Editing Trans-Neptunian object (section)

== 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, albedo, 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. The diagram illustrates known colour indices for all but the biggest objects (in slightly enhanced colour).<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>
For reference, two moons, [[Triton (moon)|Triton]] and [[Phoebe (moon)|Phoebe]], the centaur [[5145 Pholus|Pholus]] and the planet [[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:
* [[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 |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]]&thinsp;<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 ===
{{See also|Asteroid 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

===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>