Editing Kuiper belt (section)

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

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

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

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

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

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

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

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

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

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

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