Editing Kuiper belt (section)

== Origin ==
[[File:Lhborbits.png|thumb|upright=1.8|Simulation showing outer planets and Kuiper belt: (a)&nbsp;before Jupiter/Saturn 1:2 resonance, (b)&nbsp;scattering of Kuiper belt objects into the Solar System after the orbital shift of Neptune, (c)&nbsp;after ejection of Kuiper belt bodies by Jupiter]]
[[File:Outersolarsystem objectpositions labels comp.png|thumb|The Kuiper belt (green), in the Solar System's outskirts]]
The precise origins of the Kuiper belt and its complex structure are still unclear, and astronomers are awaiting the completion of several wide-field survey telescopes such as [[Pan-STARRS]] and the future [[Large Synoptic Survey Telescope|LSST]], which should reveal many currently unknown KBOs.<ref name=beyond/> These surveys will provide data that will help determine answers to these questions. Pan-STARRS 1 finished its primary science mission in 2014, and the full data from the Pan-STARRS 1 surveys were published in 2019, helping reveal many more KBOs.<ref>{{Citation|title=The Pan-STARRS1 Surveys|first1=K. C.|last1=Chambers|display-authors=etal|arxiv=1612.05560|date=29 January 2019}}</ref><ref>{{Cite journal|title=The Pan-STARRS1 Database and Data Products|first1=H. A.|last1=Flewelling|display-authors=etal|date=20 October 2020|journal=The Astrophysical Journal Supplement Series|volume=251|issue=1|page=7|doi=10.3847/1538-4365/abb82d|arxiv=1612.05243|bibcode=2020ApJS..251....7F|s2cid=119382318|doi-access=free }}</ref><ref>{{Citation|url=https://pweb.cfa.harvard.edu/news/pan-starrs-releases-largest-digital-sky-survey-world|title=Pan-STARRS Releases Largest Digital Sky Survey to the World|date=19 December 2016|publisher=Harvard-Smithsonian Center for Astrophysics|access-date=21 October 2022|archive-date=21 October 2022|archive-url=https://web.archive.org/web/20221021143334/https://pweb.cfa.harvard.edu/news/pan-starrs-releases-largest-digital-sky-survey-world|url-status=live}}</ref>

The Kuiper belt is thought to consist of [[planetesimal]]s, fragments from the original [[protoplanetary disc]] around the Sun that failed to fully coalesce into planets and instead formed into smaller bodies, the largest less than {{convert|3000|km}} in diameter. Studies of the crater counts on Pluto and [[Charon (moon)|Charon]] revealed a scarcity of small craters suggesting that such objects formed directly as sizeable objects in the range of tens of kilometers in diameter rather than being accreted from much smaller, roughly kilometer scale bodies.<ref>{{cite web |url=http://www.astronomy.com/news/year-of-pluto/2015/11/pluto-may-have-ammonia-fueled-ice-volcanoes |title=Pluto may have ammonia-fueled ice volcanoes |date=9 November 2015 |work=Astronomy Magazine |url-status=live |archive-url=https://web.archive.org/web/20160304191045/http://www.astronomy.com/news/year-of-pluto/2015/11/pluto-may-have-ammonia-fueled-ice-volcanoes |archive-date=4 March 2016}}</ref> Hypothetical mechanisms for the formation of these larger bodies include the gravitational collapse of clouds of pebbles concentrated between eddies in a turbulent protoplanetary disk<ref name="Parker_etal_2011a">{{cite journal |last1=Parker |first1=Alex H. |last2=Kavelaars |first2=J.J. |last3=Petit |first3=Jean-Marc |last4=Jones |first4=Lynne |last5=Gladman |first5=Brett |last6=Parker |first6=Joel |title=Characterization of Seven Ultra-wide Trans-Neptunian Binaries |journal=The Astrophysical Journal |date=2011 |volume=743 |issue=1 |pages=159 |doi=10.1088/0004-6256/141/5/159 |arxiv=1108.2505 |bibcode=2011AJ....141..159N|s2cid=54187134 }}</ref><ref name="Cuzzi_etal_2010">{{cite journal |last1=Cuzzi |first1=Jeffrey N. |last2=Hogan |first2=Robert C. |last3=Bottke |first3=William F. |title=Towards initial mass functions for asteroids and Kuiper Belt Objects |journal=Icarus |date=2010 |volume=208 |issue=2 |pages=518–538 |doi=10.1016/j.icarus.2010.03.005 |arxiv=1004.0270 |bibcode=2010Icar..208..518C|s2cid=31124076 }}</ref> or in [[streaming instability|streaming instabilities]].<ref name=Johansen_Jacquet_2015>{{cite book |last1=Johansen |first1=A. |last2=Jacquet |first2=E. |last3=Cuzzi |first3=J. N. |last4=Morbidelli |first4=A. |last5=Gounelle |first5=M. |date=2015 |chapter=New Paradigms For Asteroid Formation |editor1-last=Michel |editor1-first=P. |editor2-last=DeMeo |editor2-first=F. |editor3-last=Bottke |editor3-first=W. |title=Asteroids IV |pages=471 |publisher=University of Arizona Press |series=Space Science Series |arxiv=1505.02941 |bibcode=2015aste.book..471J |doi=10.2458/azu_uapress_9780816532131-ch025 |isbn=978-0-8165-3213-1|s2cid=118709894 }}</ref> These collapsing clouds may fragment, forming binaries.<ref name="Nesvorny_etal_2010a">{{cite journal |last1=Nesvorný |first1=David |last2=Youdin |first2=Andrew N. |last3=Richardson |first3=Derek C. |title=Formation of Kuiper Belt Binaries by Gravitational Collapse |journal=The Astronomical Journal |date=2010 |volume=140 |issue=3 |pages=785–793 |doi=10.1088/0004-6256/140/3/785 |arxiv=1007.1465 |bibcode=2010AJ....140..785N|s2cid=118451279 }}</ref>

Modern [[Nice model|computer simulations]] show the Kuiper belt to have been strongly influenced by [[Jupiter]] and [[Neptune]], and also suggest that neither [[Uranus]] nor Neptune could have formed in their present positions, because too little primordial matter existed at that range to produce objects of such high mass. Instead, these planets are estimated to have formed closer to Jupiter. Scattering of planetesimals early in the Solar System's history would have led to [[planetary migration|migration]] of the orbits of the giant planets: [[Saturn]], Uranus, and Neptune drifted outwards, whereas Jupiter drifted inwards. Eventually, the orbits shifted to the point where Jupiter and Saturn reached an exact 1:2 resonance; Jupiter orbited the Sun twice for every one Saturn orbit. The gravitational repercussions of such a resonance ultimately destabilized the orbits of Uranus and Neptune, causing them to be scattered outward onto high-eccentricity orbits that crossed the primordial planetesimal disc.<ref name="Levison2008"/><ref>{{cite web |last1=Hansen |first1=K. |date=7 June 2005 |title=Orbital shuffle for early solar system |url=http://www.geotimes.org/june05/WebExtra060705.html |work=[[Geotimes]] |access-date=26 August 2007 |archive-date=27 September 2007 |archive-url=https://web.archive.org/web/20070927212314/http://www.geotimes.org/june05/WebExtra060705.html |url-status=live }}</ref><ref name="Tsiganis05">{{cite journal |last1=Tsiganis |first1=K. |last2=Gomes |first2=R. |last3=Morbidelli |first3=Alessandro |last4=Levison |first4=Harold F. |date=2005 |title=Origin of the orbital architecture of the giant planets of the Solar System |journal=[[Nature (journal)|Nature]] |volume=435 |issue=7041 |pages=459–461 |bibcode=2005Natur.435..459T |doi=10.1038/nature03539 |pmid=15917800|s2cid=4430973 }}</ref>

While Neptune's orbit was highly eccentric, its mean-motion resonances overlapped and the orbits of the planetesimals evolved chaotically, allowing planetesimals to wander outward as far as Neptune's 1:2 resonance to form a dynamically cold belt of low-inclination objects. Later, after its eccentricity decreased, Neptune's orbit expanded outward toward its current position. Many planetesimals were captured into and remain in resonances during this migration, others evolved onto higher-inclination and lower-eccentricity orbits and escaped from the resonances onto stable orbits.<ref>{{cite journal |last1=Thommes |first1=E.W. |last2=Duncan |first2=M.J. |last3=Levison |first3=Harold F. |date=2002 |title=The Formation of Uranus and Neptune among Jupiter and Saturn |journal=[[The Astronomical Journal]] |volume=123 |issue=5 |pages=2862–2883 |arxiv=astro-ph/0111290 |bibcode=2002AJ....123.2862T |doi=10.1086/339975|s2cid=17510705 }}</ref> Many more planetesimals were scattered inward, with small fractions being captured as Jupiter trojans, as irregular satellites orbiting the giant planets, and as outer belt asteroids. The remainder were scattered outward again by Jupiter and in most cases ejected from the Solar System reducing the primordial Kuiper belt population by 99% or more.<ref name="Levison2008">{{cite journal |last1=Levison |first1=Harold F. |last2=Morbidelli |first2=Alessandro |last3=Van Laerhoven |first3=Christa |last4=Gomes |first4=R. |date=2008 |title=Origin of the structure of the Kuiper belt during a dynamical instability in the orbits of Uranus and Neptune |journal=[[Icarus (journal)|Icarus]] |volume=196 |issue=1 |pages=258–273 |arxiv=0712.0553 |bibcode=2008Icar..196..258L |doi=10.1016/j.icarus.2007.11.035|s2cid=7035885 }}</ref>

The original version of the currently most popular model, the "[[Nice model#Formation of the Kuiper belt|Nice model]]", reproduces many characteristics of the Kuiper belt such as the "cold" and "hot" populations, resonant objects, and a scattered disc, but it still fails to account for some of the characteristics of their distributions. The model predicts a higher average eccentricity in classical KBO orbits than is observed (0.10–0.13 versus 0.07) and its predicted inclination distribution contains too few high inclination objects.<ref name="Levison2008"/> In addition, the frequency of binary objects in the cold belt, many of which are far apart and loosely bound, also poses a problem for the model. These are predicted to have been separated during encounters with Neptune,<ref name="Parker_Kavelaars_2010">{{cite journal |last1=Parker |first1=Alex H. |last2=Kavelaars |first2=J.J. |year=2010 |title=Destruction of Binary Minor Planets During Neptune Scattering |journal=The Astrophysical Journal Letters |volume=722 |issue=2 |pages=L204–L208 |doi=10.1088/2041-8205/722/2/L204 |bibcode=2010ApJ...722L.204P |arxiv=1009.3495|s2cid=119227937 }}</ref> leading some to propose that the cold disc formed at its current location, representing the only truly local population of small bodies in the solar system.<ref>{{cite journal |last1=Lovett |first1=R. |date=2010 |title=Kuiper Belt may be born of collisions |journal=[[Nature (journal)|Nature]] |doi=10.1038/news.2010.522}}</ref>

A [[Five-planet Nice model|recent modification]] of the Nice model has the Solar System begin with five giant planets, including an additional [[ice giant]], in a chain of mean-motion resonances. About 400&nbsp;million years after the formation of the Solar System the resonance chain is broken. Instead of being scattered into the disc, the ice giants first migrate outward several AU.<ref name="Nesvorny_Morbidelli_2012">{{cite journal |last1=Nesvorný |first1=David |last2=Morbidelli |first2=Alessandro |title=Statistical Study of the Early Solar System's Instability with Four, Five, and Six Giant Planets |journal=The Astronomical Journal |date=2012 |volume=144 |issue=4 |page=117 |arxiv=1208.2957 |doi=10.1088/0004-6256/144/4/117 |bibcode=2012AJ....144..117N|s2cid=117757768 }}</ref> This divergent migration eventually leads to a resonance crossing, destabilizing the orbits of the planets. The extra ice giant encounters Saturn and is scattered inward onto a Jupiter-crossing orbit and after a series of encounters is ejected from the Solar System. The remaining planets then continue their migration until the planetesimal disc is nearly depleted with small fractions remaining in various locations.<ref name="Nesvorny_Morbidelli_2012"/>

As in the original Nice model, objects are captured into resonances with Neptune during its outward migration. Some remain in the resonances, others evolve onto higher-inclination, lower-eccentricity orbits, and are released onto stable orbits forming the dynamically hot classical belt. The hot belt's inclination distribution can be reproduced if Neptune migrated from 24&nbsp;AU to 30&nbsp;AU on a 30&nbsp;Myr timescale.<ref name="Nesvorny_2015a">{{cite journal |last1=Nesvorný |first1=David |title=Evidence for slow migration of Neptune from the inclination distribution of Kuiper belt objects |journal=The Astronomical Journal |date=2015 |volume=150 |issue=3 |page=73 |doi=10.1088/0004-6256/150/3/73 |arxiv=1504.06021 |bibcode=2015AJ....150...73N|s2cid=119185190 }}</ref> When Neptune migrates to 28&nbsp;AU, it has a gravitational encounter with the extra ice giant. Objects captured from the cold belt into the 1:2 mean-motion resonance with Neptune are left behind as a local concentration at 44&nbsp;AU when this encounter causes Neptune's semi-major axis to jump outward.<ref name="Nesvorny_2015b">{{cite journal |last1=Nesvorný |first1=David |title=Jumping Neptune Can Explain the Kuiper Belt Kernel |journal=The Astronomical Journal |date=2015 |volume=150 |issue=3 |page=68 |doi=10.1088/0004-6256/150/3/68 |arxiv=1506.06019 |bibcode=2015AJ....150...68N|s2cid=117738539 }}</ref> The objects deposited in the cold belt include some loosely bound 'blue' binaries originating from closer than the cold belt's current location.<ref name="Fraser_etal_2017">{{cite journal |last1=Fraser |first1=Wesley |display-authors=etal <!-- |author2=and 21 others --> |title=All planetesimals born near the Kuiper belt formed as binaries|journal=Nature Astronomy |date=2017 |volume=1 |issue=4 |page=0088 |doi=10.1038/s41550-017-0088 |arxiv=1705.00683 |bibcode=2017NatAs...1E..88F|s2cid=118924314 }}</ref> If Neptune's eccentricity remains small during this encounter, the chaotic evolution of orbits of the original Nice model is avoided and a primordial cold belt is preserved.<ref name="Wolff_etal_2012">{{cite journal |last1=Wolff |first1=Schuyler |last2=Dawson |first2=Rebekah I. |last3=Murray-Clay |first3=Ruth A. |title=Neptune on Tiptoes: Dynamical Histories that Preserve the Cold Classical Kuiper Belt |journal=The Astrophysical Journal |date=2012 |volume=746 |issue=2 |page=171 |doi=10.1088/0004-637X/746/2/171 |arxiv=1112.1954 |bibcode=2012ApJ...746..171W|s2cid=119233820 }}</ref> In the later phases of Neptune's migration, a slow sweeping of mean-motion resonances removes the higher-eccentricity objects from the cold belt, truncating its eccentricity distribution.<ref name="Morbidelli_etal_2014">{{cite journal |last1=Morbidelli |first1=A. |last2=Gaspar |first2=H.S. |last3=Nesvorny |first3=D. |year=2014 |title=Origin of the peculiar eccentricity distribution of the inner cold Kuiper belt |journal=Icarus |volume=232 |pages=81–87 |doi=10.1016/j.icarus.2013.12.023 |arxiv=1312.7536 |bibcode=2014Icar..232...81M|s2cid=119185365 }}</ref>