Comets are small Solar System bodies, usually only a few kilometres across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate and ionise, creating a coma: a long tail of gas and dust often visible to the naked eye.
Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets, such as Halley's Comet, are believed to originate in the Kuiper belt, while long-period comets, such as Hale-Bopp, are believed to originate in the Oort cloud. Many comet groups, such as the Kreutz Sungrazers, formed from the breakup of a single parent.[57] Some comets with hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult.[58] Old comets that have had most of their volatiles driven out by solar warming are often categorised as asteroids.[59]
Centaurs
The centaurs, which extend from 9 to 30 AU, are icy comet-like bodies that orbit in the region between Jupiter and Neptune. The largest known centaur, 10199 Chariklo, has a diameter of between 200 and 250 km.[60] The first centaur discovered, 2060 Chiron, has been called a comet since it develops a coma just as comets do when they approach the Sun.[61] Some astronomers classify centaurs as inward scattered Kuiper belt objects along with the outward scattered residents of the scattered disc.[
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