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( Oort cloud)
The Oort cloud (pronounced /'?rt/ ort, alternatively the Öpik-Oort Cloud IPA&_160;['øpik]) is a hypothetical spherical cloud of comets which may lie roughly 50&_160;000 AU, or nearly a light-year, from the Sun.[1] The 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 gravitational boundary of our Solar System [2]. 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.[1] 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 and many of the Centaurs and Jupiter-family comets as well.[3] The outer Oort cloud is only loosely bound to the Solar System, and thus is easily affected by the gravitational pull both of passing stars and of the Milky Way galaxy itself. These forces occasionally dislodge comets from their orbits within the cloud and send them towards the inner Solar System.[1] Based on their orbits, most of the short-period comets may come from the scattered disc, but some may still have originated from the Oort Cloud.[1][3] Although the Kuiper belt and the farther scattered disc have been observed and mapped, only three currently known trans-Neptunian objects, 90377 Sedna, 2000 CR105, and 2006 SQ372 are considered possible members of the inner Oort cloud.[4] In 1932, Estonian astronomer Ernst Öpik postulated that long-period comets originated in an orbiting cloud at the outermost edge of the Solar System.[5] In 1950, the idea was independently revived by Dutch astronomer Jan Hendrik Oort as a means to resolve a paradox[6] over the course of the Solar System's existence, the orbits of comets are unstable; eventually, dynamics dictate that a comet must either collide with the Sun or a planet, or else be ejected from the Solar System by planetary perturbations. Moreover, their volatile composition means that as they repeatedly approach the Sun, radiation gradually boils off the volatiles until the comet splits or develops an insulating crust that prevents further outgassing. Thus, reasoned Oort, a comet could not have formed on its current orbit, and must have been held in an outer reservoir for almost all of its existence.[6][7][8]
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