View source for Astronomy:Trans-Neptunian object

{{Short description|Solar system objects beyond Neptune}}
{{TNO imagemap}}
{{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 [[Astronomy:Minor planet|minor planet]] in the [[Astronomy:Solar System|Solar System]] that [[Physics:Orbit|orbit]]s the [[Astronomy:Sun|Sun]] at a greater average distance than [[Astronomy:Neptune|Neptune]], which has an orbital semi-major axis of 30.1 [[Astronomy:Astronomical unit|astronomical unit]]s (au).

Typically, TNOs are further divided into the [[Astronomy:Classical Kuiper belt object|classical]] and resonant objects of the [[Astronomy:Kuiper belt|Kuiper belt]], the [[Astronomy:Scattered disc|scattered disc]] and [[Astronomy:Detached object|detached object]]s with the [[Astronomy:Sednoid|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 October 2020, the [[Astronomy:List of minor planets#Main index|catalog of minor planets]] contains [[Astronomy:List of trans-Neptunian objects|678 numbered]] and more than [[Astronomy:List of unnumbered trans-Neptunian objects|2,000 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"/>

The first trans-Neptunian object to be [[Astronomy:Planets beyond Neptune|discovered was Pluto]] in 1930. It took until 1992 to discover a second trans-Neptunian object orbiting the Sun directly, [[Astronomy:15760 Albion|15760 Albion]]. The most massive TNO known is [[Astronomy:Eris (dwarf planet)|Eris]], followed by [[Astronomy:Pluto|Pluto]], {{dp|Haumea}}, {{dp|Makemake}}, and {{dp|Gonggong}}. More than [[Astronomy:Minor-planet moon#Trans-Neptunian objects|80 satellites]] have been discovered in orbit of trans-Neptunian objects. TNOs vary in color and are either grey-blue (BB) or very red (RR). They are thought to be composed of mixtures of rock, [[Chemistry:Amorphous carbon|amorphous carbon]] and volatile ices such as water and [[Chemistry:Methane|methane]], coated with [[Astronomy:Tholin|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 [[Astronomy:Extreme trans-Neptunian object|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=[[Organization:Monthly Notices of the Royal Astronomical Society|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}}</ref>

== History ==

=== Discovery of Pluto ===
[[File:Pluto in True Color - High-Res.jpg|thumb|Pluto imaged by ''New Horizons'']]

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 [[Astronomy:Planets beyond Neptune|planets beyond Neptune]]. The search for these led to the discovery of Pluto in February 1930, which was too small to explain the discrepancies. Revised estimates of Neptune's mass from the ''[[Astronomy:Voyager 2|Voyager 2]]'' flyby in 1989 showed that the problem was spurious.<ref>{{cite web|url=http://www.nasaspaceflight.com/2011/08/thirty-four-years-voyager-2-continues-explore/|title=Thirty-four years after launch, Voyager 2 continues to explore|date=August 20, 2011 |author1=Chris Gebhardt |author2=Jeff Goldader |work=NASASpaceflight}}</ref> Pluto was easiest to find because it has the highest [[Astronomy:Apparent magnitude|apparent magnitude]] of all known trans-Neptunian objects. It also has a lower inclination to the [[Astronomy:Ecliptic|ecliptic]] than most other large TNOs.

=== Subsequent discoveries ===
After Pluto's discovery, American astronomer 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 a planet, was the only major object beyond Neptune. Only after the 1992 discovery of a second TNO, [[Astronomy:15760 Albion|15760 Albion]], did systematic searches for further such objects begin. A broad strip of the sky around the [[Astronomy:Ecliptic|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. [[Astronomy:Eris (dwarf planet)|Eris]], the most massive 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. Pluto and Eris were eventually classified as [[Astronomy:Dwarf planet|dwarf planet]]s by the [[Organization:International Astronomical Union|International Astronomical Union]]. In December 2018, the discovery of {{mpl|2018 VG|18}}, nicknamed "Farout", was announced. Farout is the most distant Solar System object so-far observed and is about 120&nbsp;au away from the Sun. It takes 738 years to complete one orbit.<ref>{{cite web |title=DISCOVERY OF THE MOST DISTANT SOLAR SYSTEM OBJECT EVER OBSERVED |url=https://sites.google.com/carnegiescience.edu/sheppard/farout}}</ref>

== Classification ==
[[File:TheTransneptunians 73AU.svg|thumb|upright=1.5|Distribution of trans-Neptunian objects]]
[[File:Euler-Diagram bodies in the Solar System.jpg|thumb|[[Euler diagram|Euler diagram]] showing the types of bodies in the Solar System.]]
According to their distance from the Sun and their orbital parameters, TNOs are classified in two large groups: the [[Astronomy:Kuiper belt|Kuiper belt]] objects (KBOs) and the [[Astronomy:Scattered disc|scattered disc]] objects (SDOs).{{refn|group="nb"|name="KB"}} The diagram to the right illustrates the distribution of known trans-Neptunian objects (up to 70&nbsp;au) in relation to the orbits of the planets and the [[Astronomy:Centaur (small Solar System body)|centaurs]] for reference. Different classes are represented in different colours. Resonant objects (including [[Astronomy:Neptune trojan|Neptune trojan]]s) are plotted in red, [[Astronomy:Classical Kuiper belt object|classical Kuiper belt object]]s in blue. The scattered disc extends to the right, far beyond the diagram, with known objects at mean distances beyond 500&nbsp;au ([[Astronomy:90377 Sedna|Sedna]]) and aphelia beyond 1,000&nbsp; ({{mpl|(87269) 2000 OO|67}}).

=== KBOs ===
The Edgeworth-[[Astronomy:Kuiper belt|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 [[Astronomy:Ecliptic|ecliptic]]. Edgeworth-Kuiper belt objects are further classified into the resonant trans-Neptunian object, that are locked in an orbital resonance with [[Astronomy:Neptune|Neptune]], and the [[Astronomy:Classical Kuiper belt object|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 twotinos (1:2 resonance) and the [[Astronomy:Plutino|plutino]]s (2:3 resonance), named after their most prominent member, [[Astronomy:Pluto|Pluto]]. Members of the classical Edgeworth-Kuiper belt include [[Astronomy:15760 Albion|15760 Albion]], [[Astronomy:50000 Quaoar|50000 Quaoar]] and [[Astronomy:Makemake|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 [[Astronomy:Photometric system#Photometric letters|g-band]] [[Astronomy:Absolute magnitude|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 [[Astronomy: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 comets (JFCs), which have periods of less than 20 years.<ref>{{cite journal|display-authors=0 |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=0|last1=Brett Gladman |title=The Solar System Beyond Neptune |date=2008 |page=43}}</ref>

=== SDOs ===
The [[Astronomy:Scattered disc|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, [[Astronomy: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 [[Astronomy:Perturbation|perturbations]] from the [[Astronomy:Giant planet|giant planet]]s,<ref name="brown2004">{{cite journal
 |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 [[Astronomy:Galactic tide|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. |last2=Sheppard |first2=Scott S. |bibcode = 2014Natur.507..471T |pmid=24670765|s2cid=4393431 }}</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)
* [[Color index|colour indices]], i.e. comparisons of the [[Astronomy:Apparent magnitude|apparent magnitude]]s using different filters
* analysis of spectra, visual and [[Physics:Infrared|infrared]]

Studying colours and spectra provides insight into the objects' origin and a potential correlation with other classes of objects, namely [[Astronomy:Centaur (small Solar System body)|centaurs]] and some satellites of giant planets ([[Astronomy:Triton (moon)|Triton]], [[Astronomy:Phoebe (moon)|Phoebe]]), suspected to originate in the [[Astronomy:Kuiper belt|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, [[Astronomy:Solar wind|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 [[Chemistry:Organic compound|organic]] ([[Chemistry:Carbon|carbon]]-containing) surface material such as [[Astronomy:Tholin|tholin]], 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 [[Astronomy:Pluto|Pluto]]'s density: 1.86&nbsp;g/cm<sup>3</sup>). The composition of some small TNOs could be similar to that of [[Astronomy:Comet|comet]]s. Indeed, some centaurs undergo seasonal changes when they approach the Sun, making the boundary blurred ''(see [[Astronomy:2060 Chiron|2060 Chiron]] and [[Astronomy:7968 Elst–Pizarro|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}} ===
[[File:TheTransneptunians Color Distribution.svg|thumb|upright=1.5|Colors of trans-Neptunian objects. Yellow names in brackets are non trans-Neptunian objects added for reference. Mars and Triton are also not to scale.]]
[[File:TheTransneptunians Size Albedo Color.svg|thumb|upright=1.5|Illustration of the relative sizes, albedos and colours of some large TNOs]]

Colour indices are simple measures of the differences in the [[Astronomy:Apparent magnitude|apparent magnitude]] of an object seen through blue (B), visible (V), i.e. green-yellow, and red (R) filters. The diagram illustrates known colour indices for all but the biggest objects (in slightly enhanced colour).<ref name="Hainaut, Delsanti 2002">{{cite journal |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>
For reference, two moons, [[Astronomy:Triton (moon)|Triton]] and [[Astronomy:Phoebe (moon)|Phoebe]], the centaur [[Astronomy:5145 Pholus|Pholus]] and the planet [[Astronomy:Mars|Mars]] are plotted ''(yellow labels, size not to scale)''. Correlations between the colours and the orbital characteristics have been studied, to confirm theories of different origin of the different dynamic classes:
* [[Astronomy:Classical Kuiper belt object|Classical Kuiper belt object]] (cubewano) 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 |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 [[Astronomy:Deep Ecliptic Survey|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 |volume=183 |date=2006 |issue=1 |pages=168–178 |doi=10.1016/j.icarus.2006.01.021 |bibcode = 2006Icar..183..168G }}</ref>
*[[Astronomy:Scattered disc|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 |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 [[Astronomy:List of minor-planet groups|dynamical groups]] in the outer Solar System&thinsp;<ref name="Fornasier-2007"/>{{rp|35}}
! Color
! width= 120 | [[Astronomy:Plutino|Plutino]]s
! width= 120 | Cubewanos
! width= 120 | [[Astronomy:Centaur (minor planet)|Centaurs]]
! width= 120 | SDOs
! width= 120 | [[Astronomy:Comet|Comet]]s
! width= 120 | [[Astronomy:Jupiter trojan|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 ===

Among TNOs, as among [[Astronomy: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. [[Astronomy:90482 Orcus|Orcus]]) to '''RR''' (very red, B−V {{=}} 1.08, V−R {{=}} 0.71, e.g. [[Astronomy:90377 Sedna|Sedna]]) with '''BR''' and '''IR''' as intermediate classes. BR (intermediate blue-red) and IR (moderately red) differ mostly in the infrared [[Astronomy:Infrared astronomy#Astronomers' infrared spectrum|bands I, J and H]].

Typical models of the surface include water ice, [[Chemistry:Amorphous carbon|amorphous carbon]], [[Chemistry:Silicate|silicate]]s and organic macromolecules, named [[Astronomy:Tholin|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> ([[Chemistry:Ethane|ethane]])
* (methanol) Ice tholin II, 80% H<sub>2</sub>O, 16% CH<sub>3</sub>OH ([[Chemistry:Methanol|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

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

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

It is difficult to estimate the [[Diameter|diameter]] of TNOs. For very large objects, with very well known orbital elements (like Pluto), diameters can be precisely measured by [[Astronomy:Occultation|occultation]] of stars. For other large TNOs, diameters can be estimated by [[Physics: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 [[Earth:Albedo|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|micrometre]]s in [[Wavelength|wavelength]]. This wavelength of light is impossible to observe on the Earth's surface, but only from space using, e.g. the [[Astronomy:Spitzer Space Telescope|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 [[Astronomy: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|Astronomy:List of trans-Neptunian objects|Astronomy:List of unnumbered trans-Neptunian objects}}

{| class="wikitable sortable" style="font-size: 0.9em; width: 80%"
! width=130 | Object !! Description
|-
| [[Astronomy:Pluto|134340 Pluto]] || a dwarf planet and the first TNO discovered
|-
| [[Astronomy:15760 Albion|15760 Albion]] || the prototype [[Astronomy:Classical Kuiper belt object|cubewano]], the first Kuiper belt object discovered after Pluto
|-
| [[Astronomy:(385185) 1993 RO|(385185) 1993 RO]] || the next [[Astronomy:Plutino|plutino]] discovered after Pluto
|-
| {{mpl|(15874) 1996 TL|66}} || the first object to be identified as a [[Astronomy:Scattered disc|scattered disc]] object
|-
| {{mpl|1998 WW|31}} || the first binary Kuiper belt object discovered after Pluto
|-
| [[Astronomy:47171 Lempo|47171 Lempo]] || a plutino and triple system consisting of a central binary pair of similar size, and a third outer circumbinary satellite
|-
| [[Astronomy:20000 Varuna|20000 Varuna]] || a large cubewano, known for its rapid rotation (6.3 h) and elongated shape
|-
| [[Astronomy:28978 Ixion|28978 Ixion]] || large plutino, was considered to be among the largest Kuiper belt objects upon discovery
|-
| [[Astronomy:50000 Quaoar|50000 Quaoar]] || large cubewano with a satellite; sixth-largest-known Kuiper belt object and was considered to be among the largest Kuiper belt objects upon discovery
|-
|-
| [[Astronomy:90482 Orcus|90482 Orcus]] || The largest known plutino, after Pluto. Has a relatively large satellite.
|-
| 136108 Haumea || a dwarf planet, the third-largest-known trans-Neptunian object. 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 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=[[Organization:Monthly Notices of the Royal Astronomical Society|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
 |bibcode = 2018MNRAS.474..838D }}</ref>
|-
| 136472 Makemake || a dwarf planet, a cubewano, and the fourth-largest known trans-Neptunian object<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 trans-Neptunian object. It has one known satellite, [[Astronomy:Dysnomia (moon)|Dysnomia]]
|-
| {{mpl|612911|2004 XR|190}} || a scattered disc object following a highly inclined but nearly circular orbit
|-
| [[Astronomy:225088 Gonggong|225088 Gonggong]] || second-largest scattered-disc object with a satellite
|-
| {{mpl|(528219) 2008 KV|42}} "Drac" || the first retrograde TNO, having an orbital inclination of i = 104°
|-
| {{mpl|(471325) 2011 KT|19}} "Niku" || a TNO having 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)
|-
|[[Astronomy:486958 Arrokoth|486958 Arrokoth]] || contact binary cubewano encountered by the ''[[Astronomy:New Horizons|New Horizons]]'' spacecraft in 2019
|-
|{{mpl|2018 VG|18}} "Farout" || the first trans-Neptunian object discovered while beyond {{convert|100|au|e9km|abbr=unit}} from the Sun
|-
|{{mpl|2018 AG|37}}  "FarFarOut" || most distant observable trans-Neptunian object 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 ''[[Astronomy:New Horizons|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| date = 25 March 2015}}</ref> and [[Astronomy:486958 Arrokoth|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 {{mpl|2002 UX|25}}, {{mpl|1998 WW|31}}, and [[Astronomy:47171 Lempo|Lempo]].<ref name=":1"/>

The existence of [[Astronomy:Planets beyond Neptune|planets beyond Neptune]], ranging from less than an [[Astronomy:Earth mass|Earth mass]] ([[Astronomy:Sub-Earth|Sub-Earth]]) up to a [[Astronomy:Brown dwarf|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=[[Astronomy:The Astrophysical 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=[[Astronomy:The Astronomical 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 [[Astronomy:Kuiper belt|Kuiper belt]] and the [[Astronomy:Oort cloud|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=[[Astronomy:Celestial Mechanics and Dynamical Astronomy|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=Spaceflight|first=Leonard David 2019-01-09T11:57:34Z|website=Space.com|date=9 January 2019|language=en|access-date=2019-09-23}}</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=0|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)]]

Among the extreme trans-Neptunian objects are three high-perihelion objects classified as [[Astronomy:Sednoid|sednoid]]s: [[Astronomy:90377 Sedna|90377 Sedna]], {{mpl|2012 VP|113}}, and [[Astronomy:541132 Leleākūhonua|541132 Leleākūhonua]]. They are distant [[Astronomy:Detached object|detached object]]s with perihelia greater than 70 au. Their high perihelia keep them at a sufficient distance to avoid significant gravitational [[Astronomy:Perturbation|perturbations]] from Neptune. Previous explanations for the high perihelion of Sedna include a close encounter with an [[Astronomy: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=[[Astronomy:The Astrophysical 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>

==In fiction==
* Pluto in fiction
* Sedna is the location of key events in the book Lockstep by Karl Schroeder

==See also==
* [[Astronomy:Dwarf planet|Dwarf planet]]
* [[Astronomy:Mesoplanet|Mesoplanet]]
* [[Astronomy:Nemesis (hypothetical star)|Nemesis (hypothetical star)]]
* [[Astronomy:Planet Nine|Planet Nine]]
* [[Astronomy:Sednoid|Sednoid]]
* [[Astronomy:Small Solar System body|Small Solar System body]]
* [[Astronomy:Triton (moon)|Triton]]
* [[Astronomy:Tyche (hypothetical planet)|Tyche (hypothetical planet)]]

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

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

<ref name="MPC-List-TNOs">{{cite web
 |title      = List Of Transneptunian Objects
 |work       = Minor Planet Center
 |url        = https://minorplanetcenter.net/iau/lists/TNOs.html
 |access-date = 23 October 2018}}</ref>

<ref name="MPC-List-Cen-and-SDO">{{cite web
 |title      = List Of Centaurs and Scattered-Disk Objects
 |work       = Minor Planet Center
 |date       = 8 October 2018
 |url        = https://minorplanetcenter.net/iau/lists/Centaurs.html
 |access-date = 23 October 2018}}</ref>

<ref name="TNO-list-Johnstons">{{cite web
 |title      = List of Known Trans-Neptunian Objects
 |work       = Johnston's Archive
 |date       = 7 October 2018
 |url        = http://www.johnstonsarchive.net/astro/tnoslist.html
 |access-date = 23 October 2018}}</ref>

<ref name="JPL-TNO-query-I">{{cite web
 |title=JPL Small-Body Database Search Engine: orbital class (TNO)
 |publisher=JPL Solar System Dynamics
 |url=http://ssd.jpl.nasa.gov/sbdb_query.cgi?obj_group=all;obj_kind=all;obj_numbered=all;ast_orbit_class=TNO;OBJ_field=0;ORB_field=0;table_format=HTML;max_rows=100;format_option=comp;c_fields=AcBhBgBjBiBnBsCjCpAi;.cgifields=format_option;.cgifields=obj_kind;.cgifields=obj_group;.cgifields=obj_numbered;.cgifields=ast_orbit_class;.cgifields=table_format;.cgifields=com_orbit_class&query=1&c_sort=AiA
 |access-date=2014-07-10}}</ref>

<ref name="JPL-TNO-query-II">{{cite web
 |title=JPL Small-Body Database Search Engine: orbital class (TNO) and q > 30.1 (au)
 |url=http://ssd.jpl.nasa.gov/sbdb_query.cgi?obj_group=all;obj_kind=all;obj_numbered=all;ast_orbit_class=TNO;OBJ_field=0;ORB_field=0;c1_group=ORB;c1_item=Bi;c1_op=%3E;c1_value=30.1;table_format=HTML;max_rows=100;format_option=comp;c_fields=AcBhBgBjBiBnBsCjCpAi;.cgifields=format_option;.cgifields=obj_kind;.cgifields=obj_group;.cgifields=obj_numbered;.cgifields=ast_orbit_class;.cgifields=table_format;.cgifields=com_orbit_class&query=1&c_sort=AiA
 |access-date=2014-07-11}}</ref>

<ref name="Fornasier-2007">{{Cite journal
 |display-authors = 6
 |first1     = S. |last1 = Fornasier
 |first2     = E. |last2 = Dotto
 |first3     = O. |last3 = Hainaut
 |first4     = F. |last4 = Marzari
 |first5     = H. |last5 = Boehnhardt
 |first6     = F. |last6 = De Luise
 |first7     = M. A. |last7 = Barucci
 |date       = October 2007
 |title      = Visible spectroscopic and photometric survey of Jupiter Trojans: Final results on dynamical families
 |journal    = Icarus
 |volume     = 190
 |issue      = 2
 |pages      = 622–642
 |bibcode    = 2007Icar..190..622F 
 |doi        = 10.1016/j.icarus.2007.03.033
 |arxiv      = 0704.0350|s2cid = 12844258 }}</ref>

}} <!-- end of reflist -->

== External links ==
* 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}}
{{DEFAULTSORT:Trans-Neptunian Object}}
[[Category:Trans-Neptunian objects| ]]
[[Category:Distant minor planets]]
[[Category:Neptune]]

{{Sourceattribution|Trans-Neptunian object}}