centaur

Centaur is a term for a particular class of small solar system body in the outer solar system: various criteria are used, but generally, they are specified as those with orbits roughly between those of Jupiter and Neptune, with orbits crossing or grazing that of a planet, but without showing a resonance with the orbit of a planet. They show both minor-planet and comet characteristics, a dual nature that inspired the term centaur (a mythological being that is part horse and part human). Their orbits are considered unstable over the long term due to their proximity to the orbits of the giant planets, and their current orbits are assumed to be millions of years in age rather than billions. Being small and distant, they are a challenge to detect. The first example (Chiron) was discovered in 1977, the second (Pholus) in 1992, with currently on the order of a thousand known, the largest being 10199 Chariklo. Estimates of their total number vary from 44,000 to 10 million.