Oort cloud

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The Oort cloud is a hypothesized spherical cloud of comets that may lie nearly a light-year from the Sun. It is thought to comprise two separate regions: a spherical outer Oort cloud and a disc-shaped inner Oort cloud, or Hills cloud; the outer extent of the cloud defines the boundary of the Solar System. Objects in the Oort cloud are largely composed of ices, such as water, ammonia, and methane and are thought to have formed close to the Sun, later being scattered into space by the gravitational effects of the giant planets early in the Solar System's evolution. Although no confirmed direct observations of the Oort cloud have been made, astronomers believe that it is the source of all long-period and Halley-type comets entering the inner Solar System, as well as many of the Centaurs and Jupiter-family comets.

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http://en.wikipedia.org/wiki/Oort_cloud