Centaur (minor planet)

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Positions of known outer Solar System objects. The centaurs are those objects (in orange) that lie generally inwards of the Kuiper belt (in green) and outside the Jupiter Trojans (pink).

Centaurs are small Solar System bodies with a semi-major axis between those of the outer planets. They have unstable orbits that cross or have crossed the orbits of one or more of the giant planets, and have dynamic lifetimes of a few million years.[1] Centaurs typically behave with characteristics of both asteroids and comets. They are named after the mythological beings that were a mixture of horse and human, centaurs. It has been estimated that there are around 44,000 centaurs in the Solar System with diameters larger than 1 km.[1]

The first centaur to be discovered was 944 Hidalgo in 1920. However, they were not recognized as a distinct population until the discovery of 2060 Chiron in 1977. The largest known centaur is 10199 Chariklo which, at 260 km in diameter, is as big as a mid-sized main-belt asteroid. It was discovered in 1997.

No centaur has been photographed up close, 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.

As of 2008, three centaurs have been found to display cometary comas: Chiron, 60558 Echeclus, and 166P/NEAT. Chiron and Echeclus are therefore classified as both asteroids and comets. Other centaurs such as 52872 Okyrhoe are suspected of showing cometary activity. Any centaur that is perturbed close enough to the Sun is expected to become a comet.




  • The Minor Planet Center (MPC) defines centaurs as having a perihelion beyond the orbit of Jupiter and a semi-major axis less than that of Neptune.[2]
  • The Jet Propulsion Laboratory (JPL) similarly defines centaurs as having a semi-major axis, a, between those of Jupiter and Neptune (5.5 AU < a < 30.1 AU).[3]

The collection The Solar System Beyond Neptune (2008) uses the traditional definition of centaurs, limited to semi-major axes smaller than that of Neptune, classifying the objects on unstable orbits beyond this limit as members of the scattered disc.[5] Yet, other astronomers still prefer to define centaurs as objects that are non-resonant with a perihelion inside the orbit of Neptune that can be shown to likely cross the Hill sphere of a gas giant within the next 10 million years.[6] Thus centaurs can be thought of as inward scattered objects that interact more aggressively and scatter more quickly than typical scattered disc objects.

The JPL Small-Body Database lists 183 centaurs.[7] There are an additional 40 trans-Neptunian objects with a semi-major axis further than Neptune (a > 30.1 AU) and perihelion closer than the orbit of Uranus (q < 19 AU).[7] The Committee on Small Body Nomenclature of the International Astronomical Union has not formally weighed in on either side of the debate. Instead, it has adopted the following naming convention for such objects: befitting their centaur-like transitional orbits between TNOs and comets, "objects on unstable, non-resonant, giant-planet-crossing orbits with semimajor axes greater than Neptune's" are to be named for other hybrid and shape-shifting mythical creatures. Thus far, only the binary objects Ceto and Phorcys and Typhon and Echidna have been named according to the new policy.[8]

Other objects caught between these differences in classification methods include (44594) 1999 OX3, which has a semi-major axis of 32 AU but crosses the orbits of both Uranus and Neptune. Among the inner centaurs, 2005 VD, with a perihelion distance very near Jupiter, is listed as a centaur by both JPL and DES.

Centaurs with measured diameters listed as possible dwarf planets according to Mike Brown's website include 10199 Chariklo, 2060 Chiron, and 54598 Bienor.[9]



Orbits of known centaurs[note 1]

The diagram at right illustrates the orbits of all known centaurs in relation to the orbits of the planets. For selected objects, the eccentricity of the orbits is represented by red segments (extending from perihelion to aphelion).

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).

To illustrate the range of the orbits' parameters, a few objects with very unusual orbits are plotted in yellow on the diagram:

  • Damocles is among a few centaurs on orbits with extreme inclination (prograde i>70°, e.g. 2007 DA61, 2004 YH32, retrograde i<120° e.g. 2005 JT50; not shown)
  • 2004 YH32 follows such a highly inclined orbit (nearly 80°) that, while it crosses from the distance of the asteroid belt from the Sun to past the distance of Saturn, if its orbit is projected onto the plane of Jupiter's orbit, it does not even go out as far as Jupiter.

A dozen known centaurs, including Dioretsa ("asteroid" spelled backwards), follow retrograde orbits.

Changing orbits[edit]



Physical characteristics[edit]

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.[11]


Colour distribution of centaurs

Centaurs display a puzzling diversity of colour that challenges any simple model of surface composition.[12] In the side-diagram, 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:




Delsanti et al. suggest multiple competing processes: reddening by the radiation, and blushing by collisions.[14][15]



Water ice signatures have been confirmed on a number of centaurs[11] (including 2060 Chiron, 10199 Chariklo and 5145 Pholus). In addition to the water ice signature, a number of other models have been put forward:


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.[18][19][20]

Similarities to comets[edit]



Theories of origin[edit]

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 disc objects would be dynamically the best candidates[23] for such expulsions, 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.[24] Further developments are expected with more physical data on KBOs.

Notable centaurs[edit]

Well-known centaurs include:

Name Year Discoverer Half-life[1]
55576 Amycus 2002 NEAT at Palomar 11.1 Myr UE
54598 Bienor 2000
10370 Hylonome 1995 Mauna Kea Observatory 6.3 Myr UN
10199 Chariklo 1997 Spacewatch 10.3 Myr U
8405 Asbolus 1995 Spacewatch (James V. Scotti) 0.86 Myr SN
7066 Nessus 1993 Spacewatch (David L. Rabinowitz) 4.9 Myr SE
5145 Pholus 1992 Spacewatch (David L. Rabinowitz) 1.28 Myr SN
2060 Chiron 1977 Charles T. Kowal 1.03 Myr SU

See also[edit]


  1. ^ For the purpose of this diagram, an object is classified as a centaur if its semi-major axis lies between Jupiter and Neptune. Last update: October 2008


  1. ^ a b c d e Horner, J.; Evans, N.W.; Bailey, M. E. (2004). "Simulations of the Population of Centaurs I: The Bulk Statistics". Monthly Notices of the Royal Astronomical Society 354 (3): 798–810. arXiv:astro-ph/0407400. Bibcode:2004MNRAS.354..798H. doi:10.1111/j.1365-2966.2004.08240.x. 
  4. ^ Elliot, J.L.; Kern, Buie, Trilling; et al. (2005). "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 (2): 1117–1162. Bibcode:2005AJ....129.1117E. doi:10.1086/427395. Retrieved 2008-09-22. 
  5. ^ B. Gladman, B. Marsden, C. VanLaerhoven (2008). "Nomenclature in the Outer Solar System". In The Solar System Beyond Neptune, ISBN 978-0-8165-2755-7. 
  6. ^ Chaing, Eugene; Buie, Grundy, Holman; et al. (2007). "A Brief History of Transneptunian Space". Protostars and Planets V, B. Reipurth, D. Jewitt, and K. Keil (eds.), University of Arizona Press, Tucson: 895–911. arXiv:astro-ph/0601654. Bibcode:2006astro.ph..1654C. 
  7. ^ a b "JPL Small-Body Database Search Engine". JPL Solar System Dynamics. Retrieved 2010-12-27. 
  8. ^ Grundy, Will; Stansberry, J.A.; Noll, K; Stephens, D.C.; Trilling, D.E.; Kern, S.D.; Spencer, J.R.; Cruikshank, D.P. et al. (2007). "The orbit, mass, size, albedo, and density of (65489) Ceto/Phorcys: A tidally-evolved binary Centaur". Icarus 191 (1): 286–297. arXiv:0704.1523. Bibcode:2007Icar..191..286G. doi:10.1016/j.icarus.2007.04.004. 
  9. ^ Michael E. Brown. "How many dwarf planets are there in the outer solar system? (updates daily)". California Institute of Technology. Retrieved 2012-01-16. 
  10. ^ "Three clones of Centaur 8405 Asbolus making passes within 450Gm". Retrieved 2009-05-02.  (Solex 10)
  11. ^ a b c Jewitt, David C.; A. Delsanti (2006). "The Solar System Beyond The Planets". Solar System Update : Topical and Timely Reviews in Solar System Sciences. Springer-Praxis Ed. ISBN 3-540-26056-0.  (Preprint version (pdf))
  12. ^ M. A. Barucci, A. Doressoundiram, and D. P. Cruikshank, "Physical Characteristics of TNOs and Centaurs" (2003), available on the web (accessed 3/20/2008)
  17. ^ "JPL Close-Approach Data: 38P/Stephan-Oterma". 1981-04-04 last obs. Retrieved 2009-05-07. 
  18. ^ 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)
  19. ^ Luu, Jane X.; Jewitt, David; Trujillo, C. A. (2000). "Water Ice on 2060 Chiron and its Implications for Centaurs and Kuiper Belt Objects". The Astrophysical Journal 531 (2): L151–L154. arXiv:astro-ph/0002094. Bibcode:2000ApJ...531L.151L. doi:10.1086/312536. PMID 10688775. 
  20. ^ Fernandez, Y. R.; Jewitt, D. C.; Sheppard, S. S. (2002). "Thermal Properties of Centaurs Asbolus and Chiron". The Astronomical Journal 123 (2): 1050–1055. arXiv:astro-ph/0111395. Bibcode:2002AJ....123.1050F. doi:10.1086/338436. 
  21. ^ Y-J. Choi, P.R. Weissman, and D. Polishook (60558) 2000 EC_98, IAU Circ., 8656 (Jan. 2006), 2.
  22. ^ Mazzotta Epifani, E.; Palumbo; Capria; Cremonese;; et al. (2006). "The dust coma of the active Centaur P/2004 A1 (LONEOS): a CO-driven environment?". Astronomy & Astrophysics 460 (3): 935–944. Bibcode:2006A&A...460..935M. doi:10.1051/0004-6361:20065189. Retrieved 2009-05-08. 
  23. ^ for instance, the centaurs could be part of an "inner" scattered disc of objects perturbed inwards from the Kuiper belt [1].
  24. ^ Wan, X.-S; Huang, T.-Y. (2001). "The orbit evolution of 32 plutinos over 100 million year". Astronomy and Astrophysics 368 (2): 700–705. Bibcode:2001A&A...368..700W. doi:10.1051/0004-6361:20010056. 

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