Editing Planets beyond Neptune (section)

===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>