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A comet is a small object in the solar system orbiting the Sun. The orbit of a comet is very excentric - generally traveling from very close to the sun to outside of Pluto. Furthermore the orbits are changing constantly because of the gravitation of the planets when passing their way. So sometimes comets are caught by the sun, others are thrown out of the solar system fading in space.
Comets are consisting of rock, dust, and ices. When coming close to the sun, Comets ejects a coma and/or a tail. This is the effect of the radiation of the sun upon the comet, so the tail is always heading away from the sun despite of the comets direction.
It is believed that most comets have their origin in the Oort cloud (see trans-Neptunian objects), but some comets with hyperbolic orbits may come from outside of the solar system.