Kuiper belt#.22Kuiper cliff.22

File:Pluto-Charon-v2-10-1-15.jpg

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

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

File:Gerard Kuiper 1964b.jpg, after whom the Kuiper belt is named]]

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

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

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

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

Further evidence for the existence of the Kuiper belt later emerged from the study of comets. That comets have finite lifespans has been known for some time. As they approach the Sun, its heat causes their volatile surfaces to sublimate into space, gradually dispersing them. In order for comets to continue to be visible over the age of the Solar System, they must be replenished frequently.{{cite journal |author=David Jewitt |title=From Kuiper Belt Object to Cometary Nucleus: The Missing Ultrared Matter |journal=The Astronomical Journal |volume=123 |issue=2 |pages=1039–1049 |date=2002 |doi=10.1086/338692 |bibcode=2002AJ....123.1039J |s2cid=122240711 |doi-access=free }} A proposal for such an area of replenishment is the Oort cloud, possibly a spherical swarm of comets extending beyond 50,000 AU from the Sun first hypothesised by Dutch astronomer Jan Oort in 1950.{{cite journal |bibcode=1950BAN....11...91O |title=The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin |last1=Oort |first1=J. H. |volume=11 |date=1950 |page=91 |journal=Bull. Astron. Inst. Neth.}} The Oort cloud is thought to be the point of origin of long-period comets, which are those, like Hale–Bopp, with orbits lasting thousands of years.{{rp|page=105}}File:Julio A. Fernandez (recortado).jpg predicted the existence of a belt. It has been said that because the words "Kuiper" and "comet belt" appeared in the opening sentence of Fernandez's paper, this hypothetical region was referred to as the "Kuiper belt".{{cite web|access-date=16 August 2020|date=8 October 2019|language=en-US|title=Kuiper Belt {{!}} Facts, Information, History & Definition|url=https://nineplanets.org/kuiper-belt/|website=The Nine Planets|archive-date=16 May 2021|archive-url=https://web.archive.org/web/20210516004242/https://nineplanets.org/kuiper-belt/|url-status=live}}]]There is another comet population, known as short-period or periodic comets, consisting of those comets that, like Halley's Comet, have orbital periods of 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.{{rp|page=39}} For an Oort cloud object to become a short-period comet, it would first have to be captured by the giant planets. In a paper published in Monthly Notices of the Royal Astronomical Society in 1980, Uruguayan astronomer 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 AU would be required to account for the observed number of comets.{{cite journal |title=On the existence of a comet belt beyond Neptune |author=J.A. Fernández |bibcode=1980MNRAS.192..481F |date=1980 |volume=192 |issue=3 |pages=481–491 |journal=Monthly Notices of the Royal Astronomical Society |doi=10.1093/mnras/192.3.481 |doi-access=free}} 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.{{cite journal |title=The origin of short-period comets |author=M. Duncan |author2=T. Quinn |author3=S. Tremaine |name-list-style=amp |date=1988 |bibcode=1988ApJ...328L..69D |volume=328 |pages=L69 |journal=Astrophysical Journal |doi=10.1086/185162|doi-access=free }} 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".{{rp|page=191}}

File:Mauna Kea Summit 2021-06-16 33 (cropped).jpg. The Kuiper belt was discovered with UH88, which is the fourth from the left.]]

In 1987, astronomer David Jewitt, then at MIT, became increasingly puzzled by "the apparent emptiness of the outer Solar System".{{cite journal |doi=10.1038/362730a0 |title=Discovery of the candidate Kuiper belt object 1992 QB1 |date=1993 |last1=Jewitt |first1=David |last2=Luu |first2=Jane |journal=Nature |volume=362 |issue=6422 |pages=730–732 |bibcode=1993Natur.362..730J|s2cid=4359389 }} 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."{{rp|page=50}} Using telescopes at the Kitt Peak National Observatory in Arizona and the Cerro Tololo Inter-American Observatory in Chile, Jewitt and Luu conducted their search in much the same way as Clyde Tombaugh and Charles Kowal had, with a blink comparator.{{rp|page=50}} Initially, examination of each pair of plates took about eight hours,{{rp|page=51}} but the process was sped up with the arrival of electronic charge-coupled devices or CCDs, which, though their field of view was narrower, were not only more efficient at collecting light (they retained 90% of the light that hit them, rather than the 10% achieved by photographs) but allowed the blinking process to be done virtually, on a computer screen. Today, CCDs form the basis for most astronomical detectors.{{rp|pages=52, 54, 56}} In 1988, Jewitt moved to the Institute of Astronomy at the University of Hawaii. Luu later joined him to work at the University of Hawaii's 2.24 m telescope at Mauna Kea.{{rp|pages=57, 62}} Eventually, the field of view for CCDs had increased to 1024 by 1024 pixels, which allowed searches to be conducted far more rapidly.{{rp|page=65}} Finally, after five years of searching, Jewitt and Luu announced on 30 August 1992 the "Discovery of the candidate Kuiper belt object 1992 QB₁". This object would later be named 15760 Albion. Six months later, they discovered a second object in the region, (181708) 1993 FW.{{cite journal |title=1993 FW |author1=Marsden, B.S. |place=Minor Planet Center |bibcode=1993IAUC.5730....1L |date=1993 |author2=Jewitt, D. |author3=Marsden, B.G. |issue=5730 |page=1 |journal=IAU Circ.}} By 2018, over 2000 Kuiper belts objects had been discovered.{{Cite web |url=https://solarsystem.nasa.gov/news/792/10-things-to-know-about-the-kuiper-belt |title=10 Things to Know About the Kuiper Belt |last=Dyches |first=Preston |website=NASA Solar System Exploration |date=14 December 2018 |access-date=2019-12-01 |archive-date=10 January 2019 |archive-url=https://web.archive.org/web/20190110003110/https://solarsystem.nasa.gov/news/792/10-things-to-know-about-the-kuiper-belt |url-status=live }}

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

Studies conducted since the trans-Neptunian region was first charted have shown that the region now called the Kuiper belt is not the point of origin of short-period comets, but that they instead derive from a linked population called the scattered disc. The scattered disc was created when Neptune 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.

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

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

At its fullest extent (but excluding the scattered disc), including its outlying regions, the Kuiper belt stretches from roughly 30–55 AU. The main body of the belt is generally accepted to extend from the 2:3 mean-motion resonance (see below) at 39.5 AU to the 1:2 resonance at roughly 48 AU.{{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 }} The Kuiper belt is quite thick, with the main concentration extending as much as ten degrees outside the ecliptic plane and a more diffuse distribution of objects extending several times farther. Overall it more resembles a torus or doughnut than a belt.{{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 }} Its mean position is inclined to the ecliptic by 1.86 degrees.{{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 }}

The presence of Neptune has a profound effect on the Kuiper belt's structure due to orbital resonances. 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 gaps 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.{{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 }}

File:KBO diagram eccentricity.png

{{Main|Classical Kuiper belt object}}

Between the 2:3 and 1:2 resonances with Neptune, at approximately 42–48 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, and its members comprise roughly two thirds of KBOs observed to date.{{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 }}

{{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}} Because the first modern KBO discovered (Albion, but long called (15760) 1992 QB₁), is considered the prototype of this group, classical KBOs are often referred to as cubewanos ("Q-B-1-os").{{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}}{{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 }} The guidelines established by the IAU demand that classical KBOs be given names of mythological beings associated with creation.{{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 }}

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.{{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 }} 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.{{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 |volume=426 |issue=6965 |pages=419–421 |doi=10.1038/nature02120 |pmid=14647375 |bibcode=2003Natur.426..419L|s2cid=4395099 }} 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,{{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 }} has a different size distribution, and lacks very large objects.{{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 }} The mass of the dynamically cold population is roughly 30 times less than the mass of the hot. 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 migration of the giant planets.{{cite arXiv

|last1=Morbidelli |first1=Alessandro

|date=2005

|title=Origin and Dynamical Evolution of Comets and their Reservoirs

|eprint=astro-ph/0512256

}} 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. 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.

{{Main|Resonant trans-Neptunian object}}

File:KBOs and resonances.pngs (blue), Resonant trans-Neptunian objects (red), Sednoids (yellow) and scattered objects (grey)]]

File:TheKuiperBelt classes-en.svg)]]

When an object's orbital period is an exact ratio of Neptune's (a situation called a 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 AU. This 2:3 resonance is populated by about 200 known objects,{{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 }} 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, 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.{{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}} IAU guidelines dictate that all plutinos must, like Pluto, be named for underworld deities. The 1:2 resonance (whose objects complete half an orbit for each of Neptune's) corresponds to semi-major axes of ~47.7 AU, and is sparsely populated.{{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 }} Its residents are sometimes referred to as twotinos. Other resonances also exist at 3:4, 3:5, 4:7, and 2:5.{{rp|page=104}} Neptune has a number of trojan objects, which occupy its Lagrangian points, 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 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.{{rp|page=107}}

File:Hot and cold KBO.svg

The 1:2 resonance at 47.8 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 resonance at roughly 55 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.

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 AU,{{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 }} 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 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. Patryk Lykawka of Kobe University claimed that the gravitational attraction of an unseen large planetary object, perhaps the size of Earth or Mars, might be responsible.{{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 }}{{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 }} 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.{{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 }}

File:Lhborbits.png

File:Outersolarsystem objectpositions labels comp.png

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. Pan-STARRS 1 finished its primary science mission in 2014, and the full data from the Pan-STARRS 1 surveys were published in 2019, helping reveal many more KBOs.{{Citation|title=The Pan-STARRS1 Surveys|first1=K. C.|last1=Chambers|display-authors=etal|arxiv=1612.05560|date=29 January 2019}}{{Cite journal|title=The Pan-STARRS1 Database and Data Products|first1=H. A.|last1=Flewelling|display-authors=etal|date=20 October 2020|journal=The Astrophysical Journal Supplement Series|volume=251|issue=1|page=7|doi=10.3847/1538-4365/abb82d|arxiv=1612.05243|bibcode=2020ApJS..251....7F|s2cid=119382318|doi-access=free }}{{Citation|url=https://pweb.cfa.harvard.edu/news/pan-starrs-releases-largest-digital-sky-survey-world|title=Pan-STARRS Releases Largest Digital Sky Survey to the World|date=19 December 2016|publisher=Harvard-Smithsonian Center for Astrophysics|access-date=21 October 2022|archive-date=21 October 2022|archive-url=https://web.archive.org/web/20221021143334/https://pweb.cfa.harvard.edu/news/pan-starrs-releases-largest-digital-sky-survey-world|url-status=live}}

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

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

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

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

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

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

File:2003 UB313 near-infrared spectrum.png

Being distant from the Sun and major planets, Kuiper belt objects are thought to be relatively unaffected by the processes that have shaped and altered other Solar System objects; thus, determining their composition would provide substantial information on the makeup of the earliest Solar System. 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 lines) appear where the substances within it have absorbed that particular wavelength of light. Every element or compound has its own unique spectroscopic signature, and by reading an object's full spectral "fingerprint", astronomers can determine its composition.

Analysis indicates that Kuiper belt objects are composed of a mixture of rock and a variety of ices such as water, methane, and ammonia. The temperature of the belt is only about 50 K,{{cite journal |title=Crystalline water ice on the Kuiper belt object (50000) Quaoar |journal=Nature |volume=432 |issue=7018 |pages=731–3 |author=David C. Jewitt |author2=Jane Luu |name-list-style=amp |url=http://www2.ess.ucla.edu/~jewitt/papers/50000/Quaoar.pdf |date=2004 |access-date=21 June 2007 |archive-url=https://web.archive.org/web/20070621182808/http://www.ifa.hawaii.edu/~jewitt/papers/50000/Quaoar.pdf |archive-date=21 June 2007|bibcode=2004Natur.432..731J |doi=10.1038/nature03111 |pmid=15592406 |s2cid=4334385 }} so many compounds that would be gaseous closer to the Sun remain solid. The densities and rock–ice fractions are known for only a small number of objects for which the diameters and the masses have been determined. The diameter can be determined by imaging with a high-resolution telescope such as the Hubble Space Telescope, by the timing of an occultation when an object passes in front of a star or, most commonly, by using the albedo of an object calculated from its infrared emissions. The masses are determined using the semi-major axes and periods of satellites, which are therefore known only for a few binary objects. The densities range from less than 0.4 to 2.6 g/cm³. The least dense objects are thought to be largely composed of ice and have significant porosity. The densest objects are likely composed of rock with a thin crust of ice. There is a trend of low densities for small objects and high densities for the largest objects. One possible explanation for this trend is that ice was lost from the surface layers when differentiated objects collided to form the largest objects.{{cite journal |last1=Brown |first1=Michael E. |title=The Compositions of Kuiper Belt Objects |journal=Annual Review of Earth and Planetary Sciences |date=2012 |volume=40 |issue=1 |pages=467–494 |doi=10.1146/annurev-earth-042711-105352 |arxiv=1112.2764 |bibcode=2012AREPS..40..467B|s2cid=14936224 }}

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

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

Although to date most KBOs still appear spectrally featureless due to their faintness, there have been a number of successes in determining their composition. In 1996, Robert H. Brown et al. acquired spectroscopic data on the KBO 1993 SC, which revealed that its surface composition is markedly similar to that of Pluto, as well as Neptune's moon Triton, with large amounts of methane ice.{{cite journal |doi=10.1126/science.276.5314.937 |title=Surface Composition of Kuiper Belt Object 1993SC |date=1997 |last1=Brown |first1=R. H. |journal=Science |volume=276 |issue=5314 |pages=937–9 |pmid=9163038 |last2=Cruikshank |first2=DP |last3=Pendleton |first3=Y |last4=Veeder |first4=GJ |bibcode=1997Sci...276..937B |s2cid=45185392 }} For the smaller objects, only colors and in some cases the albedos have been determined. These objects largely fall into two classes: gray with low albedos, or very red with higher albedos. The difference in colors and albedos is hypothesized to be due to the retention or the loss of hydrogen sulfide (H₂S) on the surface of these objects, with the surfaces of those that formed far enough from the Sun to retain H₂S being reddened due to irradiation.{{cite journal|last1=Wong|first1=Ian|last2=Brown|first2=Michael E.|date=2017|title=The bimodal color distribution of small Kuiper Belt objects|arxiv=1702.02615|doi=10.3847/1538-3881/aa60c3|volume=153|issue=4|journal=The Astronomical Journal|page=145|bibcode = 2017AJ....153..145W |s2cid=30811674 |doi-access=free }}

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

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

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

File:TheKuiperBelt PowerLaw2.svg

The size distributions of the Kuiper belt objects follow a number of power laws. A power law describes the relationship between N(D) (the number of objects of diameter greater than D) and D, and is referred to as brightness slope. The number of objects is inversely proportional to some power of the diameter D:

:$\backslash frac\{d\; N\}\{d\; D\}\; \backslash propto\; D^\{-q\},$ which yields (assuming q is not 1):$N\backslash propto\; D^\{1-q\}+\backslash text\{a\; constant\}.$

(The constant may be non-zero only if the power law doesn't apply at high values of D.)

Early estimates that were based on measurements of the apparent magnitude distribution found a value of q = 4 ± 0.5,{{cite journal |last1=Bernstein |first1=G. M. |last2=Trilling |first2=D. E. |last3=Allen |first3=R. L. |last4=Brown |first4=K. E. |last5=Holman |first5=M. |last6=Malhotra |first6=R. |title=The size distribution of transneptunian bodies |journal=The Astronomical Journal |volume=128 |issue=3 |pages=1364–1390 |doi=10.1086/422919 |arxiv=astro-ph/0308467 |bibcode=2004AJ....128.1364B |date=2004|s2cid=13268096 }} which implied that there are 8 (=2³) times more objects in the 100–200 km range than in the 200–400 km range.

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

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

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

File:TheKuiperBelt Projections 100AU Classical SDO.svg

{{Main|Scattered disc|Centaur (minor planet)}}

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

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

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

{{Main|Triton (moon)}}

File:Triton moon mosaic Voyager 2 (large).jpg's moon Triton]]

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

{{see also|List of the brightest Kuiper belt objects}}

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

{{Main|Pluto}}

The discovery of these large KBOs in orbits similar to Pluto's led many to conclude that, aside from its relative size, Pluto was not particularly different from other members of the Kuiper belt. Not only are these objects similar to Pluto in size, but many also have natural satellites, and are of similar composition (methane and carbon monoxide have been found both on Pluto and on the largest KBOs). Thus, just as Ceres was considered a planet before the discovery of its fellow asteroids, some began to suggest that Pluto might also be reclassified.

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

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

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

{{Main|New Horizons}}

File:KBO 2014 MU69 HST.jpg (green circles), the selected target for the New Horizons Kuiper belt object mission]]

On 19 January 2006, the first spacecraft to explore the Kuiper belt, New Horizons, was launched, which flew by Pluto on 14 July 2015. Beyond the Pluto flyby, the mission's goal was to locate and investigate other, farther objects in the Kuiper belt.{{cite web |url=http://discoverynewfrontiers.nasa.gov/missions/missions_nh.cfml |title=New Frontiers Program: New Horizons Science Objectives |work=NASA – New Frontiers Program |access-date=15 April 2015 |url-status=dead |archive-url=https://web.archive.org/web/20150415224640/http://discoverynewfrontiers.nasa.gov/missions/missions_nh.cfml |archive-date=15 April 2015 }}

File:2014 MU69 orbit.jpg

File:Ultima_Thule_New_Horizons_CA06_vertical.png

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

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

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

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

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

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

{{Main|Debris disc}}

File:Kuiper belt remote.jpg and HD 53143 – black circle from camera hiding stars to display discs.]]

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

• Asteroid belt
• List of possible dwarf planets
• List of trans-Neptunian objects
• Planet Nine

{{notelist|1}}

{{reflist}}

{{Commons category|Kuiper belt objects}}

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

{{Small Solar System bodies}}

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