Vast spherical cloud of small, icy bodies orbiting the Sun at distances ranging from about 0.3 light-year to one light-year or more that is probably the source of most long-period comets. In 1950 the Dutch astronomer Jan Hendrik Oort (1900–92) noted that no comets have orbits that would indicate an interstellar origin.
He proposed that the Sun is surrounded by billions of these objects, which are only occasionally detectable when they enter the inner solar system. The Oort cloud is believed to be composed of primordial bodies dating from the formation of the solar system (&see; solar nebula). Whether the Oort cloud merges, in its inner region, into the disk-shaped Kuiper belt is not known.
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