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Centaur is a term used for a minor planet or comet orbiting among the outer solar system planets, beyond Jupiter but not as far as the Kuiper Belt, most of which show some comet characteristics, a dual nature that inspired the term "centaur" (a mythological being that is part horse and part man). Over 44,000 centaurs are known. Centaurs' orbits are considered unstable over the long term due to their proximity to the orbits of the giant planets. For this reason, their current orbits are assumed to be millions of years in age rather than billions.