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The Oort cloud is a spherical cloud of comets believed to lie roughly 50,000 AU, or nearly a light-year, from the Sun; this distance places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun. The Kuiper belt and scattered disc, the other two known reservoirs of trans-Neptunian objects, are less than one thousandth the Oort cloud's distance. The outer extent of the Oort cloud defines the boundary of our Solar System.
The Oort cloud is thought to comprise two separate regions: a spherical outer Oort cloud and a disc-shaped inner Oort cloud, or Hills cloud. Objects in the Oort cloud are largely composed of ices such as water, ammonia and methane. Astronomers believe that the matter comprising the Oort cloud formed closer to the Sun, and was scattered far out into space by the gravitational effects of the giant planets early in the Solar System's evolution. While no confirmed direct observations have been made, the Oort cloud is likely the source of comets. The only known trans-Neptunian objects, 90377 Sedna and 2000 CR105, are considered possible members of the inner Oort cloud.
