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Illustration of the Kuiper belt of the outer solar system. The Kuiper belt consists of a collection of small icy bodies that mostly orbit beyond the planets, but in the same plane as them. The Kuiper belt extends outwards from the orbit of Neptune, past the orbit of Pluto to about two times the Sun-Pluto distance. Although a similar size to asteroids, Kuiper belt objects comprise more ices (frozen methane, ammonia and water) due to their greater distance from the Sun. The Kuiper belt is believed to be the source of short-period comets. (KEYSTONE/SCIENCE PHOTO LIBRARY/NICOLLE R. FULLER)