Kleene star: Difference between revisions

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en>Jochen Burghardt
moved (and shortened) trivial section "When Kleene plus is equivalent to Kleene star" into "Examples", also deleting ' "sets and strings" ' which was not a quote from the article
en>Jason Quinn
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{{Use mdy dates|date=October 2011}}
== After a little silence ==
{{Featured article}}
[[File: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 colored green, whereas scattered objects are colored orange. The four outer planets are blue. Neptune's few known [[Neptune trojan|trojans]] are yellow, whereas Jupiter's are pink. The scattered objects between Jupiter's orbit and the Kuiper belt are known as [[Centaur (minor planet)|centaurs]]. The scale is in [[astronomical unit]]s. The pronounced gap at the bottom is due to difficulties in detection against the background of the plane of the [[Milky Way]].]]
{{TNO}}
The '''Kuiper belt''' {{IPAc-en|'|k|ai|p|ər}}, sometimes called the '''Edgeworth–Kuiper belt''' (after the astronomers [[Kenneth Edgeworth]] and [[Gerard Kuiper]]), is a region of the [[Solar System]] beyond the planets, extending from the [[orbit]] of [[Neptune]] (at 30 [[Astronomical unit|AU]]) to approximately 50 [[Astronomical unit|AU]] from the [[Sun]].<ref>{{cite journal | 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 | last2=Colwell | first2=Joshua E. | bibcode=1997ApJ...490..879S}}</ref> It is similar to the [[asteroid belt]], but it is far larger—20 times as wide and 20 to 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://www2.ess.ucla.edu/~jewitt/papers/2006/DJ06.pdf|accessdate=March 9, 2007|archiveurl = http://web.archive.org/web/20070925203400/http://www.ifa.hawaii.edu/faculty/jewitt/papers/2006/DJ06.pdf |archivedate = September 25, 2007}}</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. | 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}}</ref> Like the asteroid belt, it consists mainly of [[small Solar System body|small bodies]], or remnants from the Solar System's formation. Although some asteroids are composed primarily of [[Rock (geology)|rock]] and metal, most Kuiper belt objects are composed largely of frozen [[volatiles]] (termed "ices"), such as [[methane]], [[ammonia]] and water. The classical belt is home to at least three [[dwarf planet]]s: <!-- 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 --> [[Pluto]], [[Haumea (dwarf planet)|Haumea]], and [[Makemake (dwarf planet)|Makemake]]. Some of the Solar System's [[Natural satellite|moon]]s, such as Neptune's [[Triton (moon)|Triton]] and [[Saturn]]'s [[Phoebe (moon)|Phoebe]], are also believed to 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 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|year=2006| accessdate=June 20, 2006|archiveurl = http://web.archive.org/web/20070621182809/http://www.es.ucsc.edu/~cagnor/papers_pdf/2006AgnorHamilton.pdf |archivedate = June 21, 2007|deadurl=yes}}</ref>


Since the belt was discovered in 1992,<ref name=qbee/> the number of known Kuiper belt objects (KBOs) has increased to over a thousand, and more than 100,000 KBOs over {{convert|100|km|0|abbr=on}} in diameter are believed to exist.<ref>[http://pluto.jhuapl.edu/overview/piPerspective.php?page=piPerspective_08_24_2012 NEW HORIZONS ''The PI's Perspective'']</ref> The Kuiper belt was initially thought 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 classical 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
Some other mercenaries, is followed by a large army ran up, they would like to see, what is the story behind, could have qualified to lead such a large-scale siege.<br><br>'catch Xiao Yan, the guy who has a mysterious order Gong!'<br>When<br>to chase, some are Muslim snake looked at the forest to watch the mercenary team, appeared on the face touch Yin Xiao, tear the throat suddenly shouted.<br><br>hear the head of such a call, Langtou behind the mercenary group members, but also very clever in unison shouted up, suddenly, Xiao Yan was pregnant with big roar mysterious order power law, and that is vast, 'swing' casio 腕時計 スタンダード 'Dang 'The spread of the mountains.<br><br>'mysterious カシオの時計 order Gong?' the words one ear, 腕時計 メンズ casio almost all of the mercenary team are stopped her work and looked at カシオ 腕時計 激安 each other, eyes flashed a hint of all is greed.<br><br>After a little silence, someone finally lost its casio 腕時計 データバンク mysterious power law brought order lure
| title = Encyclopedia of the Solar System
相关的主题文章：
| chapter = Comet Populations and Cometary Dynamics
<ul>
| author=Harold F. Levison, Luke Donnes
 
| publisher=Academic Press
  <li>?aid=29</li>
| year = 2007
 
| editor = Lucy Ann Adams McFadden, Paul Robert Weissman, Torrence V. Johnson
  <li>http://cgi.www5f.biglobe.ne.jp/~mangetsu/cgi-bin/yybbs/yybbs.cgi</li>
| edition = 2nd
 
| publication-place = Amsterdam; Boston
  <li>?tid=435531&extra=</li>
| isbn = 0-12-088589-1
 
| pages = 575–588}}</ref> scattered disc objects such as [[Eris (dwarf planet)|Eris]] have extremely [[Orbital eccentricity|eccentric]] orbits that take them as far as 100 AU from the Sun.{{#tag:ref|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 a single 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
</ul>
|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=2011-01-03
|author=IAU: Minor Planet Center
|accessdate=2011-01-03}}</ref> the current editorial choice for Wikipedia articles on the trans-Neptunian region is to make this distinction as well. This choice means that, on Wikipedia, Eris, the largest known trans-Neptunian object, is not part of the Kuiper belt, and so Pluto becomes the largest Kuiper belt object.|group="nb"}}


The Kuiper belt should not be confused with the [[hypothetical|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=June 1, 2007|archiveurl = http://web.archive.org/web/20070529003558/http://www.astrosurf.com/aude/map/us/AstFamilies2004-05-20.htm |archivedate = May 29, 2007|deadurl=yes}}</ref>
== Xiao Yan state or not to go ==


Pluto is the largest known member of the Kuiper belt, and one of the two largest known TNOs, together with scattered-disc object [[Eris (dwarf planet)|Eris]].{{ref_label|nb 1|nb 1|none}} 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.
Vindictive, though, Emperor of India must display their demand for power is definitely not the soul, so he did not like that faint 'fan' status similar to using a Buddha lotus anger after emergence.<br><br>Even so, at the moment, Xiao Yan state or not to go, face 'color' pale, weary breath to turn his current strength cast Haiyin, they are still somewhat reluctant.<br>Xiao Yan Zi カシオ gps 時計 research and his eyes are all closely watching<br>colorful moment of that カシオ ソーラー 腕時計 energy spread to the sky, and that law enforcement sturdy iron Medusa so aggressive suffered a blow, I am afraid that will not be good カシオ 掛け時計 to get there casio 腕時計 説明書 go, but no matter what, this guy, Xiao Yan is holding Betrader heart, but now he also recognized his cast by Indian decision, in order to relieve some trouble later, then we absolutely can not let him leave peacefully!<br><br>mind flashed so the idea, Xiao Yan eyes カシオの時計 of passing touch of awe-inspiring is the intention to kill this person, you can not leave!
 
相关的主题文章：
== History ==
<ul>
After the discovery of Pluto, 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.
 
 
  <li>?aid=130</li>
=== Hypotheses ===
 
The first [[astronomer]] to suggest the existence of a trans-Neptunian population was [[Frederick C. Leonard]]. In 1930, soon after Pluto's discovery by [[Clyde Tombaugh]], Leonard 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.icq.eps.harvard.edu/kb.html|work=International Comet Quarterly|accessdate=October 24, 2010}}</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|author=J. K. Davies, J. McFarland, M. E. Bailey, B. G. Marsden, W. I. Ip|title=The Solar System Beyond Neptune|editor=M. Antonietta Baracci, Hermann Boenhardt, Dale Cruikchank, Alissandro Morbidelli|pages=11–23|publisher=University of Arizona Press|url=http://www.arm.ac.uk/preprints/2008/522.pdf|year=2008|chapter=The Early Development of Ideas Concerning the Transneptunian Region}}</ref>
  <li>http://yumegomori.xsrv.jp/hositei/cgi_bbs/fantasy.cgi</li>
 
 
[[File:GerardKuiper.jpg|thumb|150 px|Astronomer [[Gerard Kuiper]], after whom the Kuiper belt is named]]
  <li>?mod=space&uid=994042</li>
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 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]].
 
 
  </ul>
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 today.<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|accessdate=June 14, 2007}}</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> whereas 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 journal |journal=Proceedings of the National Academy of Sciences |volume= 51 |issue=5 |page=771 |url=http://www.pnas.org/cgi/reprint/51/5/711.pdf |year=1964|bibcode = 1964PNAS...51..771R |doi = 10.1073/pnas.51.5.771 |title=Decomposition of Vector Measures |last1=Rao |first1=M. M. }}</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 journal|title=The discovery and orbit of /2060/ Chiron |author=CT Kowal, W Liller, BG Marsden |place=Hale Observatories, Harvard–Smithsonian Center for Astrophysics |year=1977 |bibcode=1979IAUS...81..245K|volume=81|page=245|journal=In: Dynamics of the solar system; Proceedings of the Symposium|last2=Liller|last3=Marsden}}</ref> In 1992, another object, [[5145 Pholus]], was discovered in a similar orbit.<ref>{{cite journal|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|bibcode=1992IAUC.5434....1S|year=1992|volume=5434|page=1|journal=IAU Circ.|last2=Rabinowitz|last3=Shoemaker|last4=Shoemaker|last5=Levy|last6=King|last7=Helin|last8=Alu|last9=Lawrence}}</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 | 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 }}</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 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 evaporating them. In order to still 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 | year=2002 | doi=10.1086/338692 | bibcode=2002AJ....123.1039J}}</ref> One such area of replenishment is the [[Oort cloud]], a spherical swarm of comets extending beyond 50,000 [[Astronomical unit|AU]] from the Sun first hypothesised by 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 |author1=Oort |first1=J. H. |volume=11 |year=1950 |page=91 |journal=Bull. Astron. Inst. Neth.}}</ref> It is believed to be the point of origin of [[long-period comet]]s, those, like [[Comet Hale–Bopp|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, like [[Halley's Comet|Halley]], 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 their 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]], 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 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 journal|title=On the existence of a comet belt beyond Neptune|author=JA Fernández|bibcode=1980MNRAS.192..481F|year=1980|volume=192|page=481|journal=Monthly Notices of the Royal Astronomical Society}}</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, T. Quinn, and S. Tremaine|year=1988|bibcode=1988ApJ...328L..69D|volume=328|pages=L69|journal=Astrophysical Journal|doi=10.1086/185162}}</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>Davies p. 191</ref>
 
=== Discovery ===
[[File:Maunatele.jpg|thumb|250px|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 journal |doi=10.1038/362730a0 |title=Discovery of the candidate Kuiper belt object 1992 QB1 |year=1993 |last1=Jewitt |first1=David |last2=Luu |first2=Jane |journal=Nature |volume=362 |issue=6422 |page=730|bibcode = 1993Natur.362..730J }}</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&nbsp;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 journal|title=1993 FW|author=BS Marsden|place=Minor Planet Center|bibcode=1993IAUC.5730....1L|year=1993|author2=Jewitt, D.|author3=Marsden, B. G.|volume=5730|page=1|journal=IAU Circ.}}</ref>
 
Studies 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 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. However, [[Brian 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>Davies p. 199</ref> Conversely, David Jewitt comments that, "If anything&nbsp;... 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 a [[synonym]] though, as TNOs include all objects orbiting the Sun past the orbit of [[Neptune]], not just those in the Kuiper belt.
 
== Origins ==
[[File: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 {{convert|3000|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 their present positions, 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. 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 while Jupiter drifted inwards. Eventually, the orbits shifted to the point where Jupiter and Saturn reached an exact 2:1 resonance; Jupiter orbited the Sun twice for every one Saturn orbit. The gravitational repercussions of such a resonance ultimately disrupted the orbits of Uranus and Neptune, causing Neptune's orbit to become more eccentric and move outward into the primordial planetesimal disk, which sent the disk into temporary chaos.<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]]
|accessdate=2007-08-26
}}</ref><ref name="Tsiganis05">
{{cite journal
|last1=Tsiganis |first1=K.
|last2=Gomes |first2=R.
|last3=Morbidelli |first3=A.
|last4=Levison |first4=H. F.
|year=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
}}</ref><ref name="Levison2008"/> As Neptune's orbit expanded, it excited and scattered many TNO planetesimals into higher and more eccentric orbits.<ref>
{{cite journal
|last1=Thommes |first1=E. W.
|last2=Duncan |first2=M. J.
|last3=Levison |first3=H. F.
|year=2002
|title=The Formation of Uranus and Neptune among Jupiter and Saturn
|journal=[[The Astronomical Journal]]
|volume=123 |issue=5 |page=2862
|arxiv=astro-ph/0111290
|bibcode=2002AJ....123.2862T
|doi=10.1086/339975
}}</ref> Many more were scattered inward, often to be scattered again and in some cases ejected by Jupiter. The process is thought to have reduced the primordial Kuiper belt population by 99% or more, and to have shifted the distribution of the surviving members outward.<ref name="Levison2008">
{{cite journal
|last=Levison |first=H. F.
|last2=Morbidelli |first2=A.
|last3=Van Laerhoven |first3=C.
|last4=Gomes |first4=R.
|year=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
}}</ref>
 
However, this currently most popular model, the "[[Nice model#Formation of the Kuiper belt|Nice model]]", still fails to account for some of the characteristics of the distribution and, quoting one of the scientific articles,<ref>
{{cite journal
|last1=Malhotra |first1=R.
|year=1994
|title=Nonlinear Resonances in the Solar System
|journal=[[Physica D]]
|volume=77 |issue= |page=289
|arxiv=chao-dyn/9406004
|bibcode=1994PhyD...77..289M
|doi=10.1016/0167-2789(94)90141-4
}}</ref> the problems "continue to challenge analytical techniques and the fastest numerical modeling hardware and software". The model predicts a higher average eccentricity in classical KBO orbits than is observed (0.10–0.13 versus 0.07).<ref name="Levison2008"/> The frequency of paired objects, many of which are far apart and loosely bound, also poses a problem for the model.<ref>
{{cite journal
|last1=Lovett |first1=R.
|year=2010
|title=Kuiper Belt may be born of collisions
|journal=[[Nature (journal)|Nature]]
|doi=10.1038/news.2010.522
}}</ref>
 
== Structure ==
[[File:Dust Models Paint Alien's View of Solar System.ogv|thumb|[[Cosmic dust|Dust]] in the Kuiper belt creates a faint [[infrared]] disk.]]
 
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
|author=M. C. De Sanctis, M. T. Capria, and A. Coradini
|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
}}</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=June 23, 2007|archiveurl = http://web.archive.org/web/20080305191925/http://www.americanscientist.org/template/AssetDetail/assetid/25723/page/2;jsessionid=aaa5LVF0 |archivedate = March 5, 2008|deadurl=yes}}</ref> Its mean position is inclined to the ecliptic by 1.86 degrees.<ref>{{cite journal | 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 | bibcode=2004AJ....127.2418B}}</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 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|year=1998|accessdate=June 23, 2007}}</ref>
 
=== 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 influence of Neptune is negligible, and objects can exist with their orbits essentially 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
|last1=Lunine |first1=J.
|year=2003
|title=The Kuiper Belt
|url=http://www.gsmt.noao.edu/gsmt_swg/SWG_Apr03/The_Kuiper_Belt.pdf
|accessdate=2007-06-23
}}</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
|accessdate=2007-06-23
|archiveurl=http://web.archive.org/web/20070609094740/http://www.ifa.hawaii.edu/~jewitt/kb/kb-classical.html
|archivedate=2007-06-09
}}</ref> Because the first modern KBO discovered, {{mpl|(15760) 1992 QB|1}}, is considered the prototype of this group, classical KBOs are often referred to as [[cubewanos]] ("Q-B-1-os").<ref>
{{cite journal| last1 = Murdin| first1 = P.| title = The Encyclopedia of Astronomy and Astrophysics| year = 2000| isbn = 0-333-75088-8| doi = 10.1888/0333750888/5403| bibcode = 2000eaa..bookE5403.| chapter = Cubewano }}</ref><ref>
{{cite journal
|last1=Elliot |first1=J. L.
|coauthors=''et al.''
|year=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= |pages=1117–1162
|arxiv=
|bibcode=2005AJ....129.1117E
|doi=10.1086/427395
}}</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]]
|accessdate=2008-11-17
}}</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 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=H. F
|last2=Morbidelli |first2=A.
| year=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 }}</ref> The two populations not only possess different orbits, but different colors; the cold population is markedly redder than the hot. If this is a reflection of different compositions, it suggests they formed in different regions. 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, has been proposed to have formed more or less in its current position, although it might also have been later swept outwards by Neptune during its [[Planetary migration|migration]],<ref name=beyond /><ref name="Morbidelli2005">{{cite arXiv
|last1=Morbidelli |first1=A.
| year=2005
| title=Origin and Dynamical Evolution of Comets and their Reservoirs
| class=astro-ph
| eprint=astro-ph/0512256
}}</ref> particularly if Neptune's eccentricity was transiently increased.<ref name="Levison2008"/> 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:TheKuiperBelt 75AU All.svg|thumb|Distribution of [[cubewano]]s (blue), [[Resonant trans-Neptunian object]]s (red) and near [[scattered disk|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 is in just the right kind of orbit so that it orbits the Sun two times 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 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.minorplanetcenter.org/iau/lists/TNOs.html|accessdate=June 23, 2007}}</ref> including [[Pluto]] together with its moons. In recognition of this, the members of this family are known as [[plutinos]]. Many plutinos, including Pluto, have orbits that cross that of Neptune, though 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 | year=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 | display-authors=1 }}</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 ~47.7AU, 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=June 23, 2007}}</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 typically have very stable orbits.
 
Additionally, there is a relative absence of objects with semi-major axes below 39 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>Davies p. 107</ref>
 
=== "Kuiper cliff" ===
[[File:ExampleUpdatedHistogramOfTNOsemimajoraxii.png|thumb|Graph showing the numbers of KBOs for a given distance from the Sun. The plutinos are the "spike" at 39 AU, whereas the classicals are between 42 and 47 AU, the twotinos are at 48 AU, and the 5:2 resonance is at 55 AU.]]
The [[Twotino|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|year=1999|accessdate=July 1, 2007}}</ref> so this sudden drastic falloff, known as the "Kuiper cliff", was completely unexpected, and its cause, to date, is unknown. In 2003, Bernstein and Trilling ''et al.'' 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 was too scarce or too scattered to accrete into large objects, or that subsequent processes removed or destroyed those that did.<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|journal=The Astrophysical Journal|year = 2004| doi = 10.1086/422919 | volume=128 | page=1364|arxiv = astro-ph/0308467 |bibcode = 2004AJ....128.1364B|issue=3 }}</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=June 23, 2007}}</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=February 8, 2008}}</ref>
 
== Composition ==
[[File:2003 UB313 near-infrared spectrum.gif|thumb|200 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]],<ref name="physical">{{cite book|title=Encyclopedia of the Solar System|editor=Lucy-Ann McFadden ''et al.'' |chapter=Kuiper Belt Objects: Physical Studies|author=Stephen C. Tegler|pages=605–620|year=2007}}</ref> a composition they share with [[comets]].<ref>{{cite journal|doi=10.1023/A:1005256607402|year=1999|last1=Altwegg|first1=K.|last2=Balsiger|first2=H.|last3=Geiss|first3=J.|journal=Space Science Reviews|volume=90|page=3|bibcode = 1999SSRv...90....3A }}</ref> The low densities observed in those KBOs whose diameter is known, (less than 1 g cm<sup>−3</sup>) is consistent with an icy makeup.<ref name=physical/> 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://www2.ess.ucla.edu/~jewitt/papers/50000/Quaoar.pdf|year=2004|accessdate=June 21, 2007|archiveurl = http://web.archive.org/web/20070621182808/http://www.ifa.hawaii.edu/~jewitt/papers/50000/Quaoar.pdf |archivedate = June 21, 2007}}</ref> so many compounds that would be gaseous closer to the Sun remain 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 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 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 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|year=2004|accessdate=June 21, 2007|archiveurl = http://web.archive.org/web/20070609094911/http://www.ifa.hawaii.edu/~jewitt/kb/kb-colors.html |archivedate = June 9, 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|year=1998|last1=Jewitt|first1=David|last2=Luu|first2=Jane|journal=The Astronomical Journal|volume=115|issue=4|page=1667|bibcode = 1998AJ....115.1667J }}</ref> This suggested that their surfaces were composed of a wide range of compounds, from dirty ices to hydrocarbons.<ref name=color /> 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 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 |year=2001 |last1=Jewitt |first1=David C. |last2=Luu |first2=Jane X. |journal=The Astronomical Journal |volume=122 |issue=4 |page=2099|arxiv = astro-ph/0107277 |bibcode = 2001AJ....122.2099J }}</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 journal|doi=10.1126/science.276.5314.937|title=Surface Composition of Kuiper Belt Object 1993SC|year=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 }}</ref>
 
Water ice has been detected in several KBOs, including {{mpl-|19308|1996 TO|66}},<ref>{{cite journal |doi=10.1086/317277 |title=Near-Infrared Spectroscopy of the Bright Kuiper Belt Object 2000 EB173 |year=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 }}</ref> [[38628 Huya]] and [[20000 Varuna]].<ref>{{cite journal |year= 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 }}</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 ==
[[File: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 total mass is estimated to range between a 25th and 10th the mass of the Earth<ref name="g01">{{cite journal|last=Gladman|first=Brett|coauthors=et al|title=The structure of the Kuiper belt|journal=Astronomical Journal|date=05/April/2001|year=2001|month=August|volume=122|pages=1051–1066|doi=10.1086/322080|bibcode=2001AJ....122.1051G|issue=2}}</ref> 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|bibcode=2007MNRAS.tmp...24I|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 {{convert|100|km|0|abbr=on}} in diameter. If the Kuiper belt had always had its current low density these large objects simply could not have formed.<ref name=beyond /> 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. Although 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=Morbidelli2005/>
 
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 between ''N''(''D'') (the number of objects of diameter greater than ''D'') and ''D'', 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} \propto D^{-q}</math> where the current measures<ref name="Bernstein et al. 2004">{{cite journal |last=Bernstein |first=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 |year=2004}}</ref> give q = 4 ±0.5.
 
This implies that
:<math>N\propto D^{1-q}+\text{a constant}.</math>
 
Less formally, there are for instance 8 (=2<sup>3</sup>) 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 {{convert|1000|km|0|abbr=on}} there should be around 1000 (=10<sup>3</sup>) objects with diameter of {{convert|100|km|0|abbr=on}}.
 
If ''q'' is 4 or less, the law would imply an infinite mass in the Kuiper belt. The true function ''N''(''D'') obviously assumes only integer values, so the fractional values it gives for ''N'' at large ''D'' cannot be accurate. 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"/>
 
Of course, only the magnitude is actually known, the size is inferred assuming [[albedo]] (not a safe assumption for larger objects).
 
Since January 2010, the smallest Kuiper belt object discovered to date spans 980&nbsp;m across.<ref>{{cite news|last={{!}}|first=Eric|title=Hubble Finds Smallest Kuiper Belt Object|url=http://www.meteoritesusa.com/meteorite-photos/hubble-finds-smallest-kuiper-belt-object/|accessdate=January 16, 2011|newspaper=MeteoritesUSA|date=January 13, 2010}}</ref>
 
== Scattered objects ==
[[File:TheKuiperBelt Projections 100AU Classical SDO.svg|left|thumb|240px|Comparison of the orbits of scattered disc objects (black), classical KBOs (blue), and 2:5 resonant objects (green). Orbits of other KBOs are gray.]]
{{Main|Scattered disc|Centaur (minor planet)}}
The scattered disc is a sparsely populated region, overlapping with the Kuiper belt but 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, whereas 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. Their dynamic orbits occasionally force them into the inner Solar System, becoming first [[Centaur (astronomy)|centaurs]], and then 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="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|accessdate=October 27, 2010}}</ref> However, in some scientific circles the term "Kuiper belt object" has become synonymous with any icy minor planet 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-disc 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://www2.ess.ucla.edu/~jewitt/kb/big_kbo.html| accessdate=July 16, 2006}}</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 [[Centaur (minor planet)|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="cen_sdo" />
 
=== Triton ===
[[File:Triton moon mosaic Voyager 2 (large).jpg|thumb|200 px|Neptune's moon Triton]]
{{Main|Triton (moon)}}
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, Saturn, and Uranus, 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 some mechanism to slow the object down enough to be snared by the larger object's gravity. Triton may have encountered Neptune as part of a binary (many KBOs are members of binaries; see [[Kuiper belt#Satellites|below]]); ejection of the other member of the binary by Neptune could then explain Triton's capture.<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|year=2006|accessdate=October 29, 2007|archiveurl = http://web.archive.org/web/20070621182809/http://www.es.ucsc.edu/~cagnor/papers_pdf/2006AgnorHamilton.pdf |archivedate = June 21, 2007|deadurl=yes}}</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 book| last1 = Encrenaz| first1 = Thérèse| last2 = Kallenbach| first2 = R.| last3 = Owen| first3 = T.| last4 = Sotin| first4 = C.| title = TRITON, PLUTO, CENTAURS, AND TRANS-NEPTUNIAN BODIES| url = http://books.google.com/?id=MbmiTd3x1UcC&pg=PA421| accessdate = June 23, 2007| year = 2004| publisher = Springer| isbn = 978-1-4020-3362-9| work = NASA Ames Research Center }}</ref>
 
== Largest KBOs ==
{{see also|List of the brightest KBOs}}
{{TNO imagemap}}
<!-- 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 the year 2000, a number of KBOs with diameters of between 500 and {{convert|1500|km|0|abbr=on}}, more than half that of Pluto, have been discovered. [[50000 Quaoar]], a classical KBO discovered in 2002, is over 1,200&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 {{convert|500|km|0|abbr=on}} across.<ref name=beyond />
 
=== Pluto ===
<!-- 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 -->
{{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=June 14, 2007}}</ref> 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 |year=2006 }}</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.
 
Although Pluto is currently the largest KBO,<!-- Please do not add Eris here. Eris is often called a Kuiper belt object but Wiki convention treats the scattered disc as distinct from the Kuiper belt.--> there are two known larger objects currently outside the Kuiper belt that probably originated in the Kuiper belt. These are Eris and Neptune's moon [[Triton (moon)|Triton]] (which, as explained above, is probably a captured KBO).
 
As of 2008, only five objects in the Solar System, Ceres, Eris, and the KBOs Pluto, Makemake and Haumea, are listed as dwarf planets by the IAU. However, [[90482 Orcus]], 28978 Ixion and [[List of possible dwarf planets|many other Kuiper-belt objects]] are large enough to be in hydrostatic equilibrium; most of them will probably qualify when more is known about them.<ref>{{cite web|title=Ixion|work=eightplanets.net|url=http://ixion.eightplanets.net/|accessdate=June 23, 2007}}</ref><ref name="albedo">{{cite arXiv |year= 2007|title= Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope|class= astro-ph|eprint=0702538 |author1= John Stansberry |author2= Will Grundy |author3= Mike Brown |author4= Dale Cruikshank |author5= John Spencer |author6= David Trilling |author7= Jean-Luc Margot}}</ref><ref>{{cite web|title=IAU Draft Definition of Planet|work=IAU|url=http://www.iau.org/iau0601.424.0.html|year=2006|accessdate=October 26, 2007}}</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 larger KBOs possess satellites than the smaller objects in the Kuiper belt, suggesting that a different formation mechanism was responsible.<ref>{{cite doi |10.1086/501524 }}</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 percent of KBOs exist in binaries.<ref>{{cite journal|first=C.B.|last=Agnor|first2=D.P.|last2=Hamilton|title=Neptune's capture of its moon Triton in a binary-planet gravitational encounter|journal=Nature|volume=441|year=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}}</ref>
 
== Exploration ==
[[File:New horizons Pluto.jpg|thumb|left|150 px|Artist's conception of ''New Horizons'' at Pluto]]
{{Main|New Horizons}}
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 color.<ref>{{cite web|title=New Horizons mission timeline|work=NASA|url=http://pluto.jhuapl.edu/mission/mission_timeline.php|accessdate=March 19, 2011}}</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=August 13, 2007}}</ref> The Pan-STARRS project, partially operational since May 2010,<ref>{{cite web|title=PS1 goes Operational and begins Science Mission, May 2010|author=Institute for Astronomy, University of Hawai|url=http://pan-starrs.ifa.hawaii.edu/public/project-status/PS1%20operational.html|year=2010|accessdate=August 30, 2010}}</ref> will, when fully online, 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=August 13, 2007}}</ref> To speed up the detection process, the New Horizons team established [[Zooniverse (citizen science project)#Ice Hunters|Ice Hunters]], a [[citizen science]] project that allowed members of the public to participate in the search for suitable KBO targets;<ref>{{cite web| title=Ice Hunters web site| url=http://www.icehunters.org | publisher=[[Zooniverse (citizen science project)|Zooniverse.Org]] | accessdate=July 8, 2011}}</ref><ref name="nasaice">{{cite web|title=Citizen Scientists: Discover a New Horizons Flyby Target|url=http://solarsystem.nasa.gov/news/display.cfm?News_ID=37726|publisher=[[NASA]]|accessdate=August 23, 2011|date=Jun 21, 2011}}</ref><ref name = "exciting">{{cite web
| last = Lakdawalla | first = Emily
| title = The most exciting citizen science project ever (to me, anyway)
| publisher=[[The Planetary Society]] | date = June 21, 2011
| url = http://planetary.org/blog/article/00003073/
| accessdate =August 31, 2011}}</ref> the project has subsequently been transferred to another site, [[Ice Investigators]],<ref>{{cite web
| title = Ice Investigators
| work = web site
| publisher = CosmoQuest
| year = 2012
| url = http://cosmoquest.org/iceinvestigators/
| accessdate = 2012-05-23}}</ref> produced by [[CosmoQuest]].<ref name = "Finding">{{cite web
| title = Finding Ice
| work = [http://cosmoquest.org/ CosmoQuest web site]
| publisher = CosmoQuest
| date = 2012-05-20
| url = http://cosmoquest.org/blog/2012/05/finding-ice/
  | accessdate = 2012-05-23}}</ref>
 
[[File:Kuiper belt remote.jpg|thumb|250px|The debris disks around the stars [[HD 139664]] and [[HD 53143]]. 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.]]
 
== Other Kuiper belts ==
{{Main|Debris disk}}
By 2006, astronomers had 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 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 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 |date=February 2008 | bibcode = 2008ApJ...674.1086T | doi = 10.1086/525514 | journal=The Astrophysical Journal|arxiv = 0710.5498 | last2 = Bryden | last3 = Beichman | last4 = Rieke | last5 = Su | last6 = Stansberry | last7 = Blaylock | last8 = Stapelfeldt | last9 = Beeman | last10 = Haller }}</ref> Most known [[debris disk|debris discs]] 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 |author1=Kalas |first1=Paul |last2=Graham |first2=James R. |last3=Clampin |first3=Mark C. |last4=Fitzgerald |first4=Michael P. |volume=637 |year=2006 |pages=L57 |journal=The Astrophysical Journal |doi=10.1086/500305}}</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=July 1, 2007}}</ref> [[Supercomputer]] 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.; Stark, C. C. | volume = 140 | issue = 4 | pages = 1007–1019 | year = 2010 | bibcode = 2010AJ....140.1007K | doi = 10.1088/0004-6256/140/4/1007 | journal=The Astronomical Journal|arxiv = 1008.0904 | last2 = Stark }}</ref>
 
== See also ==
{{Portal|Solar System}}
{{Wikipedia books|Solar System}}
* [[List of possible dwarf planets]]
* [[List of trans-Neptunian objects]]
 
== Notes ==
{{Reflist|group="nb"}}
 
== References ==
{{Reflist|30em}}
 
== External links and data sources ==
{{Commons category|Kuiper belt}}
* [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.icehunters.org/ Ice Hunters], a citizen science project to help discover a KBO to serve as a follow-up ''New Horizons'' mission target after Pluto
* [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://www.minorplanetcenter.org/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], taken from {{cite journal |doi=10.1038/nature01725 |title=The evolution of comets in the Oort cloud and Kuiper belt |year=2003 |last1=Alan Stern |first1=S. |journal=Nature |volume=424 |issue=6949 |pages=639–42 |pmid=12904784}}
* SPACE.com: [http://www.space.com/scienceastronomy/060814_tno_found.html Discovery Hints at a Quadrillion Space Rocks Beyond Neptune] (Sara Goudarzi) August 15, 2006 06:13&nbsp;am ET
* [http://www.astronomycast.com/astronomy/episode-64-pluto-and-the-icy-outer-solar-system/ The Outer Solar System] [[Astronomy Cast]] episode No.&nbsp;64, includes full transcript.
* [http://365daysofastronomy.org/2009/12/08/december-8th-what-is-the-kuiper-belt/ The Kuiper belt] at 365daysofastronomy.org
 
{{Small Solar System bodies}}
{{Plutoids}}
{{Solar System}}
 
{{DEFAULTSORT:Kuiper Belt}}
[[Category:Kuiper belt objects| ]]
[[Category:Trans-Neptunian region]]
 
{{Link FA|ar}}
{{Link FA|cs}}
{{Link FA|fr}}
{{Link FA|pl}}

Latest revision as of 10:33, 21 December 2014

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