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A trans-Neptunian object (TNO; also written transneptunian object) is any object in the Solar System that orbits the Sun at a greater distance on average than Neptune. The Kuiper belt, scattered disk, and Oort cloud are three divisions of this volume of space.[1]

The first trans-Neptunian object to be discovered was Pluto in 1930. It took more than 60 years to discover, in 1992, a second trans-Neptunian object, (15760) 1992 QB1, with only the discovery of Pluto's moon Charon before that in 1978. Since then however, over 1,000 trans-Neptunian objects have been discovered, differing in sizes, orbits, and surface composition. 198 of these (as of November, 2009) have their orbit well enough determined that they are given a permanent minor planet designation.[2][3]

The largest known trans-Neptunian object is Eris (discovered in 2005), followed by Pluto, Makemake and Haumea.


Discovery of Pluto

The orbit of each of the planets is affected by the gravitational influences of all 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 1930. However, Pluto was too small to explain the discrepancies, and revised estimates of Neptune's mass showed that the problem was spurious.

Pluto was easiest to find because it has the highest apparent magnitude of all known trans-Neptunian objects. It also has a lower inclination to the ecliptic than most other large TNO's.

Discovery of other trans-Neptunian objects

After Pluto's discovery, no one searched for further TNOs for a long time. Indeed, it was generally believed that Pluto was the only major object of the Kuiper belt. Only after the discovery of a second TNO, (15760) 1992 QB1, in 1992, systematic searches for further such objects began. 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 2500 kilometers.


Eris is the largest known trans-Neptunian object. Currently lying at 97 AU away, Eris is one of the farthest known objects in the solar system, and the third brightest of the TNOs. Classified as a scattered disk object (SDO), Eris follows an orbit at 10 billion kilometres from the Sun, completing it in 560 years at an unusual 45-degree angle.

Distribution and classification

According to their distance from the Sun and their orbit parameters, TNOs are classified in two large groups:
Distribution of trans-Neptunian Objects.

* The Kuiper belt contains objects with an average distance to the sun of 30 to about 55 AU, usually having close-to-circular orbits with a small inclination from the ecliptic. Kuiper belt objects are further classified into the following two groups:
o Resonant objects are locked in an orbital resonance with Neptune. Objects with a 1:2 resonance are also called twotinos, and objects with a 2:3 resonance are called plutinos, after their most prominent member, Pluto.
o Classical Kuiper belt objects (also called cubewanos) have no such resonance, moving on almost circular orbits, unperturbed by Neptune. Examples are 1992 QB1, 50000 Quaoar and Makemake.

* The scattered disk contains objects further from the Sun, usually with very irregular orbits (i.e. very elliptical and having a strong inclination from the ecliptic). A typical example is the largest known TNO, Eris.

The diagram to the right illustrates the distribution of known trans-Neptunian objects (up to 70 AU) in relation to the orbits of the planets and the Centaurs for reference. Different classes are represented in different colours. Resonant objects (including Neptune Trojans) are plotted in red, cubewanos in blue. The scattered disk extends to the right, far beyond the diagram, with known objects at mean distances beyond 500 AU (Sedna) and aphelia beyond 1000 AU ((87269) 2000 OO67).

Notable trans-Neptunian objects
Comparison of Eris, Pluto, Makemake, Haumea, Sedna, Orcus, 2007 OR10, Quaoar, and Earth (all to scale)

* 134340 Pluto, a dwarf planet.

* Charon, the largest moon of Pluto.

* (15760) 1992 QB1, the prototype cubewano, the first Kuiper belt object discovered after Pluto and Charon.
* 1998 WW31, the first binary Kuiper belt object discovered after Pluto and Charon.
* (15874) 1996 TL66, the first object to categorized as a scattered disk object.
* (48639) 1995 TL8 has a very large satellite and is the earliest discovered scattered disc object.
* 1993 RO, the next plutino discovered after Pluto.
* 20000 Varuna and 50000 Quaoar, large cubewanos.
* 90482 Orcus and 28978 Ixion, large plutinos.
* 90377 Sedna, a distant object, classified in a new category named Extended scattered disc (E-SDO),[4] detached objects,[5] Distant Detached Objects (DDO)[6] or Scattered-Extended in the formal classification by DES[7]
* 136108 Haumea[8], a dwarf planet, the fourth largest known trans-Neptunian object. Notable for its two known satellites and unusually short rotation period (3.9 h).[9]
* 136199 Eris, dwarf planet, a scattered disk object, currently the largest known trans-Neptunian object. One known satellite, Dysnomia.
* 136472 Makemake [10], dwarf planet, a cubewano, the third largest known trans-Neptunian object
* 2004 XR190, a scattered disk object following unusual, highly inclined but circular orbit.
* (87269) 2000 OO67 and (148209) 2000 CR105, remarkable for their eccentric orbits and large aphelia.
* IAU Minor Planet Center provisional designation 2008 KV42 and temporary nickname "Drac". The first retrograde TNO. Orbital inclination i = 104 deg, semimajor axis is a sime 42 AU, the perihelion distance is q~21 AU. Discovered 31st May, 2008

A fuller list of objects is being compiled in the List of trans-Neptunian objects.

Physical characteristics

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 indices i.e. comparisons of the apparent magnitudes using different filters
* analysis of spectra, visual and infrared

Studying colors and spectra provides insight into the objects' origin and a potential correlation with other classes of objects, namely centaurs and some satellites of giant planets (Triton, 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 (carbon-containing) surface material such as tholin, detected in their spectra. On the other hand, the high density of Haumea, 2.6-3.3 g/cm3, suggests a very high non-ice content (compare with Pluto's density: 2.0 g/cm3).

The composition of some small TNO could be similar to that of comets. Indeed, some Centaurs undergo seasonal changes when they approach the Sun, making the boundary blurred (see 2060 Chiron and 133P/Elst-Pizarro). However, population comparisons between Centaurs and TNO are still object of controversy.[11]

Colours of the Transneptunians.

Like Centaurs, TNO display a wide range of colors from blue-grey to very red but unlike the centaurs, clearly re-grouped into two classes, the distribution appears to be uniform.[11]

Color indices are simple measures of the differences of the apparent magnitude of an object seen through blue (B), visible (V) i.e. green-yellow and red (R) filters. The diagram illustrates known color indices for all but the biggest objects (in slightly enhanced color).[12] For reference, two moons: Triton and Phoebe, the centaur Pholus and planet Mars are plotted (yellow labels, size not to scale).

Correlations between the colors and the orbital characteristics have been studied, to confirm theories of different origin of the different dynamic classes.

Classical objects

Classical objects seem to be composed of two different color populations: so called cold (inclination <5°) displaying only red colors and hot (higher inclination) population displaying the whole range of colors from blue to very red.[13]

A recent analysis based on the data from Deep Ecliptic Survey confirms this difference of colours between low inclination objects (named Core) and high inclination (named Halo). Red colors of the Core objects together with their unperturbed orbits suggest that these objects could be a relic of the original population of the Belt.[14]

Scattered disk objects

Scattered disk objects show color resemblances with hot classical objects pointing to a common origin.

The largest objects
Illustration of the relative sizes, albedos and colours of the largest TNOs.

Characteristically, big (bright) objects are typically on inclined orbits, while the invariable plane re-groups mostly small and dim objects. With the exception of Sedna, all big TNOs: Eris, Makemake, Haumea, Charon, and Orcus display neutral colour (infrared index V-I < 0.2), while the relatively dimmer bodies (50000 Quaoar, Ixion, 2002 AW197, and Varuna), as well as the population as the whole, are reddish (V-I in 0.3 to 0.6 range). This distinction leads to suggestion that the surface of the largest bodies is covered with ices, hiding the redder, darker areas underneath.[9]

The diagram illustrates the relative sizes, albedos and colours of the biggest TNOs. Also shown, are the known satellites and the exceptional shape of Haumea (2003 EL61) resulting from its rapid rotation. The arc around Makemake (2005 FY9) represents uncertainty given its unknown albedo. The size of Eris follows Michael Brown’s measure (2400 km) based on HST point spread model.[15] The arc around it represents the thermal measure (3000 km) by Bertoldi (see the related section of the article for the references).


The objects present wide range of spectra, differing in reflectivity in visible red and near infrared. Neutral objects present a flat spectrum, reflecting as much red and infrared as visible spectrum.[16] 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, average B-V=0.70, V-R=0.39 e.g. Orcus) to RR (very red, B-V=1.08, V-R=0.71, e.g. Sedna) with BR and IR as intermediate classes. BR and IR differ mostly in the infrared bands I, J and H.

Typical models of the surface include water ice, amorphous carbon, silicates and organic macromolecules, named tholins, 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% N2 and 10% CH4 (gaseous 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% C2H6 (ethane)
* (methanol) Ice tholin II, 80% H2O, 16% CH3OH (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%N2, 26% methanol, 33% methane
* for Orcus (BB, grey/blue): 85% amorphous carbon +4% titan tholin, 11% H20 ice

Size determination

It is difficult to estimate the diameter of TNOs. For very large objects, with very well known orbital elements (namely, Pluto and Charon), diameters can be precisely measured by occultation of stars.

For other large TNOs, diameters can be estimated by 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 the 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).

Unfortunately, TNOs are so far from the Sun that they are very cold, hence produce black-body radiation around 60 micrometres in wavelength. This wavelength of light is impossible to observe on the Earth's surface, but only 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, as example for magnitude of 1.0, in uncertainty from 1200 – 3700 km!.[17]

Largest discoveries

Main article: Plutoid

A trans-Neptunian object is called a plutoid if it is large enough to be in hydrostatic equilibrium (i.e. if it is a dwarf planet). There are currently four TNOs designated as plutoids by the IAU. These are listed here together with Pluto's largest moon Charon, which would probably be a plutoid on its own if it did not orbit Pluto:

Name Provisional
Absolute magnitude Albedo Equatorial diameter
Semimajor axis
Class Discovery date Discoverer(s) Diameter method
Eris 2003 UB313 −1.2 ~0.86 ± 0.07 2400 ± 100 67.7 SDO 2005 M. Brown, C. Trujillo & D. Rabinowitz thermal
Pluto −1.0 0.49 to 0.66 2306 ± 20 39.4 KBO 1930 C. Tombaugh occultation
Makemake 2005 FY9 -0.48 2005 M. Brown et al.
Haumea 2003 EL61 0.17 2005 disputed
Charon S/1978 P 1 1 0.36 to 0.39 1205 ± 2 39.4 KBO satellite 1978 J. Christy occultation

Plutoid candidates
Size comparison between Earth's Moon, Neptune's moon Triton, and several large TNOs

Prime plutoid candidates[18]
Name Category Estimated diameter (km) Absolute
( × 1020 kg)
by [15] by [19] by [20] by [21]
Orcus plutino
(1 moon)
1,100 909 946 1,500 2.3 6.32 ± 0.05 39.2
Huya plutino 480 480 4.7 0.6–1.8? 39.4
Pluto 39.4
Ixion plutino 980 570 650 1,065 3.2 ~3? 39.6
Varuna cubewano 780 874 500 900 3.7 ~3.7? 42.9
Haumea 43.3
Quaoar cubewano
(1 moon)
1,290 1,260 844 1,200 2.7 21–29 43.5
Makemake 45.3
(55565) 2002 AW197 cubewano 940 793 735 890 3.2 ~4.1? 47.0
(84522) 2002 TC302 5:2 SDO 710 1,200 1,150 3.8 15? 55.4
(225088) 2007 OR10 10:3? SDO 1200? 1.9  ? 67.3
Eris 68.0
(15874) 1996 TL66 SDO 632 460–690 5.4 2? 83.9
Sedna Detached object 1,800 1,500 < 1,600 < 1,500 1.5 8–70? 509

Estimated diameter is greatly affected by surface albedo which has often been assumed, not measured. Some potentially large Kuiper belt objects have not been included.


External links

* Minor Planet Center List of Transneptunian Objects
* Nine planets, University of Arizona
* David Jewitt's Kuiper Belt site
o Large KBO page
* A list of the estimates of the diameters from johnstonarchive with references to the original papers

See also

* List of trans-Neptunian objects
* Triton
* Nemesis
* Dwarf planet
* Mesoplanet
* Small solar system body


1. ^ Remo, John L. (2007). "Classifying Solid Planetary Bodies". AIP Conference Proceedings 886: 284–302. doi:10.1063/1.2710063. http://adsabs.harvard.edu/abs/2007AIPC..886..284R.
2. ^ List of Transneptunian objects
3. ^ List of Centaurs and Scattered Disc objects
4. ^ Evidence for an Extended Scattered Disk?
5. ^ D.Jewitt, A.Delsanti The Solar System Beyond The Planets in Solar System Update : Topical and Timely Reviews in Solar System Sciences , Springer-Praxis Ed., ISBN 3-540-26056-0 (2006) Preprint of the article (pdf)
6. ^ Rodney S. Gomes, John J. Matese, and Jack J. Lissauer A Distant Planetary-Mass Solar Companion May Have Produced Distant Detached Objects To appear in Icarus (2006). Preprint
7. ^ J. L. Elliot, S. D. Kern, K. B. Clancy, A. A. S. Gulbis, R. L. Millis, M. W. Buie, L. H. Wasserman, E. I. Chiang, A. B. Jordan, D. E. Trilling, and K. J. Meech The Deep Ecliptic Survey: A Search for Kuiper Belt Objects and Centaurs. II. Dynamical Classification, the Kuiper Belt Plane, and the Core Population. The Astronomical Journal, 129 (2006), pp. preprint
8. ^ "Distant object found orbiting Sun". BBC News. 2005-07-29. http://news.bbc.co.uk/1/hi/sci/tech/4726733.stm. Retrieved 2010-03-28.
9. ^ a b David L. Rabinowitz, K. M. Barkume, Michael E. Brown, H. G. Roe, M. Schwartz, S. W. Tourtellotte, C. A. Trujillo (2005), Photometric Observations Constraining the Size, Shape, and Albedo of 2003 El61, a Rapidly Rotating, Pluto-Sized Object in the Kuiper Belt, Astrophysical Journal, submitted Preprint on arXiv
10. ^ http://www.cfa.harvard.edu/mpec/K05/K05O42.html
11. ^ a b N. Peixinho, A. Doressoundiram, A. Delsanti, H. Boehnhardt, M. A. Barucci, and I. Belskaya Reopening the TNOs Color Controversy: Centaurs Bimodality and TNOs Unimodality Astronomy and Astrophysics, 410, L29-L32 (2003). Preprint on arXiv
12. ^ O. R. Hainaut & A. C. Delsanti (2002) Color of Minor Bodies in the Outer Solar System Astronomy & Astrophysics, 389, 641 datasource
13. ^ A. Doressoundiram, N. Peixinho, C. de Bergh, S. Fornasier, Ph. Thébault, M. A. Barucci and C. Veillet The color distribution in the Edgeworth-Kuiper Belt The Astronomical Journal, 124, pp. 2279-2296. Preprint on arXiv
14. ^ Gulbis, Amanda A. S.; Elliot, J. L.; Kane, Julia F. The color of the Kuiper belt Core Icarus, 183 (July 2006), Issue 1, p. 168-178.
15. ^ a b Michael E. Brown. "The Dwarf Planets". California Institute of Technology, Department of Geological Sciences. http://web.gps.caltech.edu/~mbrown/dwarfplanets/. Retrieved 2008-01-26.
16. ^ A. Barucci Trans Neptunian Objects’ surface properties, IAU Symposium #229, Asteroids, Comets, Meteors, Aug 2005, Rio de Janeiro
17. ^ http://www.cfa.harvard.edu/iau/lists/Sizes.html cfa.harvard.edu
18. ^ All bodies with estimated diameters of 900 km or more, with 3 additional prime suspects (Huya, 2007 OR10, 1996 TL66) cited in Tancredi & Favre. "Which are the dwarfs in the Solar system?". Asteroids, Comets, Meteors. http://www.lpi.usra.edu/meetings/acm2008/pdf/8261.pdf. Retrieved 2008-09-20.
19. ^ Johnston, Robert (2007-11-24). "List of Known Trans-Neptunian Objects". Johnston's Archive.net. http://www.johnstonsarchive.net/astro/tnoslist.html. Retrieved 2008-01-26.
20. ^ Barucci, M.A.; Stansberry, John; Grundy, Will; Brown, Mike; Cruikshank, Dale; Spencer, John; Trilling, David; Margot, Jean-Luc (2007). "Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope". The Solar System beyond Neptune (University of Arizona Press). http://arxiv.org/pdf/astro-ph/0702538. Retrieved 2008-01-26.
21. ^ David C. Jewitt. "Kuiper Belt: The 1000 km Scale KBOs". University of Hawaii, Institute for Astronomy. http://www2.ess.ucla.edu/~jewitt/kb.html. Retrieved 2008-02-10.
22. ^ Grundy et al. Diverse Albedos of Small Trans-Neptunian Objects Icarus Notes. Preprint on arXiv
23. ^ Dale P. Cruikshank et al. Albedos, Diameters (and a Density) of Kuiper Belt and Centaur Objects from a session of the 37th meeting of the Division for Planetary Sciences of the American Astronomical Society and the Royal Astronomical Society (September 2005, Cambridge, UK) Abstract
24. ^ The original press release announcing the measuring of the albedo of 2003 UB313 by Bertoldi et al.
25. ^ MPC Circular 2006-A28 for 2003 MW12 data

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