The first centaur-like object 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, discovered in 1997.
No centaur has yet 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 2006, three centaurs have been found to display cometary comas, Chiron, 60558 Echeclus, and 166P/NEAT 2001 T4. Chiron and Echeclus are therefore classified as both asteroids and comets. Any centaur that is perturbed close enough to the Sun is expected to become a comet.
Among the inner centaurs, 2005 VD, with a perihelion distance very near Jupiter, is listed as a centaur by both JPL and DES.
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). The inclination is represented on the Y axis.
Small inserts show histograms for orbit inclinations (i), eccentricity (e) and semi-major axis (a).
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:
A dozen known centaurs, including Dioretsa ("asteroid" spelled backwards), follow retrograde orbits.
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:
There are numerous theories to explain this colour difference, but they can be divided broadly into two categories:
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.
A. Delsanti et al suggest multiple competing processes: reddening by the radiation, and blushing by collisions.
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:
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.
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 candidates 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. Further developments are expected with more physical data on KBOs.
Well-known centaurs include: