View source for Kuiper belt

{{short description|Area of the Solar System beyond the planets, comprising small bodies}}
{{Redirect|KBOs|other uses|KBOS (disambiguation)}}
{{Use dmy dates|date=March 2023}}
[[File:Kuiper belt plot objects of outer solar system.png|upright=1.3|thumb|Known objects in the Kuiper belt beyond the orbit of Neptune. (Scale in [[Astronomical unit|AU]]; [[epoch (astronomy)|epoch]] as of January 2015.)
{|
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{{legend2|#FFFF00|[[Sun]]}}<br />
{{legend2|#666666|[[Jupiter trojan]]s}}<br />
{{legend2|#FF3333|[[Giant planet]]s: {{hlist|class=inline|[[Jupiter|J]]|[[Saturn|S]]|[[Uranus|U]]|[[Neptune|N]]}}}}<br />
{{legend2|#006700|[[Centaur (small Solar System body)|Centaurs]]}}
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{{legend2|#870087|[[Neptune trojan]]s}}<br />
{{legend2|#FFD68F|'''Resonant Kuiper belt'''}}<br />
{{legend2|#009EE6|'''Classical Kuiper belt'''}}<br />
{{legend2|#545400|[[Scattered disc]]}}
|}
<div style="font-size: 0.9em; margin-top: 4px; line-height: 1.2em;">''Distances but not sizes are to scale. The yellow disk is about the size of Mars' orbit.<br />Source: [[Minor Planet Center]], {{URL|https://www.cfeps.net/}} and others''</div>
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{{TNO}}

The '''Kuiper belt''' ({{IPAc-en|'|k|aɪ|p|ər|audio=LL-Q1860 (eng)-Flame, not lame-Kuiper belt.wav}} {{respell|KY|pər}})<ref>{{Cite dictionary |url=http://www.lexico.com/definition/Kuiper+belt |archive-url=https://web.archive.org/web/20211126211845/https://www.lexico.com/definition/kuiper_belt?s=t |url-status=dead |archive-date=2021-11-26 |title=Kuiper belt |dictionary=[[Lexico]] UK English Dictionary |publisher=[[Oxford University Press]]}}</ref> is a [[circumstellar disc]] in the outer [[Solar System]], extending from the [[orbit]] of [[Neptune]] at 30 [[astronomical unit]]s (AU) to approximately 50 AU from the [[Sun]].<ref>{{cite journal |first1=Alan |last1=Stern |title=Collisional erosion in the primordial Edgeworth-Kuiper belt and the generation of the 30–50&nbsp;AU Kuiper gap |journal=The Astrophysical Journal |volume=490 |issue=2 |pages=879–882 |date=1997 |doi=10.1086/304912 |last2=Colwell |first2=Joshua E. |bibcode=1997ApJ...490..879S|doi-access=free }}</ref> It is similar to the [[asteroid belt]], but is far larger—20&nbsp;times as wide and 20–200&nbsp;times as massive.<ref name="beyond">{{cite book |title=The Solar System beyond the Planets |first1=Audrey |last1=Delsanti |first2=David |last2=Jewitt |name-list-style=amp |department=Institute for Astronomy |publisher=University of Hawaii |url=http://www2.ess.ucla.edu/~jewitt/papers/2006/DJ06.pdf |access-date=9 March 2007 |archive-url=https://web.archive.org/web/20070925203400/http://www.ifa.hawaii.edu/faculty/jewitt/papers/2006/DJ06.pdf |archive-date=25 September 2007 |bibcode=2006ssu..book..267D |year=2006}}</ref><ref>{{cite journal |author1-link=Georgij A. Krasinsky |first1=G. A. |last1=Krasinsky |last2=Pitjeva |first2=E. V. |bibcode=2002Icar..158...98K |title=Hidden Mass in the Asteroid Belt |journal=Icarus |volume=158 |issue=1 |pages=98–105 |date=July 2002 |doi=10.1006/icar.2002.6837 |author2-link=Elena V. Pitjeva |last3=Vasilyev |first3=M.V. |last4=Yagudina |first4=E.I.}}</ref> Like the asteroid belt, it consists mainly of [[small Solar System body|small bodies]] or remnants from when the [[Formation and evolution of the Solar System|Solar System formed]]. While many [[asteroid]]s are composed primarily of [[rock (geology)|rock]] and [[metal]], most Kuiper belt objects are composed largely of frozen [[Volatile (astrogeology)|volatiles]] (termed "ices"), such as [[methane]], [[ammonia]], and [[water]]. The Kuiper belt is home to most of the objects that astronomers generally accept as [[dwarf planet]]s: <!-- Please do not add Eris and Gonggong here. They are often called Kuiper belt objects but Wiki convention treats them strictly as a scattered-disc object --> [[90482 Orcus|Orcus]], [[Pluto]],<ref name="pluto_dwarf">{{cite web |last1=Christensen |first1=Lars Lindberg |title=IAU 2006 General Assembly: Result of the IAU Resolution votes |url=https://www.iau.org/news/pressreleases/detail/iau0603/ |publisher=IAU |access-date=25 May 2021 |archive-date=29 April 2014 |archive-url=https://web.archive.org/web/20140429212224/https://www.iau.org/news/pressreleases/detail/iau0603/ |url-status=live }}</ref> [[Haumea]],<ref name="Dwarf_5">{{cite web |last1=Christensen |first1=Lars Lindberg |title=IAU names fifth dwarf planet Haumea |url=https://www.iau.org/news/pressreleases/detail/iau0807/ |publisher=IAU |access-date=25 May 2021 |archive-date=25 April 2014 |archive-url=https://web.archive.org/web/20140425065601/http://iau.org/public_press/news/detail/iau0807/ |url-status=live }}</ref> [[50000 Quaoar|Quaoar]], and [[Makemake]].<ref name="Dwarf_4">{{cite web |last1=Christensen |first1=Lars Lindberg |title=Fourth dwarf planet named Makemake |url=https://www.iau.org/news/pressreleases/detail/iau0806/ |publisher=IAU |access-date=25 May 2021 |archive-date=16 June 2019 |archive-url=https://web.archive.org/web/20190616154815/https://www.iau.org/news/pressreleases/detail/iau0806/ |url-status=live }}</ref> Some of the Solar System's [[natural satellite|moons]], such as Neptune's [[Triton (moon)|Triton]] and [[Saturn]]'s [[Phoebe (moon)|Phoebe]], may have originated in the region.<ref>Johnson, Torrence V.; and Lunine, Jonathan I.; ''Saturn's moon Phoebe as a captured body from the outer Solar System'', Nature, Vol. 435, pp. 69–71</ref><ref>{{cite journal |title=Neptune's capture of its moon Triton in a binary-planet gravitational encounter |author1=Craig B. Agnor |author2=Douglas P. Hamilton |name-list-style=amp |journal=Nature |volume=441 |issue=7090 |pages=192–4 |url=http://www.es.ucsc.edu/~cagnor/papers_pdf/2006AgnorHamilton.pdf |date=2006 |access-date=20 June 2006 |archive-url=https://web.archive.org/web/20070621182809/http://www.es.ucsc.edu/~cagnor/papers_pdf/2006AgnorHamilton.pdf |archive-date=21 June 2007 |url-status=dead |bibcode=2006Natur.441..192A |doi=10.1038/nature04792 |pmid=16688170|s2cid=4420518 }}</ref>

The Kuiper belt is named in honor of the Dutch astronomer [[Gerard Kuiper]], who conjectured the existence of the belt in 1951.<ref name = "Kuiper">{{cite book |author1=Kuiper, G.P. |editor1-last=Hynek |editor1-first=J.A. |title=Astrophysics: A Topical Symposium |date=1951 |publisher=McGraw-Hill |location=New York City, New York, US |pages=357–424 |chapter=On the origin of the solar system}}</ref> There were researchers before and after him who also speculated on its existence, such as [[Kenneth Edgeworth]] in the 1930s.<ref>{{cite web | url=https://science.nasa.gov/solar-system/kuiper-belt/facts/ | title=Kuiper Belt: Facts – NASA Science }}</ref> The astronomer [[Julio Ángel Fernández|Julio Angel Fernandez]] published a paper in 1980 suggesting the existence of a comet belt beyond Neptune<ref name=":0">{{cite journal|url=|title=On the existence of a comet belt beyond Neptune|author=JA Fernandez|date=1980|journal=Observatorio Astronomico Nacional, Madrid|volume=192|issue=3|pages=481–491|bibcode=1980MNRAS.192..481F|doi=10.1093/mnras/192.3.481|pmid=|doi-access=free}}</ref><ref>{{Cite journal |last1=Morbidelli |first1=A. |last2=Thomas |first2=F. |last3=Moons |first3=M. |date=1995-12-01 |title=The Resonant Structure of the Kuiper Belt and the Dynamics of the First Five Trans-Neptunian Objects |url=https://www.sciencedirect.com/science/article/pii/S0019103585711943 |journal=Icarus |language=en |volume=118 |issue=2 |pages=322–340 |doi=10.1006/icar.1995.1194 |bibcode=1995Icar..118..322M |issn=0019-1035}}</ref> which could serve as a source for short-period comets.<ref>{{Cite web |last=gunjan.sogani |date=2022-09-10 |title=The Discovery of the Kuiper Belt and Its Members |url=https://www.wondriumdaily.com/the-discovery-of-the-kuiper-belt-and-its-members/ |access-date=2023-08-01 |website=Wondrium Daily |language=en-US |archive-date=1 August 2023 |archive-url=https://web.archive.org/web/20230801190803/https://www.wondriumdaily.com/the-discovery-of-the-kuiper-belt-and-its-members/ |url-status=dead }}</ref><ref>{{Cite web |title=Julio A. Fernández |url=http://nationalacademyofsciences.org/member-directory/members/20038988.html |access-date=2023-08-01 |website=nationalacademyofsciences.org |archive-date=1 August 2023 |archive-url=https://web.archive.org/web/20230801190801/http://nationalacademyofsciences.org/member-directory/members/20038988.html |url-status=live }}</ref>

In 1992, [[15760 Albion|minor planet (15760)&nbsp;Albion]] was discovered, the first Kuiper belt object (KBO) since Pluto (in 1930) and [[Charon (moon)|Charon]] (in 1978).<ref name="qbee" /> Since its discovery, the number of known KBOs has increased to thousands, and more than 100,000&nbsp;KBOs over {{convert|100|km|0|abbr=on}} in diameter are thought to exist.<ref>{{cite web |url=http://pluto.jhuapl.edu/overview/piPerspective.php?page=piPerspective_08_24_2012 |title=The PI's Perspective |archive-url=https://web.archive.org/web/20141113225430/http://pluto.jhuapl.edu/overview/piPerspective.php?page=piPerspective_08_24_2012 |archive-date=13 November 2014 |website=New Horizons |date=24 August 2012}}</ref> The Kuiper belt was initially thought to be the main repository for [[periodic comet]]s, those with orbits lasting less than 200&nbsp;years. Studies since the mid-1990s have shown that the belt is dynamically stable and that comets' true place of origin is the [[scattered disc]], a dynamically active zone created by the outward motion of Neptune 4.5&nbsp;billion years ago;<ref name="book">{{cite book |title=Encyclopedia of the Solar System |chapter=Comet Populations and Cometary Dynamics |first1=Harold F. |last1=Levison |first2=Luke |last2=Donnes |publisher=Academic Press |date=2007 |editor=Lucy Ann Adams McFadden |editor2=Paul Robert Weissman |editor3=Torrence V. Johnson |edition=2nd |location=Amsterdam; Boston |isbn=978-0-12-088589-3 |pages=[https://archive.org/details/encyclopediaofso0000unse_u6d1/page/575 575–588] |chapter-url=https://archive.org/details/encyclopediaofso0000unse_u6d1/page/575 }}</ref> scattered disc objects such as [[Eris (dwarf planet)|Eris]] have extremely [[orbital eccentricity|eccentric]] orbits that take them as far as 100&nbsp;AU from the Sun.{{efn|The literature is inconsistent in the usage of the terms ''scattered disc'' and ''Kuiper belt''. For some, they are distinct populations; for others, the scattered disc is part of the Kuiper belt. Authors may even switch between these two uses in one publication.<ref>Weissman and Johnson, 2007, ''Encyclopedia of the solar system'', footnote p. 584</ref> Because the [[International Astronomical Union]]'s [[Minor Planet Center]], the body responsible for cataloguing [[minor planet]]s in the Solar System, makes the distinction,<ref>{{cite web |url=http://www.minorplanetcenter.org/iau/lists/Centaurs.html |title=List Of Centaurs and Scattered-Disk Objects |publisher=Central Bureau for Astronomical Telegrams, Harvard-Smithsonian Center for Astrophysics |date=3 January 2011 |author=IAU: Minor Planet Center |access-date=3 January 2011 |archive-date=29 June 2017 |archive-url=https://web.archive.org/web/20170629210646/http://www.minorplanetcenter.org/iau/lists/Centaurs.html |url-status=live }}</ref> the editorial choice for Wikipedia articles on the trans-Neptunian region is to make this distinction as well. On Wikipedia, Eris, the most massive known trans-Neptunian object, is not part of the Kuiper belt and this makes Pluto the most massive Kuiper belt object.|name=PlutoSize}}

The Kuiper belt is distinct from the [[scientific theory|hypothesized]] [[Oort cloud]], which is believed to be a thousand times more distant and mostly spherical. The objects within the Kuiper belt, together with the members of the scattered disc and any potential [[Hills cloud]] or Oort cloud objects, are collectively referred to as [[trans-Neptunian object]]s (TNOs).<ref>{{cite web |title=Description of the System of Asteroids as of May 20, 2004 |author=Gérard FAURE |url=http://www.astrosurf.com/aude/map/us/AstFamilies2004-05-20.htm |date=2004 |access-date=1 June 2007 |archive-url=https://web.archive.org/web/20070529003558/http://www.astrosurf.com/aude/map/us/AstFamilies2004-05-20.htm |archive-date=29 May 2007 |url-status=dead}}</ref> Pluto is the largest and most massive member of the Kuiper belt and the largest and the second-most-massive known TNO, surpassed only by Eris in the scattered disc.{{efn|name=PlutoSize}} Originally considered a planet, Pluto's status as part of the Kuiper belt caused it to be reclassified as a dwarf planet in 2006. It is compositionally similar to many other objects of the Kuiper belt, and its orbital period is characteristic of a class of KBOs, known as "[[plutino]]s," that share the same 2:3 [[orbital resonance|resonance]] with Neptune.

The Kuiper belt and Neptune may be treated as a marker of the extent of the Solar System, alternatives being the [[Heliopause (astronomy)|heliopause]] and the distance at which the Sun's gravitational influence is matched by that of other stars (estimated to be between {{val|50000|u=AU}} and {{val|125000|u=AU}}).<ref>{{cite web |url=https://svs.gsfc.nasa.gov/12639 |title=Where is the Edge of the Solar System? |date=5 September 2017 |website=Goddard Media Studios |publisher=NASA's Goddard Space Flight Center |access-date=2019-09-22 |archive-date=16 December 2021 |archive-url=https://web.archive.org/web/20211216043038/https://svs.gsfc.nasa.gov/12639 |url-status=live }}</ref>

== History ==
[[File:Pluto-Charon-v2-10-1-15.jpg|thumb|Pluto and Charon]]

After the discovery of Pluto in 1930, many speculated that it might not be alone. The region now called the Kuiper belt was hypothesized in various forms for decades. It was only in 1992 that the first direct evidence for its existence was found. The number and variety of prior speculations on the nature of the Kuiper belt have led to continued uncertainty as to who deserves credit for first proposing it.<ref name=Randall_2015>{{cite book |title=Dark Matter and the Dinosaurs |publisher=Ecco/HarperCollins Publishers |location=New York |first=Lisa |last=Randall |date=2015 |isbn=978-0-06-232847-2 |title-link=Dark Matter and the Dinosaurs}}
</ref>{{rp|page=106}}

=== Hypotheses ===
The first [[astronomer]] to suggest the existence of a trans-Neptunian population was [[Frederick C. Leonard]]. Soon after Pluto's discovery by [[Clyde Tombaugh]] in 1930, Leonard pondered whether it was "likely that in Pluto there has come to light the ''first'' of a ''series'' of ultra-Neptunian bodies, the remaining members of which still await discovery but which are destined eventually to be detected".<ref>{{cite web |title=What is improper about the term "Kuiper belt"? (or, Why name a thing after a man who didn't believe its existence?) |url=http://www.icq.eps.harvard.edu/kb.html |work=International Comet Quarterly |access-date=24 October 2010 |archive-date=8 October 2019 |archive-url=https://web.archive.org/web/20191008115030/http://www.icq.eps.harvard.edu/kb.html |url-status=live }}</ref> That same year, astronomer [[Armin Otto Leuschner|Armin O. Leuschner]] suggested that Pluto "may be one of many long-period planetary objects yet to be discovered."<ref name=lauch>{{cite book |first1=John K. |last1=Davies |first2=J. |last2=McFarland |first3=Mark E. |last3=Bailey |first4=Brian G. |last4=Marsden |first5=W. I. |last5=Ip |title=The Solar System Beyond Neptune |editor=M. Antonietta Baracci |editor2=Hermann Boenhardt |editor3=Dale Cruikchank |editor4=Alessandro Morbidelli |pages=11–23 |publisher=University of Arizona Press |chapter-url=http://www.arm.ac.uk/preprints/2008/522.pdf |date=2008 |chapter=The Early Development of Ideas Concerning the Transneptunian Region |access-date=5 November 2014 |archive-url=https://web.archive.org/web/20150220182134/http://www.arm.ac.uk/preprints/2008/522.pdf |archive-date=20 February 2015 |url-status=dead }}</ref>

[[File:Gerard Kuiper 1964b.jpg|thumb|upright|Astronomer [[Gerard Kuiper]], after whom the Kuiper belt is named]]

In 1943, in the ''[[Journal of the British Astronomical Association]]'', [[Kenneth Edgeworth]] hypothesized that, in the region beyond [[Neptune]], the material within the [[primordial element|primordial]] [[solar nebula]] was too widely spaced to condense into planets, and so rather condensed into a myriad smaller bodies. From this he concluded that "the outer region of the solar system, beyond the orbits of the planets, is occupied by a very large number of comparatively small bodies"<ref name=Davies_2001>{{cite book |title=Beyond Pluto: Exploring the outer limits of the solar system |first=John K. |last=Davies |publisher=Cambridge University Press |date=2001 }}</ref>{{rp|page=xii}} and that, from time to time, one of their number "wanders from its own sphere and appears as an occasional visitor to the inner solar system",<ref name=Davies_2001/>{{rp|page=2}} becoming a [[comet]].

In 1951, in a paper in ''Astrophysics: A Topical Symposium'', [[Gerard Kuiper]] speculated on a similar disc having formed early in the Solar System's evolution and concluded that the disc consisted of "remnants of original clusterings which have lost many members that became stray asteroids, much as has occurred with open galactic clusters dissolving into stars."<ref name = "Kuiper"/> In another paper, based upon a lecture Kuiper gave in 1950, also called ''On the Origin of the Solar System'', Kuiper wrote about the "outermost region of the solar nebula, from 38 to 50 astr. units (i.e., just outside proto-Neptune)" where "condensation products (ices of H20, NH3, CH4, etc.) must have formed, and the flakes must have slowly collected and formed larger aggregates, estimated to range up to 1 km. or more in size." He continued to write that "these condensations appear to account for the comets, in size, number and composition." According to Kuiper "the planet Pluto, which sweeps through the whole zone from 30 to 50 astr. units, is held responsible for having started the scattering of the comets throughout the solar system."<ref>{{cite journal |first1=Gerard |last1=Kuiper |title=On the Origin of the Solar System|journal=Proceedings of the National Academy of Sciences |volume=37 |issue=1 |date=1951 |pages=1–14 |doi=10.1073/pnas.37.4.233 |pmid=16588984 |pmc=1063291 |doi-access= free}}</ref> It is said that Kuiper was operating on the assumption, common in his time, that [[Pluto]] was the size of Earth and had therefore scattered these bodies out toward the [[Oort cloud]] or out of the Solar System; there would not be a Kuiper belt today if this were correct.<ref name="Jewitt">{{cite web |title=WHY "KUIPER" BELT? |author=David Jewitt |work=University of Hawaii |url=http://www2.ess.ucla.edu/~jewitt/kb/gerard.html |access-date=14 June 2007 |archive-date=12 February 2019 |archive-url=https://web.archive.org/web/20190212100219/http://www2.ess.ucla.edu/~jewitt/kb/gerard.html |url-status=live }}</ref>

The hypothesis took many other forms in the following decades. In 1962, physicist [[Alastair GW Cameron|Al G.W. Cameron]] postulated the existence of "a tremendous mass of small material on the outskirts of the solar system".<ref name=Davies_2001/>{{rp|page=14}} In 1964, [[Fred Whipple]], who popularised the famous "[[dirty snowball]]" hypothesis for cometary structure, thought that a "comet belt" might be massive enough to cause the purported discrepancies in the orbit of [[Uranus]] that had sparked the search for [[Planet X]], or, at the very least, massive enough to affect the orbits of known comets.<ref>{{cite journal |journal=Proceedings of the National Academy of Sciences |volume=51 |issue=5 |pages=771–774 |url=http://www.pnas.org/cgi/reprint/51/5/711.pdf |date=1964 |bibcode=1964PNAS...51..771R |doi=10.1073/pnas.51.5.771 |title=Decomposition of Vector Measures |last1=Rao |first1=M. M. |pmid=16591174 |pmc=300359 |doi-access=free |access-date=20 June 2007 |archive-date=3 June 2016 |archive-url=https://web.archive.org/web/20160603080459/http://www.pnas.org/content/51/5/711.full.pdf |url-status=live }}</ref> Observation ruled out this hypothesis.<ref name=Davies_2001/>{{rp|page=14}}

In 1977, [[Charles Kowal]] discovered [[2060 Chiron]], an icy planetoid with an orbit between Saturn and Uranus. He used a [[blink comparator]], the same device that had allowed Clyde Tombaugh to discover Pluto nearly 50 years before.<ref>{{cite journal |title=The discovery and orbit of /2060/ Chiron |author=CT Kowal |author2=W Liller |author3=BG Marsden |place=Hale Observatories, Harvard–Smithsonian Center for Astrophysics |date=1977 |bibcode=1979IAUS...81..245K |volume=81 |page=245 |journal=In: Dynamics of the Solar System; Proceedings of the Symposium}}</ref> In 1992, another object, [[5145 Pholus]], was discovered in a similar orbit.<ref>{{cite journal |title=1992 AD |author=JV Scotti |author2=DL Rabinowitz |author3=CS Shoemaker |author4=EM Shoemaker |author5=DH Levy |author6=TM King |author7=EF Helin |author8=J Alu |author9=K Lawrence|author10=RH McNaught |author11=L Frederick |author12=D Tholen |author13=BEA Mueller |bibcode=1992IAUC.5434....1S |date=1992 |volume=5434 |page=1 |journal=IAU Circ.}}</ref> Today, an entire population of comet-like bodies, called the [[centaur (planetoid)|centaurs]], is known to exist in the region between Jupiter and Neptune. The centaurs' orbits are unstable and have dynamical lifetimes of a few million years.<ref name="Horner2004a">{{cite journal |last1=Horner |first1=J. |last2=Evans |first2=N. W. |last3=Bailey |first3=Mark E. |title=Simulations of the Population of Centaurs I: The Bulk Statistics |date=2004 |volume=354 |pages=798–810 |journal=MNRAS |arxiv=astro-ph/0407400 |bibcode=2004MNRAS.354..798H |doi=10.1111/j.1365-2966.2004.08240.x |issue=3|doi-access=free |s2cid=16002759 }}</ref> From the time of Chiron's discovery in 1977, astronomers have speculated that the centaurs therefore must be frequently replenished by some outer reservoir.<ref name=Davies_2001/>{{rp|page=38}}

Further evidence for the existence of the Kuiper belt later emerged from the study of comets. That comets have finite lifespans has been known for some time. As they approach the Sun, its heat causes their [[volatility (physics)|volatile]] surfaces to sublimate into space, gradually dispersing them. In order for comets to continue to be visible over the age of the Solar System, they must be replenished frequently.<ref name="matter">{{cite journal |author=David Jewitt |title=From Kuiper Belt Object to Cometary Nucleus: The Missing Ultrared Matter |journal=[[The Astronomical Journal]] |volume=123 |issue=2 |pages=1039–1049 |date=2002 |doi=10.1086/338692 |bibcode=2002AJ....123.1039J |s2cid=122240711 |doi-access=free }}</ref> A proposal for such an area of replenishment is the [[Oort cloud]], possibly a spherical swarm of comets extending beyond 50,000&nbsp;AU from the Sun first hypothesised by Dutch astronomer [[Jan Oort]] in 1950.<ref>{{cite journal |bibcode=1950BAN....11...91O |title=The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin |last1=Oort |first1=J. H. |volume=11 |date=1950 |page=91 |journal=Bull. Astron. Inst. Neth.}}</ref> The Oort cloud is thought to be the point of origin of [[long-period comet]]s, which are those, like [[Comet Hale–Bopp|Hale–Bopp]], with orbits lasting thousands of years.<ref name=Randall_2015/>{{rp|page=105}}[[File:Julio A. Fernandez (recortado).jpg|thumb|In 1980, astronomer [[Julio Ángel Fernández|Julio Fernandez]] predicted the existence of a belt. It has been said that because the words "Kuiper" and "comet belt" appeared in the opening sentence of Fernandez's paper, this hypothetical region was referred to as the "Kuiper belt".<ref>{{cite web|access-date=16 August 2020|date=8 October 2019|language=en-US|title=Kuiper Belt {{!}} Facts, Information, History & Definition|url=https://nineplanets.org/kuiper-belt/|website=The Nine Planets|archive-date=16 May 2021|archive-url=https://web.archive.org/web/20210516004242/https://nineplanets.org/kuiper-belt/|url-status=live}}<!-- auto-translated by Module:CS1 translator --></ref>]]There is another comet population, known as [[short-period comet|short-period]] or [[periodic comet]]s, consisting of those comets that, like [[Halley's Comet]], have [[orbital period]]s of less than 200&nbsp;years. By the 1970s, the rate at which short-period comets were being discovered was becoming increasingly inconsistent with their having emerged solely from the Oort cloud.<ref name=Davies_2001/>{{rp|page=39}} For an Oort cloud object to become a short-period comet, it would first have to be [[asteroid capture|captured]] by the giant planets. In a paper published in ''[[Monthly Notices of the Royal Astronomical Society]]'' in 1980, Uruguayan astronomer [[Julio Ángel Fernández|Julio Fernández]] stated that for every short-period comet to be sent into the inner Solar System from the Oort cloud, 600 would have to be ejected into [[Outer space#Interstellar space|interstellar space]]. He speculated that a comet belt from between 35 and 50&nbsp;AU would be required to account for the observed number of comets.<ref>{{cite journal |title=On the existence of a comet belt beyond Neptune |author=J.A. Fernández |bibcode=1980MNRAS.192..481F |date=1980 |volume=192 |issue=3 |pages=481–491 |journal=Monthly Notices of the Royal Astronomical Society |doi=10.1093/mnras/192.3.481 |doi-access=free}}</ref> Following up on Fernández's work, in 1988 the Canadian team of Martin Duncan, Tom Quinn and [[Scott Tremaine]] ran a number of computer simulations to determine if all observed comets could have arrived from the Oort cloud. They found that the Oort cloud could not account for all short-period comets, particularly as short-period comets are clustered near the plane of the Solar System, whereas Oort-cloud comets tend to arrive from any point in the sky. With a "belt", as Fernández described it, added to the formulations, the simulations matched observations.<ref>{{cite journal |title=The origin of short-period comets |author=M. Duncan |author2=T. Quinn |author3=S. Tremaine |name-list-style=amp |date=1988 |bibcode=1988ApJ...328L..69D |volume=328 |pages=L69 |journal=Astrophysical Journal |doi=10.1086/185162|doi-access=free }}</ref> Reportedly because the words "Kuiper" and "comet belt" appeared in the opening sentence of Fernández's paper, Tremaine named this hypothetical region the "Kuiper belt".<ref name=Davies_2001/>{{rp|page=191}}

=== Discovery ===
[[File:Mauna Kea Summit 2021-06-16 33 (cropped).jpg|thumb|upright=1.25|Telescopes atop [[Mauna Kea Observatories|Mauna Kea]]. The Kuiper belt was discovered with [[UH88]], which is the fourth from the left.]]
In 1987, astronomer [[David Jewitt]], then at [[MIT]], became increasingly puzzled by "the apparent emptiness of the outer Solar System".<ref name="qbee">{{cite journal |doi=10.1038/362730a0 |title=Discovery of the candidate Kuiper belt object 1992 QB1 |date=1993 |last1=Jewitt |first1=David |last2=Luu |first2=Jane |journal=Nature |volume=362 |issue=6422 |pages=730–732 |bibcode=1993Natur.362..730J|s2cid=4359389 }}</ref> He encouraged then-graduate student [[Jane Luu]] to aid him in his endeavour to locate another object beyond [[Pluto]]'s orbit, because, as he told her, "If we don't, nobody will."<ref name=Davies_2001/>{{rp|page=50}} Using telescopes at the [[Kitt Peak National Observatory]] in Arizona and the [[Cerro Tololo Inter-American Observatory]] in Chile, Jewitt and Luu conducted their search in much the same way as Clyde Tombaugh and Charles Kowal had, with a [[blink comparator]].<ref name=Davies_2001/>{{rp|page=50}} Initially, examination of each pair of plates took about eight hours,<ref name=Davies_2001/>{{rp|page=51}} but the process was sped up with the arrival of electronic [[charge-coupled device]]s or CCDs, which, though their field of view was narrower, were not only more efficient at collecting light (they retained 90% of the light that hit them, rather than the 10% achieved by photographs) but allowed the blinking process to be done virtually, on a computer screen. Today, CCDs form the basis for most astronomical detectors.<ref name=Davies_2001/>{{rp|pages=52, 54, 56}} In 1988, Jewitt moved to the Institute of Astronomy at the [[University of Hawaii]]. Luu later joined him to work at the University of Hawaii's 2.24&nbsp;m telescope at [[Mauna Kea]].<ref name=Davies_2001/>{{rp|pages=57, 62}} Eventually, the field of view for CCDs had increased to 1024 by 1024 pixels, which allowed searches to be conducted far more rapidly.<ref name=Davies_2001/>{{rp|page=65}} Finally, after five years of searching, Jewitt and Luu announced on 30 August 1992 the "Discovery of the candidate Kuiper belt object [[15760 Albion|1992 QB<sub>1</sub>]]".<ref name=qbee/> This object would later be named 15760 Albion. Six months later, they discovered a second object in the region, [[(181708) 1993 FW]].<ref>{{cite journal |title=1993 FW |author1=Marsden, B.S. |place=Minor Planet Center |bibcode=1993IAUC.5730....1L |date=1993 |author2=Jewitt, D. |author3=Marsden, B.G. |volume=5730 |page=1 |journal=IAU Circ.}}</ref> By 2018, over 2000&nbsp;Kuiper belts objects had been discovered.<ref name="Dyches">{{Cite web |url=https://solarsystem.nasa.gov/news/792/10-things-to-know-about-the-kuiper-belt |title=10&nbsp;Things to Know About the Kuiper Belt |last=Dyches |first=Preston |website=NASA Solar System Exploration |access-date=2019-12-01 |archive-date=10 January 2019 |archive-url=https://web.archive.org/web/20190110003110/https://solarsystem.nasa.gov/news/792/10-things-to-know-about-the-kuiper-belt |url-status=live }}</ref>

Over one thousand bodies were found in a belt in the twenty years (1992–2012), after finding {{mp|1992 QB|1}} (named in 2018, 15760 Albion), showing a vast belt of bodies in addition to Pluto and Albion.<ref name=":2">{{cite web |url=https://www.astrobio.net/also-in-news/the-kuiper-belt-at-20/ |title=The Kuiper Belt at 20 |date=2012-09-01 |website=Astrobiology Magazine |access-date=2019-12-01 |archive-url=https://web.archive.org/web/20201030081029/https://www.astrobio.net/also-in-news/the-kuiper-belt-at-20/ |archive-date=2020-10-30 |url-status=dead}}</ref> Even in the 2010s the full extent and nature of Kuiper belt bodies was largely unknown.<ref name=":2"/> Finally, the unmanned spacecraft [[New Horizons]] conducted the first KBO flybys, providing much closer observations of the Plutonian system (2015) and then [[Arrokoth]] (2019).<ref>{{cite web |url=https://www.science.org/content/article/surviving-encounter-beyond-pluto-nasa-probe-begins-relaying-view-kuiper-belt-object |title=Surviving encounter beyond Pluto, NASA probe begins relaying view of Kuiper belt object |last=Voosen |first=Paul |date=2019-01-01 |website=Science |publisher=AAAS |access-date=2019-12-01 |archive-date=8 October 2022 |archive-url=https://web.archive.org/web/20221008185849/https://www.science.org/content/article/surviving-encounter-beyond-pluto-nasa-probe-begins-relaying-view-kuiper-belt-object |url-status=live }}</ref>

Studies conducted since the trans-Neptunian region was first charted have shown that the region now called the Kuiper belt is not the point of origin of short-period comets, but that they instead derive from a linked population called the [[scattered disc]]. The scattered disc was created when Neptune [[Nice model|migrated outward]] into the proto-Kuiper belt, which at the time was much closer to the Sun, and left in its wake a population of dynamically stable objects that could never be affected by its orbit (the Kuiper belt proper), and a population whose [[apsis|perihelia]] are close enough that Neptune can still disturb them as it travels around the Sun (the scattered disc). Because the scattered disc is dynamically active and the Kuiper belt relatively dynamically stable, the scattered disc is now seen as the most likely point of origin for periodic comets.<ref name=book/>

=== Name ===
Astronomers sometimes use the alternative name Edgeworth–Kuiper belt to credit Edgeworth, and KBOs are occasionally referred to as EKOs. [[Brian G. Marsden]] claims that neither deserves true credit: "Neither Edgeworth nor Kuiper wrote about anything remotely like what we are now seeing, but [[Fred Whipple]] did".<ref name=Davies_2001/>{{rp|page=199}} David Jewitt comments: "If anything ... [[Julio Ángel Fernández|Fernández]] most nearly deserves the credit for predicting the Kuiper Belt."<ref name=Jewitt />

KBOs are sometimes called "kuiperoids", a name suggested by [[Clyde Tombaugh]].<ref>Clyde Tombaugh, "The Last Word", Letters to the Editor, ''Sky & Telescope'', December 1994, p. 8</ref> The term "[[trans-Neptunian object]]" (TNO) is recommended for objects in the belt by several scientific groups because the term is less controversial than all others—it is not an exact [[synonym]] though, as TNOs include all objects orbiting the Sun past the orbit of [[Neptune]], not just those in the Kuiper belt.<ref>{{cite web|url=http://www.icq.eps.harvard.edu/kb.html|title=What is improper about the term "Kuiper belt"?|website=International Comet Quarterly|access-date=19 December 2021|archive-date=8 October 2019|archive-url=https://web.archive.org/web/20191008115030/http://www.icq.eps.harvard.edu/kb.html|url-status=live}}</ref>

== Structure ==
At its fullest extent (but excluding the scattered disc), including its outlying regions, the Kuiper belt stretches from roughly 30–55&nbsp;AU. The main body of the belt is generally accepted to extend from the 2:3 mean-motion resonance ([[#Resonances|see below]]) at 39.5&nbsp;AU to the 1:2 resonance at roughly 48&nbsp;AU.<ref>{{cite journal |author1=M. C. de Sanctis |author2=M. T. Capria |author3=A. Coradini |name-list-style=amp |year=2001 |title=Thermal Evolution and Differentiation of Edgeworth-Kuiper Belt Objects |journal=The Astronomical Journal |volume=121 |issue=5 |pages=2792–2799 |bibcode=2001AJ....121.2792D |doi=10.1086/320385|doi-access=free }}</ref> The Kuiper belt is quite thick, with the main concentration extending as much as ten degrees outside the [[plane of the ecliptic|ecliptic plane]] and a more diffuse distribution of objects extending several times farther. Overall it more resembles a [[torus]] or doughnut than a belt.<ref>{{cite web |title=Discovering the Edge of the Solar System |website=American Scientists.org |url=http://www.americanscientist.org/template/AssetDetail/assetid/25723/page/2;jsessionid=aaa5LVF0 |year=2003 |access-date=23 June 2007 |archive-url=https://web.archive.org/web/20090315010603/http://www.americanscientist.org/template/AssetDetail/assetid/25723/page/2%3Bjsessionid%3Daaa5LVF0 |archive-date=15 March 2009 |url-status=dead }}</ref> Its mean position is inclined to the ecliptic by 1.86 degrees.<ref>{{cite journal |author1=Michael E. Brown |author2=Margaret Pan |title=The Plane of the Kuiper Belt |journal=[[The Astronomical Journal]] |volume=127 |issue=4 |pages=2418–2423 |date=2004 |doi=10.1086/382515 |bibcode=2004AJ....127.2418B|s2cid=10263724 |url=http://pdfs.semanticscholar.org/4fc0/a6282a93a0ccedd858586ac95857528cf26b.pdf |archive-url=https://web.archive.org/web/20200412143632/http://pdfs.semanticscholar.org/4fc0/a6282a93a0ccedd858586ac95857528cf26b.pdf |url-status=dead |archive-date=2020-04-12 }}</ref>

The presence of [[Neptune]] has a profound effect on the Kuiper belt's structure due to [[orbital resonance]]s. Over a timescale comparable to the age of the Solar System, Neptune's gravity destabilises the orbits of any objects that happen to lie in certain regions, and either sends them into the inner Solar System or out into the [[scattered disc]] or interstellar space. This causes the Kuiper belt to have pronounced gaps in its current layout, similar to the [[Kirkwood gap]]s in the [[asteroid belt]]. In the region between 40 and 42 AU, for instance, no objects can retain a stable orbit over such times, and any observed in that region must have migrated there relatively recently.<ref>{{cite journal |title=Large Scattered Planetesimals and the Excitation of the Small Body Belts |journal=Icarus |volume=141 |issue=2 |pages=367 |first1=Jean-Marc |last1=Petit |first2=Alessandro |last2=Morbidelli |first3=Giovanni B. |last3=Valsecchi |url=http://www.obs-nice.fr/morby/papers/6166a.pdf |date=1998 |access-date=23 June 2007 |archive-url=https://web.archive.org/web/20070809103014/http://www.obs-nice.fr/morby/papers/6166a.pdf |archive-date=9 August 2007 |url-status=dead |bibcode=1999Icar..141..367P |doi=10.1006/icar.1999.6166 }}</ref>

[[File:KBO diagram eccentricity.png|upright=3|thumb|The various dynamical classes of trans-Neptunian objects.]]

=== Classical belt ===
{{Main|Classical Kuiper belt object}}
Between the 2:3 and 1:2 resonances with Neptune, at approximately 42–48&nbsp;AU, the gravitational interactions with Neptune occur over an extended timescale, and objects can exist with their orbits essentially unaltered. This region is known as the [[Classical Kuiper belt object|classical Kuiper belt]], and its members comprise roughly two thirds of KBOs observed to date.<ref>{{cite web |last1=Lunine |first1=Jonathan I. |date=2003 |title=The Kuiper Belt |url=http://www.gsmt.noao.edu/gsmt_swg/SWG_Apr03/The_Kuiper_Belt.pdf |access-date=23 June 2007 |archive-date=9 August 2007 |archive-url=https://web.archive.org/web/20070809103013/http://www.gsmt.noao.edu/gsmt_swg/SWG_Apr03/The_Kuiper_Belt.pdf |url-status=dead }}</ref><ref>
{{cite web |last1=Jewitt |first1=D. |date=February 2000 |title=Classical Kuiper Belt Objects (CKBOs) |url=http://www2.ess.ucla.edu/~jewitt/kb/kb-classical.html |access-date=23 June 2007 |archive-url=https://web.archive.org/web/20070609094740/http://www.ifa.hawaii.edu/~jewitt/kb/kb-classical.html |archive-date=9 June 2007}}</ref> Because the first modern KBO discovered ([[15760 Albion|Albion]], but long called (15760) 1992 QB<sub>1</sub>), is considered the prototype of this group, classical KBOs are often referred to as [[cubewanos]] ("Q-B-1-os").<ref>{{cite book |last1=Murdin |first1=P. |title=The Encyclopedia of Astronomy and Astrophysics |date=2000 |isbn=978-0-333-75088-9 |doi=10.1888/0333750888/5403 |bibcode=2000eaa..bookE5403. |article=Cubewano}}</ref><ref>{{cite journal |last1=Elliot |first1=J. L. |display-authors=etal |date=2005 |title=The Deep Ecliptic Survey: A Search for Kuiper Belt Objects and Centaurs. II. Dynamical Classification, the Kuiper Belt Plane, and the Core Population |url=http://occult.mit.edu/_assets/documents/publications/Elliot2005AJ129.1117.pdf |journal=[[The Astronomical Journal]] |volume=129 |issue=2 |pages=1117–1162 |bibcode=2005AJ....129.1117E |doi=10.1086/427395 |doi-access=free |access-date=18 August 2012 |archive-date=21 July 2013 |archive-url=https://web.archive.org/web/20130721180750/http://occult.mit.edu/_assets/documents/publications/Elliot2005AJ129.1117.pdf |url-status=live }}</ref> The [[Committee on Small Body Nomenclature|guidelines]] established by the [[IAU]] demand that classical KBOs be given names of mythological beings associated with creation.<ref name="clas">{{cite web |title=Naming of Astronomical Objects: Minor Planets |url=http://www.iau.org/public_press/themes/naming/#minorplanets |publisher=[[International Astronomical Union]] |access-date=17 November 2008 |archive-date=16 December 2008 |archive-url=https://web.archive.org/web/20081216024716/http://www.iau.org/public_press/themes/naming/#minorplanets |url-status=live }}</ref>

The classical Kuiper belt appears to be a composite of two separate populations. The first, known as the "dynamically cold" population, has orbits much like the planets; nearly circular, with an [[orbital eccentricity]] of less than 0.1, and with relatively low inclinations up to about 10° (they lie close to the plane of the Solar System rather than at an angle). The cold population also contains a concentration of objects, referred to as the kernel, with semi-major axes at 44–44.5 AU.<ref name="Petit 2011">{{cite journal |last1=Petit |first1=J.-M. |last2=Gladman |first2=B. |last3=Kavelaars |first3=J.J. |last4=Jones |first4=R.L. |last5=Parker |first5=J. |title=Reality and origin of the Kernel of the classical Kuiper Belt |journal=EPSC-DPS Joint Meeting |date=2011 |issue=2–7 October 2011 |url=http://meetingorganizer.copernicus.org/EPSC-DPS2011/EPSC-DPS2011-722.pdf |access-date=4 May 2016 |archive-date=4 March 2016 |archive-url=https://web.archive.org/web/20160304194450/http://meetingorganizer.copernicus.org/EPSC-DPS2011/EPSC-DPS2011-722.pdf |url-status=live }}</ref> The second, the "dynamically hot" population, has orbits much more inclined to the ecliptic, by up to 30°. The two populations have been named this way not because of any major difference in temperature, but from analogy to particles in a gas, which increase their relative velocity as they become heated up.<ref name="Levison2003">{{cite journal |last1=Levison |first1=Harold F. |last2=Morbidelli |first2=Alessandro |date=2003 |title=The formation of the Kuiper belt by the outward transport of bodies during Neptune's migration |journal=[[Nature (journal)|Nature]] |volume=426 |issue=6965 |pages=419–421 |doi=10.1038/nature02120 |pmid=14647375 |bibcode=2003Natur.426..419L|s2cid=4395099 }}</ref> Not only are the two populations in different orbits, the cold population also differs in color and [[albedo]], being redder and brighter, has a larger fraction of binary objects,<ref name="Stephen_Noll_2006">{{cite journal |last1=Stephens |first1=Denise C. |last2=Noll |first2=Keith S. |title=Detection of Six Trans-Neptunian Binaries with NICMOS: A High Fraction of Binaries in the Cold Classical Disk |journal=The Astronomical Journal |date=2006 |volume=130 |issue=2 |pages=1142–1148 |doi=10.1086/498715 |arxiv=astro-ph/0510130 |bibcode=2006AJ....131.1142S|s2cid=204935715 }}</ref> has a different size distribution,<ref name="Fraser_etal_2014"/> and lacks very large objects.<ref name="Levison_Stern_2001">{{cite journal |last1=Levison |first1=Harold F. |last2=Stern |first2=S. Alan |title=On the Size Dependence of the Inclination Distribution of the Main Kuiper Belt |journal=The Astronomical Journal |date=2001 |volume=121 |issue=3 |pages=1730–1735 |doi=10.1086/319420 |arxiv=astro-ph/0011325 |bibcode=2001AJ....121.1730L|s2cid=14671420 }}</ref> The mass of the dynamically cold population is roughly 30 times less than the mass of the hot.<ref name="Fraser_etal_2014"/> The difference in colors may be a reflection of different compositions, which suggests they formed in different regions. The hot population is proposed to have formed near Neptune's original orbit and to have been scattered out during the [[planetary migration|migration]] of the giant planets.<ref name=beyond/><ref name="Morbidelli2005">{{cite arXiv
 |last1=Morbidelli |first1=Alessandro
 |date=2005
 |title=Origin and Dynamical Evolution of Comets and their Reservoirs
 |eprint=astro-ph/0512256
}}</ref> The cold population, on the other hand, has been proposed to have formed more or less in its current position because the loose binaries would be unlikely to survive encounters with Neptune.<ref name="Parker_etal_2011a"/> Although the Nice model appears to be able to at least partially explain a compositional difference, it has also been suggested the color difference may reflect differences in surface evolution.<ref name="Levison2008"/>

=== Resonances ===
{{Main|Resonant trans-Neptunian object}}
[[File:KBOs and resonances.png|thumb|upright=1.75|Distribution of [[cubewano]]s (blue), [[Resonant trans-Neptunian object]]s (red), [[Sednoid]]s (yellow) and [[scattered disc|scattered objects]] (grey)]]
[[File:TheKuiperBelt classes-en.svg|thumb|Orbit classification (schematic of [[semi-major axis|semi-major axes]])]]

When an object's orbital period is an exact ratio of Neptune's (a situation called a [[orbital resonance|mean-motion resonance]]), then it can become locked in a synchronised motion with Neptune and avoid being perturbed away if their relative alignments are appropriate. If, for instance, an object orbits the Sun twice for every three Neptune orbits, and if it reaches perihelion with Neptune a quarter of an orbit away from it, then whenever it returns to perihelion, Neptune will always be in about the same relative position as it began, because it will have completed {{frac|1|1|2}} orbits in the same time. This is known as the 2:3 (or 3:2) resonance, and it corresponds to a characteristic [[semi-major axis]] of about 39.4&nbsp;AU. This 2:3 resonance is populated by about 200&nbsp;known objects,<ref>{{cite web |title=List Of Transneptunian Objects |work=Minor Planet Center |url=http://www.minorplanetcenter.org/iau/lists/TNOs.html |access-date=23 June 2007 |archive-date=27 October 2010 |archive-url=https://archive.today/20101027133511/http://www.minorplanetcenter.org/iau/lists/TNOs.html |url-status=live }}</ref> including [[Pluto]] together with [[Moons of Pluto|its moons]]. In recognition of this, the members of this family are known as [[plutino]]s. Many plutinos, including Pluto, have orbits that cross that of Neptune, although their resonance means they can never collide. Plutinos have high orbital eccentricities, suggesting that they are not native to their current positions but were instead thrown haphazardly into their orbits by the migrating Neptune.<ref name="trojan">{{cite journal |author=Chiang |title=Resonance Occupation in the Kuiper Belt: Case Examples of the 5:2 and Trojan Resonances |journal=[[The Astronomical Journal]] |volume=126 |issue=1 |pages=430–443 |date=2003 |doi=10.1086/375207 |last2=Jordan |first2=A. B. |last3=Millis |first3=R. L. |last4=Buie |first4=M. W. |last5=Wasserman |first5=L. H. |last6=Elliot |first6=J. L. |last7=Kern |first7=S. D. |last8=Trilling |first8=D. E. |last9=Meech |first9=K. J. |bibcode=2003AJ....126..430C |arxiv=astro-ph/0301458 |s2cid=54079935 |display-authors=6}}</ref> IAU guidelines dictate that all plutinos must, like Pluto, be named for underworld deities.<ref name=clas/> The 1:2 resonance (whose objects complete half an orbit for each of Neptune's) corresponds to semi-major axes of ~47.7&nbsp;AU, and is sparsely populated.<ref>{{cite web |title=Trans-Neptunian Objects |author=Wm. Robert Johnston |url=http://www.johnstonsarchive.net/astro/tnos.html |date=2007 |access-date=23 June 2007 |archive-date=19 October 2019 |archive-url=https://web.archive.org/web/20191019015108/http://www.johnstonsarchive.net/astro/tnos.html |url-status=live }}</ref> Its residents are sometimes referred to as [[twotino]]s. Other resonances also exist at 3:4, 3:5, 4:7, and 2:5.<ref name=Davies_2001/>{{rp|page=104}} Neptune has a number of [[Neptune trojan|trojan objects]], which occupy its [[Lagrangian point]]s, gravitationally stable regions leading and trailing it in its orbit. Neptune trojans are in a 1:1 mean-motion resonance with Neptune and often have very stable orbits.

Additionally, there is a relative absence of objects with semi-major axes below 39&nbsp;AU that cannot apparently be explained by the present resonances. The currently accepted hypothesis for the cause of this is that as Neptune migrated outward, unstable orbital resonances moved gradually through this region, and thus any objects within it were swept up, or gravitationally ejected from it.<ref name=Davies_2001/>{{rp|page=107}}

=== {{anchor|.22Kuiper_cliff.22}}Kuiper cliff ===
[[File:Hot and cold KBO.svg|thumb|upright=1.5|Histogram of the semi-major axes of Kuiper belt objects with inclinations above and below 5&nbsp;degrees. Spikes from the plutinos and the 'kernel' are visible at 39–40&nbsp;AU and 44&nbsp;AU.]]

The [[Twotino|1:2 resonance]] at 47.8&nbsp;AU appears to be an edge beyond which few objects are known. It is not clear whether it is actually the outer edge of the classical belt or just the beginning of a broad gap. Objects have been detected at the 2:5&nbsp;resonance at roughly 55&nbsp;AU, well outside the classical belt; predictions of a large number of bodies in classical orbits between these resonances have not been verified through observation.<ref name=trojan/>

Based on estimations of the primordial mass required to form [[Uranus]] and Neptune, as well as bodies as large as Pluto ''(see {{Section link||Mass and size distribution}})'', earlier models of the Kuiper belt had suggested that the number of large objects would increase by a factor of two beyond 50&nbsp;AU,<ref name="Brown 1999">{{cite journal |author=E.I. Chiang |author2=M.E. Brown |name-list-style=amp |title=Keck pencil-beam survey for faint Kuiper belt objects |journal=The Astronomical Journal |volume=118 |issue=3 |pages=1411 |url=http://www.gps.caltech.edu/~mbrown/papers/ps/kbodeep.pdf |date=1999 |access-date=1 July 2007 |bibcode=1999AJ....118.1411C |arxiv=astro-ph/9905292 |doi=10.1086/301005 |s2cid=8915427 |archive-date=12 June 2012 |archive-url=https://web.archive.org/web/20120612065305/http://www.gps.caltech.edu/~mbrown/papers/ps/kbodeep.pdf |url-status=live }}</ref> so this sudden drastic falloff, known as the ''Kuiper cliff'', was unexpected, and to date its cause is unknown. Bernstein, Trilling, et al. (2003) found evidence that the rapid decline in objects of 100&nbsp;km or more in radius beyond 50&nbsp;AU is real, and not due to [[observational bias]]. Possible explanations include that material at that distance was too scarce or too scattered to accrete into large objects, or that subsequent processes removed or destroyed those that did.<ref name="Bernstein et al. 2004"/> Patryk Lykawka of [[Kobe University]] claimed that the gravitational attraction of an [[Planets beyond Neptune|unseen large planetary object]], perhaps the size of Earth or [[Mars]], might be responsible.<ref>{{cite web |title=13 Things that do not make sense |author=Michael Brooks |work=NewScientistSpace.com |url=https://www.newscientist.com/article/mg18524911-600-13-things-that-do-not-make-sense/ |date=2005 |access-date=12 October 2018 |archive-date=12 October 2018 |archive-url=https://web.archive.org/web/20181012134912/https://www.newscientist.com/article/mg18524911-600-13-things-that-do-not-make-sense/ |url-status=live }}</ref><ref>{{cite web |title=The mystery of Planet X |date=2008 |author=Govert Schilling |work=New Scientist |url=https://www.newscientist.com/article/mg19726381.600-the-mystery-of-planet-x.html |access-date=8 February 2008 |archive-date=20 April 2015 |archive-url=https://web.archive.org/web/20150420025754/http://www.newscientist.com/article/mg19726381.600-the-mystery-of-planet-x.html |url-status=live }}</ref> An analysis of the TNO data available prior to September 2023 shows that the distribution of objects at the outer rim of the classical Kuiper belt resembles that of the outer main asteroid belt with a gap at about 72 AU, far from any mean-motion resonances with Neptune; the outer main asteroid belt exhibits a gap induced by the 5:6 mean-motion resonance with Jupiter at 5.875 AU.<ref name="KuiperGap">{{cite journal |author=C. de la Fuente Marcos |author2=R. de la Fuente Marcos |name-list-style=amp |title=Past the outer rim, into the unknown: structures beyond the Kuiper Cliff |journal=Monthly Notices of the Royal Astronomical Society Letters |volume=527 |issue=1 |pages=L110–L114 |url=https://academic.oup.com/mnrasl/article-abstract/527/1/L110/7280408 |publication-date=20 September 2023 |date=January 2024 |access-date=28 September 2023 |bibcode=2024MNRAS.527L.110D |arxiv=2309.03885 |doi=10.1093/mnrasl/slad132 |doi-access=free |s2cid= |archive-date=28 October 2023 |archive-url=https://web.archive.org/web/20231028132004/https://academic.oup.com/mnrasl/article-abstract/527/1/L110/7280408 |url-status=live }}</ref>

== 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>

== Composition ==
[[File:2003 UB313 near-infrared spectrum.png|thumb|upright=1.35|The infrared spectra of both Eris and Pluto, highlighting their common methane absorption lines]]

Being distant from the Sun and major planets, Kuiper belt objects are thought to be relatively unaffected by the processes that have shaped and altered other Solar System objects; thus, determining their composition would provide substantial information on the makeup of the earliest Solar System.<ref name="Brown_2012a"/> Due to their small size and extreme distance from Earth, the chemical makeup of KBOs is very difficult to determine. The principal method by which astronomers determine the composition of a celestial object is [[spectroscopy]]. When an object's light is broken into its component colors, an image akin to a rainbow is formed. This image is called a [[spectrum]]. Different substances absorb light at different wavelengths, and when the spectrum for a specific object is unravelled, dark lines (called [[absorption line]]s) appear where the substances within it have absorbed that particular wavelength of light. Every [[element (chemistry)|element]] or [[compound (chemistry)|compound]] has its own unique spectroscopic signature, and by reading an object's full spectral "fingerprint", astronomers can determine its composition.

Analysis indicates that Kuiper belt objects are composed of a mixture of rock and a variety of ices such as water, [[methane]], and [[ammonia]]. The temperature of the belt is only about 50 [[Kelvin|K]],<ref name="Quaoar">{{cite journal |title=Crystalline water ice on the Kuiper belt object (50000) Quaoar |journal=Nature |volume=432 |issue=7018 |pages=731–3 |author=David C. Jewitt |author2=Jane Luu |name-list-style=amp |url=http://www2.ess.ucla.edu/~jewitt/papers/50000/Quaoar.pdf |date=2004 |access-date=21 June 2007 |archive-url=https://web.archive.org/web/20070621182808/http://www.ifa.hawaii.edu/~jewitt/papers/50000/Quaoar.pdf |archive-date=21 June 2007|bibcode=2004Natur.432..731J |doi=10.1038/nature03111 |pmid=15592406 |s2cid=4334385 }}</ref> so many compounds that would be gaseous closer to the Sun remain solid. The densities and rock–ice fractions are known for only a small number of objects for which the diameters and the masses have been determined. The diameter can be determined by imaging with a high-resolution telescope such as the [[Hubble Space Telescope]], by the timing of an [[occultation]] when an object passes in front of a star or, most commonly, by using the [[albedo]] of an object calculated from its infrared emissions. The masses are determined using the semi-major axes and periods of satellites, which are therefore known only for a few binary objects. The densities range from less than 0.4 to 2.6 g/cm<sup>3</sup>. The least dense objects are thought to be largely composed of ice and have significant porosity. The densest objects are likely composed of rock with a thin crust of ice. There is a trend of low densities for small objects and high densities for the largest objects. One possible explanation for this trend is that ice was lost from the surface layers when differentiated objects collided to form the largest objects.<ref name="Brown_2012a">{{cite journal |last1=Brown |first1=Michael E. |title=The Compositions of Kuiper Belt Objects |journal=Annual Review of Earth and Planetary Sciences |date=2012 |volume=40 |issue=1 |pages=467–494 |doi=10.1146/annurev-earth-042711-105352 |arxiv=1112.2764 |bibcode=2012AREPS..40..467B|s2cid=14936224 }}</ref>

[[File:Artist’s impression of exiled asteroid 2004 EW95.jpg|left|thumb|Artist's impression of plutino and possible former [[C-type asteroid]] {{mpl|120216|2004 EW|95}}<ref>{{cite web|title=Exiled Asteroid Discovered in Outer Reaches of Solar System – ESO telescopes find first confirmed carbon-rich asteroid in Kuiper Belt|url=https://www.eso.org/public/news/eso1814/|website=www.eso.org|access-date=May 12, 2018|archive-date=31 May 2019|archive-url=https://web.archive.org/web/20190531211838/https://www.eso.org/public/news/eso1814/|url-status=live}}</ref>]]

Initially, detailed analysis of KBOs was impossible, and so astronomers were only able to determine the most basic facts about their makeup, primarily their color.<ref name="KBOKBO">{{cite web |title=Surfaces of Kuiper Belt Objects |author=Dave Jewitt |work=University of Hawaii |url=http://www2.ess.ucla.edu/~jewitt/kb/kb-colors.html |date=2004 |access-date=21 June 2007 |archive-url=https://web.archive.org/web/20070609094911/http://www.ifa.hawaii.edu/~jewitt/kb/kb-colors.html |archive-date=9 June 2007}}</ref> These first data showed a broad range of colors among KBOs, ranging from neutral grey to deep red.<ref name="color">{{cite journal |doi=10.1086/300299 |title=Optical-Infrared Spectral Diversity in the Kuiper Belt |date=1998 |last1=Jewitt |first1=David |last2=Luu |first2=Jane |journal=The Astronomical Journal |volume=115 |issue=4 |pages=1667–1670 |bibcode=1998AJ....115.1667J|s2cid=122564418 |url=http://pdfs.semanticscholar.org/1749/a1869f99bc927872bebce8fac8682feaf3e6.pdf |archive-url=https://web.archive.org/web/20200412143631/http://pdfs.semanticscholar.org/1749/a1869f99bc927872bebce8fac8682feaf3e6.pdf |url-status=dead |archive-date=2020-04-12 }}</ref> This suggested that their surfaces were composed of a wide range of compounds, from dirty ices to [[hydrocarbon]]s.<ref name=color/> This diversity was startling, as astronomers had expected KBOs to be uniformly dark, having lost most of the volatile ices from their surfaces to the effects of [[cosmic ray]]s.<ref name=Davies_2001/>{{rp|page=118}} Various solutions were suggested for this discrepancy, including resurfacing by impacts or [[outgassing]].<ref name=KBOKBO/> Jewitt and Luu's spectral analysis of the known Kuiper belt objects in 2001 found that the variation in color was too extreme to be easily explained by random impacts.<ref>{{cite journal |doi=10.1086/323304 |title=Colors and Spectra of Kuiper Belt Objects |date=2001 |last1=Jewitt |first1=David C. |last2=Luu |first2=Jane X. |journal=The Astronomical Journal |volume=122 |issue=4 |pages=2099–2114 |arxiv=astro-ph/0107277 |bibcode=2001AJ....122.2099J|s2cid=35561353 }}</ref> The radiation from the Sun is thought to have chemically altered methane on the surface of KBOs, producing products such as [[tholin]]s. [[Makemake]] has been shown to possess a number of hydrocarbons derived from the radiation-processing of methane, including [[ethane]], [[ethylene]] and [[acetylene]].<ref name="Brown_2012a"/>

Although to date most KBOs still appear spectrally featureless due to their faintness, there have been a number of successes in determining their composition.<ref name=Quaoar/> In 1996, Robert H. Brown et al. acquired spectroscopic data on the KBO 1993 SC, which revealed that its surface composition is markedly similar to that of [[Pluto]], as well as Neptune's moon [[Triton (moon)|Triton]], with large amounts of methane ice.<ref name="rbrown">{{cite journal |doi=10.1126/science.276.5314.937 |title=Surface Composition of Kuiper Belt Object 1993SC |date=1997 |last1=Brown |first1=R. H. |journal=Science |volume=276 |issue=5314 |pages=937–9 |pmid=9163038 |last2=Cruikshank |first2=DP |last3=Pendleton |first3=Y |last4=Veeder |first4=GJ |bibcode=1997Sci...276..937B |s2cid=45185392 }}</ref> For the smaller objects, only colors and in some cases the albedos have been determined. These objects largely fall into two classes: gray with low albedos, or very red with higher albedos. The difference in colors and albedos is hypothesized to be due to the retention or the loss of [[hydrogen sulfide]] (H<sub>2</sub>S) on the surface of these objects, with the surfaces of those that formed far enough from the Sun to retain H<sub>2</sub>S being reddened due to irradiation.<ref name="Wong_Brown_2016">{{cite journal|last1=Wong|first1=Ian|last2=Brown|first2=Michael E.|date=2017|title=The bimodal color distribution of small Kuiper Belt objects|arxiv=1702.02615|doi=10.3847/1538-3881/aa60c3|volume=153|issue=4|journal=The Astronomical Journal|page=145|bibcode = 2017AJ....153..145W |s2cid=30811674 |doi-access=free }}</ref>

The largest KBOs, such as Pluto and [[Quaoar]], have surfaces rich in volatile compounds such as methane, [[solid nitrogen|nitrogen]] and [[carbon monoxide]]; the presence of these molecules is likely due to their moderate vapor pressure in the 30–50&nbsp;K temperature range of the Kuiper belt. This allows them to occasionally boil off their surfaces and then fall again as snow, whereas compounds with higher boiling points would remain solid. The relative abundances of these three compounds in the largest KBOs is directly related to their [[surface gravity]] and ambient temperature, which determines which they can retain.<ref name="Brown_2012a"/> Water ice has been detected in several KBOs, including members of the Haumea family such as {{mpl-|19308|1996 TO|66}},<ref>{{cite journal |doi=10.1086/317277 |title=Near-Infrared Spectroscopy of the Bright Kuiper Belt Object 2000 EB173 |date=2000 |last1=Brown |first1=Michael E. |last2=Blake |first2=Geoffrey A. |last3=Kessler |first3=Jacqueline E. |journal=The Astrophysical Journal |volume=543 |issue=2 |pages=L163 |bibcode=2000ApJ...543L.163B|citeseerx=10.1.1.491.4308 |s2cid=122764754 }}</ref> mid-sized objects such as [[38628 Huya]] and [[20000 Varuna]],<ref>{{cite journal |date=2001 |title=NICS-TNG infrared spectroscopy of trans-neptunian objects 2000 EB173 and 2000 WR106 |author1=Licandro |author2=Oliva |author3=Di MArtino |doi=10.1051/0004-6361:20010758 |journal=Astronomy and Astrophysics |volume=373 |issue=3 |pages=L29 |arxiv=astro-ph/0105434 |bibcode=2001A&A...373L..29L|s2cid=15690206 }}</ref> and also on some small objects.<ref name="Brown_2012a"/> The presence of crystalline ice on large and mid-sized objects, including [[50000 Quaoar]] where [[ammonia]] [[hydrate]] has also been detected,<ref name=Quaoar/> may indicate past tectonic activity aided by melting point lowering due to the presence of ammonia.<ref name="Brown_2012a"/>

== Mass and size distribution ==

Despite its vast extent, the collective [[mass]] of the Kuiper belt is relatively low. The total mass of the dynamically hot population is estimated to be 1% the [[Earth mass|mass of the Earth]]. The dynamically cold population is estimated to be much smaller with only 0.03% the mass of the Earth.<ref name="Fraser_etal_2014"/><ref name="g01">{{cite journal |last=Gladman |first=Brett |display-authors=etal |title=The structure of the Kuiper belt |journal=Astronomical Journal |date=August 2001 |volume=122 |pages=1051–1066 |doi=10.1086/322080 |bibcode=2001AJ....122.1051G |issue=2|s2cid=54756972 |doi-access= }}</ref> While the dynamically hot population is thought to be the remnant of a much larger population that formed closer to the Sun and was scattered outward during the migration of the giant planets, in contrast, the dynamically cold population is thought to have formed at its current location. The most recent estimate (2018) puts the total mass of the Kuiper belt at {{val|1.97e-2|0.30}} Earth masses based on the influence that it exerts on the motion of planets.<ref name=Pitjeva2018>{{cite journal |last1=Pitjeva |first1=E. V. |last2=Pitjev |first2=N. P. |title=Masses of the Main Asteroid Belt and the Kuiper Belt from the Motions of Planets and Spacecraft |journal=Astronomy Letters |date=30 October 2018 |volume=44 |issue=89 |pages=554–566 |doi=10.1134/S1063773718090050|arxiv=1811.05191 |bibcode=2018AstL...44..554P |s2cid=119404378 }}</ref>

The small total mass of the dynamically cold population presents some problems for models of the [[Formation and evolution of the Solar System|Solar System's formation]] because a sizable mass is required for accretion of KBOs larger than {{convert|100|km|0|abbr=on}} in diameter.<ref name=beyond/> If the cold classical Kuiper belt had always had its current low density, these large objects simply could not have formed by the collision and mergers of smaller planetesimals.<ref name=beyond/> Moreover, the eccentricity and inclination of current orbits make the encounters quite "violent" resulting in destruction rather than accretion. The removal of a large fraction of the mass of the dynamically cold population is thought to be unlikely. Neptune's current influence is too weak to explain such a massive "vacuuming", and the extent of mass loss by collisional grinding is limited by the presence of loosely bound binaries in the cold disk, which are likely to be disrupted in collisions.<ref name="Nesvorny_etal_2011a">{{cite journal |last1=Nesvorný |first1=David |last2=Vokrouhlický |first2=David |last3=Bottke |first3=William F. |last4=Noll |first4=Keith |last5=Levison |first5=Harold F. |title=Observed Binary Fraction Sets Limits on the Extent of Collisional Grinding in the Kuiper Belt |journal=The Astronomical Journal |date=2011 |volume=141 |issue=5 |page=159 |doi=10.1088/0004-6256/141/5/159 |arxiv=1102.5706 |bibcode=2011AJ....141..159N|s2cid=54187134 }}</ref> Instead of forming from the collisions of smaller planetesimals, the larger object may have formed directly from the collapse of clouds of pebbles.<ref name="Morbidelli_Nesvorny_2019">{{cite book |last1=Morbidelli |first1=Alessandro |last2=Nesvorny |first2=David |title=The Trans-Neptunian Solar System |chapter=Kuiper belt: formation and evolution |year=2020 |pages=25–59 |doi=10.1016/B978-0-12-816490-7.00002-3 |arxiv=1904.02980|isbn=9780128164907 |s2cid=102351398 }}</ref>

[[File:TheKuiperBelt PowerLaw2.svg|thumb|upright=1.25|Illustration of the power law]]
The size distributions of the Kuiper belt objects follow a number of [[power law]]s. A power law describes the relationship between ''N''(''D'') (the number of objects of diameter greater than ''D'') and ''D'', and is referred to as brightness slope. The number of objects is inversely proportional to some power of the diameter ''D'':
:<math> \frac{d N}{d D} \propto D^{-q},</math> which yields (assuming ''q'' is not 1):<math>N\propto D^{1-q}+\text{a constant}.</math>
(The constant may be non-zero only if the power law doesn't apply at high values of ''D''.)<!--If it were negative, that would imply negative numbers about a certain size.-->

Early estimates that were based on measurements of the apparent magnitude distribution found a value of q = 4 ± 0.5,<ref name="Bernstein et al. 2004">{{cite journal |last1=Bernstein |first1=G. M. |last2=Trilling |first2=D. E. |last3=Allen |first3=R. L. |last4=Brown |first4=K. E. |last5=Holman |first5=M. |last6=Malhotra |first6=R. |title=The size distribution of transneptunian bodies |journal=[[The Astronomical Journal]] |volume=128 |issue=3 |pages=1364–1390 |doi=10.1086/422919 |arxiv=astro-ph/0308467 |bibcode=2004AJ....128.1364B |date=2004|s2cid=13268096 }}</ref> which implied that there are 8 (=2<sup>3</sup>) times more objects in the 100–200&nbsp;km range than in the 200–400&nbsp;km range. 
<!--If ''q'' was 1 or less, the law would imply an infinite number and mass of large objects in the Kuiper belt. If 1<''q''≤4 there will be a finite number of objects greater than a given size, but the [[expected value]] of their combined mass would be infinite. If ''q'' is 4 or more, the law would imply an infinite mass of small objects. More accurate models find that the "slope" parameter ''q'' is in effect greater at large diameters and lesser at small diameters.<ref name="Bernstein et al. 2004"/> It seems that [[Pluto]] is somewhat unexpectedly large, having several percent of the total mass of the Kuiper belt. It is not expected that anything larger than Pluto exists in the Kuiper belt, and in fact, most of the brightest (largest) objects at inclinations less than 5° have probably been found.<ref name="Bernstein et al. 2004"/>

For most TNOs, only the [[absolute magnitude]] is actually known, the size is inferred assuming a given [[albedo]] (not a safe assumption for larger objects).-->

Recent research has revealed that the size distributions of the hot classical and cold classical objects have differing slopes. The slope for the hot objects is q = 5.3 at large diameters and q = 2.0 at small diameters with the change in slope at 110&nbsp;km. The slope for the cold objects is q = 8.2 at large diameters and q = 2.9 at small diameters with a change in slope at 140&nbsp;km.<ref name="Fraser_etal_2014">{{cite journal |last1=Fraser |first1=Wesley C. |last2=Brown |first2=Michael E. |last3=Morbidelli |first3=Alessandro |last4=Parker |first4=Alex |last5=Batygin |first5=Konstantin |title=The Absolute Magnitude Distribution of Kuiper Belt Objects |journal=The Astrophysical Journal |date=2014 |volume=782 |issue=2 |page=100 |doi=10.1088/0004-637X/782/2/100 |bibcode=2014ApJ...782..100F |arxiv=1401.2157|s2cid=2410254 }}</ref> The size distributions of the [[scattering object]]s, the plutinos, and the Neptune trojans have slopes similar to the other dynamically hot populations, but may instead have a divot, a sharp decrease in the number of objects below a specific size. This divot is hypothesized to be due to either the collisional evolution of the population, or to be due to the population having formed with no objects below this size, with the smaller objects being fragments of the original objects.<ref name="Shankman_etal_2016a">{{cite journal |last1=Shankman |first1=C. |last2=Kavelaars |first2=J. J. |last3=Gladman |first3=B. J. |last4=Alexandersen |first4=M. |last5=Kaib |first5=N. |last6=Petit |first6=J.-M. |last7=Bannister |first7=M. T. |last8=Chen |first8=Y.-T. |last9=Gwyn |first9=S.|last10=Jakubik|first10=M. |last11=Volk |first11=K. |title=OSSOS. II. A Sharp Transition in the Absolute Magnitude Distribution of the Kuiper Belt's Scattering Population |journal=The Astronomical Journal |date=2016 |volume=150 |issue=2 |page=31 |doi= 10.3847/0004-6256/151/2/31 |arxiv= 1511.02896 |bibcode= 2016AJ....151...31S |s2cid=55213074 |doi-access=free }}</ref><ref name="Alexandersen_etal_2014a">{{cite journal |last1= Alexandersen |first1= Mike |last2= Gladman |first2=Brett |last3= Kavelaars |first3=J.J. |last4=Petit |first4= Jean-Marc |last5=Gwyn |first5=Stephen |last6= Shankman |first6= Cork |title=A carefully characterised and tracked Trans-Neptunian survey, the size-distribution of the Plutinos and the number of Neptunian Trojans |journal= The Astronomical Journal |volume= 152 |issue= 5 |pages= 111 |date=2014 |arxiv= 1411.7953 |doi= 10.3847/0004-6256/152/5/111 |s2cid= 119108385 |doi-access= free }}</ref>

The smallest known Kuiper belt objects with radii below 1&nbsp;km have only been detected by [[stellar occultation]]s, as they are far too dim ([[apparent magnitude|magnitude]] 35) to be seen directly by telescopes such as the [[Hubble Space Telescope]].<ref>{{cite news |title=Hubble Finds Smallest Kuiper Belt Object Ever Seen |url=https://hubblesite.org/contents/news-releases/2009/news-2009-33.html |access-date=29 June 2015 |publisher=HubbleSite |date=December 2009 |archive-date=25 January 2021 |archive-url=https://web.archive.org/web/20210125162801/https://hubblesite.org/contents/news-releases/2009/news-2009-33.html |url-status=live }}</ref> The first reports of these occultations were from Schlichting et al. in December 2009, who announced the discovery of a small, sub-kilometre-radius Kuiper belt object in archival ''Hubble'' [[photometry (astronomy)|photometry]] from March 2007. With an estimated radius of {{val|520|60|u=m}} or a diameter of {{val|1040|120|u=m}}, the object was detected by ''Hubble''{{'s}} star tracking system when it briefly occulted a star for 0.3 seconds.<ref name="Schlichting2009">{{cite journal |last1= Schlichting |first1= H. E. |last2= Ofek |first2= E. O.|last3= Wenz |first3= M. |last4= Sari |first4= R. |last5=Gal-Yam |first5=A. |last6= Livio |first6= M. |display-authors=etal |title=A single sub-kilometre Kuiper belt object from a stellar occultation in archival data |journal=Nature |volume= 462 |issue= 7275 |pages= 895–897 |date=December 2009 |arxiv= 0912.2996 |doi= 10.1038/nature08608 |pmid= 20016596 |bibcode=2009Natur.462..895S|s2cid= 205219186 }}</ref> In a subsequent study published in December 2012, Schlichting et al. performed a more thorough analysis of archival ''Hubble'' photometry and reported another occultation event by a sub-kilometre-sized Kuiper belt object, estimated to be {{val|530|70|u=m}} in radius or {{val|1060|140|u=m}} in diameter. From the occultation events detected in 2009 and 2012, Schlichting et al. determined the Kuiper belt object size distribution slope to be q = 3.6 ± 0.2 or q = 3.8 ± 0.2, with the assumptions of a single power law and a uniform [[ecliptic latitude]] distribution. Their result implies a strong deficit of sub-kilometer-sized Kuiper belt objects compared to  extrapolations from the population of larger Kuiper belt objects with diameters above 90&nbsp;km.<ref name="Schlichting2012">{{cite journal |last1= Schlichting |first1= H. E. |last2= Ofek |first2= E. O.|last3= Wenz |first3= M. |last4= Sari |first4= R. |last5=Gal-Yam |first5=A. |last6= Livio |first6= M. |display-authors=etal |title=Measuring the Abundance of Sub-kilometer-sized Kuiper Belt Objects Using Stellar Occultations |journal=The Astrophysical Journal |volume= 761 |issue= 2 |pages= 10 |id= 150 |date=December 2012 |arxiv= 1210.8155  |doi= 10.1088/0004-637X/761/2/150 |bibcode=2012ApJ...761..150S  |s2cid= 31856299 |doi-access= free}}</ref>

Observations made by NASA's ''[[New Horizons]]'' Venetia Burney Student Dust Counter showed "higher than model-predicted dust fluxes" as far as 55 au, not explained by any existing model.<ref>{{cite journal |last1=Doner |first1=Alex |last2=Horányi |first2=Mihály |last3=Bagenal |first3=Fran |last4=Brandt |first4=Pontus |last5=Grundy |first5=Will |last6=Lisse |first6=Carey |last7=Parker |first7=Joel |last8=Poppe |first8=Andrew R. |last9=Singer |first9=Kelsi N. |last10=Stern |first10=S. Alan |last11=Verbiscer |first11=Anne |title=New Horizons Venetia Burney Student Dust Counter Observes Higher than Expected Fluxes Approaching 60 au |journal=The Astrophysical Journal Letters |date=1 February 2024 |volume=961 |issue=2 |pages=L38 |doi=10.3847/2041-8213/ad18b0|doi-access=free |arxiv=2401.01230 |bibcode=2024ApJ...961L..38D }}</ref>

== Scattered objects ==
[[File:TheKuiperBelt Projections 100AU Classical SDO.svg|left|thumb|upright=1.25|Comparison of the orbits of scattered disc objects (black), classical KBOs (blue), and 2:5 resonant objects (green). Orbits of other KBOs are gray. (Orbital axes have been aligned for comparison.)]]
{{Main|Scattered disc|Centaur (minor planet)}}

The scattered disc is a sparsely populated region, overlapping with the Kuiper belt but extending to beyond 100 AU. Scattered disc objects (SDOs) have very elliptical orbits, often also very inclined to the ecliptic. Most models of Solar System formation show both KBOs and SDOs first forming in a primordial belt, with later gravitational interactions, particularly with Neptune, sending the objects outward, some into stable orbits (the KBOs) and some into unstable orbits, the scattered disc.<ref name=book/> Due to its unstable nature, the scattered disc is suspected to be the point of origin of many of the Solar System's short-period comets. Their dynamic orbits occasionally force them into the inner Solar System, first becoming [[centaur (minor planet)|centaurs]], and then short-period comets.<ref name=book/>

According to the [[Minor Planet Center]], which officially catalogues all trans-Neptunian objects, a KBO is any object that orbits exclusively within the defined Kuiper belt region regardless of origin or composition. Objects found outside the belt are classed as scattered objects.<ref name="cen_sdo">{{cite web |url=http://www.minorplanetcenter.org/iau/lists/Centaurs.html |title=List Of Centaurs and Scattered-Disk Objects |work=IAU: Minor Planet Center |access-date=27 October 2010 |archive-date=29 June 2017 |archive-url=https://web.archive.org/web/20170629210646/http://www.minorplanetcenter.org/iau/lists/Centaurs.html |url-status=live }}</ref> In some scientific circles the term "Kuiper belt object" has become synonymous with any icy minor planet native to the outer Solar System assumed to have been part of that initial class, even if its orbit during the bulk of Solar System history has been beyond the Kuiper belt (e.g. in the scattered-disc region). They often describe scattered disc objects as "scattered Kuiper belt objects".<ref>{{cite web |date=2005 |author=David Jewitt |title=The 1000 km Scale KBOs |work=University of Hawaii |url=http://www2.ess.ucla.edu/~jewitt/kb/big_kbo.html |access-date=16 July 2006 |archive-date=2 July 2017 |archive-url=https://web.archive.org/web/20170702173042/http://www2.ess.ucla.edu/~jewitt/kb/big_kbo.html |url-status=live }}</ref> [[Eris (dwarf planet)|Eris]], which is known to be more massive than Pluto, is often referred to as a KBO, but is technically an SDO.<ref name="cen_sdo"/> A consensus among astronomers as to the precise definition of the Kuiper belt has yet to be reached, and this issue remains unresolved.

The centaurs, which are not normally considered part of the Kuiper belt, are also thought to be scattered objects, the only difference being that they were scattered inward, rather than outward. The Minor Planet Center groups the centaurs and the SDOs together as scattered objects.<ref name="cen_sdo"/>

=== Triton ===
{{Main|Triton (moon)}}
[[File:Triton moon mosaic Voyager 2 (large).jpg|thumb|upright|[[Neptune]]'s moon [[Triton (moon)|Triton]]]]

During its period of migration, Neptune is thought to have captured a large KBO, [[Triton (moon)|Triton]], which is the only large moon in the Solar System with a [[retrograde orbit]] (that is, it orbits opposite to Neptune's rotation). This suggests that, unlike the large [[moons of Jupiter]], [[Moons of Saturn|Saturn]] and [[Moons of Uranus|Uranus]], which are thought to have coalesced from rotating discs of material around their young parent planets, Triton was a fully formed body that was captured from surrounding space. Gravitational capture of an object is not easy: it requires some mechanism to slow down the object enough to be caught by the larger object's gravity. A possible explanation is that Triton was part of a binary when it encountered Neptune. (Many KBOs are members of binaries. See [[#Satellites|below]].) Ejection of the other member of the binary by Neptune could then explain Triton's capture.<ref>{{cite journal |title=Neptune's capture of its moon Triton in a binary-planet gravitational encounter |author=Craig B. Agnor |author2=Douglas P. Hamilton |name-list-style=amp |journal=Nature |volume=441 |issue=7090 |pages=192–194 |url=http://www.es.ucsc.edu/~cagnor/papers_pdf/2006AgnorHamilton.pdf |date=2006 |access-date=29 October 2007 |archive-url=https://web.archive.org/web/20070621182809/http://www.es.ucsc.edu/~cagnor/papers_pdf/2006AgnorHamilton.pdf |archive-date=21 June 2007 |url-status=dead|bibcode=2006Natur.441..192A |doi=10.1038/nature04792 |pmid=16688170 |s2cid=4420518 }}</ref> Triton is only 14% larger than Pluto, and spectral analysis of both worlds shows that their surfaces are largely composed of similar materials, such as [[methane]] and [[carbon monoxide]]. All this points to the conclusion that Triton was once a KBO that was captured by Neptune during its [[Nice model|outward migration]].<ref>{{cite book |last1=Encrenaz |first1=Thérèse |author1-link=Thérèse Encrenaz|last2=Kallenbach |first2=R. |last3=Owen |first3=T. |last4=Sotin |first4=C. |title=Triton, Pluto, Centaurs, and Trans-Neptunian Bodies |url=https://books.google.com/books?id=MbmiTd3x1UcC&pg=PA421 |access-date=23 June 2007 |date=2004 |publisher=Springer |isbn=978-1-4020-3362-9}}</ref>

== Largest KBOs ==
{{see also|List of the brightest Kuiper belt objects}}
<!-- Please do not add Eris here. Eris is often called a Kuiper belt object but Wiki convention treats it strictly as a scattered disc object -->

Since 2000, a number of KBOs with diameters of between 500 and {{convert|1500|km|0|abbr=on}}, more than half that of Pluto (diameter 2370&nbsp;km), have been discovered. [[50000 Quaoar|Quaoar]], a classical KBO discovered in 2002, is over 1,200&nbsp;km across. {{dp|Makemake}} and {{dp|Haumea}}, both announced on 29 July 2005, are larger still. Other objects, such as [[28978 Ixion]] (discovered in 2001) and [[20000 Varuna]] (discovered in 2000), measure roughly {{convert|600–700|km|0|abbr=on}} across.<ref name=beyond/>

=== Pluto ===
<!-- Please do not add Eris here. Eris is often called a Kuiper belt object but Wikipedia convention treats it strictly as a scattered disc object -->
{{Main|Pluto}}
The discovery of these large KBOs in orbits similar to Pluto's led many to conclude that, aside from its relative size, [[Pluto]] was not particularly different from other members of the Kuiper belt. Not only are these objects similar to Pluto in size, but many also have [[natural satellites]], and are of similar composition (methane and carbon monoxide have been found both on Pluto and on the largest KBOs).<ref name=beyond/> Thus, just as [[Ceres (dwarf planet)|Ceres]] was considered a planet before the discovery of its fellow [[asteroid]]s, some began to suggest that Pluto might also be reclassified.

The issue was brought to a head by the discovery of [[Eris (dwarf planet)|Eris]], an object in the [[scattered disc]] far beyond the Kuiper belt, that is now known to be 27% more massive than Pluto.<ref>{{cite web |title=Dysnomia, the moon of Eris |author=Mike Brown |work=Caltech |url=http://www.gps.caltech.edu/~mbrown/planetlila/moon/index.html |date=2007 |access-date=14 June 2007 |archive-date=17 July 2012 |archive-url=https://web.archive.org/web/20120717010420/http://www.gps.caltech.edu/~mbrown/planetlila/moon/index.html |url-status=live }}</ref> (Eris was originally thought to be larger than Pluto by volume, but the ''[[New Horizons]]'' mission found this not to be the case.) In response, the [[International Astronomical Union]] (IAU) was forced to [[Definition of planet|define what a planet is]] for the first time, and in so doing included in their definition that a planet must have "[[Clearing the neighborhood|cleared the neighbourhood]] around its orbit".<ref>{{cite web |url=http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf |title=Resolution B5 and B6 |publisher=International Astronomical Union |date=2006 |access-date=2 September 2011 |archive-date=20 June 2009 |archive-url=https://web.archive.org/web/20090620102000/http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf |url-status=live }}</ref> As Pluto shares its orbit with many other sizable objects, it was deemed not to have cleared its orbit and was thus reclassified from a planet to a [[dwarf planet]], making it a member of the Kuiper belt.

It is not clear how many KBOs are large enough to be dwarf planets. Consideration of the surprisingly low densities of many dwarf-planet candidates suggests that not many are.<ref name="Grundy2019">{{cite journal |last1=Grundy |first1=W.M. |last2=Noll |first2=K.S. |last3=Buie |first3=M.W. |last4=Benecchi |first4=S.D. |last5=Ragozzine |first5=D. |last6=Roe |first6=H.G. |title=The mutual orbit, mass, and density of transneptunian binary Gǃkúnǁʼhòmdímà ({{mp|(229762) 2007 UK|126}}) |journal=Icarus |date=December 2019 |volume=334 |pages=30–38 |doi=10.1016/j.icarus.2018.12.037 |s2cid=126574999 |url=http://www2.lowell.edu/users/grundy/abstracts/preprints/2019.G-G.pdf |url-status=live |archive-url=https://web.archive.org/web/20190407045339/http://www2.lowell.edu/~grundy/abstracts/preprints/2019.G-G.pdf |archive-date=2019-04-07 }}</ref> {{dp|Orcus}}, Pluto, [[Haumea]], {{dp|Quaoar}}, and [[Makemake]] are accepted by most astronomers; some have proposed other bodies, such as {{dp|Salacia}}, {{mpl|2002 MS|4}},<ref name="BrownList">Mike Brown, [http://www.gps.caltech.edu/~mbrown/dps.html 'How many dwarf planets are there in the outer solar system?'] {{webarchive |url=https://web.archive.org/web/20111018154917/http://www.gps.caltech.edu/~mbrown/dps.html |date=18 October 2011 }} Accessed 15 November 2013</ref> {{mpl|2002 AW|197}}, and {{dp|Ixion}}.<ref name=Tancredi2008>{{Cite journal |last1=Tancredi |first1=G. |last2=Favre |first2=S. A. |doi=10.1016/j.icarus.2007.12.020 |title=Which are the dwarfs in the Solar System? |journal=Icarus |volume=195 |issue=2 |pages=851–862 |year=2008 |bibcode=2008Icar..195..851T}}</ref>

=== Satellites ===
The six largest TNOs ([[Eris (dwarf planet)|Eris]], [[Pluto]], [[225088 Gonggong|Gonggong]], [[Makemake]], [[Haumea]] and [[50000 Quaoar|Quaoar]]) are all known to have satellites, and two of them have more than one. A higher percentage of the larger KBOs have satellites than the smaller objects in the Kuiper belt, suggesting that a different formation mechanism was responsible.<ref>{{cite journal |doi=10.1086/501524 |last1=Brown |first1=M. E. |author-link=Michael E. Brown |last2=Van Dam |first2=M. A. |last3=Bouchez |first3=A. H. |last4=Le Mignant |first4=D. |last5=Campbell |first5=R. D. |last6=Chin |first6=J. C. Y. |last7=Conrad |first7=A. |last8=Hartman |first8=S. K. |last9=Johansson |first9=E. M. |last10=Lafon |first10=R. E. |last11=Rabinowitz |first11=D. L. Rabinowitz |last12=Stomski |first12=P. J. Jr. |last13=Summers |first13=D. M. |last14=Trujillo |first14=C. A. |last15=Wizinowich |first15=P. L. |year=2006 |title=Satellites of the Largest Kuiper Belt Objects |journal=The Astrophysical Journal |volume=639 |issue=1 |pages=L43–L46 |arxiv=astro-ph/0510029 |bibcode=2006ApJ...639L..43B |s2cid=2578831 |url=http://web.gps.caltech.edu/~mbrown/papers/ps/gab.pdf |access-date=19 October 2011 |ref={{sfnRef|Brown Van Dam et al.|2006}} |archive-date=28 September 2018 |archive-url=https://web.archive.org/web/20180928185647/http://web.gps.caltech.edu/~mbrown/papers/ps/gab.pdf |url-status=live }}</ref> There are also a high number of binaries (two objects close enough in mass to be orbiting "each other") in the Kuiper belt. The most notable example is the Pluto–Charon binary, but it is estimated that around 11% of KBOs exist in binaries.<ref>{{cite journal |first1=C.B. |last1=Agnor |first2=D.P. |last2=Hamilton |title=Neptune's capture of its moon Triton in a binary-planet gravitational encounter |journal=Nature |volume=441 |date=2006 |pages=192–4 |url=http://www.astro.umd.edu/~hamilton/research/reprints/AgHam06.pdf |doi=10.1038/nature04792 |pmid=16688170 |issue=7090 |bibcode=2006Natur.441..192A |s2cid=4420518 |access-date=9 July 2010 |archive-date=3 November 2013 |archive-url=https://web.archive.org/web/20131103235809/http://www.astro.umd.edu/~hamilton/research/reprints/AgHam06.pdf |url-status=live }}</ref>

== Exploration ==
{{Main|New Horizons}}
[[File:KBO 2014 MU69 HST.jpg|thumb|upright|The KBO [[486958 Arrokoth]] (green circles), the selected target for the ''New Horizons'' Kuiper belt object mission]]
On 19 January 2006, the first spacecraft to explore the Kuiper belt, ''[[New Horizons]]'', was launched, which flew by [[Pluto]] on 14 July 2015. Beyond the Pluto flyby, the mission's goal was to locate and investigate other, farther objects in the Kuiper belt.<ref>{{cite web |url=http://discoverynewfrontiers.nasa.gov/missions/missions_nh.cfml |title=New Frontiers Program: New Horizons Science Objectives |work=NASA – New Frontiers Program |access-date=15 April 2015 |url-status=dead |archive-url=https://web.archive.org/web/20150415224640/http://discoverynewfrontiers.nasa.gov/missions/missions_nh.cfml |archive-date=15 April 2015  }}</ref>

[[File:2014 MU69 orbit.jpg|thumb|left|upright=1|Diagram showing the location of 486958 Arrokoth and trajectory for rendezvous]]
[[File:Ultima_Thule_New_Horizons_CA06_vertical.png|thumb|upright|''New Horizons'' grayscale image of Arrokoth, its surface likely covered in organic compounds.<ref name="NASA-20190516">{{cite web|url=https://www.nasa.gov/feature/nasa-s-new-horizons-team-publishes-first-kuiper-belt-flyby-science-results|title=NASA's New Horizons Team Publishes First Kuiper Belt Flyby Science Results|publisher=NASA|date=16 May 2019|access-date=16 May 2019|archive-date=16 December 2019|archive-url=https://web.archive.org/web/20191216073527/https://www.nasa.gov/feature/nasa-s-new-horizons-team-publishes-first-kuiper-belt-flyby-science-results/|url-status=live}}</ref> So far, it is the only KBO besides Pluto and its satellites to be visited by a spacecraft.]]

On 15 October 2014, it was revealed that ''Hubble'' had uncovered three potential targets, provisionally designated PT1 ("potential target 1"), PT2 and PT3 by the ''New Horizons'' team.<ref>{{cite web |title=NASA's Hubble Telescope Finds Potential Kuiper Belt Targets for New Horizons Pluto Mission |work=press release |publisher=Johns Hopkins [[Applied Physics Laboratory]] |date=15 October 2014 |access-date=16 October 2014 |archive-date=16 October 2014 |url=http://www.jhuapl.edu/newscenter/pressreleases/2014/141015_2.asp |archive-url=https://web.archive.org/web/20141016023345/http://www.jhuapl.edu/newscenter/pressreleases/2014/141015_2.asp}}</ref><ref>{{cite web |author=Buie, Marc |author-link=Marc W. Buie |title=New Horizons HST KBO Search Results: Status Report |url=http://www.stsci.edu/institute/stuc/oct-2014/New-Horizons.pdf |publisher=[[Space Telescope Science Institute]] |date=15 October 2014 |page=23 |access-date=29 August 2015 |archive-date=27 July 2015 |archive-url=https://wayback.archive-it.org/all/20150727213348/http://www.stsci.edu/institute/stuc/oct-2014/New-Horizons.pdf |url-status=dead }}</ref> The objects' diameters were estimated to be in the 30–55&nbsp;km range; too small to be seen by ground telescopes, at distances from the Sun of 43–44 AU, which would put the encounters in the 2018–2019 period.<ref name = "Lakdawalla2014">{{cite web |last=Lakdawalla |first=Emily |author-link=Emily Lakdawalla |title=Finally! New Horizons has a second target |work=Planetary Society blog |publisher=[[Planetary Society]] |date=15 October 2014 |access-date=15 October 2014 |archive-date=15 October 2014 |url=http://www.planetary.org/blogs/emily-lakdawalla/2014/10151024-finally-new-horizons-has-a-kbo.html |archive-url=https://web.archive.org/web/20141015230432/http://www.planetary.org/blogs/emily-lakdawalla/2014/10151024-finally-new-horizons-has-a-kbo.html |url-status=live}}</ref> The initial estimated probabilities that these objects were reachable within ''New Horizons''{{'}} fuel budget were 100%, 7%, and 97%, respectively.<ref name="Lakdawalla2014"/> All were members of the "cold" (low-[[orbital inclination|inclination]], low-[[orbital eccentricity|eccentricity]]) [[classical Kuiper belt]], and thus very different from Pluto. [[2014 MU69|PT1]] (given the temporary designation "1110113Y" on the HST web site<ref>{{cite web |title=Hubble to Proceed with Full Search for New Horizons Targets |work=HubbleSite news release |publisher=[[Space Telescope Science Institute]] |date=1 July 2014 |access-date=15 October 2014 |url=http://hubblesite.org/newscenter/archive/releases/2014/35/image/a/ |archive-date=12 May 2015 |archive-url=https://web.archive.org/web/20150512031618/http://hubblesite.org/newscenter/archive/releases/2014/35/image/a/ |url-status=live }}</ref>), the most favorably situated object, was magnitude 26.8, 30–45&nbsp;km in diameter, and was encountered in January 2019.<ref name="VOX-20150414">{{cite web |last=Stromberg |first=Joseph |title=NASA's New Horizons probe was visiting Pluto — and just sent back its first color photos |url=https://www.vox.com/2015/4/14/8412031/pluto-new-horizons |date=14 April 2015 |work=[[Vox (website)|Vox]] |access-date=14 April 2015 |archive-date=6 April 2020 |archive-url=https://web.archive.org/web/20200406132908/https://www.vox.com/2015/4/14/8412031/pluto-new-horizons |url-status=live }}</ref> Once sufficient orbital information was provided, the [[Minor Planet Center]] gave official designations to the three target KBOs: {{mpl|2014 MU|69}} (PT1), {{mpl|2014 OS|393}} (PT2), and {{mpl|2014 PN|70}} (PT3). By the fall of 2014, a possible fourth target, {{mpl|2014 MT|69}}, had been eliminated by follow-up observations. PT2 was out of the running before the Pluto flyby.<ref>{{cite web |author=Corey S. Powell |title=Alan Stern on Pluto's Wonders, New Horizons' Lost Twin, and That Whole "Dwarf Planet" Thing |url=http://blogs.discovermagazine.com/outthere/2015/03/29/alan-stern-on-plutos-wonders/ |magazine=[[Discover (magazine)|Discover]] |date=29 March 2015 |author-link=Corey S. Powell |access-date=29 August 2015 |archive-date=16 November 2019 |archive-url=https://web.archive.org/web/20191116104116/http://blogs.discovermagazine.com/outthere/2015/03/29/alan-stern-on-plutos-wonders/ |url-status=dead }}</ref><ref>{{cite journal |url=http://www.hou.usra.edu/meetings/lpsc2015/pdf/1301.pdf |title=Orbits and Accessibility of Potential New Horizons KBO Encounter Targets |issue=1832 |pages=1301 |date=2015 |journal=USRA-Houston |archive-url=https://web.archive.org/web/20160303182211/http://www.hou.usra.edu/meetings/lpsc2015/pdf/1301.pdf |archive-date=3 March 2016|bibcode=2015LPI....46.1301P |last1=Porter |first1=S. B. |last2=Parker |first2=A. H. |last3=Buie |first3=M. |last4=Spencer |first4=J. |last5=Weaver |first5=H. |last6=Stern |first6=S. A. |last7=Benecchi |first7=S. |last8=Zangari |first8=A. M. |last9=Verbiscer |first9=A. |last10=Gywn |first10=S. |last11=Petit |first11=J. -M. |last12=Sterner |first12=R. |last13=Borncamp |first13=D. |last14=Noll |first14=K. |last15=Kavelaars |first15=J. J. |last16=Tholen |first16=D. |last17=Singer |first17=K. N. |last18=Showalter |first18=M. |last19=Fuentes |first19=C. |last20=Bernstein |first20=G. |last21=Belton |first21=M. }}</ref>

On 26 August 2015, the first target, {{mp|2014 MU|69}} (nicknamed "Ultima Thule" and later named [[486958 Arrokoth]]), was chosen. Course adjustment took place in late October and early November 2015, leading to a flyby in January 2019.<ref>{{cite web |last=McKinnon |first=Mika |url=http://space.gizmodo.com/new-horizons-locks-onto-next-target-lets-explore-the-k-1727298103 |title=New Horizons Locks Onto Next Target: Let's Explore the Kuiper Belt! |date=28 August 2015 |url-status=live |archive-url=https://web.archive.org/web/20151231190843/http://space.gizmodo.com/new-horizons-locks-onto-next-target-lets-explore-the-k-1727298103 |archive-date=31 December 2015}}</ref> On 1 July 2016, NASA approved additional funding for ''New Horizons'' to visit the object.<ref>{{cite web|title=New Horizons Receives Mission Extension to Kuiper Belt, Dawn to Remain at Ceres|url=https://www.nasa.gov/feature/new-horizons-receives-mission-extension-to-kuiper-belt-dawn-to-remain-at-ceres|date=1 July 2016|author=Dwayne Brown / Laurie Cantillo|publisher=NASA|access-date=15 May 2017|archive-date=20 August 2016|archive-url=https://web.archive.org/web/20160820075642/http://www.nasa.gov/feature/new-horizons-receives-mission-extension-to-kuiper-belt-dawn-to-remain-at-ceres/|url-status=live}}</ref>

On 2 December 2015, ''New Horizons'' detected what was then called {{mpl|1994 JR|1}} (later named ''15810 Arawn'') from {{convert|170|e6mi|e6km|order=flip}} away.<ref>[http://www.spacedaily.com/reports/New_Horizons_catches_a_wandering_Kuiper_Belt_Object_not_far_off_999.html New Horizons' catches a wandering Kuiper Belt Object not far off] {{Webarchive|url=https://web.archive.org/web/20211126230455/http://www.spacedaily.com/reports/New_Horizons_catches_a_wandering_Kuiper_Belt_Object_not_far_off_999.html |date=26 November 2021 }} spacedaily.com Laurel MD (SPX). 7 December 2015.</ref>

On 1 January 2019, ''New Horizons'' successfully flew by Arrokoth, returning data showing Arrokoth to be a [[Contact binary (small Solar System body)|contact binary]] 32&nbsp;km long by 16&nbsp;km wide.<ref name="NYT-20190210">{{cite news |last=Corum |first=Jonathan |title=New Horizons Glimpses the Flattened Shape of Ultima Thule – NASA's New Horizons spacecraft flew past the most distant object ever visited: a tiny fragment of the early solar system known as 2014 MU69 and nicknamed Ultima Thule. – Interactive |url=https://www.nytimes.com/interactive/2018/12/31/science/new-horizons-ultima-thule-flyby.html |date=10 February 2019 |work=[[The New York Times]] |access-date=11 February 2019 |archive-date=24 December 2021 |archive-url=https://web.archive.org/web/20211224050632/https://www.nytimes.com/interactive/2018/12/31/science/new-horizons-ultima-thule-flyby.html |url-status=live }}</ref> The [[Ralph (New Horizons)|Ralph]] instrument aboard ''New Horizons'' confirmed Arrokoth's red color. Data from the fly-by will continue to be downloaded over the next 20 months.

No follow-up missions for ''New Horizons'' are planned, though at least two concepts for missions that would return to orbit or land on Pluto have been studied.<ref>{{Cite news|url=https://www.nasa.gov/directorates/spacetech/niac/2017_Phase_I_Phase_II/Fusion_Enabled_Pluto_Orbiter_and_Lander|title=Fusion-Enabled Pluto Orbiter and Lander|last=Hall|first=Loura|date=2017-04-05|work=NASA|access-date=2018-07-13|archive-date=21 April 2017|archive-url=https://web.archive.org/web/20170421033505/https://www.nasa.gov/directorates/spacetech/niac/2017_Phase_I_Phase_II/Fusion_Enabled_Pluto_Orbiter_and_Lander/|url-status=live}}</ref><ref>{{Cite web|url=https://www.eurekalert.org/pub_releases/2017-09/gac-gac092117.php|title=Global Aerospace Corporation to present Pluto lander concept to NASA|website=EurekAlert!|access-date=2018-07-13|archive-date=21 January 2019|archive-url=https://web.archive.org/web/20190121154920/https://www.eurekalert.org/pub_releases/2017-09/gac-gac092117.php|url-status=live}}</ref> Beyond Pluto, there exist many large KBOs that cannot be visited with ''New Horizons'', such as the dwarf planets [[Makemake]] and [[Haumea]]. New missions would be tasked to explore and study these objects in detail. [[Thales Alenia Space]] has studied the logistics of an orbiter mission to Haumea,<ref>{{Cite journal|date=2011-03-01|title=A preliminary assessment of an orbiter in the Haumean system: How quickly can a planetary orbiter reach such a distant target?|journal=Acta Astronautica|volume=68|issue=5–6|pages=622–628|doi=10.1016/j.actaastro.2010.04.011|issn=0094-5765|bibcode=2011AcAau..68..622P|last1=Poncy|first1=Joel|last2=Fontdecaba Baig|first2=Jordi|last3=Feresin|first3=Fred|last4=Martinot|first4=Vincent}}</ref> a high priority scientific target due to its status as the parent body of a collisional family that includes several other TNOs, as well as Haumea's ring and two moons. The lead author, Joel Poncy, has advocated for new technology that would allow spacecraft to reach and orbit KBOs in 10–20 years or less.<ref>{{Cite web|url=https://www.centauri-dreams.org/2009/07/15/haumea-technique-and-rationale/|title=Haumea: Technique and Rationale|website=www.centauri-dreams.org|access-date=2018-07-13|archive-date=13 July 2018|archive-url=https://web.archive.org/web/20180713143305/https://www.centauri-dreams.org/2009/07/15/haumea-technique-and-rationale/|url-status=live}}</ref> ''New Horizons'' Principal Investigator Alan Stern has informally suggested missions that would flyby the planets Uranus or Neptune before visiting new KBO targets,<ref>{{Cite news|url=https://www.space.com/40453-chasing-new-horizons-alan-stern-interview.html|title=New Horizons' Dramatic Journey to Pluto Revealed in New Book|work=Space.com|access-date=2018-07-13|archive-date=13 July 2018|archive-url=https://web.archive.org/web/20180713133732/https://www.space.com/40453-chasing-new-horizons-alan-stern-interview.html|url-status=live}}</ref> thus furthering the exploration of the Kuiper belt while also visiting these [[ice giant]] planets for the first time since the ''[[Voyager 2]]'' flybys in the 1980s.

===Design studies and concept missions===
[[50000 Quaoar|Quaoar]] has been considered as a flyby target for a probe tasked with exploring the [[interstellar medium]], as it currently lies near the [[Heliosphere|heliospheric]] nose; Pontus Brandt at [[Johns Hopkins University|Johns Hopkins]] [[Applied Physics Laboratory]] and his colleagues have studied a probe that would flyby Quaoar in the 2030s before continuing to the interstellar medium through the heliospheric nose.<ref name="exploration">{{Citation|last=TVIW|title=22. Humanity's First Explicit Step in Reaching Another Star: The Interstellar Probe Mission|date=2017-11-04|url=https://www.youtube.com/watch?v=Ailuk9ou0YI| archive-url=https://ghostarchive.org/varchive/youtube/20211030/Ailuk9ou0YI| archive-date=2021-10-30|access-date=2018-07-24}}{{cbignore}}</ref><ref>{{Cite news|url=https://agu.confex.com/agu/2018tess/meetingapp.cgi/Paper/334466|title=Triennial Earth Sun-Summit|access-date=2018-07-24|archive-date=3 August 2020|archive-url=https://web.archive.org/web/20200803050843/https://agu.confex.com/agu/2018tess/meetingapp.cgi/Paper/334466|url-status=live}}</ref> Among their interests in Quaoar include its likely disappearing methane atmosphere and [[Cryovolcano|cryovolcanism]].<ref name="exploration"/> The mission studied by Brandt and his colleagues would launch using [[Space Launch System|SLS]] and achieve 30&nbsp;km/s using a Jupiter flyby. Alternatively, for an orbiter mission, a study published in 2012 concluded that [[28978 Ixion|Ixion]] and [[38628 Huya|Huya]] are among the most feasible targets.<ref>{{Cite conference|last1=Gleaves|first1=Ashley|last2=Allen|first2=Randall|last3=Tupis|first3=Adam|last4=Quigley|first4=John|last5=Moon|first5=Adam|last6=Roe|first6=Eric|last7=Spencer|first7=David|last8=Youst|first8=Nicholas|last9=Lyne|first9=James|date=2012-08-13|title=A Survey of Mission Opportunities to Trans-Neptunian Objects – Part II, Orbital Capture|conference=AIAA/AAS Astrodynamics Specialist Conference, Minneapolis, Minnesota|location=Reston, Virginia|publisher=American Institute of Aeronautics and Astronautics|doi=10.2514/6.2012-5066|isbn=9781624101823|s2cid=118995590}}</ref> For instance, the authors calculated that an orbiter mission could reach Ixion after 17 years cruise time if launched in 2039.

== Extrasolar Kuiper belts ==
{{Main|Debris disc}}
[[File:Kuiper belt remote.jpg|thumb|upright=1.25|Debris discs around the stars [[HD 139664]] and [[HD 53143]] – black circle from [[coronograph|camera]] hiding stars to display discs.]]

By 2006, astronomers had resolved dust discs thought to be Kuiper belt-like structures around nine stars other than the Sun. They appear to fall into two categories: wide belts, with radii of over 50 AU, and narrow belts (tentatively like that of the Solar System) with radii of between 20 and 30 AU and relatively sharp boundaries.<ref name="Kalas et al. 2006"/> Beyond this, 15–20% of solar-type stars have an observed [[infrared excess]] that is suggestive of massive Kuiper-belt-like structures.<ref>{{cite journal |title=Debris Disks around Sun-like Stars |author=Trilling, D. E. |author2=Bryden, G. |author3=Beichman, C. A. |author4=Rieke, G. H. |author5=Su, K. Y. L. |author6=Stansberry, J. A. |author7=Blaylock, M. |author8=Stapelfeldt, K. R. |author9=Beeman, J. W. |author10=Haller, E. E. |volume=674 |issue=2 |pages=1086–1105 |date=February 2008 |bibcode=2008ApJ...674.1086T |doi=10.1086/525514 |journal=The Astrophysical Journal |arxiv=0710.5498|s2cid=54940779 }}</ref> Most known [[debris disc]]s around other stars are fairly young, but the two images on the right, taken by the Hubble Space Telescope in January 2006, are old enough (roughly 300 million years) to have settled into stable configurations. The left image is a "top view" of a wide belt, and the right image is an "edge view" of a narrow belt.<ref name="Kalas et al. 2006">{{cite journal |bibcode=2006ApJ...637L..57K |arxiv=astro-ph/0601488 |title=First Scattered Light Images of Debris Disks around HD 53143 and HD 139664 |last1=Kalas |first1=Paul |last2=Graham |first2=James R. |last3=Clampin |first3=Mark C. |last4=Fitzgerald |first4=Michael P. |volume=637 |issue=1 |date=2006 |pages=L57 |journal=The Astrophysical Journal |doi=10.1086/500305|s2cid=18293244 }}</ref><ref>{{cite web |title=Dusty Planetary Disks Around Two Nearby Stars Resemble Our Kuiper Belt |url=http://hubblesite.org/newscenter/archive/releases/2006/05/image/a |date=2006 |access-date=1 July 2007 |archive-date=9 July 2016 |archive-url=https://web.archive.org/web/20160709094354/http://hubblesite.org/newscenter/archive/releases/2006/05/image/a/ |url-status=live }}</ref> Computer simulations of dust in the Kuiper belt suggest that when it was younger, it may have resembled the narrow rings seen around younger stars.<ref>{{cite journal |title=Collisional Grooming Models of the Kuiper Belt Dust Cloud |author=Kuchner, M. J. |author2=Stark, C. C. |volume=140 |issue=4 |pages=1007–1019 |date=2010 |bibcode=2010AJ....140.1007K |doi=10.1088/0004-6256/140/4/1007 |journal=The Astronomical Journal |arxiv=1008.0904|s2cid=119208483 }}</ref>

== See also ==
* [[Asteroid belt]]
* [[List of possible dwarf planets]]
* [[List of trans-Neptunian objects]]
* [[Planet Nine]]

== Notes ==
{{notelist|1}}

== References ==
{{reflist}}

== External links ==
{{Commons category|Kuiper belt objects}}
* [http://www2.ess.ucla.edu/~jewitt/kb.html Dave Jewitt's page @ UCLA]
** [http://www2.ess.ucla.edu/~jewitt/kb/gerard.html The belt's name]
* [http://www.physics.ucf.edu/~yfernandez/cometlist.html List of short period comets by family]
* [https://web.archive.org/web/20021119032639/http://solarsystem.nasa.gov/planets/profile.cfm?Object=KBOs Kuiper Belt Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
* [http://www.boulder.swri.edu/ekonews/ The Kuiper Belt Electronic Newsletter]
* [http://www.johnstonsarchive.net/astro/tnos.html Wm. Robert Johnston's TNO page]
* [http://www.minorplanetcenter.org/iau/lists/OuterPlot.html Minor Planet Center: Plot of the Outer Solar System], illustrating Kuiper gap
* [http://www.minorplanetcenter.net/iau/lists/TNOs.html List of TNOS]

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