Editing Planets beyond Neptune (section)

== 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}}, 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 />

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}}, 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.

===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/>