Editing Planets beyond Neptune (section)

{{Short description|Hypothetical planets further than Neptune}}
{{about|hypothetical planets|known objects beyond Neptune|Trans-Neptunian object}}
{{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>

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