‡ Trans-Neptunian dwarf planets are
The Centaurs are a class of icy planetoids named after the mythical race of centaurs. Centaurs orbit the Sun between Jupiter and Neptune, crossing the orbits of the large gas giant planets. The first centaur to be discovered was 2060 Chiron in 1977, while the largest is 10199 Chariklo discovered in 1997.
No centaur has yet been photographed up close by a spacecraft, although there is evidence that Saturn's moon Phoebe, imaged by the Cassini probe in 2004, may be a captured centaur. In addition, the Hubble Space Telescope has gleaned some information about the surface features of 8405 Asbolus.
Three centaurs, Chiron, 60558 Echeclus, and 166P/NEAT 2001 T4, have been found to display cometary comas. Chiron and 60558 Echeclus are now classified as both asteroids and comets. It is possible that other centaurs may also be comets, but as of March 2006 no cometary behavior has been discovered for any others.
The diagram illustrates the orbits of all known centaurs1 in relation to the orbits of the planets. Known Neptune Trojans are shown in green. For selected objects, the eccentricity of the orbits is represented by red segments (extending from perihelion to aphelion). The inclination is represented on the Y axis.
To illustrate the range of the orbits' paramters, three objects with extremely unusual orbits are plotted in yellow on the diagram:
- 2005 VB123 has the most highly inclined orbit (~39o); curiously, its orbit is also quasi-circular (the lowest eccentricity < 0.01)
- Asbolus has the most eccentric orbit (eccentricity=0.62)
- 2001 XZ25 has the lowest inclination (<3o).
Small inserts show histograms for orbit inclinations (i) (5o interval), eccentricity (e) (interval 0.05) and semi-major axis (a) (interval 2 AU). Centaurs' orbits are characterised by a wide range of eccentricity, from highly eccentric (Pholus, Asbolus, Amicus, Nessus) to more circular (Chariklo and the Saturn-crossers: Thereus, Okyrhoe).
1For the purpose of this diagram, an object is classified as a centaur if its semi-major axis is between those of Jupiter and Neptune
Because the centaurs cross the orbits of the giant planets and are not protected by orbital resonances, their orbits are unstable within a timescale of 106 –107 years. Dynamical studies of their orbits indicate that centaurs are probably an intermediate orbital state of objects transitioning from the Kuiper Belt to the Jupiter Family of short period comets. Objects may be perturbed from the Kuiper Belt, whereupon they become Neptune-crossing and interact gravitationally with that planet (see theories of origin). They then become classed as centaurs, but their orbits are chaotic, evolving relatively rapidly as the centaur makes repeated close approaches to one or more of the outer planets. Some centaurs will evolve into Jupiter-crossing orbits whereupon their perihelia may become reduced into the inner solar system and they may be reclassified as active comets in the Jupiter Family if they display cometary activity. Centaurs will thus ultimately collide with the Sun or a planet or else they may be ejected into interstellar space after a close approach to one of the planets, particularly Jupiter.
The relatively small size of centaurs precludes surface observations, but colour indices and spectra can indicate possible surface composition and can provide insight into the origin of the bodies.
Centaurs display a puzzling diversity of colour that challenges any simple model of surface composition. In the diagram on the right, the colour indices are measures of apparent magnitude of an object through blue (B), visible (V) i.e. green-yellow and red (R) filters. The diagram illustrates these differences (in enhanced colour) for all centaurs with known colour indices. For reference, two moons: Triton and Phoebe, and planet Mars are plotted (yellow labels, size not to scale).
Centaurs appear to be grouped into two classes:
- very red, for example 5145 Pholus
- blue (or blue-grey, according to some authors), for example 2060 Chiron
There are numerous theories to explain this colour difference, but they can be divided broadly into two categories:
- The colour difference results from a difference in the origin and/or composition of the centaur (see origin below)
- The colour difference reflects a different level of space weathering from radiation and/or cometary activity.
As examples of the second category, the reddish colour of Pholus has been explained as a possible mantle of irradiated red organics, whereas Chiron has instead had its ice exposed due to its periodic cometary activity, giving it a blue/grey index. The correlation with activity and color is not certain, however, as the active Centaurs span the range of colors from blue (Chiron) to red (166P/NEAT 2001 T4). Alternatively, Pholus may have been only recently expelled from the Kuiper Belt, so that surface transformation processes have not yet taken place.
The interpretation of spectra is often ambiguous, related to particle sizes and other factors, but the spectra offer an insight into surface composition. As with the colours, the observed spectra can fit a number of models of the surface.
Water ice signatures have been confirmed on a number of centaurs (including 2060 Chiron, 10199 Chariklo and 5145 Pholus). In addition to the water ice signature, a number of other models have been put forward:
- Chariklo's surface has been suggested to be a mixture of tholins (like those detected on Titan and Triton) with amorphous carbon)
- Pholus has been suggested to be covered by a mixture of Titan-like tholins, carbon black, olivine1 and methanol ice.
- The surface of (52872) Okyrhoe has been suggested to be a mixture of kerogens, olivines and small percentage of water ice.
- 8405 Asbolus is been suggested to be a mixture of 15% Triton-like tholins, 8% Titan-like tholin, 37% amorphous carbon and 40% ice tholin.
Chiron, the only centaur with known cometary activity, appears to be the most complex. The spectra observed vary depending on the period of the observation. Water ice signature was detected during a period of low activity and disappeared during high activity.   
1A class of Magnesium Iron Silicates (Mg, Fe)2SiO4, common components of igneous rocks.
Similarities to comets
Observations of Chiron in 1988 and 1989 near its perihelion found it to display a coma (a cloud gas and dust evaporating from its surface). It is thus now officially classified as both a comet and an asteroid, although it is far larger than a typical comet and there is some lingering controversy. Other centaurs are being monitored for comet-like activity: so far two, 60558 Echeclus, and 166P/NEAT 2001 T4 have shown such behavior. 166P/NEAT 2001 T4 was discovered while it exhibited a coma, and so is classified as a comet, though its orbit is that of a Centaur. 60558 Echeclus was discovered without a coma but recently became active , and so it is now accordingly also classified as both a comet and an asteroid.
Theories of origin
The study of centaur development is rich in recent developments but still hampered by limited physical data. Different models have been put forward for possible origin of centaurs.
Simulations indicate that the orbit of some Kuiper Belt objects can be perturbed, resulting in the object's expulsion so that it becomes a centaur. Scattered disk objects would be dynamically the best candidates1 for such explusions, but their colours do not fit the bicoloured nature of the centaurs. Plutinos are a class of Kuiper Belt Object that display a similar bicoloured nature, and there are suggestions that not all plutinos' orbits are as stable as initially thought, due to perturbation by Pluto. Further developments are expected with more physical data on KBOs.
Well-known centaurs include:
|8405 Asbolus||1995||Spacewatch (James V. Scotti)|
|7066 Nessus||1993||Spacewatch (David L. Rabinowitz)|
|5145 Pholus||1992||Spacewatch (David L. Rabinowitz)|
|2060 Chiron||1977||Charles T. Kowal|
- List of Centaurs and Scattered-Disk Objects
- Centaurs from The Encyclopedia of Astrobiology Astronomy and Spaceflight
- D.Jewitt,A.Delsanti The Solar System Beyond The Planets,to appear in the book Solar System Update, Springer-Praxis Ed., Horwood, Blondel and Mason, 2006. Preprint version (pdf)
- Bauer, J. M., Fernández, Y. R., & Meech, K. J. 2003. "An Optical Survey of the Active Centaur C/NEAT (2001 T4)", Publication of the Astronomical Society of the Pacific", 115, 981 
- N. Peixinho1, A. Doressoundiram1, 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(pdf)
- Hainaut & Delsanti (2002) Color of Minor Bodies in the Outer Solar System Astronomy & Astrophysics, 389, 641 datasource
- Dotto, E; Barucci, M A; De Bergh, C, Colours and composition of the centaurs, Earth, Moon, and Planets, 92, no. 1-4, pp. 157-167. (June 2003)
- Jane X. Luu, David Jewitt and C. A. Trujillo Water Ice on 2060 Chiron and its Implications for Centaurs and Kuiper Belt Objects, The Astrophysical Journal, 531 (2000),L151-L154. Preprint on arXiv.
- Y. R. Fernandez, D. C. Jewitt, S. S. Sheppard TThermal Properties of Centaurs Asbolus and Chiron,The Astronomical Journal, 123 (Feb. 2002),1050–1055. Preprint on arXiv.
- Y-J. Choi, P.R. Weissman, and D. Polishook (60558) 2000 EC_98, IAU Circ., 8656 (Jan. 2006), 2.
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