Planets beyond Neptune
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{{Redirect|Tenth planet|the Doctor Who serial|The Tenth Planet}}
{{Redirect6|Planet X|the "Planet X" 2003 / 2012 doomsday scare|ZetaTalk}}
[[Image:Percival Lowell-observing Mars from the Lowell Observatory.jpg|thumb|right|250px|[[Percival Lowell]], originator of the Planet X hypothesis]]
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Since the discovery of the [[planet]] [[Neptune]] in 1846, there had been considerable speculation that a '''ninth planet''' might exist beyond its orbit. The search began in the early 20th century with [[Percival Lowell]]'s quest for '''Planet X'''. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the [[orbit]]s of the gas giants, particularly of [[Uranus]] and [[Neptune]],<ref name=bower/> speculating that the gravity of a large unseen planet could have perturbed it enough to account for the irregularities.<ref name=tombaugh>{{cite journal|title=The Search for the Ninth Planet, Pluto|author=CW Tombaugh|year=1946|journal=Astronomical Society of the Pacific Leaflets|volume= 5|pages=73-80|url=http://articles.adsabs.harvard.edu/full/1946ASPL....5...73T|accessdate=2008-07-13}}</ref>
[[Clyde Tombaugh]]'s discovery of [[Pluto]] in 1930 initially appeared to validate Lowell's hypothesis, and Pluto was considered the ninth planet until 2006. However, in 1978, Pluto was found to be too small for its gravity to affect the gas giants, resulting in the 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 an incorrect measurement of Neptune's mass.<ref name="standage">{{cite book | title=The Neptune File: A Story of Astronomical Rivalry and the Pioneers of Planet Hunting | author=Tom Standage | pages=p. 188 | isbn=0802713637 | year=2000 | publisher=Walker}}</ref> After 1992, the discovery of numerous small icy [[Trans-Neptunian object|objects within or near Pluto's orbit]] led to a debate over whether Pluto should remain a planet, or whether it and its neighbours should, like the [[asteroid]]s, be given their own separate classification. Although many of the larger members of this group were initially described as planets, in 2006 the [[International Astronomical Union]] reclassified Pluto and its largest neighbours as [[dwarf planet]]s, leaving only eight planets 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=IAU|date=2006-08-24|publisher=International Astronomical Union}}</ref>
Today, the astronomical community widely agrees that Planet X, as originally envisioned, does not exist. However, the concept of Planet X has been revived by a number of astronomers to explain other anomalies observed in the outer [[Solar System]]. In popular culture, and even among some astronomers,<ref name=varuna>{{cite journal|title=Almost Planet X|journal=Nature|volume=411|pages=423–424 |date=2001-05-24 | doi=10.1038/35078164|author=S. C. Tegler & W. Romanishin|url=http://www.nature.com/nature/journal/v411/n6836/full/411423a0.html|accessdate=2008-06-26}}</ref> "Planet X" has become a stand-in term for any undiscovered planet in the outer Solar System, regardless of its gravitational effect. Other trans-Neptunian planets have also been suggested, based on different evidence.
==Planet X==
=== Origin of the search ===
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{{seealso|Discovery of Neptune}}
In the 1840s, using [[Newtonian mechanics]], [[Urbain Le Verrier]] predicted the position of the then-undiscovered planet [[Neptune]] after analysing perturbations in the orbit of [[Uranus]]. Hypothesising that the perturbations were caused by the gravitational pull of another planet, Le Verrier sent his calculations to German astronomer [[Johann Gottfried Galle]]. On [[September 23]], [[1846]], the night following his receipt of the letter, Galle and his student [[Heinrich d'Arrest]] found [[Neptune]] exactly where Le Verrier had predicted.<ref name=cros2>{{cite book
| title = Planet Quest: The Epic Discovery of Alien Solar Systems
| author = Ken Croswell
| publisher = The Free Press
| year = 1997
| pages = 32–55
| id = ISBN 978-0684832524}}</ref>
After the discovery of Neptune, however, some slight discrepancies remained in the gas giants' orbits. These were taken to indicate the existence of yet another planet orbiting beyond Neptune. In 1906, [[Percival Lowell]], a wealthy Bostonian who had founded the [[Lowell Observatory]] in [[Flagstaff, Arizona]] in 1894, started an extensive project in search of a possible ninth planet,<ref name="pluto guide">{{cite web
| url = http://www.space.com/spacewatch/050311_pluto_guide.html
| title = Finding Pluto: Tough Task, Even 75 Years Later
| author = J. Rao
| publisher = SPACE.com
| date=2005-03-11
| accessdate=2006-09-08
}}</ref><ref name=tombaugh/> which he termed "Planet X". The ''"X"'' in the name represents an unknown and is pronounced as the letter, as opposed to the [[Roman numeral]] for ten, as it would not, at the time, have been the tenth planet. Lowell's hope in tracking down Planet X was to establish his scientific credibility, which had been dented by his widely derided belief that channel-like features visible on the surface of [[Mars]] were in fact [[Martian canal|canals constructed by an intelligent civilisation]].<ref name=cros2/>
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, Lowell manually examined over 200 three-hour exposures with a magnifying glass. However, no planet was found (Pluto was at that time too far above the ecliptic to be imaged by the survey).<ref name=tombaugh/> After revising his prediction for where it should be found, Lowell conducted a second search from 1913 to 1915. In that year, Lowell published his ''Memoir of a Trans-Neptunian Planet'', in which he concluded that Planet X had a mass roughly 7 times that of the Earth (or half that of Neptune) and a mean distance from the Sun of 43 AU. Lowell assumed that, like the gas giants, Planet X would have a low density (meaning large size) and a high [[albedo]]. As the result it would show a disc with diameter of about one arcsecond and an [[apparent magnitude]] of between 12 and 13—bright enough to be spotted.<ref name=tombaugh/><ref>{{cite book | title=Planets Beyond: Discovering the Outer Solar System | author=Mark Littman | year=1990 | pages=70 | isbn=047151053X | publisher=Wiley }}</ref>
=== Discovery of Pluto ===
{{main|Pluto}}
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[[Image:ClydeTombaugh.jpg|thumb|right|200 px|Clyde William Tombaugh]]
Lowell's sudden death in 1916 temporarily halted the search for Planet X. His failure to find the planet, according to one friend, "virtually killed him".<ref name=cros2/> Constance Lowell, Percival Lowell's widow, subsequently embroiled the observatory in a long legal battle to secure its million-dollar portion of Lowell's legacy for herself, which meant that its search for Planet X could not resume for several years.<ref name=cros2/> In 1925, the observatory obtained the glass discs for a 13-inch wide-field telescope to continue the search, constructed with funds from [[George Lowell]], 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 taken two weeks apart. He then placed the two images of each section in a machine called a [[blink comparator]], which created a [[time lapse]] illusion which exposed the movement of any planetary bodies. 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 also decided to image the entire zodiac, rather than focus on those regions suggested by Lowell.<ref name=tombaugh/>
[[Image:Pluto discovery plates.png|thumb|left|300 px|Discovery photographs of Pluto]]
By the beginning of 1930, Tombaugh's search had reached the constellation of Gemini.<ref name=tombaugh/> On [[February 18]], [[1930]], after searching for nearly a year and examining nearly 2 million stars,<ref name=tombaugh/> Tombaugh discovered a moving object on photographic plates taken on [[January 23]] and [[January 29]] of that year. 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 X."<ref name=cros2/> The object lay just six degrees from one of two locations for Planet 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 would later be found on photographs dating back to [[March 19]], [[1915]].<ref name="Hoyt">{{cite journal
| title = W. H. Pickering's Planetary Predictions and the Discovery of Pluto
| author = W. G. Hoyt
| journal = Isis
| volume = 67
| issue = 4
| year = 1976
| pages = 551–564.
| url = http://links.jstor.org/sici?sici=0021-1753(197612)67%3A4%3C551%3AWHPPPA%3E2.0.CO%3B2-I
| accessdate = 2007-06-27
| doi = 10.1086/351668
}}</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
| accessdate =2007-03-25 }}</ref>
===Pluto loses Planet X title===
To the observatory's disappointment and surprise, Pluto showed no visible disc; it appeared fuzzy and indistinct, 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/> Since astronomers thought Pluto was massive enough to [[Perturbation (astronomy)|perturb]] planets, they assumed that it should have an [[Albedo#Astronomical_albedo|albedo]] of 0.07; about as dark as asphalt and similar to that of [[Mercury (planet)|Mercury]], the least reflective planet known.<ref name=bower>{{cite web|title=Bulletin Number 437 - On the orbit and mass of Pluto with an ephemeris for 1931-1932|author=Ernest Clare Bower|year=1030|journal=Lick Observatory Bulletin|volume=15|pages=171-178|url=http://adsabs.harvard.edu/full/1930LicOB..15..171B|accessdate=2008-07-14}}</ref> This would give Pluto an assumed diameter of about 8000 km.<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) |author=Dan Bruton |url=http://www.physics.sfasu.edu/astro/asteroids/sizemagnitude.html |accessdate=2008-06-16}} (Pluto: H=−1 with Albedo=0.07 would be 7961km diameter)</ref>
Throughout the mid-20th century, estimates of Pluto's mass were revised continuously downward. In 1931, Nicholson and Mayall calculated its mass, based on its supposed effect on the gas giants, as roughly that of the Earth,<ref>{{cite journal|title= Council note on the discovery of Pluto|journal=Monthly Notices of the Royal Astronomical Society|volume=91|pages380-386|url=http://articles.adsabs.harvard.edu/full/seri/MNRAS/0091/0000385.000.html|accessdate=2008-07-13}}</ref> while in 1949, measurements of Pluto's diameter concluded that it was midway in size between Mercury and Mars and that its mass was most probably about 0.1 Earth mass.<ref>{{cite journal|title=The Diameter of Pluto|author=Gerard Kuiper|journal=Publications of the Astronomical Society of the Pacific|volume=62|number=366|pages=133-137|year=1950|url=http://articles.adsabs.harvard.edu/full/1950PASP...62..133K
|accessdate=2008-07-13}}</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]] [[volatiles|ice]], which is highly reflective. This meant that Pluto, far from being dark, must in fact be very bright, and thus could not have been more than 0.01 Earth mass.<ref name=cros />
In 1978, [[James Christy]] (of the Lowell Observatory, as it happened) discovered Pluto's moon [[Charon (moon)|Charon]]. Christy and Robert Harrington's observations of the moon's speed around Pluto allowed the direct measurement of the mass of the Pluto-Charon system for the first time. Pluto's mass was determined to be roughly 0.002 that of the Earth;<ref name=Christy>{{cite journal | bibcode=1978AJ.....83.1005C | author = J. W. Christy; R. S. Harrington | title = The satellite of Pluto | journal = Astronomical Journal | year = 1978 | volume = 83 | pages = 1005 | url = http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1978AJ.....83.1005C&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf | format=PDF }}</ref> far too small to account for the observed discrepancies. Lowell's "prediction" had been a coincidence; if there was a Planet X, it was not Pluto.<ref name=cros />
=== Further searches for Planet(s) X ===
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After discovering Pluto, Tombaugh continued to search the [[ecliptic]] for other distant objects. He found hundreds of [[Clyde Tombaugh#Asteroids discovered|asteroids]] and [[variable star]]s, as well as two [[comet]]s, but no more planets.<ref>{{cite web|title=Clyde W. Tombaugh|work=New Mexico Museum of Space History |url=http://www.nmspacemuseum.org/halloffame/detail.php?id=51|accessdate=2008-06-29}}</ref> However, a number of astronomers kept up the search for Lowell's Planet X, convinced that, since Pluto was no longer a viable candidate, an unseen tenth planet must be perturbing the outer planets.<ref name=cros />
In the 1980s and 1990s, astronomer [[Robert Sutton Harrington]] of the [[US Naval Observatory]], who had first calculated that Pluto was too small to be Planet X, led a search to determine the real cause of the apparent irregularities.<ref name=cros /> He calculated that any Planet X would be at roughly three times the distance from the sun of Neptune's orbit, highly [[elliptic]]al, and far below the [[ecliptic]] (the planet's orbit would be at roughly a 90-degree angle from the orbital plane of the other known planets).<ref>{{cite journal | title = Planet X – The current status | author = P. K. Seidelmann, R. S. Harrington | journal = Journa Celestial Mechanics and Dynamical Astronomy | volume = 43 | year = 1987
| pages = 55 | doi = 10.1007/BF01234554}}</ref> This hypothesis was met with a mixed reception. Noted Planet X sceptic [[Brian Marsden]] of Harvard University's [[Minor Planet Center]] pointed out that these discrepancies were a hundred times smaller than those noticed by Le Verrier, and could easily be due to observational error.<ref name=cros>Croswell pp. 56–71</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 that is in a [[Retrograde and direct motion|retrograde]] orbit around the Sun. However, both Marsden and Planet 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 X would have severely affected the orbits of the outer planets.<ref name=cros />
While its mission did not involve a search for "Planet 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|url=http://spider.ipac.caltech.edu/staff/tchester/iras/washington_post_mystery_object.html|title=Mystery Heavenly Body Discovered|publisher=Washington Post|publisherlink=Washington Post|accessdate=2008-01-28|author=Thomas O'Toole|page=A1|date=1983-12-30 }}</ref> However, further analysis revealed that of several unidentified objects, nine were distant galaxies and the tenth was "[[infrared cirrus|intergalactic cirrus]]"; none were found to be Solar System bodies.<ref>{{cite journal|title=Unidentified IRAS sources: Ultra-High Luminosity Galaxies|author= JR Houck, DP Schneider, DE Danielson et. al.|journal=The Astrophysical Journal|volume=290|year=1985|url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1985ApJ...290L...5H&db_key=AST&high=3ccf23290006822|pages=5-8|doi=10.1086/184431|accessdate=2008-07-14}}</ref>
=== Planet X disproved ===
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Harrington died in 1993, having never found Planet X.<ref name=cros /> That same year, Myles Standish used data from ''[[Voyager 2]]'''s 1989 flyby of [[Neptune]], which had revised the planet's total mass downward by 0.5 percent (an amount comparable to the mass of [[Mars]]),<ref name=cros /> to recalculate its gravitational effect on Uranus.<ref>{{cite news | title=Hopes Fade in hunt for Planet X | author=Ken Croswell | date=1993-01-30 | url=http://kencroswell.com/HopesFadeInHuntForPlanetX.html |accessdate=2007-11-04 | work=New Scientist | page= 18 }}</ref> When the newly determined mass was used in the [[Jet Propulsion Laboratory Developmental Ephemeris]] (JPL DE), the supposed discrepancies in its orbit vanished.<ref name="standage"/> Also, to date there are no discrepancies in the trajectories of any space probes (''[[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>Littmann, p. 204</ref> Today the overwhelming consensus among astronomers is that Planet X, as Lowell defined it, does not exist.
==Observed trans-Neptunian objects==
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{{main|trans-Neptunian objects}}
<imagemap>
Image:EightTNOs.png|thumb|410px|right|Sedna compared with Eris, Pluto, Makemake, (136108) 2003 EL<sub>61</sub>, Varuna, Orcus, Quaoar, and Earth
#Earth
rect 646 1714 2142 1994 [[Earth|The Earth]]
#Eris and Dysnomia
circle 226 412 16 [[Dysnomia (moon)|Dysnomia]]
circle 350 626 197 [[Eris (dwarf planet)|(136199) Eris]]
#Pluto and Charon
circle 1252 684 86 [[Charon (moon)|Charon]]
circle 1038 632 188 [[Pluto|(134340) Pluto]]
#2005 FY9
circle 1786 614 142 [[Makemake (dwarf planet)|Makemake]]
#2003 EL61
circle 2438 616 155 [[(136108) 2003 EL61]]
#Sedna
circle 342 1305 137 [[90377 Sedna|(90377) Sedna]]
#Orcus
circle 1088 1305 114 [[90482 Orcus|(90482) Orcus]]
#Quaoar
circle 1784 1305 97 [[50000 Quaoar|(50000) Quaoar]]
#Varuna
circle 2420 1305 58 [[20000 Varuna|(20000) Varuna]]
desc none
# - setting this to "bottom-right" will display a (rather large) icon linking to the graphic, if desired
#Notes:
#Details on the new coding for clickable images is here: [[mw:Extension:ImageMap]]
#While it may look strange, it's important to keep the codes for a particular system in order. The clickable coding treats the first object created in an area as the one on top.
#Moons should be placed on "top" so that their smaller circles won't disappear "under" their respective primaries.
</imagemap>
After Pluto and [[Charon (moon)|Charon]], no more [[trans-Neptunian object]]s were found until the discovery of {{mpl|(15760) 1992 QB|1}} in 1992.<ref>{{cite web|author=Minor Planet Center|title=Circular No. 5611|year=1992|url=http://cfa-www.harvard.edu/iau/iauc/05600/05611.html|accessdate=2008-07-02}}</ref> Since that time, hundreds of trans-Neptunian objects have been discovered. The objects are now recognised as mostly belonging to the [[Kuiper belt]]: a swarm of icy bodies orbiting in the [[ecliptic plane]] beyond Neptune, which are left over from the formation of the Solar System. Some of these distant trans-Neptunian objects, such as [[90377 Sedna|Sedna]], were initially described in the media as "the tenth planet", though none were as large as Pluto.<ref>{{cite web|title=Astronomers discover 'new planet'|work=BBC News|year=2004|url=http://news.bbc.co.uk/1/hi/sci/tech/3511678.stm|accessdate=2008-06-20}}</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),<ref>{{cite web|title=Circular No. 8747|author=Central Bureau for Astronomical Telegrams, International Astronomical Union|url=http://www.cfa.harvard.edu/iau/special/08747.pdf|year=2006|accessdate=2007-02-23}}</ref> a trans-Neptunian object larger than Pluto. Soon after the announcement, NASA's [[Jet Propulsion Laboratory]] put out a press release describing the object as the "tenth planet".<ref>{{cite web|title=NASA-Funded Scientists Discover Tenth Planet|work=Jet Propulsion Laboratory|url=http://www.jpl.nasa.gov/news/news-print.cfm?release=2005-126| year=2005|accessdate=2007-02-22}}</ref> However, Eris is no longer considered a planet. The [[2006 definition of planet]] redefined both Eris and Pluto as [[dwarf planet]]s because they have not [[cleared the neighborhood]] of their orbits.<ref name=IAU/> In other words, they did 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 second largest dwarf planet after Eris. A number of astronomers, most notably [[Alan Stern]], head of NASA's [[New Horizons]] mission to Pluto, contend that the IAU's definition is flawed, and that not only Pluto and Eris, but all large trans-Neptunian objects, such as {{Dp|Makemake}}, {{Dp|Sedna}}, {{Dp|Quaoar}} and {{Dp|Varuna}}, should be considered planets in their own right.<ref>{{cite web|title=Unabashedly Onward to the Ninth Planet|author=Alan Stern|year=2006|work=NASA|url=http://www.spacedaily.com/reports/Unabashedly_Onward_to_the_Ninth_Planet_999.html
|accessdate=2008-06-25}}</ref> Regardless, Eris is not Planet X, as it is far too small to have significantly affected the orbits of the outer planets.<ref>{{cite web|title=David Jewitt:Planet X|author=David Jewitt (University of Hawaii) | work=Personal web site |year=2006|url=http://www.ifa.hawaii.edu/~jewitt/kb/planetx.html|accessdate=2008-05-21}}</ref>
== Subsequent proposed trans-Neptunian planets ==
{{seealso|Kuiper belt#"Kuiper cliff"|l1=The "Kuiper cliff"}}
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Though it is unlikely that Lowell's Planet X exists, the idea that a large unseen planet could create observable gravitational effects in the outer Solar System has been revived by a number of astronomers. These hypothetical objects are often referred to as "Planet X", although their link to that world is purely conceptual.<ref>{{cite journal|journal=Monthly Notices of the Royal Astronomical Society|volume=335|issue=3|pages=641–654|date= September 2002|title=Biases in cometary catalogues and Planet X|author=J. Horner and N. W. Evans|url=http://www.blackwell-synergy.com/doi/abs/10.1046/j.1365-8711.2002.05649.x|doi=10.1046/j.1365-8711.2002.05649.x|
work=Oxford University|accessdate=2008-06-27}}</ref><ref name=planetx /> The Kuiper belt terminates suddenly at a distance of 48 [[astronomical units]] (AU) from the Sun (for comparison, Neptune lies 30 AU from the Sun),<ref name="neptune_fact">{{cite web | url = http://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html | title = Neptune Fact Sheet | date =2004-09-01 | publisher = NASA | accessdate = 2007-08-14 | author=David R. Williams}}</ref> and there is some speculation this sudden drop-off, known as the "Kuiper cliff", may be caused by the presence of an object with a mass between that of Mars and Earth located beyond 48 AU.<ref name=Brunini2002>{{cite journal|author=A. Brunini and M.D. Melita|title=The Existence of a Planet beyond 50 AU and the Orbital Distribution of the Classical Edgeworth–Kuiper-Belt Objects|journal=Icarus|year=2002|volume=160|pages=32–43|doi=10.1006/icar.2002.6935| url=http://adsabs.harvard.edu/abs/2002Icar..160...32B}}</ref> However, the possibility of a Mars-like planetoid at 60 AU can be safely excluded; such an object would be so bright that it would have been discovered by now.<ref name=lykawka/><ref name=Brunini2002/> In addition, its influence leads to a TNO population incompatible with observations. For instance, it would severally deplete the [[plutino]] population.<ref name=lykawka/> However, astronomers have not excluded the possibility of a more massive Earth-like planetoid located further than 100 AU with an eccentric and inclined orbit. Computer simulations have suggested that a body roughly the size of Earth, ejected outward by Neptune early in the Solar System's formation and currently in an elongated orbit between 80 and 170 AU from the Sun, could explain not only the Kuiper cliff but also the peculiar "detached" [[TNO]]s such as Sedna.<ref name=lykawka>{{cite journal | author= P. S. Lykawka; T. Mukai | title=An Outer Planet Beyond Pluto and the Origin of the Trans-Neptunian Belt Architecture | journal=Astronomical Journal | year = 2008 | volume=135 | pages=1161 | doi=10.1088/0004-6256/135/4/1161 | url=http://arxiv.org/pdf/0712.2198| id={{arXiv|0712.2198}} }}</ref> While some astronomers have cautiously supported these claims, others have dismissed them as "contrived".<ref name=planetx>{{cite news | work=New Scientist | url=http://space.newscientist.com/article/mg19726381.600-the-mystery-of-planet-x.html | date=2008-01-11 | title=The Mystery of Planet X | pages=30–33 | accessdate=2008-06-25 | author=Govert Schilling | format=subscription required }}</ref>
There is, however, a different hypothesis, which tries to explain some percularities in the distribution of the [[long period comet]]s. According to it, comets originating from a specific region of the sky were disturbed by an unseen object at least as large as [[Jupiter]] or possibly a [[brown dwarf]]. The hypothetical planet—or companion of the Sun—would be located in the outer part of the [[Oort cloud]].<ref>{{cite journal | author = J. B. Murray| title = Arguments for the presence of a distant large undiscovered Solar system planet | journal = Monthly Notices of the Royal Astronomical Society | year = 1999 | volume = 309 | pages=31 | url=http://adsabs.harvard.edu/abs/1999MNRAS.309...31M | laysource=Houston Chronicle | laysummary=http://www.chron.com/content/interactive/space/astronomy/news/1999/solarsys/991012.html | laydate=1999-10-11 | doi = 10.1046/j.1365-8711.1999.02806.x }}</ref><ref>{{cite journal | author = J. J. Matese; P. G. Whitman; D. P. Whitmire | title = Cometary Evidence of a Massive Body in the Outer Oort Clouds | journal = Icarus | year = 1999 | volume = 141 | pages = 354 | doi = 10.1006/icar.1999.6177 | url = http://adsabs.harvard.edu/abs/1999Icar..141..354M }}</ref>
Besides supposed gravitational effects, probability arguments have also been used to suggest the existence of planet-sized objects in the outer Solar System. Sedna's 12,000-year orbit is so extremely eccentric that, according to Mike Brown, who discovered it in 2004, there was only a one-in-sixty chance of it ever having been observed. This means, loosely, that unless its discovery was a freak accident, there are probably sixty objects roughly Sedna's diameter in its orbital region that have yet to be observed. Mike Brown noted in the 2007 Lowell Lecture that, "Sedna is about three-quarters the size of Pluto. If there are sixty objects three-quarters the size of Pluto out through here, 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|title=Lowell Lectures in Astronomy|author=Mike Brown|work=WGBH|url=http://forum.wgbh.org/content/forum/3710-2007_04_11.mp3|year=2007|accessdate=2008-07-13}}</ref> However, Brown contends that, should such an object be found, even though it might approach the Earth in size, it would still be a dwarf planet by the current definition, since it will not have cleared its neighborhood sufficiently.<ref name=wgbh/>
== See also ==
* [[Fifth planet (hypothetical)]], historical speculation about a planet between the orbits of Mars and Jupiter.
* [[Hypothetical planetary object]]
* [[Oort cloud]]
* [[Outer planets of the Solar System in fiction]], fictional trans-Neptunian planets
* [[Nemesis (star)]]
* [[Vulcan (hypothetical planet)|Vulcan]], a hypothetical cis-Mercurian planet
==References==
{{reflist|2}}
== Further reading ==
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This article is NOT about the "Planet X" 2012 doomsday panic. For that, go to the article ZetaTalk.
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* [http://www.nineplanets.org/hypo.html#planetx SEDS on Planet X]
* {{cite journal | author = Standish, E. M. | title = Planet X - No dynamical evidence in the optical observations | journal = Astronomical Journal | year = 1993 | volume = 105 | pages = 2000 | doi = 10.1086/116575 | url = http://adsabs.harvard.edu/abs/1993AJ....105.2000S}}
*{{cite journal
| title = Planet X: a myth exposed
| author = G. D. Quinlan
| journal = Nature
| volume = 363
| year = 1985
| pages = 18–19
| doi = 10.1038/363018b0}}
*{{cite journal
| title = Periodic comet showers and planet X
| author = D. P. Whitmire, J. J. Matese
| journal = Nature
| volume = 313
| year = 1985
| pages = 36–38
| doi = 10.1038/313036a0}}
[[Category:Hypothetical bodies of the Solar System|X]]
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[[zh:假設的海王星外天體]]