in 1878, [[CHF Peters]], director of the [[Hamilton College]] Observatory in [[New York (state)|New York]], showed that the star had not in fact vanished, and that the previous results had been due to human error.<ref name=grosser/>

In 1879, [[Camille Flammarion]] noted that the comets [[Comet Swift–Tuttle|1862 III]] and [[177P/Barnard|1889 III]] had [[Apsis|aphelia]] of 47 and 49&nbsp;[[Astronomical unit|AU]], respectively, suggesting that they might mark the orbital radius of an unknown planet that had dragged them into an elliptical orbit.<ref name=grosser/> Astronomer [[George Forbes (scientist)|George Forbes]] concluded on the basis of this evidence that two planets must exist beyond Neptune. He calculated, based on the fact that four comets possessed aphelia at around 100&nbsp;AU and a further six with aphelia clustered at around 300&nbsp;AU, the orbital elements of a pair of hypothetical trans-Neptunian planets. These elements accorded suggestively with those made independently by another astronomer named [[David Peck Todd]], suggesting to many that they might be valid.<ref name=grosser/> However, sceptics argued that the orbits of the comets involved were still too uncertain to produce meaningful results.<ref name=grosser/> Some have considered Forbes's hypothesis a precursor to [[Planet Nine]].<ref>{{cite journal|title=Constraints on Planet Nine's Orbit and Sky Position within a Framework of Mean Motion Resonances|journal=The Astronomical Journal|volume=153|issue=3|pages=91|year=2017|last1=Millholland|first1=Sarah|last2=Laughlin|first2=Gregory|doi=10.3847/1538-3881/153/3/91|arxiv=1612.07774|bibcode=2017AJ....153...91M|s2cid=119325788 |doi-access=free }}</ref>

In 1900 and 1901, [[Harvard College Observatory]] director [[William Henry Pickering]] led two searches for trans-Neptunian planets. The first was begun by Danish astronomer [[Hans E. Lau|Hans Emil Lau]] who, after studying the data on the orbit of Uranus from 1690 to 1895, concluded that one trans-Neptunian planet alone could not account for the discrepancies in its orbit, and postulated the positions of two planets he believed were responsible. The second was launched when Gabriel Dallet suggested that a single trans-Neptunian planet lying at 47&nbsp;AU could account for the motion of Uranus. Pickering agreed to examine plates for any suspected planets. In neither case were any found.<ref name=grosser/>

In 1902, after observing the orbits of comets with aphelia beyond Neptune, Theodor Grigull of [[Münster]], Germany proclaimed the existence of a Uranus-sized planet at 50 AU with a 360-year period, which he named Hades, cross-checking with the deviations in the orbit of Uranus. In 1921, Grigull revised his orbital period to 310–330 years, to better fit the observed deviations.<ref>{{Cite web |last=Schlyter |first=Paul |title=Hypothetical Planets |url=http://solarviews.com/eng/hypothet.htm |access-date=2019-12-07 |website=The Swedish Amateur Astronomical Society}}</ref>

In 1909, [[Thomas Jefferson Jackson See]], an astronomer with a reputation as an egocentric contrarian, opined "there is certainly one, most likely two and possibly three planets beyond Neptune".<ref name="see">{{Cite journal |last=Sherrill |first=Thomas J. |date=February 1999 |title=A Career of Controversy: The Anomaly of T.J.J. See |journal=Journal for the History of Astronomy |language=en |volume=30 |issue=1 |pages=25–50 |bibcode=1999JHA....30...25S |doi=10.1177/002182869903000102 |issn=0021-8286 |s2cid=117727302}}</ref> Tentatively naming the first planet "Oceanus", he placed their respective distances at 42, 56 and 72 AU from the Sun. He gave no indication as to how he determined their existence, and no known searches were mounted to locate them.<ref name=see/>

In 1911, Indian astronomer [[Venkatesh P. Ketakar]] suggested the existence of two trans-Neptunian planets, which he named after the Hindu gods [[Brahma]] and [[Vishnu]], by reworking the patterns observed by [[Pierre-Simon Laplace]] in the planetary satellites of [[Jupiter]] and applying them to the outer planets.<ref name=ketakar/> The three inner [[Galilean moons]] of Jupiter, [[Io (moon)|Io]], [[Europa (moon)|Europa]] and [[Ganymede (moon)|Ganymede]], are locked in a complicated 1:2:4 resonance called a [[Orbital resonance#The Laplace resonance|Laplace resonance]].<ref name="Musotto2002">{{Cite journal |last1=Musotto |first1=Susanna |last2=Varadi |first2=Ferenc |last3=Moore |first3=William |last4=Schubert |first4=Gerald |date=October 2002 |title=Numerical Simulations of the Orbits of the Galilean Satellites |journal=Icarus |language=en |volume=159 |issue=2 |pages=500–504 |bibcode=2002Icar..159..500M |doi=10.1006/icar.2002.6939}}</ref> Ketakar suggested that Uranus, Neptune and his hypothetical trans-Neptunian planets were also locked in Laplace-like resonances. This is incorrect; Uranus and Neptune, while in a near-2:1 resonance, are not in full resonance.<ref>{{Cite journal |last=Malhotra |first=Renu |author-link=Renu Malhotra |year=1998 |title=Orbital Resonances and Chaos in the Solar System |url=https://www.lpl.arizona.edu/~renu/malhotra_preprints/rio97.pdf |journal=Solar System Formation and Evolution ASP Conference Series |volume=149 |page=37 |bibcode=1998ASPC..149...37M}}</ref> His calculations predicted a mean distance for Brahma of 38.95&nbsp;AU and an orbital period of 242.28 Earth years (3:4 resonance with Neptune). When Pluto was discovered 19 years later, its mean distance of 39.48&nbsp;AU and orbital period of 248 Earth years were close to Ketakar's prediction (Pluto in fact has a [[Plutino|2:3 resonance with Neptune]]). Ketakar made no predictions for the orbital elements other than mean distance and period. It is not clear how Ketakar arrived at these figures, and his second planet, Vishnu, was never located.<ref name="ketakar">{{Cite journal |last1=Chhabra |first1=J. G. |last2=Sharma |first2=S. D. |last3=Khanna |first3=Manju |date=1984 |title=Prediction of Pluto by V. P. Ketakar |url=http://www.new.dli.ernet.in/rawdataupload/upload/insa/INSA_1/20005abd_18.pdf |url-status=dead |journal=Indian Journal of History of Science |volume=19 |issue=1 |pages=18–26 |bibcode=1984InJHS..19...18C |archive-url=https://web.archive.org/web/20090225135119/http://www.new.dli.ernet.in/rawdataupload/upload/insa/INSA_1/20005abd_18.pdf |archive-date=2009-02-25 |access-date=2008-09-04}}</ref>

== Planet X ==
{{Redirect|Planet X|the conspiracy theory|Nibiru cataclysm|the hypothetical planet first proposed in 2014|Planet Nine||Planet X (disambiguation)}}
In 1894, with the help of William Pickering, [[Percival Lowell]] (a wealthy Bostonian) founded the [[Lowell Observatory]] in [[Flagstaff, Arizona]]. In 1906, convinced he could resolve the conundrum of Uranus's orbit, he began an extensive project to search for a trans-Neptunian planet,<ref name=tombaugh/> which he named ''Planet&nbsp;X'', a name previously used by Gabriel Dallet.<ref name=grosser/> The ''X'' in the name represents an unknown and is pronounced as the letter, as opposed to the [[Roman numeral]] for 10 (at the time, Planet&nbsp;X would have been the ninth planet). Lowell's hope in tracking down Planet&nbsp;X was to establish his scientific credibility, which had eluded him due to his widely derided belief that channel-like features visible on the surface of [[Mars]] were [[Martian canals|canals constructed by an intelligent civilization]].<ref>Croswell (1997), p. 43.</ref>

Lowell's first search focused on the [[ecliptic]], the plane encompassed by the [[zodiac]] where the other planets in the Solar System lie. Using a 5-inch photographic camera, he manually examined over 200 three-hour exposures with a magnifying glass, and found no planets. At that time Pluto was too far above the ecliptic to be imaged by the survey.<ref name=tombaugh>Tombaugh (1946).</ref>  After revising his predicted possible locations, Lowell conducted a second search from 1914 to 1916.<ref name=tombaugh/> In 1915, he published his ''Memoir of a Trans-Neptunian Planet'', in which he concluded that Planet&nbsp;X had a mass roughly seven times that of Earth—about half that of Neptune<ref name="ley195608">{{Cite magazine  |last=Ley  |first=Willy  |date=August 1956  |title=The Demotion of Pluto  |department=For Your Information  |url=https://archive.org/stream/galaxymagazine-1956-08#page/n79/mode/2up  |magazine=Galaxy Science Fiction  |pages=79–91  }}</ref>—and a mean distance from the Sun of 43&nbsp;AU. He assumed Planet&nbsp;X would be a large, low-density object with a high [[albedo]], like the giant planets. As a result, it would show a disc with diameter of about one arcsecond and an [[apparent magnitude]] between 12 and 13—bright enough to be spotted.<ref name=tombaugh/><ref>Littman (1990), p. 70.</ref>

{{anchor|Planet O}}Separately, in 1908, Pickering announced that, by analysing irregularities in Uranus's orbit, he had found evidence for a ninth planet. His hypothetical planet, which he termed "Planet&nbsp;O" (because it came after "N", i.e. Neptune),<ref>{{cite book|title=The Hunt For Planet X|author=Govert Schilling |date=2009 |publisher=Springer |page=34 | isbn= 978-0387778044 }}</ref> possessed a mean orbital radius of 51.9&nbsp;AU and an orbital period of 373.5 years.<ref name=grosser/> Plates taken at his observatory in [[Arequipa]], Peru, showed no evidence for the predicted planet, and British astronomer [[Philip Herbert Cowell|P. H. Cowell]] showed that the irregularities observed in Uranus's orbit virtually disappeared once the planet's displacement of longitude was taken into account.<ref name=grosser/> Lowell himself, despite his close association with Pickering, dismissed Planet&nbsp;O out of hand, saying, "This planet is very properly designated "O", [for it] is nothing at all."<ref name=cros50>Croswell p. 50</ref> Unbeknownst to Pickering, four of the photographic plates taken in the search for "Planet O" by astronomers at the Mount Wilson Observatory in 1919 captured images of [[Pluto]], though this was only recognised years later.<ref name="Hoyt" /> Pickering went on to suggest many other possible trans-Neptunian planets up to the year 1932, which he named ''P'', ''Q'', ''R'', ''S'', ''T'', and ''U''; none were ever detected.<ref name=ketakar/>

=== Discovery of Pluto ===
{{main|Pluto}}
[[File:Clyde W. Tombaugh.jpeg|thumb|right|upright|Clyde William Tombaugh]]
Lowell's sudden death in 1916 temporarily halted the search for Planet&nbsp;X. Failing to find the planet, according to one friend, "virtually killed him".<ref>Croswell (1997), p. 49.</ref> Lowell's widow, Constance, engaged in a legal battle with the observatory over Lowell's legacy which halted the search for Planet&nbsp;X for several years.<ref name=cros2>Croswell (1997), pp. 32–55.</ref> In 1925, the observatory obtained glass discs for a new {{convert|13|in|cm|abbr=on}} wide-field telescope to continue the search, constructed with funds from [[Abbott Lawrence Lowell]],<ref>{{Cite web |last=Schindler |first=Kevin |date=2015-05-14 |title=Percival Lowell's three early searches for Planet X |url=https://www.astronomy.com/science/percival-lowells-three-early-searches-for-planet-x/ |access-date=2024-09-11 |website=Astronomy Magazine |language=en-US}}</ref> Percival's brother.<ref name=tombaugh/> In 1929 the observatory's director, [[Vesto Melvin Slipher]], summarily handed the job of locating the planet to [[Clyde Tombaugh]], a 22-year-old Kansas farm boy who had only just arrived at the Lowell Observatory after Slipher had been impressed by a sample of his astronomical drawings.<ref name=cros2/>

Tombaugh's task was to systematically capture sections of the night sky in pairs of images. Each image in a pair was taken two weeks apart. He then placed both images of each section in a machine called a [[blink comparator]], which by exchanging images quickly created a [[Time-lapse photography|time lapse]] illusion of the movement of any planetary body. To reduce the chances that a faster-moving (and thus closer) object be mistaken for the new planet, Tombaugh imaged each region near its opposition point, 180 degrees from the Sun, where the [[apparent retrograde motion]] for objects beyond Earth's orbit is at its strongest. He also took a third image as a control to eliminate any false results caused by defects in an individual plate. Tombaugh decided to image the entire zodiac, rather than focus on those regions suggested by Lowell.<ref name=tombaugh/>

[[File:Pluto discovery plates.png|thumb|left|Discovery photographs of Pluto]]
By the beginning of 1930, Tombaugh's search had reached the constellation of Gemini. On 18 February 1930, after searching for nearly a year and examining nearly 2&nbsp;million stars, Tombaugh discovered a moving object on photographic plates taken on 23 January and 29 January of that year.<ref>Tombaugh (1946), p. 79</ref> A lesser-quality photograph taken on January 21 confirmed the movement.<ref name=cros2/> Upon confirmation, Tombaugh walked into Slipher's office and declared, "Doctor Slipher, I have found your Planet&nbsp;X."<ref name=cros2/> The object was just six degrees from one of two locations for Planet&nbsp;X  Lowell had suggested; thus it seemed he had at last been vindicated.<ref name=cros2/> After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the [[Harvard College Observatory]] on March 13, 1930. The new object was later [[Precovery|precovered]] on photographs dating back to 19 March 1915.<ref name="Hoyt">{{Cite journal |last=Hoyt |first=William Graves |date=December 1976 |title=W. H. Pickering's Planetary Predictions and the Discovery of Pluto |journal=Isis |language=en |volume=67 |issue=4 |pages=551–564 |doi=10.1086/351668 |issn=0021-1753 |jstor=230561 |pmid=794024 |s2cid=26512655}} p. 563.</ref> The decision to name the object ''Pluto'' was intended in part to honour Percival Lowell, as his initials made up the word's first two letters.<ref>{{cite web|title= NASA's Solar System Exploration: Multimedia: Gallery: Pluto's Symbol |url= http://sse.jpl.nasa.gov/multimedia/display.cfm?IM_ID=263 |publisher=NASA |access-date=2007-03-25 |url-status=dead |archive-url= https://web.archive.org/web/20061001015053/http://sse.jpl.nasa.gov/multimedia/display.cfm?IM_ID=263 |archive-date=2006-10-01 }}</ref> After discovering Pluto, Tombaugh continued to search the ecliptic for other distant objects. He found hundreds of [[variable star]]s and [[Clyde Tombaugh#Asteroids discovered|asteroids]], as well as two [[comet]]s, but no further planets.<ref>{{cite web|title=Clyde W. Tombaugh|publisher=New Mexico Museum of Space History |url=http://www.nmspacemuseum.org/halloffame/detail.php?id=51|access-date=2008-06-29}}</ref>

=== Pluto loses Planet X title ===
[[File:Charon Discovery.jpg|thumb|Discovery image of [[Charon (moon)|Charon]]]]
To the observatory's disappointment and surprise, Pluto showed no visible disc; it appeared as a point, no different from a star, and, at only 15th magnitude, was six times dimmer than Lowell had predicted, which meant it was either very small, or very dark.<ref name=tombaugh/> Because of Lowell's predictions, astronomers thought that Pluto would be massive enough to [[Perturbation (astronomy)|perturb]] planets. This led them to assume that its [[Albedo#Astronomical albedo|albedo]] could be no less than 0.07 (meaning that, at minimum, it would reflect 7% of the light that hit it), which would have made Pluto about as dark as asphalt, and similar in reflectivity to the least reflective planet, which is [[Mercury (planet)|Mercury]].<ref name="bower">{{Cite journal |last=Bower |first=Ernest Clare |date=1931 |title=On the orbit and mass of Pluto with an ephemeris for 1931-1932 |journal=Lick Observatory Bulletins |language=en |volume=15 |issue=437 |pages=171–178 |bibcode=1931LicOB..15..171B |doi=10.5479/ADS/bib/1931LicOB.15.171B |issn=0075-9317}}</ref> This would have given Pluto an estimated mass of no more than 70% that of Earth.<ref name=bower/> Observations also revealed that Pluto's orbit was very elliptical, far more than that of any other planet.<ref name="lauch">{{Cite book |last1=Davies |first1=John K. |title=The Solar System Beyond Neptune |last2=McFarland |first2=John |last3=Bailey |first3=Mark E. |last4=Marsden |first4=Brian G. |last5=Ip |first5=Wing-Huen |date=2008 |publisher=University of Arizona Press |editor-last=Baracci |editor-first=M. Antonietta |pages=11–23 |chapter=The Early Development of Ideas Concerning the Transneptunian Region |display-authors=4 |access-date=2014-11-05 |editor-last2=Boenhardt |editor-first2=Hermann |editor-last3=Cruikchank |editor-first3=Dale |editor-last4=Morbidelli |editor-first4=Alissandro |chapter-url=http://www.arm.ac.uk/preprints/2008/522.pdf |archive-url=https://web.archive.org/web/20150220182134/http://www.arm.ac.uk/preprints/2008/522.pdf |archive-date=2015-02-20 |url-status=dead}}</ref>

Almost immediately, some astronomers questioned Pluto's status as a planet. Barely a month after its discovery was announced, on April 14, 1930, in an article in ''[[The New York Times]]'', [[Armin Otto Leuschner|Armin O. Leuschner]] suggested that Pluto's dimness and high orbital eccentricity made it more similar to an asteroid or comet: "The Lowell result confirms the possible high eccentricity announced by us on April 5. Among the possibilities are a large asteroid greatly disturbed in its orbit by close approach to a major planet such as Jupiter, or it may be one of many long-period planetary objects yet to be discovered, or a bright cometary object."<ref name=lauch/><ref name=nyt/> In that same article, [[Harvard Observatory]] director [[Harlow Shapley]] wrote that Pluto was a "member of the Solar System not comparable with known asteroids and comets, and perhaps of greater importance to cosmogony than would be another major planet beyond Neptune."<ref name=nyt>{{cite news|journal=The New York Times|date=April 14, 1930|url= https://timesmachine.nytimes.com/timesmachine/1930/04/14/96096499.pdf|title="Planet X" Orbit Raises More Doubt}}</ref> In 1931, after examining the structure of the [[Errors and residuals|residuals]] of Uranus' longitude using a trigonometric formula, [[Ernest W. Brown]] asserted (in agreement with [[E. C. Bower]]) that the presumed irregularities in the orbit of Uranus could not be due to the gravitational effect of a more distant planet, and thus that Lowell's supposed prediction was "purely accidental".<ref>{{Cite journal |last=Brown |first=Ernest W. |date=November 1931 |title=On a Criterion for the Prediction of an Unknown Planet |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=92 |issue=1 |pages=80–100 |bibcode=1931MNRAS..92...80B |doi=10.1093/mnras/92.1.80 |issn=0035-8711 |doi-access=free}}</ref>

Throughout the mid-20th century, estimates of Pluto's mass were revised downward. In 1931, Nicholson and Mayall calculated its mass, based on its supposed effect on the giant planets, as roughly that of Earth;<ref name="The Discovery of Pluto">{{cite journal|title= The Discovery of Pluto |journal= Monthly Notices of the Royal Astronomical Society |volume=91 |issue= 4 |date=February 1931 |pages= 380–385 |bibcode= 1931MNRAS..91..380. |doi=10.1093/mnras/91.4.380|doi-access= free }}</ref> a value somewhat in accord with the 0.91 Earth mass calculated in 1942 by [[Lloyd R. Wylie]] at the [[US Naval Observatory]], using the same assumptions.<ref name="wylie">{{Cite book |last=Weintraub |first=David A. |url=https://books.google.com/books?id=dW1_AwAAQBAJ |title=Is Pluto a Planet?: A Historical Journey through the Solar System |date=2014 |publisher=Princeton University Press |isbn=978-1-4008-5297-0 |page=141 |language=en}}</ref> In 1949, [[Gerard Kuiper]]'s measurements of Pluto's diameter with the 200-inch telescope at [[Mount Palomar Observatory]] led him to the conclusion that it was midway in size between Mercury and Mars and that its mass was most probably about 0.1 Earth mass.<ref name="Gerard P. Kuiper 1950 133–137">{{cite journal|title=The Diameter of Pluto |first=Gerard P. |last=Kuiper |journal=Publications of the Astronomical Society of the Pacific |volume=62 |pages= 133–137 |date=August 1950 |bibcode= 1950PASP...62..133K|doi= 10.1086/126255 |issue=366|doi-access=free }}</ref>

In 1973, based on the similarities in the periodicity and amplitude of brightness variation with [[Triton (moon)|Triton]], Dennis Rawlins conjectured Pluto's mass must be similar to Triton's. In retrospect, the conjecture turns out to have been correct; it had been argued by astronomers [[Walter Baade]] and E.C. Bower as early as 1934.<ref>{{Cite journal |last=Baade |first=W. |author-link=Walter Baade |year=1934 |title=The Photographic Magnitude and Color Index of Pluto |journal=Publications of the Astronomical Society of the Pacific |language=en |volume=46 |page=218 |bibcode=1934PASP...46..218B |doi=10.1086/124467 |issn=0004-6280 |doi-access=free |number=272}}</ref> However, because Triton's mass was then believed to be roughly 2.5% of the Earth–Moon system (more than ten times its actual value), Rawlins's determination for Pluto's mass was similarly incorrect. It was nonetheless a meagre enough value for him to conclude Pluto was not Planet X.<ref name="Mass and Position Limits for an Hypothetical Tenth Planet of the Solar System">{{Cite journal |last1=Rawlins |first1=D. |last2=Hammerton |first2=M. |date=June 1973 |title=Mass and Position Limits for an Hypothetical Tenth Planet of the Solar System |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=162 |issue=3 |pages=261–270 |bibcode=1973MNRAS.162..261R |doi=10.1093/mnras/162.3.261 |issn=0035-8711 |doi-access=free}} Rawlins also took into account Pluto's stellar occultation failure as reported by {{Cite journal |last1=Halliday |first1=Ian |last2=Hardie |first2=R. H. |last3=Franz |first3=Otto G. |last4=Priser |first4=John B. |year=1966 |title=An Upper Limit for the Diameter of Pluto |journal=Publications of the Astronomical Society of the Pacific |language=en |volume=78 |issue=461 |pages=113 |bibcode=1966PASP...78..113H |doi=10.1086/128307 |issn=0004-6280 |s2cid=121483531}}</ref> In 1976, Dale Cruikshank, Carl Pilcher, and David Morrison of the [[University of Hawaii]] analysed spectra from Pluto's surface and determined that it must contain [[methane]] [[Volatile (astrogeology)|ice]], which is highly reflective. This meant that Pluto, far from being dark, was in fact exceptionally bright, and thus was probably no more than {{frac|1|100}}&nbsp;Earth mass.<ref>{{cite journal |doi=10.1126/science.194.4267.835-a |pmid=17744185 |journal=Science |volume=194 |issue=4267 |pages=835–837 |year=1976 |title=Pluto: Evidence for methane frost }}</ref><ref name="Croswell 1997, p. 57">Croswell (1997), p. 57.</ref>

<div style="float:right; margin:2px;">
{| class=wikitable style="text-align:center; font-size:11px"
|+ Mass estimates for Pluto
!Year!! Mass!!Notes
|-
|1931 || 1 Earth ||Nicholson & Mayall<ref name="The Discovery of Pluto"/>
|-
|1942 || 0.91 Earth || Wylie<ref name=wylie/>
|-
|1948||  0.1 (1/10 Earth) || Kuiper<ref name="Gerard P. Kuiper 1950 133–137"/>
|-
|1973|| 0.025 (1/40 Earth) || Rawlins<ref name="Mass and Position Limits for an Hypothetical Tenth Planet of the Solar System" />
|-
|1976||0.01 (1/100 Earth)||Cruikshank, Pilcher, & Morrison<ref name="Croswell 1997, p. 57"/>
|-
|1978||0.002 (1/500 Earth) || Christy & Harrington<ref name="Christy">{{Cite journal |last1=Christy |first1=J. W. |last2=Harrington |first2=R. S. |name-list-style=amp |date=August 1978 |title=The satellite of Pluto |journal=The Astronomical Journal |volume=83 |issue=8 |pages=1005 |bibcode=1978AJ.....83.1005C |doi=10.1086/112284}}</ref>
|-
|2006||0.00218 (1/459 Earth) || Buie et al.<ref name="Buie">{{Cite journal |last1=Buie |first1=Marc W. |last2=Grundy |first2=William M. |last3=Young |first3=Eliot F. |last4=Young |first4=Leslie A. |last5=Stern |first5=S. Alan |name-list-style=amp |date=July 2006 |title=Orbits and Photometry of Pluto's Satellites: Charon, S/2005 P1, and S/2005 P2 |journal=The Astronomical Journal |language=en |volume=132 |issue=1 |pages=290–298 |arxiv=astro-ph/0512491 |bibcode=2006AJ....132..290B |doi=10.1086/504422 |issn=0004-6256 |s2cid=119386667}}</ref>
|}
</div>

Pluto's size was finally determined conclusively in 1978, when American astronomer [[James W. Christy]] discovered its moon [[Charon (moon)|Charon]]. This enabled him, together with [[Robert Sutton Harrington]] of the U.S. Naval Observatory, to measure the mass of the Pluto–Charon system directly by observing the moon's orbital motion around Pluto.<ref name="Christy" /> They determined Pluto's mass to be 1.31×10<sup>22</sup>&nbsp;kg; roughly one five-hundredth that of Earth or one-sixth that of the Moon, and far too small to account for the observed discrepancies in the orbits of the outer planets. Lowell's prediction had been a coincidence: If there was a Planet&nbsp;X, it was not Pluto.<ref>Croswell (1997), pp. 57–58.</ref>

=== Further searches for Planet X ===
After 1978, a number of astronomers kept up the search for Lowell's Planet&nbsp;X, convinced that, because Pluto was no longer a viable candidate, an unseen tenth planet must have been perturbing the outer planets.<ref name=cros />

In the 1980s and 1990s, Robert Harrington led a search to determine the real cause of the apparent irregularities.<ref name=cros>Croswell, pp. 56–71</ref> He calculated that any Planet&nbsp;X would be at roughly three times the distance of Neptune from the Sun; its orbit would be highly [[orbital eccentricity|eccentric]], and strongly [[orbital inclination|inclined]] to the ecliptic—the planet's orbit would be at roughly a 32-degree angle from the orbital plane of the other known planets.<ref>{{Cite journal |last=Harrington |first=R. S. |date=October 1988 |title=The location of Planet X |journal=The Astronomical Journal |volume=96 |pages=1476 |bibcode=1988AJ.....96.1476H |doi=10.1086/114898}}</ref> This hypothesis was met with a mixed reception. Noted Planet&nbsp;X skeptic [[Brian G. Marsden]] of the [[Minor Planet Center]] pointed out that these discrepancies were a hundredth the size of those noticed by Le Verrier, and could easily be due to observational error.<ref>Croswell (1997), pp. 62–63.</ref>

In 1972, Joseph Brady of the [[Lawrence Livermore National Laboratory]] studied irregularities in the motion of [[Halley's Comet]]. Brady claimed that they could have been caused by a Jupiter-sized planet beyond Neptune at 59 AU that is in a [[retrograde orbit]] around the Sun.<ref name=Brady1972>{{cite journal |last=Brady |first=Joseph L. |title=The Effect of a Trans-Plutonian Planet on Halley's Comet |journal=Publications of the Astronomical Society of the Pacific |volume=84 |issue=498 |pages=314–322 |date=1972 |bibcode=1972PASP...84..314B |doi=10.1086/129290|s2cid=122053270 |doi-access=free }}</ref> However, both Marsden and Planet&nbsp;X proponent [[P. Kenneth Seidelmann]] attacked the hypothesis, showing that Halley's Comet randomly and irregularly ejects jets of material, causing changes to its own orbital trajectory, and that such a massive object as Brady's Planet&nbsp;X would have severely affected the orbits of known outer planets.<ref>Croswell (1997), p. 63.</ref>

Although its mission did not involve a search for Planet&nbsp;X, the [[IRAS]] space observatory made headlines briefly in 1983 due to an "unknown object" that was at first described as "possibly as large as the giant planet Jupiter and possibly so close to Earth that it would be part of this Solar System".<ref>{{Cite news |last=O'Toole |first=Thomas |date=December 29, 1983 |title=Possibly as Large as Jupiter |url=https://www.washingtonpost.com/archive/politics/1983/12/30/possibly-as-large-as-jupiter/1075b265-120a-4d40-9493-a8c523b76927/ |access-date=2024-09-11 |newspaper=[[Washington Post]] |language=en-US |issn=0190-8286 |archive-date=2023-04-01 |archive-url=https://web.archive.org/web/20230401061239/https://www.washingtonpost.com/archive/politics/1983/12/30/possibly-as-large-as-jupiter/1075b265-120a-4d40-9493-a8c523b76927/ |url-status=dead }}</ref> Further analysis revealed that of several unidentified objects, nine were distant galaxies and the tenth was "[[infrared cirrus|interstellar cirrus]]"; none were found to be Solar System bodies.<ref>{{Cite journal |last1=Houck |first1=J. R. |last2=Schneider |first2=D. P. |last3=Danielson |first3=G. E. |last4=Neugebauer |first4=G. |last5=Soifer |first5=B. T. |last6=Beichman |first6=C. A. |last7=Lonsdale |first7=C. J. |display-authors=3 |name-list-style=vanc |date=March 1985 |title=Unidentified IRAS sources - Ultrahigh-luminosity galaxies |url=https://authors.library.caltech.edu/74762/1/1985ApJ___290L___5H.pdf |journal=The Astrophysical Journal |language=en |volume=290 |pages=L5 |bibcode=1985ApJ...290L...5H |doi=10.1086/184431 |issn=0004-637X}}</ref>

In 1988, A. A. Jackson and R. M. Killen studied the stability of Pluto's resonance with Neptune by placing test "Planet X-es" with various masses and at various distances from Pluto. Pluto and Neptune's orbits  are in a 3:2 resonance, which prevents their collision or even any close approaches, regardless of their separation in the [[z axis]]. It was found that the hypothetical object's mass had to exceed 5 Earth masses to break the resonance, and the parameter space is quite large and a large variety of objects could have existed beyond Pluto without disturbing the resonance. Four test orbits of a trans-Plutonian planet have been integrated forward for four million years in order to determine the effects of such a body on the stability of the Neptune–Pluto 3:2 resonance. Planets beyond Pluto with masses of 0.1 and 1.0 Earth masses in orbits at 48.3 and 75.5 AU, respectively, do not disturb the 3:2 resonance. Test planets of 5 Earth masses with semi-major axes of 52.5 and 62.5 AU disrupt the four-million-year libration of Pluto's argument of perihelion.<ref>{{Cite journal |last1=Jackson |first1=A. A. |last2=Killen |first2=R. M. |name-list-style=amp |date=October 1988 |title=Planet X and the stability of resonances in the Neptune-Pluto system |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=235 |issue=2 |pages=593–601 |bibcode=1988MNRAS.235..593J |doi=10.1093/mnras/235.2.593 |issn=0035-8711 |doi-access=free}}</ref>

=== Planet X disproved ===
Harrington died in January 1993, without having found Planet X.<ref name="croswell66">Croswell (1997), p. 66.</ref> Six months before, [[E. Myles Standish]] had used data from ''Voyager 2'''s 1989 flyby of Neptune, which had revised the planet's total mass downward by 0.5%—an amount comparable to the mass of Mars<ref name="croswell66" />—to recalculate its gravitational effect on Uranus.<ref>{{Cite journal |last=Standish |first=E. M. |date=May 1993 |title=Planet X - No dynamical evidence in the optical observations |journal=The Astronomical Journal |volume=105 |issue=5 |pages=2000 |bibcode=1993AJ....105.2000S |doi=10.1086/116575}}</ref> When Neptune's newly determined mass was used in the [[Jet Propulsion Laboratory Developmental Ephemeris]] (JPL DE), the supposed discrepancies in the Uranian orbit, and with them the need for a Planet&nbsp;X, vanished.<ref name="standage" /> There are no discrepancies in the trajectories of any space probes such as ''[[Pioneer 10]]'', ''[[Pioneer 11]]'', ''[[Voyager 1]]'', and ''[[Voyager 2]]'' that can be attributed to the gravitational pull of a large undiscovered object in the outer Solar System.<ref>Littman (1990), p. 204.</ref> Today, most astronomers agree that Planet&nbsp;X, as Lowell defined it, does not exist.<ref>{{Cite book |last=Standage |first=Tom |url=https://archive.org/details/neptunefilestory00stan/ |title=The Neptune file: a story of astronomical rivalry and the pioneers of planet hunting |date=2000 |publisher=Walker |isbn=978-0-8027-1363-6 |location=New York |page=[https://archive.org/details/neptunefilestory00stan/page/168 168] |url-access=registration}}</ref>

== Discovery of further trans-Neptunian objects ==

{{See also|Kuiper belt#History|l1=History of the Kuiper belt}}
After the discovery of Pluto and Charon, no more [[trans-Neptunian object]]s (TNOs) were found until [[15760 Albion]] in 1992.<ref>{{cite web|author=Minor Planet Center|title=Circular No. 5611 |date= 1992 |url=http://www.cbat.eps.harvard.edu/iauc/05600/05611.html|access-date=2011-07-05|archive-url = https://web.archive.org/web/20080504183122/http://cfa-www.harvard.edu/iau/iauc/05600/05611.html |archive-date = May 4, 2008}}</ref> Since then, thousands of such objects have been discovered. Most are now recognized as part of the [[Kuiper belt]], a swarm of icy bodies left over from the Solar System's formation that orbit near the [[plane of the ecliptic|ecliptic plane]] just beyond Neptune. Though none were as large as Pluto, some of these distant trans-Neptunian objects, such as [[90377 Sedna|Sedna]], were initially described in the media as "new planets".<ref>{{cite news |title= Astronomers discover 'new planet' |work= BBC News |url=http://news.bbc.co.uk/1/hi/sci/tech/3511678.stm |access-date= 2008-06-20 | date=2004-03-15}}</ref>

In 2005, astronomer [[Michael E. Brown|Mike Brown]] and his team announced the discovery of {{mp|2003 UB|313}} (later named {{dp|Eris}} after the Greek goddess of discord and strife), a trans-Neptunian object then thought to be just barely larger than Pluto.<ref>{{cite web|title=Circular No. 8747 |author= Central Bureau for Astronomical Telegrams, International Astronomical Union |url=http://www.cbat.eps.harvard.edu/iauc/08700/08747.html |date= 2006 |access-date= 2011-07-05|archive-url = https://web.archive.org/web/20070205035336/http://www.cfa.harvard.edu/iau/special/08747.pdf |archive-date = February 5, 2007|url-status=dead}}</ref> Soon afterwards, a NASA [[Jet Propulsion Laboratory]] press release described the object as the "tenth planet".<ref>{{cite web |title=NASA-Funded Scientists Discover Tenth Planet |publisher=Jet Propulsion Laboratory |url=http://www.jpl.nasa.gov/news/news-print.cfm?release=2005-126 |date=2005 |access-date=2007-02-22 |url-status=dead |archive-url=https://web.archive.org/web/20110721191652/http://www.jpl.nasa.gov/news/news-print.cfm?release=2005-126 |archive-date=2011-07-21 }}</ref>

Eris was never officially classified as a planet, and the [[IAU definition of planet|2006 definition of planet]] defined both Eris and Pluto not as planets but as dwarf planets because they have not [[clearing the neighbourhood|cleared their neighbourhoods]].<ref name=IAU/> They do not orbit the Sun alone, but as part of a population of similarly sized objects. Pluto itself is now recognized as being a member of the Kuiper belt and the largest dwarf planet, larger than the more massive Eris.

A number of astronomers, most notably [[Alan Stern]], the head of NASA's ''[[New Horizons]]'' mission to Pluto, contend that the IAU's definition is flawed, and that Pluto and Eris, and all large trans-Neptunian objects, such as {{Dp|Makemake}}, {{Dp|Sedna}}, {{Dp|Quaoar}}, {{Dp|Gonggong}} and {{Dp|Haumea}}, should be considered planets in their own right.<ref>{{Cite web |last=Stern |first=Alan |date=September 8, 2006 |title=Unabashedly Onward to the Ninth Planet |url=http://www.spacedaily.com/reports/Unabashedly_Onward_to_the_Ninth_Planet_999.html |access-date=2008-06-25 |website=SpaceDaily.com |publisher=NASA}}</ref> However, the discovery of Eris did not rehabilitate the Planet&nbsp;X theory because it is far too small to have significant effects on the outer planets' orbits.<ref>{{Cite web |last=C. Jewitt |first=David |author-link=David C. Jewitt |date=2021 |title=Dave Jewitt: Planets IX and X |url=http://www2.ess.ucla.edu/~jewitt/kb/planetx.html |access-date=2024-09-11 |website=UCLA - Earth, Planetary & Space Sciences}}</ref>

== Subsequently proposed trans-Neptunian planets ==
Although most astronomers accept that Lowell's Planet&nbsp;X does not exist, a number have revived the idea that a large unseen planet could create observable gravitational effects in the outer Solar System. These hypothetical objects are often referred to as "Planet&nbsp;X", although the conception of these objects may differ considerably from that proposed by Lowell.<ref>{{Cite journal |last1=Horner |first1=J. |last2=Evans |first2=N. W. |name-list-style=amp |date=September 2002 |title=Biases in cometary catalogues and Planet X |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=335 |issue=3 |pages=641–654 |arxiv=astro-ph/0205150 |bibcode=2002MNRAS.335..641H |doi=10.1046/j.1365-8711.2002.05649.x |s2cid=17110153 |doi-access=free}}</ref><ref name="planetx" />

===Orbits of distant objects===
{{see also|Planet Nine}}
[[File:Sedna orbit.svg|thumb|right|The orbit of Sedna (red) set against the orbits of Jupiter (orange), Saturn (yellow), Uranus (green), Neptune (blue), and Pluto (purple)|alt=The orbit of Sedna lies well beyond these objects, and extends many times their distances from the Sun]]

====Sedna's orbit====
{{anchor|Sedna's orbit}}
When [[90377 Sedna|Sedna]] was discovered, its extreme orbit raised questions about its origin. Its perihelion is so distant (approximately {{convert|76|AU|e9km e9mi|abbr=unit}}) that no currently observed mechanism can explain Sedna's eccentric distant orbit. It is too far from the planets to have been affected by the gravity of Neptune or the other giant planets and too bound to the Sun to be affected by outside forces such as the [[galactic tide]]s. Hypotheses to explain its orbit include that it was affected by a passing star, that it was captured from another [[planetary system]], or that it was tugged into its current position by a trans-Neptunian planet.<ref name="Mike">{{Cite journal |last1=Brown |first1=Michael E. |last2=Trujillo |first2=Chadwick |last3=Rabinowitz |first3=David |date=2004 |title=Discovery of a Candidate Inner Oort Cloud Planetoid |journal=The Astrophysical Journal |language=en |volume=617 |issue=1 |pages=645–649 |arxiv=astro-ph/0404456 |bibcode=2004ApJ...617..645B |doi=10.1086/422095 |issn=0004-637X |s2cid=7738201}}</ref> The most obvious solution to determining Sedna's peculiar orbit would be to locate a number of objects in a similar region, whose various orbital configurations would provide an indication as to their history. If Sedna had been pulled into its orbit by a trans-Neptunian planet, any other objects found in its region would have a similar perihelion to Sedna (around {{convert|80|AU|e9km e9mi|abbr=unit}}).<ref name="sisters">{{cite web|title=Searching for Sedna's Sisters: Exploring the inner Oort cloud|last=Schwamb|first=Megan|date=2007-09-18|publisher=Caltech|url=http://www.astro.caltech.edu/~george/option/candex07/schwamb_report.pdf|access-date=2010-08-06|url-status=dead|archive-url=https://web.archive.org/web/20130512221422/http://www.astro.caltech.edu/~george/option/candex07/schwamb_report.pdf|archive-date=2013-05-12}}</ref>

====Excitement of Kuiper belt orbits====
In 2008, Tadashi Mukai and Patryk Sofia Lykawka suggested a distant Mars- or Earth-sized planet, currently in a highly eccentric orbit between 100 and {{val|200|u=AU}} and orbital period of 1000 years with an inclination of 20° to 40°, was responsible for the structure of the Kuiper belt. They proposed that the perturbations of this planet excited the eccentricities and inclinations of the [[trans-Neptunian object]]s, truncated the planetesimal disk at 48&nbsp;AU, and detached the orbits of objects like Sedna from Neptune. During [[Neptune#Formation|Neptune's migration]] this planet is posited to have been captured in an outer resonance of Neptune and to have evolved into a higher perihelion orbit due to the [[Kozai mechanism]] leaving the remaining trans-Neptunian objects on stable orbits.<ref name=LykawaMukai>{{cite journal |last1=Patryk S. |first1=Lykawka |last2=Tadashi |first2=Mukai |year=2008 |title=An Outer Planet Beyond Pluto and the Origin of the Trans-Neptunian Belt Architecture |journal=[[The Astronomical Journal]] |volume=135 |issue=4 |pages=1161–1200 |arxiv=0712.2198 |bibcode=2008AJ....135.1161L |doi=10.1088/0004-6256/135/4/1161|s2cid=118414447 }}</ref><ref name=spaceplanetX>{{cite web |last=Than |first=Ker |date=18 June 2008 |title=Large 'Planet X' May Lurk Beyond Pluto |url=http://www.space.com/5526-large-planet-lurk-pluto.html |website=[[Space.com]] |access-date=18 July 2016}}</ref><ref name=canwestPlanetX>{{cite web |last=Hasegawa |first=Kyoko |date=28 February 2008 |title=Japanese scientists eye mysterious 'Planet X' |url=http://www.bibliotecapleyades.net/hercolobus/esp_hercolobus_66.htm |website=BibliotecaPleyades.net |access-date=18 July 2016}}</ref>

====Elongated orbits of group of Kuiper belt objects====
In 2012, Rodney Gomes modelled the orbits of 92 Kuiper belt objects and found that six of those orbits were far more elongated than the model predicted. He concluded that the simplest explanation was the gravitational pull of a distant planetary companion, such as a Neptune-sized object at 1,500 AU. This Neptune-sized object would cause the perihelia of objects with semi-major axes greater than 300 AU to oscillate, delivering them into planet-crossing orbits like those of {{mpl|308933|2006 SQ|372}} and {{mpl|87269|2000 OO|67}} or detached orbits like Sedna's.<ref>{{cite magazine|title=New planet found in our Solar System?|magazine=National Geographic|url=http://news.nationalgeographic.com/news/2012/05/120511-new-planet-solar-system-kuiper-belt-space-science|archive-url=https://web.archive.org/web/20120514021638/http://news.nationalgeographic.com/news/2012/05/120511-new-planet-solar-system-kuiper-belt-space-science/|url-status=dead|archive-date=May 14, 2012|date=2012|access-date=2012-05-21}}</ref>

====Planet Nine====
{{main article|Planet Nine}}
Planet Nine is a [[List of hypothetical Solar System objects|hypothetical ninth planet]] in the [[outer region of the Solar System]].<ref name=P9H2019/><ref name=TAJ201601>{{cite journal |last1=Batygin |first1=Konstantin |author-link1=Konstantin Batygin |last2=Brown |first2=Michael E. |author-link2=Michael E. Brown |year=2016 |title=Evidence for a Distant Giant Planet in the Solar System |journal=[[The Astronomical Journal]] |volume=151 |issue=2 |page=22 |arxiv=1601.05438 |bibcode=2016AJ....151...22B |doi=10.3847/0004-6256/151/2/22 |s2cid=2701020 |doi-access=free}}</ref> Its gravitational effects could explain the peculiar clustering of [[orbit]]s for a group of [[extreme trans-Neptunian object]]s (ETNOs)—bodies beyond [[Neptune]] that orbit the Sun at distances averaging more than 250&nbsp;times that of the Earth, over 250&nbsp;[[astronomical unit]]s (AU). 
[[File:Planet Nine - black background.png|thumbnail|right|Prediction of hypothetical Planet Nine's orbit based on unique clustering]]
In 2014, astronomers announced the discovery of {{mpl|2012 VP|113}}, a large object with a Sedna-like 4,200-year orbit and a perihelion of roughly 80 AU,<ref name="nature letter"/> which led them to suggest that it offered evidence of a potential trans-Neptunian planet.<ref>{{cite web |url=http://phys.org/news/2014-03-edge-solar.html |title=A new object at the edge of our Solar System discovered |work=Physorg.com |date=26 March 2014}}</ref> [[Chad Trujillo|Trujillo]] and [[Scott S. Sheppard|Sheppard]] argued that the orbital clustering of [[argument of perihelion|arguments of perihelia]] for {{mp|2012 VP|113}} and other extremely distant TNOs suggests the existence of a "[[super-Earth]]" of between 2 and 15 Earth masses beyond 200 AU and possibly on an inclined orbit at 1,500 AU.<ref name="nature letter"/>

In 2014 astronomers at the [[Universidad Complutense]] in [[Madrid]] suggested that the available data actually indicates more than one trans-Neptunian planet;<ref name="Marcos2014">{{cite journal| title=Extreme trans-Neptunian objects and the Kozai mechanism: signalling the presence of trans-Plutonian planets| first1=C.| last1=de la Fuente Marcos| first2=R.| last2=de la Fuente Marcos| date=1 September 2014|journal=Monthly Notices of the Royal Astronomical Society: Letters| volume=443| issue=1| pages=L59–L63| arxiv=1406.0715| bibcode=2014MNRAS.443L..59D |doi=10.1093/mnrasl/slu084| doi-access=free| s2cid=118622180}}</ref> subsequent work further suggests that the evidence is robust enough but rather than connected with the longitudes of the ascending nodes and the arguments of perihelia, semi-major axes and nodal distances could be the signposts.<ref name=commensurabilities>{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |date=2016 |title=Commensurabilities between ETNOs: a Monte Carlo survey |journal=[[Monthly Notices of the Royal Astronomical Society Letters]] |volume=460 |issue=1 |pages=L64–L68 |arxiv=1604.05881 |bibcode=2016MNRAS.460L..64D |doi=10.1093/mnrasl/slw077|doi-access=free |s2cid=119110892 }}</ref><ref name=nodes>{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |date=11 October 2017 |title=Evidence for a possible bimodal distribution of the nodal distances of the extreme trans-Neptunian objects: avoiding a trans-Plutonian planet or just plain bias? |journal=[[Monthly Notices of the Royal Astronomical Society Letters]] |volume=471 |issue=1 |pages=L61–L65 |arxiv=1706.06981 |bibcode=2017MNRAS.471L..61D |doi=10.1093/mnrasl/slx106|doi-access=free |s2cid=55469849 }}</ref> Additional work based on improved orbits of 39 objects still indicates that more than one perturber could be present and that one of them could orbit the Sun at 300-400 AU.<ref name="randomsearch">{{cite journal |last1=de la Fuente Marcos |first1=C. |last2=de la Fuente Marcos | first2= R.|title=Memories of past close encounters in extreme trans-Neptunian space: Finding unseen planets using pure random searches  |journal=Astronomy and Astrophysics Letters |volume=646 |page=L14 (9 pp) |date=18 February 2021 |doi=10.1051/0004-6361/202140311 |arxiv=2102.02220 |bibcode=2021A&A...646L..14D |s2cid= 231802033| url=https://www.aanda.org/articles/aa/full_html/2021/02/aa40311-21/aa40311-21.html}}</ref>

On January 20, 2016, Brown and [[Konstantin Batygin]] published an article corroborating Trujillo and Sheppard's initial findings; proposing a super-Earth (dubbed [[Planet Nine]]) based on a statistical clustering of the arguments of perihelia (noted before) near zero and also [[ascending node]]s near 113° of six distant [[trans-Neptunian object]]s. They estimated it to be ten times the mass of [[Earth]] (about 60% the mass of Neptune) with a [[semimajor axis]] of approximately 400–1500 [[astronomical unit|AU]].<ref name="AJ121-2-22">{{cite journal|title=Evidence for a distant giant planet in the Solar system|first1=Konstantin|last1=Batygin|first2=Michael E.|last2=Brown|date=20 January 2016|journal=[[The Astronomical Journal]]|volume=151|number=2|doi=10.3847/0004-6256/151/2/22|pages=22|arxiv = 1601.05438 |bibcode = 2016AJ....151...22B |s2cid=2701020 |doi-access=free }}</ref><ref name="NYT-20160120">{{cite news |last=Chang |first=Kenneth |title=Ninth Planet May Exist Beyond Pluto, Scientists Report |url=https://www.nytimes.com/2016/01/21/science/space/ninth-planet-solar-system-beyond-pluto.html |date=20 January 2016 |work=[[The New York Times]] |access-date=22 January 2016 }}</ref><ref>{{Cite news|title = New evidence suggests a ninth planet lurking at the edge of the solar system|url = https://www.washingtonpost.com/news/speaking-of-science/wp/2016/01/20/new-evidence-suggests-a-ninth-planet-lurking-at-the-edge-of-the-solar-system/|newspaper = The Washington Post|date = 2016-01-20|access-date = 2016-01-20|language = en-US|first1 = Joel|last1 = Achenbach|first2 = Rachel|last2 = Feltman}}</ref>

===Probability===

Even without gravitational evidence, Mike Brown, the discoverer of Sedna, has argued that Sedna's 12,000-year orbit means that probability alone suggests that an Earth-sized object exists beyond Neptune. Sedna's orbit is so eccentric that it spends only a small fraction of its orbital period near the Sun, where it can be easily observed. This means that unless its discovery was a freak accident, there is probably a substantial population of objects roughly Sedna's diameter yet to be observed in its orbital region.<ref name="Mike" /> Mike Brown noted that
{{blockquote|Sedna is about three-quarters the size of Pluto. If there are sixty objects three-quarters the size of Pluto [out there] then there are probably forty objects the size of Pluto ... If there are forty objects the size of Pluto, then there are probably ten that are twice the size of Pluto. There are probably three or four that are three times the size of Pluto, and the biggest of these objects ... is probably the size of Mars or the size of the Earth.<ref name="wgbh">{{Cite web |last=Brown |first=Michael |date=11 April 2007 |title=Pluto and the outer solar system |url=http://forum.wgbh.org/wgbh/forum.php?lecture_id=3710 |url-status=dead |archive-url=https://web.archive.org/web/20080103202810/http://forum.wgbh.org/wgbh/forum.php?lecture_id=3710 |archive-date=2008-01-03 |access-date=2008-07-13 |series=Lowell Lectures in Astronomy |publisher=Museum of Science, Boston / WGBH |place=Boston, MA}}</ref><ref>{{cite AV media |first=Mike |last=Brown |date=August 2008 |title=Pluto, Eris, and the dwarf planets of the outer solar system |type=academic talk |publisher=Smithsonian |time=50ᵐ |url=https://www.youtube.com/watch?v=WHNO079G1i8&t=3135 |url-status=live |access-date=2 January 2019 |archive-url=https://ghostarchive.org/varchive/youtube/20211114/WHNO079G1i8 |archive-date=2021-11-14 |via=[[YouTube]]}}{{cbignore}} The argument about dwarf planet sizes beyond Neptune is 50ᵐ into his talk. The WGBH link doesn't work; view on YouTube.</ref><ref>{{cite web |title=Planet 10? Another Earth-size world may lurk in the outer solar system |date=22 June 2017 |website=[[Space.com]] |url=https://www.space.com/37295-possible-planet-10.html}}</ref>}}

However, Brown notes that even though it might approach or exceed Earth in size, should such an object be found it would still be a "dwarf planet" by the current definition, because it would not have cleared its neighbourhood sufficiently.<ref name=wgbh/>

===Kuiper cliff and "Planet Ten"===
Additionally, speculation of a possible trans-Neptunian planet has revolved around the so-called "[[Kuiper Belt#Kuiper cliff|Kuiper cliff]]". The Kuiper belt terminates suddenly at a distance of {{convert|48|AU|e9km e9mi|abbr=unit}} from the Sun. Brunini and Melita have speculated that this sudden drop-off may be attributed to the presence of an object with a mass between those of Mars and Earth located beyond 48&nbsp;AU.<ref name="Brunini2002">{{Cite journal |last1=Brunini |first1=A |last2=M.D. Melita |name-list-style=amp |date=November 2002 |title=The Existence of a Planet beyond 50 AU and the Orbital Distribution of the Classical Edgeworth–Kuiper-Belt Objects |url=https://linkinghub.elsevier.com/retrieve/pii/S0019103502969356 |journal=Icarus |volume=160 |issue=1 |pages=32–43 |bibcode=2002Icar..160...32B |doi=10.1006/icar.2002.6935 |hdl-access=free |hdl=11336/37037}}</ref>

The presence of an object with a mass similar to that of Mars in a circular orbit at {{convert|60|AU|e9km e9mi|abbr=unit}} leads to a trans-Neptunian object population incompatible with observations. For instance, it would severely deplete the [[plutino]] population.<ref name=LykawaMukai/> Astronomers have not excluded the possibility of an object with a mass similar to that of Earth located farther than {{convert|100|AU|e9km e9mi|abbr=unit}} with an [[orbital eccentricity|eccentric]] and inclined orbit. Computer simulations by Patryk Lykawka of [[Kobe University]] have suggested that an object with a mass between {{earth mass|0.3~0.7|sym=y}} and diameter of 10,000 to 16,000 km, ejected outward by Neptune early in the Solar System's formation and currently in an elongated orbit between {{convert|101|and|200|AU|e9km e9mi|abbr=unit}} from the Sun, could explain the Kuiper cliff and the peculiar [[detached object]]s such as Sedna and {{mpl|2012 VP|113}}.<ref name=LykawaMukai /> In a 2023 follow-up paper, Lykawka gave his trans-Neptunian planet, which he dubbed the "Kuiper Belt planet", a mass of {{Earth mass|1.5 to 3|sym=y}}, a semi-major axis between 250 and 500 AU, and an orbital inclination of 30 degrees, but did not attribute the Kuiper cliff (or the putative orbital clustering attributed to Planet Nine) to its existence.<ref>{{Cite journal |last1=Lykawka |first1=Patryk Sofia |last2=Ito |first2=Takashi |date=August 2023 |title=Is There an Earth-like Planet in the Distant Kuiper Belt? |journal=The Astronomical Journal |language=en |volume=166 |issue=3 |pages=118 |doi=10.3847/1538-3881/aceaf0 |doi-access=free |arxiv=2308.13765 |bibcode=2023AJ....166..118L |issn=1538-3881}}</ref>

Although some astronomers, such as Renu Malhotra and David Jewitt, have cautiously supported these claims, others, such as Alessandro Morbidelli, have dismissed them as "contrived".<ref name="planetx">{{Cite news |last=Schilling |first=Govert |date=2008-01-11 |title=The mystery of Planet&nbsp;X |url=http://allesoversterrenkunde.nl/#!/actueel/artikelen/_detail/gli/the-mystery-of-planet-x/ |access-date=2008-06-25 |work=[[New Scientist]] |pages=30–33 |df=dmy-all}}</ref> Malhotra & Volk (2017)<ref name=ARX-20170619/> argued that an unexpected variance in inclination for KBOs farther than the cliff at {{convert|50|AU|e9km e9mi|abbr=unit}} provided evidence of a possible Mars-sized planet, possibly up to {{earth mass|2.4|sym=y}}, residing at the edge of the Solar System, which many news sources began referring to as "Planet&nbsp;Ten".<ref name=NSW-20170623>{{cite news |last=Osbourne |first=Hannah |date=23 June 2017 |title=Forget Planet&nbsp;9 - there's evidence of a tenth planet lurking at the edge of the solar system |magazine=[[Newsweek]] |url=http://www.newsweek.com/planet-10-lurking-edge-solar-system-628517 |access-date=23 June 2017 }}</ref><ref name=ARX-20170619>{{cite journal |last1=Volk |first1=Kathryn |last2=Malhotra |first2=Renu |year=2017 |title=The curiously warped mean plane of the Kuiper belt |journal=[[The Astronomical Journal]] |volume=154 |issue=2 |page=62 |bibcode=2017AJ....154...62V |s2cid=5756310 |arxiv=1704.02444 |doi=10.3847/1538-3881/aa79ff |doi-access=free }}</ref><ref>{{cite news |title=Planet 10? Another Earth-size world may lurk in the outer solar system |date=22 June 2017 |website=[[Space.com]]  |url=https://www.space.com/37295-possible-planet-10.html}}</ref><ref>{{cite web |url=https://www.science.org/content/article/forget-about-planet-nine-here-s-evidence-planet-10|title=Forget about Planet Nine. Here's evidence for Planet 10 |website=www.science.org}}</ref> Shortly after it was proposed, Lorenzo Iorio showed that the hypothetical planet's existence cannot be ruled out by [[Cassini (spacecraft)|Cassini]] ranging data.<ref>{{cite journal |last1=Iorio |first1=Lorenzo |year=2017 |title=Is the recently proposed Mars-sized perturber at 65–80&nbsp;{{sc|AU}} ruled out by the Cassini ranging data? |journal=Frontiers in Astronomy and Space Sciences |volume=4 |page=28 |doi=10.3389/fspas.2017.00028 |doi-access=free |arxiv=1407.5894 |bibcode=2017FrASS...4...28I |s2cid=26844167}}</ref>

Starting in 2018, several surveys have discovered multiple objects located beyond the Kuiper Cliff. Some of these new discoveries are close to the [[Heliopause (astronomy)|heliopause]] (120 AU) or well beyond it ({{mpl|2018 VG|18}}, {{mpl|2018 AG|37}}, {{mpl|2020 BE|102}}, {{mpl|2020 MK|53}}). An analysis of the TNO data available prior to September 2023 shows that there is a gap at about 72 AU, far from any mean-motion resonances with Neptune.<ref name="KuiperGap">{{Cite journal |last1=de La Fuente Marcos |first1=C. |last2=de La Fuente Marcos |first2=R. |date=October 2023 |title=Past the outer rim, into the unknown: structures beyond the Kuiper Cliff |journal=Monthly Notices of the Royal Astronomical Society: Letters |language=en |volume=527 |issue=1 |pages=L110–L114 |doi=10.1093/mnrasl/slad132 |doi-access=free |issn=1745-3925|arxiv=2309.03885 |bibcode=2024MNRAS.527L.110D }}</ref> Such a gap may have been induced by a massive perturber located further away.

=== Captured rogue planets ===
[[Rogue planets]] are planets not gravitationally bound to any star or stellar remnant. Simulations by A. Siraj in 2023, looking at the possibility of the Solar System having captured a rogue planet earlier in its history and based on a mass function of terrestrial-mass rogue planets, suggest that the expected number of captured rogue planets in the outer Solar System with masses greater than that of Mars (a tenth the mass of the Earth) is 1.2, while 2.7 with masses comparable to Mars and 5.2 with masses comparable to [[Mercury (planet)|Mercury]] (5.5% the mass of Earth) may exist. These captured planets would have a medium distance of 1,400 AU with roughly half of them existing in the range of 600 to 3,500 AU. Assuming these rogue planets have albedos of 0.2, it is expected that the [[Legacy Survey of Space and Time]] may discover 0.9 Mars-sized captured rogue planets and 1.4 Mercury-sized ones.<ref>{{Citation |last=Siraj |first=Amir |title=Are There Terrestrial Planets Lurking in the Outer Solar System? |journal=The Astrophysical Journal |date=2023-12-13 |volume=959 |issue=2 |pages=L17 |doi=10.3847/2041-8213/ad13eb |doi-access=free |url=http://arxiv.org/abs/2312.08431 |access-date=2025-06-07 |arxiv=2312.08431 |bibcode=2023ApJ...959L..17S }}</ref><ref>{{Cite web |last=Starr |first=Michelle |date=2024-01-10 |title=There Could Be Alien, Mars-Sized Planets Lurking Beyond Pluto |url=https://www.sciencealert.com/there-could-be-alien-mars-sized-planets-lurking-beyond-pluto |access-date=2025-06-07 |website=ScienceAlert |language=en-US}}</ref>

===Other proposed planets===
[[Tyche (hypothetical planet)|Tyche]] was a hypothetical [[gas giant]] proposed to be located in the [[Solar System]]'s [[Oort cloud]]. It was first proposed in 1999 by astrophysicists John Matese, Patrick Whitman and Daniel Whitmire of the [[University of Louisiana at Lafayette]].<ref name=Independent>{{Cite news|last = Rodgers|first = Paul|title = Up telescope! Search begins for giant new planet|newspaper = [[The Independent]]|date = February 13, 2011|url =https://www.independent.co.uk/news/science/up-telescope-search-begins-for-giant-new-planet-2213119.html|access-date = February 14, 2011 }}</ref> They argued that evidence of Tyche's existence could be seen in a supposed bias in the points of origin for [[long-period comet]]s. In 2013, Matese<ref>{{cite web|url=http://www.ucs.louisiana.edu/~jjm9638/ |title=Astrophysics Homepage of John J. Matese |publisher=Ucs.louisiana.edu |date=2011-09-21 |access-date=2013-04-01}}</ref> and Whitmire<ref>{{cite web|url=http://www.ucs.louisiana.edu/~dpw9254/ |title=Daniel P. Whitmire |publisher=Ucs.louisiana.edu |access-date=2013-04-01}}</ref> re-evaluated the comet data and noted that Tyche, if it existed, would be detectable in the archive of data that was collected by [[NASA]]'s [[Wide-field Infrared Survey Explorer]] (WISE) telescope.<ref name="WISE">{{Cite web |last=Clavin |first=Whitney |date=2011-02-18 |title=Can WISE Find the Hypothetical 'Tyche'? |url=https://www.jpl.nasa.gov/news/news.php?feature=2921 |access-date=2011-02-19 |publisher=NASA/JPL}}</ref> In 2014, NASA announced that the WISE survey had ruled out any object with Tyche's characteristics, indicating that Tyche as hypothesized by Matese, Whitman, and Whitmire does not exist.<ref name="NASA-20140307">{{Cite web |last1=Clavin |first1=Whitney |last2=Harrington |first2=J.D. |date=7 March 2014 |title=NASA's WISE Survey Finds Thousands of New Stars, But No 'Planet X' |url=http://www.jpl.nasa.gov/news/news.php?release=2014-075 |access-date=7 March 2014 |website=[[NASA]]}}</ref><ref name="MateseWhitmire2011">{{Cite journal | title=Persistent evidence of a jovian mass solar companion in the Oort cloud | last1=Matese|first1=John J.|last2=Whitmire|first2=Daniel P.| journal=[[Icarus (journal)|Icarus]] | date=2011 | volume=211 | issue=2 | pages=926–938 | doi=10.1016/j.icarus.2010.11.009|arxiv=1004.4584 | bibcode=2011Icar..211..926M| s2cid=44204219}}</ref><ref>{{cite news|last=Helhoski|first=Anna|title=News 02/16/11 Does the Solar System Have Giant New Planet?|url=http://norwalk.dailyvoice.com/news/does-solar-system-have-giant-new-planet|access-date=10 July 2012|newspaper=The Norwalk Daily Voice}}</ref>

Conversely, in 1999, British astronomer John Murray theorized the existence of a Jupiter-sized planet similar to Tyche 32,000 astronomical units away from the Sun in a retrograde orbit. Murray estimates that the planet would be located in the constellation of [[Delphinus]].<ref>{{Cite web |title=BBC News {{!}} SCI/TECH {{!}} A planet beyond Pluto |url=http://news.bbc.co.uk/2/hi/science/nature/467572.stm |access-date=2024-12-16 |website=news.bbc.co.uk}}</ref> These parameters, also based on the orbits of various long-period comets, are different from those originally hypothesized by Matese, Whitman, and Whitmire for Tyche, and hence signify a different object.<ref>{{Cite web |date=2005-01-27 |title=A mystery revolves around the sun |url=https://www.nbcnews.com/id/wbna3077838 |access-date=2024-12-16 |website=NBC News |language=en}}</ref> Unlike Tyche, this putative planet lies outside the 26,000 AU limit set by mid-infrared observations by the [[Wide-field Infrared Survey Explorer|''WISE'' telescope]], but this limit can be as high as 82,000 AU based on [[albedo]]. A brown dwarf, for instance, would have a smaller albedo than a Jupiter analog.<ref name="Luhman2014" />

The oligarch theory of [[Nebular hypothesis#Formation of planets|planet formation]] states that there were hundreds of planet-sized objects, known as oligarchs, in the early stages of the Solar System's evolution. In 2005, astronomer Eugene Chiang speculated that although some of these oligarchs became the planets we know today, most would have been flung outward by gravitational interactions. Some may have escaped the Solar System altogether to become [[rogue planet|free-floating planets]], whereas others would be orbiting in a halo around the Solar System, with orbital periods of millions of years. This halo would lie at between {{convert|1,000|and|10,000|AU|e9km e9mi|abbr=unit}} from the Sun, or between a third and a thirtieth the distance to the [[Oort cloud]].<ref>''New Scientist'', 23 July 2005, issue 2509, [https://www.newscientist.com/article/mg18725091.500-farout-worlds-just-waiting-to-be-found.html Far-out worlds, just waiting to be found]</ref>

In December 2015, astronomers at the [[Atacama Large Millimeter Array]] (ALMA) detected a brief series of 350&nbsp;GHz pulses that they concluded must either be a series of independent sources, or a single, fast moving source. Deciding that the latter was the most likely, they calculated based on its speed that, were it bound to the Sun, the object, which they named "Gna" after a [[Gná and Hófvarpnir|fast-moving messenger goddess]] in Norse mythology,<ref name="sciam">{{Cite magazine |last=Billings |first=Lee |year=2015 |title=Astronomers Skeptical Over "Planet X" Claims |url=http://www.scientificamerican.com/article/astronomers-skeptical-over-planet-x-claims/ |access-date=2016-01-22 |magazine=[[Scientific American]]}}</ref> would be about 12–25 AU distant and have a [[dwarf planet]]-sized diameter of 220 to 880&nbsp;km. However, if it were a [[rogue planet]] not gravitationally bound to the Sun, and as far away as 4000 AU, it could be much larger.<ref name="alma">{{Cite arXiv |eprint=1512.02650 |class=astro-ph.SR |first1=Wouter |last1=Vlemmings |first2=S. |last2=Ramstedt |title=The serendipitous discovery of a possible new solar system object with ALMA |date=8 December 2015 |first3=M. |last3=Maercker |first4=B. |last4=Davidsson}}</ref> The paper was never formally accepted, and has been withdrawn until the detection is confirmed.<ref name=alma/> Scientists' reactions to the notice were largely sceptical; Mike Brown commented that, "If it is true that ALMA accidentally discovered a massive outer Solar System object in its tiny, tiny, tiny, field of view, that would suggest that there are something like 200,000 Earth-sized planets in the outer Solar System ... Even better, I just realized that this many Earth-sized planets existing would destabilize the entire Solar System and we would all die."<ref name=sciam/>

==Constraints on additional planets==
As of 2023 the following observations severely constrain the mass and distance of any possible additional Solar System planet:
* An analysis of mid-infrared observations with the [[Wide-field Infrared Survey Explorer|''WISE'' telescope]] have ruled out the possibility of a [[Saturn]]-sized object (95 Earth masses) out to 10,000 [[astronomical unit|AU]], and a Jupiter-sized or larger object out to 26,000 AU.<ref name="Luhman2014"/> ''WISE'' has continued to take more data since then, and NASA has invited the public to help search this data for evidence of planets beyond these limits, via the [[Backyard Worlds|Backyard Worlds: Planet 9]] citizen science project.<ref name="CNN-NASA-Search-Worlds">{{cite news|title=NASA wants you to help find a new planet|url=https://edition.cnn.com/2017/02/16/us/nasa-search-worlds-trnd/index.html|work=[[CNN]]|date=16 February 2017|access-date=11 March 2018}}</ref>
* Using modern data on the [[Anomalous perihelion precession|anomalous precession]] of the perihelia of Saturn, Earth, and Mars, Lorenzo Iorio concluded that any unknown planet with a mass of 0.7 times that of Earth must be farther than 350–400&nbsp;AU; one with a mass of 2 times that of Earth, farther than 496–570&nbsp;AU; and finally one with a mass of 15 times that of Earth, farther than 970–1,111&nbsp;AU.<ref name="Iorio2014">{{Cite journal |last=Iorio |first=L. |year=2014 |title=Planet X revamped after the discovery of the Sedna-like object 2012 VP113? |journal=Monthly Notices of the Royal Astronomical Society: Letters |language=en |volume=444 |issue=1 |pages=L78–L79 |arxiv=1404.0258 |bibcode=2014MNRAS.444L..78I |doi=10.1093/mnrasl/slu116 |issn=1745-3933 |s2cid=118554088 |doi-access=free}}</ref> Moreover, Iorio stated that the modern ephemerides of the Solar System outer planets has provided even tighter constraints: no celestial body with a mass of 15 times that of Earth can exist closer than 1,100–1,300&nbsp;AU.<ref name=Iorio2014-2>{{cite journal |arxiv=1407.5894|doi=10.3389/fspas.2017.00028|title=Is the Recently Proposed Mars-Sized Perturber at 65–80 AU Ruled Out by the Cassini Ranging Data?|journal=Frontiers in Astronomy and Space Sciences|volume=4|year=2017|last1=Iorio|first1=Lorenzo|page=28|bibcode=2017FrASS...4...28I|s2cid=26844167|doi-access=free}}</ref>  However, work by another group of astronomers using a more comprehensive model of the Solar System found that Iorio's conclusion was only partially correct. Their analysis of [[Cassini–Huygens|''Cassini'']] data on Saturn's orbital residuals found that observations were inconsistent with a planetary body with the orbit and mass similar to those of Batygin and Brown's Planet Nine having a [[true anomaly]] of −130° to −110°, or −65° to 85°. Furthermore, the analysis found that Saturn's orbit is slightly better explained if such a body is located at a true anomaly of {{val|117.8|+11|-10|u=deg}}. At this location, it would be approximately 630 AU from the Sun.<ref>{{cite journal |title=Constraints on the location of a possible 9th planet derived from the Cassini data |journal=[[Astronomy and Astrophysics]] |arxiv=1602.06116 |first1=A |last1=Fienga |first2=J |last2=Laskar |first3=H |last3=Manche |first4=M |last4=Gastineau |date=23 February 2016 |doi=10.1051/0004-6361/201628227 |volume=587 |page=L8 |bibcode=2016A&A...587L...8F|s2cid=119116589 }}</ref>
* Using public data on the orbits of the [[extreme trans-Neptunian object]]s, it has been confirmed that a statistically significant (62σ) asymmetry between the shortest mutual ascending and descending nodal distances does exist; in addition, multiple highly improbably (p < 0.0002) correlated pairs of orbits with mutual nodal distances as low as 0.2&nbsp;AU at 152&nbsp;AU from the Solar System's barycentre or 1.3&nbsp;AU at 339&nbsp;AU have been found.<ref name="de la Fuente Marcos">{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |title=Twisted extreme trans-Neptunian orbital parameter space: statistically significant asymmetries confirmed |journal=Monthly Notices of the Royal Astronomical Society Letters |url=https://academic.oup.com/mnrasl/article-abstract/512/1/L6/6524836 |volume=512 |issue=1 |pages=L6–L10 |arxiv=2202.01693 |bibcode=2022MNRAS.512L...6D |doi=10.1093/mnrasl/slac012 |date=1 May 2022|doi-access=free }}</ref> Both findings suggest that massive perturbers may exist at hundreds of AUs from the Sun and are difficult to explain within the context of a uniform distribution of orbital orientations in the outermost Solar System.<ref name="Napier_etal_2021">{{cite journal |last1=Napier |first1=J. K. |display-authors=etal |title=No Evidence for Orbital Clustering in the Extreme Trans-Neptunian Objects |journal=The Planetary Science Journal |year=2021 |volume=2 |issue=2 |page=59 |doi=10.3847/PSJ/abe53e |arxiv=2102.05601|bibcode=2021PSJ.....2...59N |s2cid=231861808 |doi-access=free }}</ref>

== See also ==
* [[Fictional planets of the Solar System]]
* [[List of hypothetical Solar System objects]]

=== Survey telescopes ===
* [[Vera C. Rubin Observatory|Large Synoptic Survey Telescope]] (LSST)
* [[Pan-STARRS]]
* [[Wide-field Infrared Survey Explorer]] (WISE)

== References ==
{{reflist|refs=

<ref name=P9H2019>{{cite journal |last1=Batygin |first1=Konstantin |last2=Adams |first2=Fred C. |last3=Brown |first3=Michael E. |last4=Becker |first4=Juliette C. |title=The Planet Nine Hypothesis |year=2019 |journal=[[Physics Reports]] |volume=805 |pages=1–53 |doi=10.1016/j.physrep.2019.01.009 |arxiv=1902.10103 |bibcode=2019PhR...805....1B |s2cid=119248548}}</ref>

}}

== Bibliography ==
* {{anchor |CITEREFCroswell1997}}{{Cite book |last=Croswell |first=Ken |url=https://books.google.com/books?id=60sPD6yjbVAC |title=Planet quest: the epic discovery of alien solar systems |date=1997 |publisher=Free Press |isbn=978-0-684-83252-4 |location=New York}}
* {{anchor |CITEREFLittman1990}}{{Cite book |last=Littmann |first=Mark |title=Planets beyond: discovering the outer solar system |date=1990 |publisher=Wiley |isbn=978-0-471-51053-6 |series=The Wiley science editions |location=New York}}
* {{anchor |CITEREFSchilling2009}}{{Cite book |last=Schilling |first=Govert |title=The hunt for Planet X: new worlds and the fate of Pluto |date=2009 |publisher=Copernicus Books |isbn=978-0-387-77804-4 |location=New York, NY}}
* {{anchor |CITEREFTombaugh1946}}{{Cite journal |last=Tombaugh |first=Clyde W. |date=1946 |title=The Search for the Ninth Planet, Pluto |journal=Astronomical Society of the Pacific Leaflets |volume=5 |issue=209 |pages=73–80 |bibcode=1946ASPL....5...73T}}

== Further reading ==
* {{Cite AV media |url=https://www.youtube.com/watch?v=xGfv3Oay_pY |title=Planet X Discovered?? |date=2016-02-17 |publisher=[[PBS Space Time]] |via=[[YouTube]]}}
* [http://www.nineplanets.org/hypo.html#planetx SEDS on Planet X]
* {{Cite journal |last=Standish |first=E. M. |date=May 1993 |title=Planet X - No dynamical evidence in the optical observations |journal=The Astronomical Journal |volume=105 |issue=5 |pages=2000 |bibcode=1993AJ....105.2000S |doi=10.1086/116575}}
* {{Cite journal |last=Quinlan |first=Gerald D. |date=May 1993 |title=Planet X: A Myth Exposed |journal=Nature |language=en |volume=363 |issue=6424 |pages=18 |bibcode=1993Natur.363...18Q |doi=10.1038/363018b0 |issn=0028-0836 |s2cid=29058579 |doi-access=free}}
* {{Cite journal |last1=Whitmire |first1=Daniel P. |last2=Matese |first2=John J. |name-list-style=amp |date=January 1985 |title=Periodic comet showers and planet X |journal=Nature |language=en |volume=313 |issue=5997 |pages=36–38 |bibcode=1985Natur.313...36W |doi=10.1038/313036a0 |issn=0028-0836 |s2cid=7658694}}
* {{Cite web |last1=Jones |first1=R. L. |last2=Brown |first2=M. E. |last3=Abell |first3=P. A. |last4=Becker |first4=A. C. |last5=Harris |first5=A. |last6=Ivezic |first6=Z. |display-authors=4 |date=2009 |title=Next steps in understanding the outer solar system: A whitepaper submitted to the 2010 Decadal Survey Committee |url=http://www8.nationalacademies.org/astro2010/DetailFileDisplay.aspx?id=288 |access-date=2009-04-10 |publisher=Submission to the National Academy of Sciences Board on Physics and Astronomy |format=PDF}}
* Andrew Coates provides a simplified summary of the [http://theconversation.com/claims-about-new-planets-that-turned-out-to-be-wrong-and-why-planet-nine-may-be-different-53573 history behind the search & claims] for [[Planet X]]
* {{cite journal|arxiv=2306.11109 |doi=10.1093/mnrasl/slad079 |title=Oort cloud (Exo)planets |date=2023 |last1=Raymond |first1=Sean N. |last2=Izidoro |first2=Andre |last3=Kaib |first3=Nathan A. |journal=Monthly Notices of the Royal Astronomical Society: Letters |volume=524 |pages=L72–L77 |doi-access=free }}

{{Pluto}}
{{Solar System}}
{{Portal bar|Physics|Stars|Spaceflight|Outer space|Science}}
{{featured article}}

{{DEFAULTSORT:Planets Beyond Neptune}}
[[Category:Astronomical controversies]]
[[Category:Hypothetical bodies of the Solar System|X]]
[[Category:Hypothetical planets]]
[[Category:Hypothetical trans-Neptunian objects]]
[[Category:Early scientific cosmologies]]
[[Category:Pluto's planethood]]
[[Category:Solar System dynamic theories]]
[[Category:Trans-Neptunian objects]]
[[Category:Planets of the Solar System]]
[[Category:Solar System]]
[[Category:Hypothetical astronomical objects]]