A centaur is a small Solar System body with either a perihelion or a semi-major axis between those of the outer planets. Centaurs generally have unstable orbits because they cross or have crossed the orbits of one or more of the giant planets; almost all their orbits have dynamic lifetimes of only a few million years, but there is one known centaur, 514107 Kaʻepaokaʻawela, which may be in a stable (though retrograde) orbit.
Centaurs typically behave with characteristics of both asteroids and comets. Observational bias toward large objects makes determination of the total centaur population difficult. Estimates for the number of centaurs in the Solar System more than 1 km in diameter range from as low as 44,000 to more than 10,000,000.
The generic definition of a centaur is a small body that orbits the Sun between Jupiter and Neptune and crosses the orbits of one or more of the giant planets. Due to the inherent long-term instability of orbits in this region, even centaurs such as 2000 GM137 and 2001 XZ255, which do not currently cross the orbit of any planet, are in gradually changing orbits that will be perturbed until they start to cross the orbit of one or more of the giant planets.
Some astronomers count only bodies with semimajor axes in the region of the outer planets to be centaurs; others accept any body with a perihelion in the region, as their orbits are similarly unstable.
The first centaur to be discovered, under the definition of the Jet Propulsion Laboratory and the one used here, was 944 Hidalgo in 1920. However, they were not recognized as a distinct population until the discovery of 2060 Chiron in 1977.
The largest confirmed centaur is 10199 Chariklo, which at 260 kilometers in diameter is as big as a mid-sized main-belt asteroid, and is known to have a system of rings. It was discovered in 1997. However, the lost centaur 1995 SN55 may be somewhat larger. The transneptunian object 2018 VG18, which is a centaur under the broader definition, may be quite a bit larger.
1 Ceres may have originated in the region of the outer planets, and if so might be considered an ex-centaur, but the centaurs seen today all originated elsewhere.
Of the objects known to occupy centaur-like orbits, approximately 30 have been found to display comet-like dust comas, with three, 2060 Chiron, 60558 Echeclus, and 29P/Schwassmann-Wachmann 1, having detectable levels of volatile production in orbits entirely beyond Jupiter. Chiron and Echeclus are therefore classified as both asteroids and comets, while Schwassmann-Wachmann 1 has always held a comet designation. Other centaurs, such as 52872 Okyrhoe, are suspected of having shown comas. Any centaur that is perturbed close enough to the Sun is expected to become a comet.
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 (For instance, the centaurs could be part of an “inner” scattered disc of objects perturbed inwards from the Kuiper belt), 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 Kuiper belt objects.