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{{Short description|Solar system objects beyond Neptune}}
{{About|solar system objects beyond Neptune|hypothetical planets| Planets beyond Neptune}}
{{Use dmy dates|date=August 2024}}
{{TNO}}

A '''trans-Neptunian object''' ('''TNO'''), also written '''transneptunian object''',<ref>{{Cite web|url=https://www.eso.org/public/images/eso9415a/|title = Transneptunian object 1994 TG2}}</ref> is any [[minor planet]] in the [[Solar System]] that [[orbit]]s the [[Sun]] at a greater average distance than [[Neptune]], which has an orbital [[semi-major axis]] of 30.1 [[astronomical unit]]s (AU).

Typically, TNOs are further divided into the [[Classical Kuiper belt object|classical]] and [[Resonant trans-Neptunian object|resonant]] objects of the [[Kuiper belt]], the [[scattered disc]] and [[detached object]]s with the [[sednoid]]s being the most distant ones.{{refn|The literature is inconsistent in the use of the phrases "scattered disc" and "Kuiper belt". For some, they are distinct populations; for others, the scattered disk is part of the Kuiper belt, in which case the low-eccentricity population is called the "classical Kuiper belt". Authors may even switch between these two uses in a single publication.<ref>McFadden, Weissman, & Johnson (2005). ''Encyclopedia of the Solar System'', footnote p. 584</ref>|group="nb"|name="KB"}} As of February 2025, the [[List of minor planets#Main index|catalog of minor planets]] contains [[List of trans-Neptunian objects|1006 numbered]] and more than [[List of unnumbered trans-Neptunian objects|4000 unnumbered TNOs]].<ref name="MPC-List-TNOs"/><ref name="MPC-List-Cen-and-SDO"/><ref name="TNO-list-Johnstons"/><ref name="JPL-TNO-query-I"/><ref name="JPL-TNO-query-II"/> However, nearly 5900 objects with semimajor axis over 30 AU are present in the MPC catalog, with 1009 being numbered.

The first trans-Neptunian object to be [[Planets beyond Neptune|discovered was Pluto]] in 1930. It took until 1992 to discover a second trans-Neptunian object orbiting the Sun directly, [[15760 Albion]]. The most massive TNO known is [[Eris (dwarf planet)|Eris]], followed by [[Pluto]], {{dp|Haumea}}, {{dp|Makemake}}, and {{dp|Gonggong}}. More than [[Minor-planet moon#Trans-Neptunian objects|80 satellites]] have been discovered in orbit of trans-Neptunian objects. TNOs vary in [[#Colours|color]] and are either grey-blue (BB) or very red (RR). They are thought to be composed of mixtures of rock, [[amorphous carbon]] and volatile ices such as water and [[methane]], coated with [[tholin]]s and other organic compounds.

Twelve minor planets with a semi-major axis greater than 150&nbsp;AU and [[perihelion]] greater than 30&nbsp;AU are known, which are called [[extreme trans-Neptunian object]]s (ETNOs).<ref name="Marcos2014">{{cite journal|author=C. de la Fuente Marcos|author2=R. de la Fuente Marcos|title=Extreme trans-Neptunian objects and the Kozai mechanism: signalling the presence of trans-Plutonian planets|journal=[[Monthly Notices of the Royal Astronomical Society]]|volume=443|issue=1|pages=L59–L63|arxiv=1406.0715|bibcode=2014MNRAS.443L..59D|doi=10.1093/mnrasl/slu084|date=September 1, 2014|doi-access=free }}</ref>

== History ==

=== Discovery of Pluto ===
[[File:Pluto in True Color - High-Res.jpg|thumb|[[Pluto]], the first known TNO, imaged by ''[[New Horizons]]'' in 2015.]]

The orbit of each of the planets is slightly affected by the [[gravitational]] influences of the other planets. Discrepancies in the early 1900s between the observed and expected orbits of Uranus and Neptune suggested that there were one or more additional [[planets beyond Neptune]]. The search for these led to the [[discovery of Pluto]] in February&nbsp;1930, which was progressively determined to be too small to explain the discrepancies. Revised estimates of Neptune's mass from the ''[[Voyager 2]]'' flyby in 1989 showed that there is no real discrepancy: The problem was an error in the expectations for the orbits.<ref>{{cite press release |first1=Chris |last1=Gebhardt |first2=Jeff |last2=Goldader |date=20 August 2011 |title=Thirty-four years after launch, Voyager&nbsp;2 continues to explore |website=NASA Spaceflight (nasaspaceflight.com) |url=http://www.nasaspaceflight.com/2011/08/thirty-four-years-voyager-2-continues-explore/ }}</ref> Pluto was easiest to find because it is the [[apparent magnitude|brightest]] of all known trans-Neptunian objects. It also has a lower inclination to the [[ecliptic]] than most other large TNOs, so its position in the sky is typically closer to the search zone in the disc of the Solar System.

=== Subsequent discoveries ===
After Pluto's discovery, American astronomer [[Tombaugh, C. W.|Clyde Tombaugh]] continued searching for some years for similar objects but found none. For a long time, no one searched for other TNOs as it was generally believed that Pluto, which up to August 2006 was classified as a planet, was the only major object beyond Neptune. Only after the 1992 discovery of a second TNO, [[15760 Albion]], did systematic searches for further such objects begin. A broad strip of the sky around the [[ecliptic]] was photographed and digitally evaluated for slowly moving objects. Hundreds of TNOs were found, with diameters in the range of 50 to 2,500 kilometers. [[Eris (dwarf planet)|Eris]], the most massive known TNO, was discovered in 2005, revisiting a long-running dispute within the scientific community over the classification of large TNOs, and whether objects like Pluto can be considered planets. In 2006, Pluto and Eris were classified as [[dwarf planet]]s by the [[International Astronomical Union]].

== Classification ==
According to their distance from the Sun and their [[orbital parameter]]s, TNOs are classified in two large groups: the [[Kuiper belt]] objects (KBOs) and the [[scattered disc]] objects (SDOs).{{refn|group="nb"|name="KB"}} The diagram below illustrates the distribution of known trans-Neptunian objects beyond the orbit of [[Neptune]] at 30.07&nbsp;AU. Different classes of TNOs are represented in different colours. The main part of the Kuiper belt is shown in orange and blue between the 2:3 and 1:2 [[orbital resonance]]s with Neptune. [[Plutinos]] (orange) are the objects in the 2:3 resonance, including the dwarf planets [[Pluto]] and [[Orcus (dwarf planet)|Orcus]]. [[Classical Kuiper belt object]]s are shown in blue, with the largest of these, including [[Haumea]], [[Makemake]], and [[Quaoar]] in the dynamically 'hot' population in light blue, and the dynamically 'cold' population, including [[486958 Arrokoth]], in low-eccentricity orbits clustered near 44&nbsp;AU in dark blue.

The scattered disc can be found beyond the Kuiper belt, shown in grey and purple. These objects, including dwarf planets [[Eris (dwarf planet)|Eris]] and [[Gonggong (dwarf planet)|Gonggong]] have been excited into eccentric orbits due to gravitational perturbations by Neptune, resulting in a concentration of their perihelia in the horizontal band between 30 and 40&nbsp;AU. Some [[detached objects]], such as {{mpl|612911|2004 XR|190}} however have higher perihelia. [[Centaur (small Solar System body)|Centaurs]], shown in green, have been perturbed from the scattered disc onto orbits crossing the outer planets. Bodies in both of these groups may be found in [[Resonant trans-Neptunian object|mean-motion resonances]] with Neptune; these are plotted in red.

Finally, [[extreme trans-Neptunian objects]] are shown at the right of the diagram, with many having orbits that extend over 1000&nbsp;AU from the sun. These can be divided into the extended scattered disc (pink), including {{mpl|768325|2015 BP|519}}, the distant detached objects (brown), including {{mpl|2017 OF|201}}, and the four known [[sednoid]]s, including [[Sedna (dwarf planet)|Sedna]] and [[541132 Leleākūhonua]].

[[File:TNOs-wide-view.svg|thumb|upright=3.2|center|Distribution of trans-Neptunian objects, with semi-major axis on the horizontal, and perihelion on the vertical axis. Scattered disc objects occupy the wide horizontal region in grey and purple, while objects that are in resonance with Neptune are in red. Extreme trans-Neptunian objects and sednoids are in pink, brown, and yellow. Finally, the classical Kuiper belt is in blue.]]

=== KBOs ===
The Edgeworth{{endash}}[[Kuiper belt]] contains objects with an average distance to the Sun of 30 to about 55&nbsp;AU, usually having close-to-circular orbits with a small inclination from the [[ecliptic]]. Edgeworth{{endash}}Kuiper belt objects are further classified into the [[resonant trans-Neptunian object]] that are locked in an orbital resonance with [[Neptune]], and the [[classical Kuiper belt object]]s, also called "cubewanos", that have no such resonance, moving on almost circular orbits, unperturbed by Neptune. There are a large number of resonant subgroups, the largest being the [[twotino]]s (1:2 resonance) and the [[plutino]]s (2:3 resonance), named after their most prominent member, [[Pluto]]. Members of the classical Edgeworth{{endash}}Kuiper belt include [[15760 Albion]], [[Quaoar]] and [[Makemake]].

Another subclass of Kuiper belt objects is the so-called scattering objects (SO). These are non-resonant objects that come near enough to Neptune to have their orbits changed from time to time (such as causing changes in semi-major axis of at least 1.5 AU in 10 million years) and are thus undergoing [[gravitational scattering]]. Scattering objects are easier to detect than other trans-Neptunian objects of the same size because they come nearer to Earth, some having perihelia around 20 AU. Several are known with [[Photometric system#Photometric letters|g-band]] [[absolute magnitude]] below 9, meaning that the estimated diameter is more than 100&nbsp;km. It is estimated that there are between 240,000 and 830,000 scattering objects bigger than [[Photometric system#Photometric letters|r-band]] absolute magnitude 12, corresponding to diameters greater than about 18&nbsp;km. Scattering objects are hypothesized to be the source of the so-called [[Jupiter-family comet]]s (JFCs), which have periods of less than 20 years.<ref>{{cite journal|display-authors=etal |last1=Cory Shankman |title=A Possible Divot in the Size Distribution of the Kuiper Belt's Scattering Objects |journal=Astrophysical Journal Letters |date=Feb 10, 2013 |volume=764 |issue=1 |pages=L2 |doi=10.1088/2041-8205/764/1/L2 |arxiv=1210.4827 |bibcode=2013ApJ...764L...2S |s2cid=118644497 |url=https://iopscience.iop.org/article/10.1088/2041-8205/764/1/L2/pdf}}</ref><ref name="Shankman_etal_2016a">{{cite journal |last1=Shankman |first1=C. |last2=Kavelaars |first2=J. J. |last3=Gladman |first3=B. J. |last4=Alexandersen |first4=M. |last5=Kaib |first5=N. |last6=Petit |first6=J.-M. |last7=Bannister |first7=M. T. |last8=Chen |first8=Y.-T. |last9=Gwyn |first9=S.|last10=Jakubik|first10=M. |last11=Volk |first11=K. |title=OSSOS. II. A Sharp Transition in the Absolute Magnitude Distribution of the Kuiper Belt's Scattering Population |journal=The Astronomical Journal |date=2016 |volume=150 |issue=2 |page=31 |doi= 10.3847/0004-6256/151/2/31 |arxiv= 1511.02896 |bibcode= 2016AJ....151...31S |s2cid=55213074 |doi-access=free }}</ref><ref>{{cite book |display-authors=etal|last1=Brett Gladman |title=The Solar System Beyond Neptune |date=2008 |page=43}}</ref>

=== SDOs ===
The [[scattered disc]] contains objects farther from the Sun, with very eccentric and inclined orbits. These orbits are non-resonant and non-planetary-orbit-crossing. A typical example is the most-massive-known TNO, [[Eris (dwarf planet)|Eris]]. Based on the [[Tisserand parameter]] relative to Neptune (T<sub>N</sub>), the objects in the scattered disc can be further divided into the "typical" scattered disc objects (SDOs, Scattered-near) with a T<sub>N</sub> of less than 3, and into the [[detached objects]] (ESDOs, Scattered-extended) with a T<sub>N</sub> greater than 3. In addition, detached objects have a time-averaged eccentricity greater than 0.2<ref name="DES_Elliot2006">{{cite journal|first1=J. L. |last1=Elliot |first2=S. D. |last2=Kern |first3=K. B. |last3=Clancy |first4=A. A. S. |last4=Gulbis |first5=R. L. |last5=Millis |first6=M. W. |last6=Buie |first7=L. H. |last7=Wasserman |first8=E. I. |last8=Chiang |first9=A. B. |last9=Jordan |first10=D. E. |last10=Trilling |first11=K. J. |last11=Meech|title=The Deep Ecliptic Survey: A Search for Kuiper Belt Objects and Centaurs. II. Dynamical Classification, the Kuiper Belt Plane, and the Core Population|journal=The Astronomical Journal|volume=129|date=2005|pages=1117–1162|doi=10.1086/427395|bibcode=2005AJ....129.1117E|issue=2|doi-access=free }}</ref> The [[sednoids]] are a further extreme sub-grouping of the detached objects with [[perihelia]] so distant that it is confirmed that their orbits cannot be explained by [[Perturbation (astronomy)|perturbations]] from the [[giant planet]]s,<ref name="brown2004">{{cite journal
 |first1=Michael E. |last1=Brown |author-link=Michael E. Brown |first2=Chadwick A. |last2=Trujillo |first3=David L. |last3=Rabinowitz
 |date=2004
 |title=Discovery of a Candidate Inner Oort Cloud Planetoid
 |url=http://www.gps.caltech.edu/classes/ge133/reading/sedna.pdf
 |archive-url=https://web.archive.org/web/20060627200056/http://www.gps.caltech.edu/classes/ge133/reading/sedna.pdf
 |archive-date=2006-06-27
 |journal=[[Astrophysical Journal]]
 |volume=617 |issue=1 |pages=645–649
 |arxiv = astro-ph/0404456
 |bibcode=2004ApJ...617..645B
 |doi=10.1086/422095
 |s2cid=7738201 |access-date=2008-04-02
}}</ref> nor by interaction with the [[galactic tide]]s.<ref name="nature letter">{{Cite journal |doi=10.1038/nature13156 |title=A Sedna-like body with a perihelion of 80 astronomical units |url=http://home.dtm.ciw.edu/users/sheppard/pub/TrujilloSheppard2014.pdf |archive-url=https://web.archive.org/web/20141216183818/http://home.dtm.ciw.edu/users/sheppard/pub/TrujilloSheppard2014.pdf |archive-date=2014-12-16 |url-status=live |journal=Nature |volume=507 |issue=7493 |pages=471–474 |year=2014 |last1=Trujillo |first1=Chadwick A. |author-link=Chadwick A. Trujillo |last2=Sheppard |first2=Scott S. |bibcode = 2014Natur.507..471T |pmid=24670765|s2cid=4393431 }}</ref> However, a [[Passing star hypothesis|passing star]] could have moved them on their orbit.<ref>{{Cite journal |last1=Pfalzner |first1=Susanne |last2=Govind |first2=Amith |last3=Portegies Zwart |first3=Simon |date=2024-09-04 |title=Trajectory of the stellar flyby that shaped the outer Solar System |journal=Nature Astronomy |volume=8 |issue=11 |language=en |pages=1380–1386 |doi=10.1038/s41550-024-02349-x |issn=2397-3366|doi-access=free |arxiv=2409.03342 |bibcode=2024NatAs...8.1380P }}</ref>

== Physical characteristics ==
[[File:Pluto crescent.jpg|thumb|Looking back at Pluto, the largest visited KBO so far]]
Given the apparent magnitude (>20) of all but the biggest trans-Neptunian objects, the physical studies are limited to the following:
* thermal emissions for the largest objects (see [[#Size determination|size determination]])
* [[color index|colour indices]], i.e. comparisons of the [[apparent magnitude]]s using different filters
* analysis of [[optical spectrum|spectra]], visual and [[infrared]]

Studying colours and spectra provides insight into the objects' origin and a potential correlation with other classes of objects, namely [[Centaur (small Solar System body)|centaurs]] and some satellites of giant planets ([[Triton (moon)|Triton]], [[Phoebe (moon)|Phoebe]]), suspected to originate in the [[Kuiper belt]]. However, the interpretations are typically ambiguous as the spectra can fit more than one model of the surface composition and depend on the unknown particle size. More significantly, the optical surfaces of small bodies are subject to modification by intense radiation, [[solar wind]] and [[micrometeorites]]. Consequently, the thin optical surface layer could be quite different from the [[regolith]] underneath, and not representative of the bulk composition of the body.

Small TNOs are thought to be low-density mixtures of rock and ice with some [[organic compound|organic]] ([[carbon]]-containing) surface material such as [[tholins]], detected in their spectra. On the other hand, the high density of {{dp|Haumea}}, 2.6–3.3&nbsp;g/cm<sup>3</sup>, suggests a very high non-ice content (compare with [[Pluto]]'s density: 1.86&nbsp;g/cm<sup>3</sup>). The composition of some small TNOs could be similar to that of [[comet]]s. Indeed, some [[Centaur (planetoid)|centaurs]] undergo seasonal changes when they approach the Sun, making the boundary blurred ''(see [[2060 Chiron]] and [[7968 Elst–Pizarro]])''. However, population comparisons between centaurs and TNOs are still controversial.<ref name="Peixinho 2003">{{cite journal |first1=N. |last1=Peixinho |first2=A. |last2=Doressoundiram |first3=A. |last3=Delsanti |first4=H. |last4=Boehnhardt |first5=M. A. |last5=Barucci |first6=I. |last6=Belskaya |title=Reopening the TNOs Color Controversy: Centaurs Bimodality and TNOs Unimodality |journal=Astronomy and Astrophysics |volume=410 |issue=3 |pages=L29–L32 |date=2003 |doi=10.1051/0004-6361:20031420 |arxiv = astro-ph/0309428 |bibcode = 2003A&A...410L..29P |s2cid=8515984 }}</ref>

=== Color indices {{anchor|Colors|Colours}} ===
{{See also|Asteroid color indices}}
[[File:Trans-Neptunians_Size_Albedo_Color.svg|thumb|upright=1.5|Comparison of sizes, albedos, and colors of various large trans-Neptunian objects with sizes of >700 km. The dark colored arcs represent uncertainties of the object's size.]]

[[Colour indices]] are simple measures of the differences in the [[apparent magnitude]] of an object seen through blue (B), visible (V), i.e. green-yellow, and red (R) filters.<ref name="Hainaut, Delsanti 2002">{{cite journal |author-link=Olivier R. Hainaut |first1=O. R. |last1=Hainaut |first2=A. C. |last2=Delsanti |date=2002 |title=Color of Minor Bodies in the Outer Solar System |journal=Astronomy & Astrophysics |volume=389 |issue=2 |pages=641–664 |doi=10.1051/0004-6361:20020431 |bibcode = 2002A&A...389..641H |doi-access=free }} [http://www.sc.eso.org/~ohainaut/MBOSS datasource]</ref>
Correlations between the colours and the orbital characteristics have been studied, to confirm theories of different origin of the different dynamic classes:
* [[Classical Kuiper belt object]]s (cubewanos) seem to be composed of two different colour populations: the so-called cold (inclination <5°) population, displaying only red colours, and the so-called hot (higher inclination) population displaying the whole range of colours from blue to very red.<ref name="Doressoundiram 2005">{{cite journal |author-link1=Alain Doressoundiram |first1=A. |last1=Doressoundiram |first2=N. |last2=Peixinho |author-link3=Catherine de Bergh |first3=C. |last3=de Bergh |author-link4=Sonia Fornasier |first4=S. |last4=Fornasier |author-link5=Philippe Thébault |first5=Ph. |last5=Thébault |author-link6=Maria A. Barucci |first6=M. A. |last6=Barucci |author-link7=Christian Veillet |first7=C. |last7=Veillet |title=The color distribution in the Edgeworth-Kuiper Belt |journal=The Astronomical Journal |volume=124 |issue=4 |pages=2279–2296 |doi=10.1086/342447 |arxiv = astro-ph/0206468 |bibcode = 2002AJ....124.2279D |year=2002 |s2cid=30565926 }}</ref> A recent analysis based on the data from [[Deep Ecliptic Survey]] confirms this difference in colour between low-inclination (named ''Core'') and high-inclination (named ''Halo'') objects. Red colours of the Core objects together with their unperturbed orbits suggest that these objects could be a relic of the original population of the belt.<ref name="Elliot2006">{{cite journal |last1=Gulbis |first1=Amanda A. S. |last2=Elliot |first2=J. L. |last3=Kane |first3=Julia F. |title=The color of the Kuiper belt Core |journal=[[Icarus (journal)|Icarus]] |volume=183 |date=2006 |issue=1 |pages=168–178 |doi=10.1016/j.icarus.2006.01.021 |bibcode = 2006Icar..183..168G }}</ref>
*[[Scattered disc]] objects show colour resemblances with hot classical objects pointing to a common origin.

While the relatively dimmer bodies, as well as the population as the whole, are reddish (V−I = 0.3–0.6), the bigger objects are often more neutral in colour (infrared index V−I < 0.2). This distinction leads to suggestion that the surface of the largest bodies is covered with ices, hiding the redder, darker areas underneath.<ref name="Rabinowitz 2005">{{cite journal |author-link=David L. Rabinowitz |first1=David L. |last1=Rabinowitz |first2=K. M. |last2=Barkume |author-link3=Michael E. Brown |first3=Michael E. |last3=Brown |first4=H. G. |last4=Roe |first5=M. |last5=Schwartz |first6=S. W. |last6=Tourtellotte |author-link7=C. A. Trujillo |first7=C. A. |last7=Trujillo |date=2006 |title=Photometric Observations Constraining the Size, Shape, and Albedo of 2003 El<sub>61</sub>, a Rapidly Rotating, Pluto-Sized Object in the Kuiper Belt |journal=Astrophysical Journal |volume=639 |issue=2 |pages=1238–1251 |doi=10.1086/499575 |arxiv = astro-ph/0509401 |bibcode = 2006ApJ...639.1238R |s2cid=11484750 }}</ref>

{| class="wikitable" style="text-align: center;"
|+ Mean-color indices of [[List of minor-planet groups|dynamical groups]] in the [[outer Solar System]]<ref name="Fornasier-2007"/>{{rp|35}}
! Color
! width= 120 | [[Plutino]]s
! width= 120 | [[Cubewano]]s
! width= 120 | [[Centaur (minor planet)|Centaurs]]
! width= 120 | [[Scattered disc object|SDOs]]
! width= 120 | [[Comet]]s
! width= 120 | [[Jupiter trojan]]s
|-
! B–V
| {{val|0.895|0.190}}
| {{val|0.973|0.174}}
| {{val|0.886|0.213}}
| {{val|0.875|0.159}}
| {{val|0.795|0.035}}
| {{val|0.777|0.091}}
|-
! V–R
| {{val|0.568|0.106}}
| {{val|0.622|0.126}}
| {{val|0.573|0.127}}
| {{val|0.553|0.132}}
| {{val|0.441|0.122}}
| {{val|0.445|0.048}}
|-
! V–I
| {{val|1.095|0.201}}
| {{val|1.181|0.237}}
| {{val|1.104|0.245}}
| {{val|1.070|0.220}}
| {{val|0.935|0.141}}
| {{val|0.861|0.090}}
|-
! R–I
| {{val|0.536|0.135}}
| {{val|0.586|0.148}}
| {{val|0.548|0.150}}
| {{val|0.517|0.102}}
| {{val|0.451|0.059}}
| {{val|0.416|0.057}}
|}

==={{Anchor|Spectra}} Spectral type from visible and near-Infrared Observations===
{{See also|Asteroid spectral types|Trans-Neptunian spectral types}}
Among TNOs, as among [[Centaur (minor planet)|centaurs]], there is a wide range of colors from blue-grey (neutral) to very red, but unlike the centaurs, bimodally grouped into grey and red centaurs, the distribution for TNOs appears to be uniform.<ref name="Peixinho 2003"/> The wide range of spectra differ in reflectivity in visible red and near infrared. Neutral objects present a flat spectrum, reflecting as much red and infrared as visible spectrum.<ref name="Barucci ACM2005">A. Barucci ''Trans Neptunian Objects’ surface properties'', [[IAU]] Symposium No. 229, Asteroids, Comets, Meteors, Aug 2005, Rio de Janeiro</ref> Very red objects present a steep slope, reflecting much more in red and infrared.
A recent attempt at classification (common with centaurs) uses the total of four classes from '''BB''' (blue, or neutral color, average B−V {{=}} 0.70, V−R {{=}} 0.39, e.g. [[90482 Orcus|Orcus]]) to '''RR''' (very red, B−V {{=}} 1.08, V−R {{=}} 0.71, e.g. [[90377 Sedna|Sedna]]) with '''BR''' and '''IR''' as intermediate classes. BR (intermediate blue-red) and IR (moderately red) differ mostly in the infrared [[Infrared astronomy#Astronomers' infrared spectrum|bands I, J and H]].

Typical models of the surface include water ice, [[amorphous carbon]], [[silicate]]s and organic macromolecules, named [[tholin]]s, created by intense radiation. Four major tholins are used to fit the reddening slope:
* Titan tholin, believed to be produced from a mixture of 90% N<sub>2</sub> (nitrogen) and 10% {{CH4}} (methane)
* Triton tholin, as above but with very low (0.1%) methane content
* (ethane) Ice tholin I, believed to be produced from a mixture of 86% {{H2O}} and 14% C<sub>2</sub>H<sub>6</sub> ([[ethane]])
* (methanol) Ice tholin II, 80% H<sub>2</sub>O, 16% CH<sub>3</sub>OH ([[methanol]]) and 3% {{CO2}}
As an illustration of the two extreme classes BB and RR, the following compositions have been suggested
* for Sedna (RR very red): 24% Triton tholin, 7% carbon, 10% N<sub>2</sub>, 26% methanol, and 33% methane
* for Orcus (BB, grey/blue): 85% amorphous carbon, +4% Titan tholin, and 11% H<sub>2</sub>O ice

=== Spectral types after the James Webb Space Telescope ===

Recent observations with the [[James Webb Space Telescope]] (JWST), and in particular its high sensitivity with the [[Near-Infrared Spectrograph|NIRSpec]] instrument in the 0.7–5.3 μm range, have led to a new spectral classification for [[Trans-Neptunian spectral types|trans-Neptunian objects]] (TNOs). This classification is based on the ''Discovering the Surface Composition of TNOs'' (DiSCo) Large program and, for the first time, incorporates not only spectral profiles but also how they relate to the surface composition of TNOs. Additionally to the discovery of species that had not been detected before, the most   
significant discoveries from the spectral study of TNOs with Webb is the prevalence of CO<sub>2</sub> on the surface for TNOs, independently of the size, albedo, and color and the non-prevalence of water ice, clearly present only in 20% of the sample.<ref name="Pinilla-Alonso-2025"/>
 
The DiSCo compositional classes <ref name=" Pinilla-Alonso-2025"/> show three distinct groups:  
* '''Bowl-type''': the only class with a clear absorption features of [[Ice|water ice]] all over the NIRSPEC spectral range, accompanied by [[silicate]]s and some [[carbon dioxide]] (CO<sub>2</sub>). Due to the high presence of refractory material on the surface these objects show the lowest geometric albedo in the population.<ref name="Pinilla-Alonso-2025"/> 
* '''Double-Dip''': reddish objects in the visible with spectra dominated by CO<sub>2</sub> (including the isotopologue <sup>13</sup>CO<sub>2</sub>) and [[carbon monoxide]] (CO) The non-icy surface component is probably dominated by tholins with aliphatic stretching absorptions at 3.2 - 3.5 μm.<ref name="Pinilla-Alonso-2025"/><ref name="DePra-2025"/><ref name="Henault-2025"/>  
* '''Cliff''': the reddest objects below 1.2 μm, with chemically evolved surfaces dominated by [[methanol]], CO<sub>2</sub>, CO, and irradiation byproducts of methanol bearing −OH, −CH and −NH groups.<ref name="Brunetto-2025"/> These spectra also display additional complex bands, likely associated with OCN<sup>−</sup> and OCS.

The distribution of these groups shows no clear relation with physical parameters or dynamical class, except for the color in the visible and the fact that all [[Classical Kuiper belt object#Orbits: 'hot' and 'cold' populations|cold classical TNOs]] belong to the Cliff class.<ref name="Pinilla-Alonso-2025"/> These three groups are also reproduced, with some differences, in [[centaur (minor planet)|centaurs]]<ref name="Licandro-2025"/>(including active ones such as [[Chiron (minor planet)|Chiron]]),<ref name="Pinilla-Alonso-2024"/> in [[Neptune Trojan]]s,<ref name="Markwardt-2025"/> and in [[Extreme trans-Neptunian object|ETNOs]],<ref name="Holler-2024"/> making them a useful reference for icy bodies throughout the [[Solar System]]. Similarities have also been noted with [[debris disk]] spectra.<ref name="Xie-2025"/>

As expected given their peculiar compositions, the [[dwarf planet]]s [[Eris (dwarf planet)|Eris]] and [[Makemake]]<ref name="Grundy-2024"/> do not fall into any of these groups. The same is true for large (~1000&nbsp;km) dwarf planet candidates such as [[Quaoar]], [[225088 Gonggong|Gonggong]], and [[90377 Sedna|Sedna]], which show distinctive spectral profiles with irradiation products of methane.<ref name="Emery-2024"/>

===Size determination and distribution ===
[[File:Selected Planemos.svg|thumb|Size comparison between the [[Moon]], Neptune's moon Triton, Pluto, several large TNOs, and the dwarf planet Ceres. Their respective shapes are not represented.]]

Characteristically, big (bright) objects are typically on inclined orbits, whereas the [[invariable plane]] regroups mostly small and dim objects.<ref name="Rabinowitz 2005"/>

It is difficult to estimate the [[diameter]] of TNOs. For very large objects, with very well known orbital elements (like Pluto), diameters can be precisely measured by [[occultation]] of stars. For other large TNOs, diameters can be estimated by [[infrared|thermal]] measurements. The intensity of light illuminating the object is known (from its distance to the Sun), and one assumes that most of its surface is in thermal equilibrium (usually not a bad assumption for an airless body). For a known [[albedo]], it is possible to estimate the surface temperature, and correspondingly the intensity of heat radiation. Further, if the size of the object is known, it is possible to predict both the amount of visible light and emitted heat radiation reaching Earth. A simplifying factor is that the Sun emits almost all of its energy in visible light and at nearby frequencies, while at the cold temperatures of TNOs, the heat radiation is emitted at completely different wavelengths (the far infrared).

Thus there are two unknowns (albedo and size), which can be determined by two independent measurements (of the amount of reflected light and emitted infrared heat radiation). TNOs are so far from the Sun that they are very cold, hence producing black-body radiation around 60 [[micrometre]]s in [[wavelength]]. This wavelength of light is impossible to observe from the Earth's surface, but can be observed from space using, e.g. the [[Spitzer Space Telescope]]. For ground-based observations, astronomers observe the tail of the black-body radiation in the far infrared. This far infrared radiation is so dim that the thermal method is only applicable to the largest KBOs. For the majority of (small) objects, the diameter is estimated by assuming an albedo. However, the albedos found range from 0.50 down to 0.05, resulting in a size range of 1,200–3,700&nbsp;km for an object of [[Absolute magnitude#Solar System bodies (H)|magnitude]] of 1.0.<ref>{{cite web|url=http://www.minorplanetcenter.org/iau/lists/Sizes.html |title=Conversion of Absolute Magnitude to Diameter |publisher=Minorplanetcenter.org |access-date=2013-10-07}}</ref>

== Notable objects ==
{{main list|List of trans-Neptunian objects|List of unnumbered trans-Neptunian objects}}

{| class="wikitable sortable" style="font-size: 0.9em; width: 80%"
! width=130 | Object !! Description
|-
| [[Pluto|134340 Pluto]] || A dwarf planet, the first and largest trans-Neptunian object (TNO) discovered. It is the only TNO known to have an atmosphere. Hosts a [[Moons of Pluto|system of five satellites]] and is the prototype [[plutino]].
|-
| [[15760 Albion]] || The prototype [[classical Kuiper belt object]] (KBO), and the first TNO discovered after Pluto.
|-
| [[(385185) 1993 RO]] || The next [[plutino]] discovered after Pluto.
|-
| {{mpl|(15874) 1996 TL|66}} || The first object identified as a [[scattered disc]] object.
|-
| {{mpl|1998 WW|31}} || The first binary KBO discovered after Pluto.
|-
| [[47171 Lempo]] || A plutino and triple system consisting of a central binary pair of similar size, and a third outer circumbinary satellite.
|-
| [[20000 Varuna]] || A large classical KBO, known for its rapid rotation (6.3 h) and elongated shape.
|-
| [[28978 Ixion]] || A large plutino, was considered to be among the largest KBOs upon discovery.
|-
| [[50000 Quaoar]] || A dwarf planet and a large classical KBO. It has an elongated shape, albeit less elongated than Haumea. It has one known moon, [[Weywot (satellite)|Weywot]], and two known rings that are both outside Quaoar's [[Roche limit]].
|-
| [[90377 Sedna]] || A distant dwarf planet, proposed for a new category named ''extended scattered disc'' (E-SDO),<ref name="Gladman">{{cite web|url=http://www.obs-nice.fr/gladman/cr105.html|title=Evidence for an Extended Scattered Disk?|work=obs-nice.fr}}</ref> ''detached objects'',<ref name="Jewitt2006">{{cite book |author-link=David Jewitt |first1=D. |last1=Jewitt |first2=A. |last2=Delsanti |chapter=The Solar System Beyond The Planets |title=Solar System Update : Topical and Timely Reviews in Solar System Sciences |edition=Springer-Praxis |isbn=978-3-540-26056-1 |date=2006 |publisher=Springer |chapter-url=http://www.ifa.hawaii.edu/faculty/jewitt/papers/2006/DJ06.pdf }}</ref> ''distant detached objects'' (DDO)<ref name="Gomez 2006">{{cite journal |first1=Rodney S. |last1=Gomes |first2=John J. |last2=Matese |first3=Jack J. |last3=Lissauer |title=A Distant Planetary-Mass Solar Companion May Have Produced Distant Detached Objects |journal=[[Icarus (journal)|Icarus]] |date=2006 |volume=184 |issue=2 |pages=589–601 |url=http://staff.on.br/rodneyg/companion/solar_companion.pdf |doi=10.1016/j.icarus.2006.05.026 |bibcode=2006Icar..184..589G |url-status=dead |archive-url=https://web.archive.org/web/20070108051810/http://staff.on.br/rodneyg/companion/solar_companion.pdf |archive-date=2007-01-08 }}</ref> or ''scattered-extended'' in the formal classification by [[Deep Ecliptic Survey|DES]].<ref name="DES_Elliot2006"/> 
|-
| [[90482 Orcus]] || A dwarf planet and the second-largest known plutino, after Pluto. Has a relatively large satellite, [[Vanth (moon)|Vanth]].
|-
| [[136108 Haumea]] || A dwarf planet, the third-largest-known TNO. Notable for its two known satellites, rings, and unusually short rotation period (3.9 h). It is the most massive known member of the [[Haumea family|Haumea collisional family]].<ref name="BrownBarkume2007">{{cite journal |last=Brown |first=Michael E. |author2=Barkume, Kristina M. |author3=Ragozzine, Darin |author4= Schaller, Emily L. |date=2007 |title=A collisional family of icy objects in the Kuiper belt |journal=Nature |volume=446 |issue=7133 |pages=294–296 |doi=10.1038/nature05619 |pmid=17361177 |bibcode = 2007Natur.446..294B |s2cid=4430027 |url=https://authors.library.caltech.edu/34346/2/nature05619-s1.pdf }}</ref><ref name=pairs>{{cite journal
 |last1=de la Fuente Marcos |first1=Carlos
 |last2=de la Fuente Marcos |first2=Raúl 
 |title=Dynamically correlated minor bodies in the outer Solar system
 |journal=[[Monthly Notices of the Royal Astronomical Society]]
 |date=11 February 2018
 |volume=474 |issue=1
 |arxiv=1710.07610
 |doi=10.1093/mnras/stx2765 
 |pages=838–846
 |doi-access=free
 |bibcode = 2018MNRAS.474..838D }}</ref>
|-
| [[136472 Makemake]] || A dwarf planet, a classical KBO, and the fourth-largest known TNO.<ref>{{cite web|url=http://www.minorplanetcenter.org/mpec/K05/K05O42.html |title=MPEC 2005-O42 : 2005 FY9 |publisher=Minorplanetcenter.org |access-date=2013-10-07}}</ref>
|-
| [[136199 Eris]] || A dwarf planet, a scattered disc object, and currently the most massive known TNO. It has one known satellite, [[Dysnomia (moon)|Dysnomia]].
|-
| {{mpl|612911|2004 XR|190}} || A [[detached object]] whose orbit is highly inclined and lies outside the classical Kuiper belt.
|-
| [[225088 Gonggong]] || A dwarf planet and the second-largest discovered scattered-disc object. Has one known satellite, [[Xiangliu (satellite)|Xiangliu]].
|-
| {{mpl|(528219) 2008 KV|42}} || The first retrograde TNO, having an unusually high orbital inclination of 104°.
|-
| [[471325 Taowu]] || Another retrograde TNO with an unusually high orbital inclination of 110°.<ref>{{cite web| url=https://www.newscientist.com/article/2100700-mystery-object-in-weird-orbit-beyond-neptune-cannot-be-explained | work=New Scientist | title=Mystery object in weird orbit beyond Neptune cannot be explained | date=2016-08-10 | access-date=2016-08-11}}</ref>
|-
| {{mpl|2012 VP|113}} || A sednoid with a large perihelion of 80 AU from the Sun (50 AU beyond Neptune).
|-
|[[486958 Arrokoth]] || A contact binary classical KBO encountered by the ''[[New Horizons]]'' spacecraft in 2019.
|-
|{{mpl|2018 VG|18}} || A scattered disc object, and the first TNO discovered while beyond {{convert|100|AU|e9km|abbr=unit}} from the Sun.
|-
|{{mpl|2018 AG|37}} || The most distant observable TNO at {{convert|132|AU|e9km|abbr=unit}} from the Sun.
|}

== Exploration ==
[[File:UltimaThule CA06 color 20190516.png|thumb|Kuiper belt object 486958 Arrokoth, in images taken by the ''New Horizons'' spacecraft]]

The only mission to date that primarily targeted a trans-Neptunian object was NASA's ''[[New Horizons]]'', which was launched in January 2006 and flew by the Pluto system in July 2015<ref>{{cite web| url = https://www.nasa.gov/mission_pages/newhorizons/overview/index.html| title = NASA New Horizons Mission Page| work = NASA| date = 25 March 2015| last1 = Talbert| first1 = Tricia}}</ref> and [[486958 Arrokoth]] in January 2019.<ref>{{Cite web|url=http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20190101|title=New Horizons: News Article?page=20190101|website=pluto.jhuapl.edu|access-date=2019-01-01}}</ref>

In 2011, a design study explored a spacecraft survey of Quaoar, Sedna, Makemake, Haumea, and Eris.<ref>{{Cite web|url=https://www.researchgate.net/publication/258495993|title=A Survey of Mission Opportunities to Trans-Neptunian Objects|website=ResearchGate|language=en|access-date=2019-09-23}}</ref>

In 2019 one mission to TNOs included designs for orbital capture and multi-target scenarios.<ref>Low-Cost Opportunity for Multiple Trans-Neptunian Object Rendezvous and Capture, AAS Paper 17-777.</ref><ref name=":1">{{Cite web|url=https://www.researchgate.net/publication/326569409|title=AAS 17-777 Low-cost Opportunity for Multiple Trans-Neptunian Object Rendezvous and Orbital Capture |website=ResearchGate|language=en|access-date=2019-09-23}}</ref>

Some TNOs that were studied in a design study paper were [[55637 Uni|Uni]], {{mpl|1998 WW|31}}, and [[47171 Lempo|Lempo]].<ref name=":1"/>

The existence of [[planets beyond Neptune]], ranging from less than an [[Earth mass]] ([[Sub-Earth]]) up to a [[brown dwarf]] has been often postulated<ref>{{cite journal|first=Fernández|last=Julio A.|title=On the Existence of a Distant Solar Companion and its Possible Effects on the Oort Cloud and the Observed Comet Population|journal=[[The Astrophysical Journal]]|date=January 2011|volume=726|issue=1|page=33|doi=10.1088/0004-637X/726/1/33|bibcode=2011ApJ...726...33F|s2cid=121392983 |doi-access=free}}</ref><ref>{{cite journal|first1=Lykawka|last1=Patryk S.|first2=Mukai|last2=Tadashi|title=An Outer Planet Beyond Pluto and the Origin of the Trans-Neptunian Belt Architecture|journal=[[The Astronomical Journal]]|date=April 2008|volume=135|issue=4|pages=1161–1200|doi=10.1088/0004-6256/135/4/1161|bibcode=2008AJ....135.1161L|arxiv=0712.2198|s2cid=118414447}}</ref> for different theoretical reasons to explain several observed or speculated features of the [[Kuiper belt]] and the [[Oort cloud]]. It was recently proposed to use ranging data from the ''New Horizons'' spacecraft to constrain the position of such a hypothesized body.<ref>{{cite journal|first=Iorio|last=Lorenzo|title=Perspectives on effectively constraining the location of a massive trans-Plutonian object with the New Horizons spacecraft: a sensitivity analysis|journal=[[Celestial Mechanics and Dynamical Astronomy]]|date=August 2013|volume=116|issue=4|pages=357–366|doi=10.1007/s10569-013-9491-x|bibcode=2013CeMDA.116..357I|arxiv=1301.3831|s2cid=119219926}}
</ref>

NASA has been working towards a dedicated Interstellar Precursor in the 21st century, one intentionally designed to reach the interstellar medium, and as part of this the flyby of objects like Sedna are also considered.<ref name=":0">{{cite web|url=https://www.space.com/42935-nasa-interstellar-probe-mission-idea.html|title=A Wild 'Interstellar Probe' Mission Idea Is Gaining Momentum|last=David |first=Leonard |website=Space.com|date=9 January 2019|language=en|access-date=2025-04-22}}</ref> Overall this type of spacecraft studies have proposed a launch in the 2020s, and would try to go a little faster than the Voyagers using existing technology.<ref name=":0"/> One 2018 design study for an Interstellar Precursor, included a visit of minor planet 50000 Quaoar, in the 2030s.<ref>{{cite web|url=https://www.hou.usra.edu/meetings/V2050/eposter/8173.pdf|title=The Interstellar Probe Mission (Graphic Poster)|last=Bradnt|first=P.C.|display-authors=etal|website=hou.usra.edu|access-date=October 13, 2019}}</ref>

== Extreme trans-Neptunian objects ==
[[File:Extreme transneptunian object eccentricity vs perihelion.png|thumb|300px|Overview of trans-Neptunian objects with extreme TNOs grouped into three categories at the top.]]
[[File:Sedna orbit.svg|thumb|upright=1.5|Sedna's orbit takes it far beyond even the Kuiper belt (30–50 AU), out to nearly 1,000 AU (Sun–Earth distance)]]
{{Main|Extreme trans-Neptunian object}}

Among the extreme trans-Neptunian objects are high-perihelion objects classified as [[sednoid]]s, four of which have been confirmed: [[90377 Sedna]], {{mpl|2012 VP|113}}, [[541132 Leleākūhonua]], and {{mpl|2023 KQ|14}}. They are distant [[detached object]]s with perihelia greater than 70 AU. Their high perihelia keep them at a sufficient distance to avoid significant gravitational [[Perturbation (astronomy)|perturbations]] from Neptune. Previous explanations for the high perihelion of Sedna include a close encounter with an [[Planets beyond Neptune|unknown planet]] on a distant orbit and a distant encounter with a random star or a member of the Sun's birth cluster that passed near the Solar System.<ref name=spaceBrown>{{cite web |last=Wall |first=Mike |date=24 August 2011 |title=A Conversation With Pluto's Killer: Q & A With Astronomer Mike Brown |url=http://www.space.com/12711-pluto-killer-mike-brown-dwarf-planets-interview.html |website=[[Space.com]] |access-date=7 February 2016}}</ref><ref name=iopsciencesedna>{{cite journal |last1=Brown |first1=Michael E. |last2=Trujillo |first2=Chadwick |last3=Rabinowitz |first3=David |date=2004 |title=Discovery of a Candidate Inner Oort Cloud Planetoid |journal=[[The Astrophysical Journal]] |volume=617 |issue=1 |pages=645–649 |arxiv=astro-ph/0404456 |bibcode=2004ApJ...617..645B |doi=10.1086/422095|s2cid=7738201 }}</ref><ref name=brownsedna>{{cite web |last=Brown |first=Michael E. |date=28 October 2010 |title=There's something out there – part 2 |url=http://www.mikebrownsplanets.com/2010/10/theres-something-out-there-part-2.html |website=Mike Brown's Planets |access-date=18 July 2016}}</ref>

==See also==
* [[Dwarf planet]]
* [[Mesoplanet]]
* [[Nemesis (hypothetical star)]]
* [[Planet Nine]]
* [[Sednoid]]
* [[Small Solar System body]]
* [[Trans-Neptunian planets in fiction]]
*[[Trans-Neptunian spectral types]]
* [[Triton (moon)|Triton]]
* [[Tyche (hypothetical planet)]]

==Notes==
<references group=nb/>

==References==
{{Reflist|30em|refs=

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<ref name="Grundy-2024">{{Cite journal  |last1=Grundy  |first1=William M. 
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}} <!-- end of reflist -->

== External links ==
{{Commons category|Trans-Neptunian objects}}
* Nine planets, [https://web.archive.org/web/20000303081140/http://seds.lpl.arizona.edu/nineplanets/nineplanets/kboc.html University of Arizona]
* David Jewitt's [http://www2.ess.ucla.edu/~jewitt/kb.html Kuiper Belt site]
** [http://www2.ess.ucla.edu/~jewitt/kb/big_kbo.html Large KBO page]
* A list of the estimates of the diameters from [http://www.johnstonsarchive.net/astro/tnodiam.html johnstonarchive] with references to the original papers

{{Trans-Neptunian objects}}
{{Dwarf planets}}
{{Small Solar System bodies}}
{{Solar System}}
{{Portal bar|Astronomy|Spaceflight|Outer space|Solar system}}

{{DEFAULTSORT:Trans-Neptunian Object}}
[[Category:Trans-Neptunian objects| ]]
[[Category:Distant minor planets]]
[[Category:Neptune]]
[[Category:Solar System]]