Editing Pluto (section)

== Origin <span class="anchor" id="Origins"></span> ==
{{Further|Kuiper belt|Nice model}}
[[File:Outersolarsystem objectpositions labels comp.png|thumb|Plot of the known Kuiper belt objects, set against the four [[giant planet]]s]]

Pluto's origin and identity had long puzzled astronomers. One early hypothesis was that Pluto was an escaped moon of Neptune<ref>{{Cite book|last=Kuiper|first=Gerard|title=Planets and Satellites|publisher=University of Chicago Press|year=1961|location=Chicago|pages=576}}</ref> knocked out of orbit by Neptune's largest moon, Triton. This idea was eventually rejected after dynamical studies showed it to be impossible because Pluto never approaches Neptune in its orbit.<ref>{{cite book|title=Pluto and Charon|author1-link=S. Alan Stern|first1=S. Alan|last1=Stern|first2=David J.|last2=Tholen|publisher=[[University of Arizona Press]]|year=1997|page=623|url=https://books.google.com/books?id=VcY7iYJwJZoC&pg=PA623|isbn=978-0-8165-1840-1|access-date=October 23, 2015|archive-date=February 26, 2024|archive-url=https://web.archive.org/web/20240226151142/https://books.google.com/books?id=VcY7iYJwJZoC&pg=PA623#v=onepage&q&f=false|url-status=live}}</ref>

Pluto's true place in the [[Solar System]] began to reveal itself only in 1992, when astronomers began to find small icy objects beyond Neptune that were similar to Pluto not only in orbit but also in size and composition. This trans-Neptunian population is thought to be the source of many [[short-period comet]]s. Pluto is the largest member of the [[Kuiper belt]],{{efn|name = wiki-kbo}} a stable belt of objects located between 30 and 50 AU from the Sun. As of 2011, surveys of the Kuiper belt to magnitude 21 were nearly complete and any remaining Pluto-sized objects are expected to be beyond 100&nbsp;AU from the Sun.<ref name="Sheppard2011" /> Like other Kuiper-belt objects (KBOs), Pluto shares features with [[comet]]s; for example, the [[solar wind]] is gradually blowing Pluto's surface into space.<ref name="pluto.jhuapl cousin" /> It has been claimed that if Pluto were placed as near to the Sun as Earth, it would develop a tail, as comets do.<ref name="Tyson1999" /> This claim has been disputed with the argument that Pluto's escape velocity is too high for this to happen.<ref>{{cite web |date=April 13, 2015 |url=http://www.philipmetzger.com/blog/nine-reasons-why-pluto-is-a-planet/ |title=Nine Reasons Why Pluto Is a Planet |archive-url=https://web.archive.org/web/20150415183323/http://www.philipmetzger.com/blog/nine-reasons-why-pluto-is-a-planet/ |archive-date=April 15, 2015 |url-status=live |author=Philip Metzger |author-link=Philip T. Metzger |website=Philip Metzger}}</ref> It has been proposed that Pluto may have formed as a result of the agglomeration of numerous comets and Kuiper-belt objects.<ref name="SP-20180524">{{cite web |last=Wall |first=Mike |title=Pluto May Have Formed from 1 Billion Comets |url=https://www.space.com/40687-pluto-formation-1-billion-comets.html |date=May 24, 2018 |work=[[Space.com]] |access-date=May 24, 2018 |archive-date=May 24, 2018 |archive-url=https://web.archive.org/web/20180524135822/https://www.space.com/40687-pluto-formation-1-billion-comets.html |url-status=live }}</ref><ref name="ARX-20180524">{{Cite journal|last1=Glein |first1=Christopher R. |last2=Waite |first2=J. Hunter Jr. |title=Primordial N2 provides a cosmochemical explanation for the existence of Sputnik Planitia, Pluto |journal=Icarus |volume=313 |issue=2018 |pages=79–92 |date=May 24, 2018 |arxiv=1805.09285|doi=10.1016/j.icarus.2018.05.007 |bibcode=2018Icar..313...79G |s2cid=102343522 }}</ref>

Though Pluto is the largest Kuiper belt object discovered,<ref name="Plutosize" /> Neptune's moon [[Triton (moon)|Triton]], which is larger than Pluto, is similar to it both geologically and atmospherically, and is thought to be a captured Kuiper belt object.<ref name="PlanetaryOrg Triton" /> [[Eris (dwarf planet)|Eris]] ([[#Classification|see above]]) is about the same size as Pluto (though more massive) but is not strictly considered a member of the Kuiper belt population. Rather, it is considered a member of a linked population called the [[scattered disc]].<ref name="GallardoBrunini">
{{cite journal
| title = On the origin of the High-Perihelion Scattered Disk: the role of the Kozai mechanism and mean motion resonances
|author1=Gomes R. S. |author2=Gallardo T. |author3=Fernández J. A. |author4=Brunini A. |s2cid=18066500 | journal = Celestial Mechanics and Dynamical Astronomy
| date = 2005
| volume = 91
| issue = 1–2
| pages = 109–129
| doi = 10.1007/s10569-004-4623-y
| bibcode = 2005CeMDA..91..109G
|hdl=11336/38379 | hdl-access = free
}}
</ref>

Like other members of the Kuiper belt, Pluto is thought to be a residual [[planetesimal]]; a component of the original [[protoplanetary disc]] around the [[Sun]] that failed to fully coalesce into a full-fledged planet. Most astronomers agree that Pluto owes its position to a [[planetary migration|sudden migration]] undergone by Neptune early in the Solar System's formation. As Neptune migrated outward, it approached the objects in the proto-Kuiper belt, setting one in orbit around itself (Triton), locking others into resonances, and knocking others into chaotic orbits. The objects in the [[scattered disc]], a dynamically unstable region overlapping the Kuiper belt, are thought to have been placed in their positions by interactions with Neptune's migrating resonances.<ref name="Hahn2005" /> A computer model created in 2004 by [[Alessandro Morbidelli (astronomer)|Alessandro Morbidelli]] of the [[Observatoire de la Côte d'Azur]] in [[Nice]] suggested that the migration of Neptune into the Kuiper belt may have been triggered by the formation of a 1:2 resonance between Jupiter and Saturn, which created a gravitational push that propelled both Uranus and Neptune into higher orbits and caused them to switch places, ultimately doubling Neptune's distance from the Sun. The resultant expulsion of objects from the proto-Kuiper belt could also explain the [[Late Heavy Bombardment]] 600&nbsp;million years after the Solar System's formation and the origin of the [[Jupiter trojan]]s.<ref name="Levison2007" /> It is possible that Pluto had a near-circular orbit about 33 AU from the Sun before Neptune's migration [[Perturbation (astronomy)|perturbed]] it into a resonant capture.<ref name="Malhotra1995" /> The [[Nice model]] requires that there were about a thousand Pluto-sized bodies in the original planetesimal disk, which included Triton and Eris.<ref name="Levison2007" />