View source for Kuiper belt

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[[Image:Outersolarsystem objectpositions labels comp.png|thumb|300px|Known objects in the Kuiper belt, derived from data from the [[Minor Planet Center]]. Objects in the main belt are coloured green, while scattered objects are coloured orange. The four outer planets are blue. Neptune's few known [[Trojan asteroid]]s are yellow, while Jupiter's are pink. The scattered objects between the Sun and the Kuiper belt are known as [[Centaur (planetoid)|centaurs]]. The scale is in [[astronomical unit]]s. The pronounced gap at the bottom is due to obscuration by the band of the [[Milky Way]].]]
{{TNO}}
The '''Kuiper belt''' ({{pron-en|ˈkaɪpər}}, rhyming with "viper"),<ref>{{cite web|title=Dutch requests|url=http://homepage.mac.com/schuffelen/dureq.html|accessdate=2007-06-01}}</ref> sometimes called the '''Edgeworth-Kuiper belt''', is a region of the [[Solar System]] beyond the planets extending from the [[orbit]] of [[Neptune]] (at 30 [[Astronomical unit|AU]]) to approximately 55 [[Astronomical unit|AU]] from the [[Sun]]. <ref>{{cite journal | url=http://www.iop.org/EJ/article/0004-637X/490/2/879/36659.html | author=Alan Stern | title=Collisional Erosion in the Primordial Edgeworth-Kuiper Belt and the Generation of the 30–50 AU Kuiper Gap | journal=The [[Astrophysical Journal]] | volume=490 | issue=2 | pages=879–882 | year=1997 | doi=10.1086/304912 }}</ref> It is similar to the [[asteroid belt]], although it is far larger—20 times as wide and 20–200 times as massive.<ref name=beyond>{{cite web|title=The Solar System Beyond The Planets|author=Audrey Delsanti and David Jewitt|work=Institute for Astronomy, University of Hawaii|url=http://www.ifa.hawaii.edu/faculty/jewitt/papers/2006/DJ06.pdf|format=PDF|accessdate=2007-03-09}}</ref><ref>{{cite journal| authorlink= Georgij A. Krasinsky | first=G. A. | last= Krasinsky | coauthors=[[Elena V. Pitjeva|Pitjeva, E. V.]]; Vasilyev, M. V.; Yagudina, E. I. | url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002Icar..158...98K&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=4326fb2cf906949| title=Hidden Mass in the Asteroid Belt| journal=Icarus| volume=158| issue=1| pages=98–105| month= July| year= 2002| doi=10.1006/icar.2002.6837}}</ref> Like the asteroid belt, it consists mainly of [[small solar system body|small bodies]] (remnants from the Solar System's formation).  While the asteroid belt is composed primarily of [[Rock (geology)|rock]] and [[metal]], the Kuiper belt objects are composed largely of frozen [[volatiles]] (termed "ices"), such as [[methane]], [[ammonia]] and [[water]]. It is home to at least three [[dwarf planet]]s – [[Pluto]], [[Haumea (dwarf planet)|Haumea]] and [[Makemake (dwarf planet)|Makemake]].

Since the first discovery in 1992, the number of known '''Kuiper belt objects''' ('''KBOs''') has increased to over a thousand, and more than 70&nbsp;000 KBOs over 100&nbsp;km in diameter are believed to reside there.<ref>{{cite web|title=Kuiper Belt Page|author= David Jewitt|url=http://www.ifa.hawaii.edu/faculty/jewitt/kb.html|accessdate=2007-10-15}}</ref> The Kuiper belt was initially believed to be the main repository for [[periodic comet]]s, those with orbits lasting less than 200 years. However, studies since the mid-1990s have shown that the Kuiper belt is dynamically stable, and that it is the farther [[scattered disc]], a dynamically active region created by the outward motion of Neptune 4.5 billion years ago, that is their true place of origin.<ref name=book>{{cite book
 | title = Encyclopedia of the Solar System
 | chapter = Comet Populations and Cometary Dynamics
 | author = Harold F. Levison, Luke Donnes
 | publisher = Academic Press
 | year = 2007
 | editor = Lucy Ann Adams McFadden, Paul Robert Weissman, Torrence V. Johnson
 | edition = 2<sup>nd</sup>
 | publication-place = Amsterdam; Boston
 | isbn = 0120885891
 | pages = 575&ndash;588}}</ref> [[Scattered disc object]]s such as [[Eris (dwarf planet)|Eris]] are KBO-like bodies with extremely large orbits that take them as far as 100 AU from the Sun. The [[centaur (planetoid)|centaurs]], comet-like bodies that orbit among the [[gas giant]]s, are believed to originate there. [[Neptune]]'s moon [[Triton (moon)|Triton]] is believed to be a captured KBO.<ref>{{cite web|title=Neptune’s capture of its moon Triton in a binary-planet gravitational encounter|author=Craig B. Agnor & Douglas P. Hamilton|work=Nature|url=http://www.es.ucsc.edu/~cagnor/papers_pdf/2006AgnorHamilton.pdf|format=PDF|year=2006| accessdate=2006-06-20}}</ref> Pluto, a [[dwarf planet]], is the largest<!-- Pluto is the largest KBO; Eris is not a KBO--> known member of the Kuiper belt. Originally considered a planet, it is similar to many other objects of the Kuiper belt, and its orbital period is identical to that of the KBOs known as "[[Plutino]]s".

The Kuiper belt should not be confused with the hypothesized [[Oort cloud]], which is a thousand times more distant. 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|year=2004|accessdate=2007-06-01}}</ref>

==History==
Since the discovery of Pluto, many have speculated that it might not be alone. The region now called the Kuiper belt had been 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.
===Hypotheses===

The first [[astronomer]] to suggest the existence of a trans-Neptunian population was [[Frederick C. Leonard]]. In 1930, soon after Pluto's discovery, he pondered whether it was "not 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.cfa.harvard.edu/icq/kb.html|Harvard Smithsonian Center for Astrophysics|accessdate=2007-06-20}}</ref> 
 
[[Image:GerardKuiper.jpg|thumb|150 px|Astronomer [[Gerard Kuiper]], after whom the Kuiper belt is named]]
In 1943, in the ''Journal of the British Astronomical Association'', [[Kenneth Edgeworth]] hypothesised that, in the region beyond [[Neptune]], the material within the primordial [[solar nebula]] was too widely spaced to condense into planets, and so rather condensed into a myriad of 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>{{cite book|title=Beyond Pluto: Exploring the outer limits of the solar system |author=John Davies|publisher=Cambridge University Press|year=2001|pages=xii|nopp=true}}</ref> 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>Davies, p. 2</ref> becoming a [[comet]].   

In 1951, in an article for the journal ''Astrophysics'', [[Gerard Kuiper]] speculated on a similar disc having formed early in the Solar System's evolution, however, he did not believe that such a belt still existed today. Kuiper was operating on the assumption common in his time, that [[Pluto]] was the size of the Earth, and had therefore scattered these bodies out toward the Oort cloud or out of the Solar System. Were Kuiper's hypothesis correct, there would not be a Kuiper belt where we now see it.<ref name=Jewitt>{{cite web|title=WHY "KUIPER" BELT?|author=David Jewitt|work=University of Hawaii|url=http://www.ifa.hawaii.edu/faculty/jewitt/kb/gerard.html|accessdate=2007-06-14}}</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=Davies2>Davies, p. 14</ref> while 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, to affect the orbits of known comets.<ref>{{cite web|title=EVIDENCE FOR A COMET BELT BEYOND NEPTUNE|author=FOR A COMET BELT BEYOND NEPTUNE
BY FRED L. WHIPPLE|work=SMITHSONIAN ASTROPHYSICAL OBSERVATORY AND HARVARD COLLEGE OBSERVATORY|url=http://www.pnas.org/cgi/reprint/51/5/711.pdf|format=PDF|year=1964|accessdate=2007-06-20}}</ref> Observation, however, ruled out this hypothesis.<ref name=Davies2 /> 

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 web|title=The discovery and orbit of /2060/ Chiron|author= CT Kowal, W Liller, BG Marsden|work=Hale Observatories, Harvard&ndash;Smithsonian Center for Astrophysics|url=http://adsabs.harvard.edu/abs/1979IAUS...81..245K|year=1977|accessdate=2007-06-20}}</ref>  In 1992, another object [[5145 Pholus]], was discovered in a similar orbit.<ref>{{cite web|title=1992 AD|author=JV Scotti, DL Rabinowitz, CS Shoemaker, EM Shoemaker, DH Levy,  TM King, EF Helin, J Alu, K Lawrence, RH McNaught, L Frederick, D Tholen, BEA Mueller|url=http://adsabs.harvard.edu/abs/1992IAUC.5434....1S|year=1992|accessdate=2007-06-20}}</ref> Today, an entire population of comet-like bodies, 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 |last=Horner |first= J. |coauthors=Evans, N.W.; Bailey, M. E. |title=Simulations of the Population of Centaurs I: The Bulk Statistics |year=2004 |url=http://arxiv.org/abs/astro-ph?papernum=0407400 |accessdate=2008-09-22 }}</ref> From the time of Chiron's discovery, astronomers speculated that they therefore must be frequently replenished by some outer reservoir.<ref>Davies p. 38</ref> 

Further evidence for the belt's existence 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, eating them gradually away. In order to still be visible over the age of the Solar System, they must be frequently replenished.<ref name=matter>{{cite journal | url=http://www.iop.org/EJ/article/1538-3881/123/2/1039/201410.text.html | 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 | year=2002 | doi=10.1086/338692 }}</ref>One such area of replenishment is the [[Oort cloud]]; the spherical swarm of comets extending beyond 50&nbsp;000 [[Astronomical unit|AU]] from the Sun first hypothesised by astronomer [[Jan Oort]] in 1950.<ref>[[Jan Oort|Oort, J. H.]], ''The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin'', Bull. Astron. Inst. Neth., ''11'', p. 91–110 (1950) [http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1950BAN....11...91O&amp;data_type=PDF_HIGH&amp;type=PRINTER&amp;filetype=.pdf  Text at Harvard server (PDF)]</ref> It is believed to be the point of origin for [[long period comet]]s, those, like [[Hale-Bopp]], with orbits lasting thousands of years. 

There is however another comet population, known as [[short-period comet|short period]] or [[periodic comet]]s; those with orbits lasting less than 200 years. By the 1970s, the rate at which short-period comets were being discovered was becoming increasingly inconsistent with them having emerged solely from the [[Oort cloud]].<ref>Davies p. 39</ref> For an Oort cloud object to become a short-period comet, it would first have to be captured by the giant planets. In 1980, in the [[Monthly Notices of the Royal Astronomical Society]], [[Julio Ángel Fernández|Julio Fernandez]] 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 interstellar space. He speculated that a comet belt from between 35 and 50 [[Astronomical Unit|AU]] would be required to account for the observed number of comets.<ref>{{cite web|title=On the existence of a comet belt beyond Neptune|author=JA Fernandez|work=Observatorio Astronomico Nacional, Madrid|url=http://adsabs.harvard.edu/abs/1980MNRAS.192..481F|year=1980|accessdate=2007-06-20}}</ref> Following up on Fernandez'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 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 Fernandez described it added to the formulations, the simulations matched observations.<ref>{{cite web|title=The origin of short-period comets|author=M. Duncan, T. Quinn, and S. Tremaine|work=The Astrophysical Journal|url=http://adsabs.harvard.edu/full/1988ApJ...328L..69D|year=1988|accessdate=2007-06-20}}</ref>  Reportedly because the words "Kuiper" and "comet belt" appeared in the opening sentence of Fernandez's paper, Tremaine named this region the "Kuiper belt."<ref>Davies p. 191</ref>

===Discovery===
[[Image:Maunatele.jpg|thumb|left|200 px|The array of telescopes atop [[Mauna Kea]], with which the Kuiper belt was discovered]]
In 1987, astronomer [[David Jewitt]], then at [[MIT]], became increasingly puzzled by "the apparent emptiness of the outer Solar System."<ref name=qbee>{{cite web|title=Discovery of the candidate Kuiper belt object 1992 QB1|author=David Jewitt, Jane Luu|url=http://www.nature.com/nature/journal/v362/n6422/abs/362730a0.html|work=Nature|year=1992|accessdate=2007-06-20}}</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=Davies3>Davies p. 50</ref> 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=Davies3 /> Initially, examination of each pair of plates took about eight hours,<ref>Davies p. 51</ref> 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 percent of the light that hit them, rather than the ten percent 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>Davies pp. 52, 54, 56</ref> 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 m telescope at Mauna Kea.<ref>Davies pp. 57, 62</ref> Eventually, the field of view for CCDs had increased to 1024 by 1024 pixels, which allowed searches to be conducted far more rapidly.<ref>Davies p. 65</ref> Finally, after five years of searching, on [[August 30]], [[1992]], Jewitt and Luu announced the "Discovery of the candidate Kuiper belt object" {{mpl|(15760) 1992 QB|1}};<ref name=qbee /> Six months later, they discovered a second object in the region, (181708) 1993 FW.<ref>{{cite web|title=1993 FW|author=BS Marsden|work=Minor Planet Center|url=http://adsabs.harvard.edu/abs/1993IAUC.5730....1L|year=1993|accessdate=2007-06-20}}</ref>

Studies since the trans-Neptunian region was first charted have shown that in fact, the region now called the Kuiper belt is not the point of origin for short-period comets, but that they instead derive from a separate but linked population called the [[scattered disc]]. The scattered disc was created when Neptune [[planetary migration|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 which could never be affected by its orbit (the Kuiper belt proper), and a separate population whose 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 will sometimes use alternative name '''Edgeworth-Kuiper belt''' to credit Edgeworth, and KBOs are occasionally referred to as EKOs. However, [[Brian Marsden]] claims neither deserve true credit; "Neither Edgeworth or Kuiper wrote about anything remotely like what we are now seeing, but [[Fred Whipple]] did."<ref>Davies p. 199</ref> Conversely, David Jewitt comments that, "If anything . . . Fernandez most nearly deserves the credit for predicting the Kuiper Belt."<ref name=Jewitt /> 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 &mdash; it is not a [[synonym]] though, as TNOs, include all objects orbiting the Sun past the orbit of [[Neptune]], not just those in the Kuiper belt.

==Origins==
 
[[Image:Lhborbits.png|thumb|400px|Simulation showing Outer Planets and Kuiper Belt: a)Before Jupiter/Saturn 2:1 resonance b)Scattering of Kuiper Belt objects into the solar system after the orbital shift of Neptune c)After ejection of Kuiper Belt bodies by Jupiter]]
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 [[LSST]], which should reveal many currently unknown KBOs. These surveys will provide data that will help determine answers to these questions.<ref name=beyond /> 

The Kuiper belt is believed to consist of [[planetesimals]]; 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 3000&nbsp;km in diameter. 

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 situ'' beyond Saturn, as too little primordial matter existed at that range to produce objects of such high mass. Instead, these planets are believed to have formed closer to Jupiter, and [[Planetary migration|migrated]] outwards during the course of the Solar System's early evolution. Eventually, the orbits shifted to the point where Jupiter and Saturn existed in an exact 2:1 resonance; Jupiter orbited the Sun twice for every one Saturn orbit. The gravitational pull from such a resonance ultimately disrupted the orbits of Uranus and Neptune, causing Neptune's orbit to move outward into the primordial planetesimal disk, which sent the disk into temporary chaos.<ref>{{cite web|title=Orbital shuffle for early solar system|author=Kathryn Hansen|work=Geotimes|url=http://www.geotimes.org/june05/WebExtra060705.html|date=June 7, 2005|accessdate=2007-08-26}}</ref> As Neptune traveled along this modified orbit, it excited and scattered many TNO planetesimals into higher and more eccentric orbits, depleting the primordial population.<ref>{{cite web|title=THE FORMATION OF URANUS AND NEPTUNE AMONG JUPITER AND SATURN|author=E. W. THOMMES, M. J. DUNCAN, H. F. LEVISON|url=http://arxiv.org/abs/astro-ph/0111290|doi=10.1086/339975|year=2001|accessdate=2007-06-24}} (arXiv:astro-ph/0111290v1)</ref>

However, the present [[Nice model#Formation of the Kuiper belt|most popular model]] still fails to account for many of the characteristics of the distribution and, quoting one of the scientific articles,<ref>{{cite web|title=Nonlinear Resonances in the Solar System|url=http://arxiv.org/abs/chao-dyn/9406004|accessdate=2007-06-03}}</ref> the problems "continue to challenge analytical techniques and the fastest numerical modeling hardware and software".

==Structure==
At its fullest extent, including its outlying regions, the Kuiper belt stretches from roughly 30 to 55 AU. However, the main body of the belt  is generally accepted to extend from the 2:3 resonance ([[#Resonances|see below]]) at 39.5 AU to the 1:2 resonance at roughly 48 AU.<ref>{{cite journal 
|url= http://www.iop.org/EJ/abstract/1538-3881/121/5/2792
|title=Thermal Evolution and Differentiation of Edgeworth-Kuiper Belt Objects
|author=M. C. De Sanctis, M. T. Capria, and A. Coradini
|journal=The Astronomical Journal
| doi= 10.1086/320385 
|volume=121 
|pages=2792–2799 
|accessdate=2008-08-28
|year=2001}}</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|work=American Scientists.org|url=http://www.americanscientist.org/template/AssetDetail/assetid/25723/page/2;jsessionid=aaa5LVF0|year=2003|accessdate=2007-06-23}}</ref> Its mean position is inclined to the ecliptic by 1.86 degrees. <ref>{{cite journal | url=http://www.iop.org/EJ/article/1538-3881/127/4/2418/203316.html | author=Michael E. Brown, Margaret Pan | title=The Plane of the Kuiper Belt | journal=The [[Astronomical Journal]] | volume=127 | issue=4 | pages=2418–2423 | year=2004 | doi=10.1086/382515 | accessdate=2009-08-15}}</ref>

[[Image:TheKuiperBelt classes-en.svg|right|thumb|250px|Orbit classification (schematic of [[semi-major axis|semi-major axes]])]]
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 which 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 possess 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 web|title=Large Scattered Planetesimals and the Excitation of the Small Body Belts|author=Jean-Marc Petit, Alessandro Morbidelli, Giovanni B. Valsecchi|url=http://www.obs-nice.fr/morby/papers/6166a.pdf|format=PDF|year=1998|accessdate=2007-06-23}}</ref>  

===Classical belt===
{{main|Classical Kuiper belt object}}
Between approximately 42&ndash;48&nbsp;AU, however, the gravitational influence of Neptune is negligible, and objects can exist with their orbits pretty much unmolested. 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|title=The Kuiper Belt|author=Jonathan Lunine|url=http://www.gsmt.noao.edu/gsmt_swg/SWG_Apr03/The_Kuiper_Belt.pdf|format=PDF|year=2003|accessdate=2007-06-23}}</ref><ref>{{cite web|title=CLASSICAL KUIPER BELT OBJECTS (CKBOs)|author=Dave Jewitt|url=http://www.ifa.hawaii.edu/~jewitt/kb/kb-classical.html|year=2004|accewssdate=2007-06-23}}</ref>  Because the first modern KBO discovered, [[1992 QB1]], is considered the prototype of this group, classical KBOs are often referred to as [[cubewanos]] ("Q-B-1-os").<ref>{{cite web|title=Cubewano|author=P Murdin|url=http://adsabs.harvard.edu/abs/2000eaa..bookE5403|year=2000|accessdate=2007-06-23}}</ref><ref>{{cite web|title=THE DEEP ECLIPTIC SURVEY: A SEARCH FOR KUIPER BELT OBJECTS AND CENTAURS. II. DYNAMICAL CLASSIFICATION, THE KUIPER BELT PLANE, AND THE CORE POPULATION|author=J. L. Elliot, S. D. Kern, K. B. Clancy, A. A. S. Gulbis, R. L. Millis, M. W. Buie,
L. H. Wasserman, E. I. Chiang, A. B. Jordan, D. E. Trilling, and K. J. Meech|url=http://alpaca.as.arizona.edu/~trilling/des2.pdf|format=PDF|year=2004|accessdate=2007-06-23}}</ref>

The classical Kuiper belt appears to be a composite of two separate populations. The first, known as "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 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>{{cite web|title=The formation of the Kuiper belt by the outward transport of bodies during Neptune’s migration|author=Harold F. Levison, Alessandro Morbidelli|url=http://www.obs-nice.fr/morby/stuff/NATURE.pdf|format=PDF|year=2003|accessdate=2007-06-25}}</ref> The two populations not only possess different orbits, but different compositions; the cold population is markedly redder than the hot, suggesting it formed in a different region. The hot population is believed to have formed near Jupiter, and to have been ejected out by movements among the gas giants. The cold population, on the other hand, is believed to have formed more or less in its current position although it may also have been later swept outwards by Neptune during its [[Planetary migration|migration]].<ref name=beyond /><ref>{{cite web|title=ORIGIN AND DYNAMICAL EVOLUTION OF COMETS AND THEIR RESERVOIRS|author=Alessandro Morbidelli|work=Observatoire de la Cˆpte d’Azur, Nice, France|year=2006|url=http://arxiv.org/abs/astro-ph/0512256v1|accessdate=2007-08-30}}</ref>

===Resonances===
{{main|Resonant trans-Neptunian object}}
[[Image:TheKuiperBelt 75AU All.svg|right|thumb|400px|Distribution of [[cubewano]]s, [[plutino]]s and near [[scattered disk|scattered objects]]]]
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 is in just the right kind of orbit so that it orbits the Sun two times for every three Neptune orbits, then whenever it returns to its original position, Neptune will always be half an orbit away from it, or in the same position as it began to it, since it will have completed 1½ 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 known objects,<ref>{{cite web|title=List Of Transneptunian Objects|work=Minor Planet Center|url=http://www.cfa.harvard.edu/iau/lists/TNOs.html|accessdate=2007-06-23}}</ref>  including [[Pluto]] together with its moons. In recognition of this, the other members of this family are known as [[Plutinos]]. Many Plutinos, including Pluto, often have orbits which cross that of Neptune, though their resonance means they can never collide. Many others, such as [[90482 Orcus]] and [[28978 Ixion]], are large enough to likely [[List of plutoid candidates|qualify as plutoids]] when more is known about them.<ref>{{cite web|title=Ixion|work=eightplanets.net|url=http://ixion.eightplanets.net/|accessdate=2007-06-23}}</ref><ref name=albedo>{{cite web|title=Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope|author=John Stansberry, Will Grundy, Mike Brown, Dale Cruikshank, John Spencer, David Trilling, Jean-Luc Margot|url=http://arxiv.org/abs/astro-ph/0702538v1|year=2007|accessdate=2007-06-23}}</ref> 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 | url=http://www.iop.org/EJ/article/1538-3881/126/1/430/203022.html | author=Chiang ''et al.'' | 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 | year=2003 | doi=10.1086/375207 | accessdate=2009-08-15}}</ref>  The 1:2 resonance (whose objects complete half an orbit for each of Neptune's) corresponds to semi-major axes of ~47.7AU, and is sparsely populated.<ref>{{cite web|title=Trans-Neptunian Objects|author=Wm. Robert Johnston|url=http://www.johnstonsarchive.net/astro/tnos.html|year=2007|accessdate=2007-06-23}}</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>Davies p. 104</ref> Neptune possesses a number of [[Neptune trojan|trojan objects]], which occupy its [[Lagrange point|L<sub>4</sub> and L<sub>5</sub> points]]; gravitationally stable regions leading and trailing it in its orbit. Neptune trojans are often described as being in a 1:1 resonance with Neptune. Neptune trojans are remarkably stable in their orbits and are unlikely to have been captured by Neptune, but rather to have formed alongside it.<ref name=trojan />  

Additionally, there is a relative absence of objects with semi-major axes below 39 AU which 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>Davies p. 107</ref>

==="Kuiper cliff"===
[[Image:Semimajorhistogramofkbos.svg|thumb|401 px|Graph showing the numbers of KBOs for a given distance from the Sun. The Plutinos are the "spike" at 40 AU, while the classicals are between 42 and 47 AU, and the twotinos are at 48 AU.]] 
The [[Resonant_trans-Neptunian_object#1:2_resonance_.28twotinos.29|1:2 resonance]] 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 resonance  at roughly 55 AU, well outside the classical belt; however, predictions of a large number of bodies in classical orbits between these resonances have not been verified through observation.<ref name=trojan />

Earlier models of the Kuiper belt had suggested that the number of large objects would increase by a factor of two beyond 50 AU;<ref name="Brown 1999">{{cite web|author=E. I. Chiang and M. E. Brown|title=KECK PENCIL-BEAM SURVEY FOR FAINT KUIPER BELT OBJECTS|url=http://www.gps.caltech.edu/~mbrown/papers/ps/kbodeep.pdf|format=PDF|year=1999|accessdate=2007-07-01}}</ref> so this sudden drastic falloff, known as the "Kuiper cliff", was completely unexpected, and its cause, to date, is unknown. Bernstein and Trilling et al. have found evidence that the rapid decline in objects of 100&nbsp;km or more in radius beyond 50 AU is real, and not due to observational bias. Possible explanations include that material at that distance is too scarce or too scattered to accrete into large objects, or that subsequent processes removed or destroyed those which did form.<ref>{{cite journal|author = G.M. Bernstein, D.E. Trilling, R.L. Allen, M.E. Brown, M. Holman and R. Malhotra|title=The Size Distribution of Trans-Neptunian Bodies|url=http://www.gps.caltech.edu/~mbrown/papers/ps/bernstein.pdf|format=PDF|journal = The Astrophysical Journal|year = 2004}}</ref> [[Patryk Lykawka]] of [[Kobe University]] has claimed that the gravitational attraction of an 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=http://space.newscientist.com/article.ns?id=mg18524911.600|year=2007|accessdate=2007-06-23}}</ref><ref>{{cite web|title=The mystery of Planet X|year=2008|author=Govert Schilling|work=New Scientist|url=http://space.newscientist.com/article/mg19726381.600-the-mystery-of-planet-x.html
|accessdate=2008-02-08}}</ref>

==Composition==
<!-- [[Image:2003 UB313 near-infrared spectrum.gif|thumb|300 px|The infrared spectra of both Eris and Pluto, highlighting their common methane absorption lines]] -->

Studies of the Kuiper belt since its discovery have generally indicated that its members are primarily composed of ices; a mixture of light hydrocarbons (such as [[methane]]), [[ammonia]], and water [[ice]], a composition they share with [[comets]].<ref>{{cite web|title=COMPOSITION OF THE VOLATILE MATERIAL IN HALLEY’S COMA FROM IN SITU MEASUREMENTS|author=K. ALTWEGG and H. BALSIGER and J. GEISS|url=http://www.springerlink.com/content/h761v5534553k608/fulltext.pdf|format=PDF|year=1999|accessdate=2007-06-23}}</ref>  The temperature of the belt is only about 50K,<ref name=Quaoar>{{cite web|title=Crystalline water ice on the Kuiper belt object (50000) Quaoar|author=David C. Jewitt & Jane Luu|url=http://www.ifa.hawaii.edu/~jewitt/papers/50000/Quaoar.pdf|format=PDF|year=2004|accessdate=2007-06-21}}</ref> so many compounds that would remain gaseous closer to the Sun are solid. 
 
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 colours, 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 what it is made of.

Initially, such detailed analysis of KBOs was impossible, and so astronomers were only able to determine the most basic facts about their makeup, primarily their colour.<ref name=KBOKBO>{{cite web|title=Surfaces of Kuiper Belt Objects|author=Dave Jewitt|work=University of Hawaii|url=http://www.ifa.hawaii.edu/~jewitt/kb/kb-colors.html|year=2004|accessdate=2007-06-21}}</ref> These first data showed a broad range of colours among KBOs, ranging from neutral grey to deep red.<ref name=colour>{{cite web|title=OPTICAL-INFRARED SPECTRAL DIVERSITY IN THE KUIPER BELT|author=DAVID JEWITT, JANE LUU|work=University of Hawaii, Harvard University|url=http://www.journals.uchicago.edu/doi/full/10.1086/300299|year=1997|accessdate=2007-06-21}}</ref>  This suggested that their surfaces were composed of a wide range of compounds, from dirty ices to hydrocarbons.<ref name=colour /> This diversity was startling, as astronomers had expected KBOs to be uniformly dark, having lost most of their volatile ices to the effects of cosmic rays.<ref>Davies p. 118</ref> Various solutions were suggested for this discrepancy, including resurfacing by impacts or outgassing.<ref name=KBOKBO /> However, Jewitt and Luu's spectral analysis of the known Kuiper belt objects in 2001 found that the variation in colour was too extreme to be easily explained by random impacts.<ref>{{cite web|title=COLORS AND SPECTRA OF KUIPER BELT OBJECTS|author=David C. Jewitt, Jane X. Luu|work=University of Hawaii, Harvard University|url=http://www.journals.uchicago.edu/doi/full/10.1086/323304|year=2001|accessdate=2007-06-21}}</ref>

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.'' obtained spectroscopic data on the KBO 1993 SC, revealing its surface composition to be markedly similar to that of [[Pluto]], as well as Neptune's moon [[Triton (moon)|Triton]], possessing large amounts of [[methane]] ice.<ref name=rbrown>{{cite web|title=Surface Composition of Kuiper Belt Object 1993SC|author=Robert H. Brown, Dale P. Cruikshank, Yvonne Pendleton, Glenn J. Veeder|work=Lunar and Planetary Laboratory and Steward Observatory, University of Arizona, Jet Propulsion Laboratory, NASA Ames Research Center|url=http://www.sciencemag.org/cgi/content/abstract/276/5314/937|year= 1997|accessdate=2007-06-21}}</ref> 

Water ice has been detected in several KBOs, including [[1996 TO66]],<ref>{{cite web|title=NEAR-INFRARED SPECTROSCOPY OF THE BRIGHT KUIPER BELT OBJECT 2000 EB173|author=Michael E. Brown, Geoffrey A. Blake,  Jacqueline E. Kessler|url=http://www.journals.uchicago.edu/doi/full/10.1086/317277|year=2000|accessdate=2007-06-21}}</ref> [[38628 Huya|2000 EB173]] and [[2000 WR106]].<ref>{{cite web|title=NICS-TNG infrared spectroscopy of trans-neptunian objects 2000 EB173 and 2000 WR106|author=J. Licandro, E. Oliva and M. Di Martino|url=http://arxiv.org/abs/astro-ph/0105434v1|year=2001|accessdate=2007-06-21}}</ref> In 2004, Mike Brown ''et al.'' determined the existence of crystalline water ice and [[ammonia]] [[hydrate]] on one of the largest known KBOs, [[50000 Quaoar]]. Both of these substances would have been destroyed over the age of the solar system, suggesting that Quaoar had been recently resurfaced, either by internal tectonic activity or by meteorite impacts.<ref name=Quaoar />

== Mass and size distribution ==
[[Image:TheKuiperBelt PowerLaw2.svg|thumb|250px|Illustration of the power law.]]
Despite its vast extent, the collective mass of the Kuiper belt is relatively low. The upper limit to the total mass is estimated at roughly a tenth the mass of the Earth,<ref name=beyond /> with some estimates placing it at a thirtieth an Earth mass.<ref>{{cite journal|title=Dynamical determination of the mass of the Kuiper Belt from motions of the inner planets of the Solar system|author=Lorenzo Iorio|journal=Monthly Notices of the Royal Astronomical Society|volume=375|pages=1311–1314|url=http://adsabs.harvard.edu/abs/2007MNRAS.tmp...24I|accessdate=2008-08-09|issue=4|doi=10.1111/j.1365-2966.2006.11384.x|year=2007}}</ref> Conversely, models of the Solar System's formation predict a collective mass for the Kuiper belt of 30 Earth masses.<ref name=beyond /> This missing >99% of the mass can hardly be dismissed, as it is required for the accretion of any KBOs larger than 100&nbsp;km in diameter. At the current low density, these objects simply should not exist. Moreover, the eccentricity and inclination of current orbits makes the encounters quite "violent," resulting in destruction rather than accretion. 
It appears that either the current residents of the Kuiper belt have been created closer to the Sun or some mechanism dispersed the original mass. Neptune’s current influence is too weak to explain such a massive "vacuuming", though the [[Nice model]] proposes that it could have been the cause of mass removal in the past. While the question remains open, the conjectures vary from a passing star scenario to grinding of smaller objects, via collisions, into dust small enough to be affected by solar radiation.<ref name="Morbidelli 2005">
Morbidelli A. ''Origin and dynamical evolution of comets and their reservoirs.''
[http://arxiv.org/pdf/astro-ph/0512256 Preprint on arXiv (pdf)]
</ref> 

Bright objects are rare compared with the dominant dim population, as expected from accretion models of origin, given that only some objects of a given size would have grown further. This relationship N(D), the population expressed as a function of the diameter, referred to as brightness slope, has been confirmed by observations. The slope is inversely proportional to some power of the diameter D. 
:<math> \frac{d N}{d D} \sim D^{-q}</math> where the current measures<ref name="Bernstein et al. 2004">Bernstein G.M., Trilling D.E., Allen R.L., Brown K.E, Holman M., Malhotra R. ''The size Distribution of transneptunian bodies.'' The Astronomical Journal, '''128''', 1364–1390. 
[http://arxiv.org/pdf/astro-ph/0308467 preprint on arXiv (pdf)] </ref> give q = 4 ±0.5.

Less formally, there are for instance 8 (=2³) times more objects in 100–200&nbsp;km range than objects in 200–400&nbsp;km range. In other words, for every object with the diameter of 1000&nbsp;km there should be around 1000 (=10³) objects with diameter of 100&nbsp;km.

The law is expressed in this differential form rather than as a cumulative cubic relationship, because only the middle part of the slope can be measured; the law must break at smaller sizes, beyond the current measure.

Of course, only the magnitude is actually known, the size is inferred assuming [[albedo]] (not a safe assumption for larger objects).
<br clear="all"/>

==Largest KBOs==
{{main|List of the brightest KBOs}}
<imagemap>
Image:EightTNOs.png|thumb|300 px|Comparison of [[Eris (dwarf planet)|Eris]], [[Pluto]], [[Makemake (dwarf planet)|Makemake]], [[Haumea (dwarf planet)|Haumea]], [[90377 Sedna|Sedna]], [[90482 Orcus|Orcus]], [[50000 Quaoar|Quaoar]], [[20000 Varuna|Varuna]], and [[Earth]] (all to scale).
#Earth
rect 646 1714 2142 1994 [[Earth|The Earth]]
#Eris and Dysnomia
circle 226 412 16 [[Dysnomia (moon)|Dysnomia]]
circle 350 626 197 [[Eris (dwarf planet)|Eris]]
#Pluto and Charon
circle 1252 684 86 [[Charon (moon)|Charon]]
circle 1038 632 188 [[Pluto]]
#Makemake
circle 1786 614 142 [[Makemake (dwarf planet)|Makemake]]
#Haumea
circle 2438 616 155 [[Haumea (dwarf planet)|Haumea]]
#Sedna
circle 342 1305 137 [[90377 Sedna|Sedna]]
#Orcus
circle 1088 1305 114 [[90482 Orcus|Orcus]]
#Quaoar
circle 1784 1305 97 [[50000 Quaoar|Quaoar]]
#Varuna
circle 2420 1305 58 [[20000 Varuna|Varuna]]
#link to image (under all other links)
rect 0 0 2749 1994 [[File:EightTNOs.png]]

desc none
# - setting this to "bottom-right" will display a (rather large) icon linking to the graphic, if desired

#Notes:
#Details on the new coding for clickable images is here: [[mw:Extension:ImageMap]]
#While it may look strange, it is important to keep the codes for a particular system in order. The clickable coding treats the first object created in an area as the one on top.
#Moons should be placed on "top" so that their smaller circles will not disappear "under" their respective primaries.
</imagemap>

Since the year 2000, a number of KBOs with diameters of between 500 and 1200&nbsp;km (about half that of Pluto) have been discovered. [[50000 Quaoar]], a classical KBO discovered in 2002, is over 1200&nbsp;km across. {{dp|Makemake}} (originally {{mp|(136472) 2005 FY|9}}, nicknamed "Easterbunny") and {{dp|Haumea}} (originally {{mp|(136108) 2003 EL|61}}, nicknamed "Santa"), both announced on July 29, 2005, are larger still. Other objects, such as [[28978 Ixion]] (discovered in 2001) and [[20000 Varuna]] (discovered in 2000) measure roughly 500&nbsp;km across.<ref name=beyond /> 

===Pluto===
{{Main|Pluto}}
The discovery of these large KBOs in similar orbits to Pluto led many to conclude that, bar its relative size, [[Pluto]] was not particularly different from other members of the Kuiper belt. Not only did these objects approach Pluto in size, but many also possessed satellites, and were 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 percent 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 |year=2007|accessdate=2007-06-14}}</ref> In response, the [[International Astronomical Union]] (IAU), was forced to [[Definition of planet|define a planet]] 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 news|url=http://www.iau.org/Resolutions_5-6.398.0.html|title=IAU 2006 General Assembly: Resolutions 5 and 6|date=24 August 2006|publisher=IAU}}</ref> As Pluto shared its orbit with so many KBOs, it was deemed not to have cleared its orbit, and was thus reclassified from a planet to a member of the Kuiper belt. 

Though Pluto is the largest KBO, a number of objects outside the Kuiper belt which may have begun their lives as KBOs are larger. Eris is the most obvious example, but Neptune's moon [[Triton (moon)|Triton]], which, as explained above, is probably a captured KBO, is also larger than Pluto.

As of 2008, only five objects in the Solar System, Ceres, Pluto, Eris, Makemake and Haumea, are considered dwarf planets. However, a number of other Kuiper belt objects are also large enough to be spherical and could be classified as dwarf planets in the future.<ref>{{cite web|title=IAU Draft Definition of Planet|work=IAU|url=http://www.iau.org/iau0601.424.0.html|year=2006|accessdate=2007-10-26}}</ref>

===Satellites===
Of the four largest TNOs, three (Eris, Pluto, and Haumea) possess satellites, and two have more than one. A higher percentage of the largest KBOs possess satellites than the smaller objects in the Kuiper belt, suggesting that a different formation mechanism was responsible.<ref>{{cite web|title=Satellites of the Largest Kuiper Belt Objects|author=M. E. Brown, M. A. van Dam, A. H. Bouchez, D. Le Mignant, R. D. Campbell, J. C. Y. Chin, A. Conrad, S. K. Hartman, E. M. Johansson, R. E. Lafon, D. L. Rabinowitz, P. J. Stomski, Jr., D. M. Summers, C. A. Trujillo, and P. L. Wizinowich|url=http://www.journals.uchicago.edu/cgi-bin/resolve?doi=10.1086/501524|year=2006|accessdate=2007-06-24}}</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 over 1 percent of KBOs (a high percentage) exist in binaries.<ref name=binary>{{cite web|title=Binary Kuiper Belt Objects|author=Dave Jewitt|url=http://www.ifa.hawaii.edu/~jewitt/kb/binaries.html|year=2005|accessdate=2007-06-24}}</ref>

==Scattered objects==
{{main|Scattered disc|Centaur (planetoid)}}
[[Image:TheKuiperBelt Projections 100AU Classical SDO.svg|left|thumb|240px|The orbits of objects in the scattered disc; the classical KBOs are blue, while the 2:5 resonant objects are green.]]
The scattered disc is a sparsely populated region beyond the Kuiper belt, extending as far as 100 AU and farther. [[Scattered disc object]]s (SDOs) travel in highly elliptical orbits, usually also highly inclined to the ecliptic. Most models of solar system formation show both KBOs and SDOs first forming in a primordial comet belt, while later gravitational interactions, particularly with Neptune, sent the objects spiraling outward; some into stable orbits  (the KBOs) and some into unstable orbits, becoming the scattered disc.<ref name=book /> Due to its unstable nature, the scattered disc is believed to be the point of origin for many of the Solar System's short-period comets.<ref name=book />

According to the [[Minor Planet Center]], which officially catalogues all trans-Neptunian objects, a KBO, strictly speaking, 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=scattered>{{cite web|url=http://cfa-www.harvard.edu/iau/lists/Centaurs.html|title=List Of Centaurs and Scattered-Disk Objects|work=IAU: Minor Planet Center|accessdate=2007-04-02}}</ref> However, in some scientific circles the term "Kuiper belt object" has become synonymous with any icy planetoid native to the outer solar system believed 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 disk region). They often describe scattered disc objects as "scattered Kuiper belt objects."<ref>{{cite web |year= 2005| author= David Jewitt| title=The 1000 km Scale KBOs| work=University of Hawaii| url=http://www.ifa.hawaii.edu/faculty/jewitt/kb/big_kbo.html| accessdate=2006-07-16}}</ref> [[Eris (dwarf planet)|Eris]], the recently discovered object now known to be larger than Pluto, is often referred to as a KBO, but is technically an SDO. A consensus among astronomers as to the precise definition of the Kuiper belt has yet to be reached, and this issue remains unresolved. 

[[Centaur (planetoid)|The centaurs]], which are not normally considered part of the Kuiper belt, are also believed 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=scattered />

=== Triton ===
{{main|Triton (moon)}}
[[Image:Triton moon mosaic Voyager 2 (large).jpg|thumb|200 px|left|Neptune's moon Triton]]
During its period of migration, Neptune is thought to have captured one of the larger KBOs and set it in orbit around itself. This is its moon [[Triton (moon)|Triton]], which is the only large moon in the Solar System to have a [[retrograde orbit]]; it orbits in the opposite direction to Neptune's rotation. This suggests that, unlike the large moons of Jupiter and Saturn, which are thought to have coalesced from spinning discs of material encircling 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 that some force act upon the object to slow it down enough to be snared by the larger object's gravity. How this happened to Triton is not well understood, though it does suggest that Triton formed as part of a large population of similar objects whose gravity could impede its motion enough to be captured.<ref>{{cite web|title=Neptune’s capture of its moon Triton in a binary-planet gravitational encounter|author=Craig B. Agnor & Douglas P. Hamilton|work=Nature|url=http://www.es.ucsc.edu/~cagnor/papers_pdf/2006AgnorHamilton.pdf|format=PDF|year=2006|accessdate=2007-10-29}}</ref> Triton is only slightly larger than Pluto, and spectral analysis of both worlds shows that they 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 web|title=TRITON, PLUTO, CENTAURS, AND TRANS-NEPTUNIAN BODIES|author=DALE P. CRUIKSHANK|work=NASA Ames Research Center|url=http://books.google.co.uk/books?hl=en&lr=&id=MbmiTd3x1UcC&oi=fnd&pg=PA421&dq=.+TRITON,+PLUTO,+CENTAURS,+AND+TRANS-NEPTUNIAN+BODIES&ots=pzwtede88A&sig=-p1FaqV0VcFtCRzwCoq9Mq8jijE|year=2004|accessdate=2007-06-23}}</ref>

==Exploration==
{{main|New Horizons}}
[[Image:New horizons Pluto.jpg|thumb|150 px|Artist's conception of ''New Horizons'' at Pluto]] 
On January 19, 2006 the first spacecraft mission to explore the Kuiper belt, ''[[New Horizons]],'' was launched. The mission, headed by [[Alan Stern]] of the [[Southwest Research Institute]], will arrive at [[Pluto]] on July 14, 2015 and, circumstances permitting, will continue on to study another as-yet undetermined KBO. Any KBO chosen will be between 25 and 55&nbsp;miles (40 to 90&nbsp;km) in diameter and, ideally, white or grey, to contrast with Pluto's reddish colour.<ref>{{cite web|title=New Horizons mission timeline|work=NASA|url=http://pluto.jhuapl.edu/mission/mission_timeline.html|accessdate=2007-08-12}}</ref> John Spencer, an astronomer on the ''New Horizons'' mission team, says that no target for a post-Pluto Kuiper belt encounter has yet been selected, as they are awaiting data from the [[Pan-STARRS]] survey project to ensure as wide a field of options as possible.<ref>{{cite web|title=The Man Who Finds Planets|author=Cal Fussman|work=Discover magazine|year=2006|url=http://discovermagazine.com/2006/may/cover/article_view?b_start:int=3&-C=|accessdate=2007-08-13}}</ref> The Pan-STARRS project, due to come fully online by 2009,<ref>{{cite web|title=Calibration of the Pan-STARRS 3π Survey|author=E. Magnier|work=Astronomical Society of the Pacific|url=http://adsabs.harvard.edu/abs/2007ASPC..364..153M|year=2007|accessdate=2007-08-13}}</ref> will survey the entire sky with four 1.4 gigapixel digital cameras to detect any moving objects, from [[near-earth object]]s to KBOs.<ref>{{cite web|title=Pan-Starrs: University of Hawaii|url=http://pan-starrs.ifa.hawaii.edu/public/home.html|year=2005|accessdate=2007-08-13}}</ref>

[[Image:Kuiper belt remote.jpg|thumb|250px|left|The debris disks around two stars ([[HD 139664]] and [[HD 53143]])]]

==Other Kuiper belts==
{{main|Debris disk}}
{{As of|2006}}, astronomers have resolved dust disks believed 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 (like our own Kuiper belt) with diameters of between 20 and 30 AU and relatively sharp boundaries. Most known [[debris disk|debris discs]] around other stars are fairly young, but the two images on the left, 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. The black central circle is produced by the camera's [[coronagraph]] which hides the central star to allow the much fainter disks to be seen.<ref name="Kalas et al. 2006">P. Kalas, J. R. Graham, M. C. Clampin, M. P. Fitzgerald (01/2006). ''First Scattered Light Images of Debris Disks Around HD 53143 And HD 139 664.'' The Astrophysical Journal, '''637''', issue 1, pp. L57–L60.  
[http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?bibcode=2006ApJ...637L..57K Article on ADS] [http://arxiv.org/abs/astro-ph/0601488 Article on Arxiv]</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|year=2006|accessdate=2007-07-01}}</ref>  Beyond this, 15-20% of solar-type stars have observed [[infrared excess]] which is believed to indicate massive Kuiper Belt like structures.<ref>{{cite journal | title = Debris Disks around Sun-like Stars | author = Trilling, D. E.; Bryden, G.; Beichman, C. A.; Rieke, G. H.; Su, K. Y. L.; Stansberry, J. A.; Blaylock, M.; Stapelfeldt, K. R.; Beeman, J. W.; Haller, E. E. | volume = 674 | issue =  2 | pages =  1086–1105 | month = February | year = 2008 | url = http://adsabs.harvard.edu/abs/2008ApJ...674.1086T | doi = 10.1086/525514 | journal = The Astrophysical Journal}}</ref>

==See also==
{{portal|Solar System|Solar system.jpg}}
* [[List of trans-Neptunian objects]]
* [[List of plutoid candidates]]

==References ==
{{reflist|3}}

==External links and data sources==
*[http://www.ifa.hawaii.edu/faculty/jewitt/kb.html Dave Jewitt's page @ University of Hawaii]
**[http://www.ifa.hawaii.edu/faculty/jewitt/kb/gerard.html The belt's name]
* [http://www.physics.ucf.edu/~yfernandez/cometlist.html List of short period comets by family]
* [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://cfa-www.harvard.edu/iau/lists/OuterPlot.html Minor Planet Center: Plot of the Outer Solar System], illustrating Kuiper gap
*[http://www.iau.org/ Website of the International Astronomical Union] (debating the status of TNOs)
*[http://www.astronomy2006.com XXVIth General Assembly 2006]
*[http://www.nature.com/nature/journal/v424/n6949/fig_tab/nature01725_F1.html nature.com article: diagram displaying inner solar system, Kuiper Belt, and Oort Cloud]
* SPACE.com: [http://www.space.com/scienceastronomy/060814_tno_found.html Discovery Hints at a Quadrillion Space Rocks Beyond Neptune] (Sara Goudarzi) 15 August 2006 06:13 a.m. ET
* [http://www.astronomycast.com/astronomy/episode-64-pluto-and-the-icy-outer-solar-system/ The Outer Solar System] [[Astronomy Cast]] episode #64, includes full transcript.

{{Small Solar System bodies}}
{{Trans-Neptunian dwarf planets}}
{{Solar System}}

[[Category:Solar System]]
[[Category:Kuiper belt objects| ]]

{{Link FA|fr}}

[[af:Kuiper-gordel]]
[[ang:Kuiperes gyrtel]]
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[[be-x-old:Пояс Койпэра]]
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[[bg:Пояс на Кайпер]]
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