A group of unusual bodies in the outer Solar System, possibly giant icy planetesimals or cometary nuclei. Group members have orbits which lie roughly between those of Jupiter and Neptune, in most cases crossing the orbits of one or more of the outer planets, and periods in the range 20–200 years. The first to be discovered were Chiron, in 1977, and Pholus, in 1992. Centaurs have diameters in the range 50–300 km, but there may be many smaller members still undiscovered. They are thought to be derived from the Kuiper Belt and are now usually classified with the so-called scattered-disk objects (SDOs) which have eccentric orbits extending well beyond Neptune. Centaurs and SDOs could be captured into Jupiter comet family or Halley-family cometary orbits. Centaurs have relatively short dynamical lifetimes, just a few million years on average, before they impact a planet or are ejected from the Solar System, so new objects must be evolving inwards all the time to replace those which are removed.