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Oort cloud

An extremely distant spherical distribution of cometary nuclei very loosely gravitationally attracted to the Sun. The Oort cloud is estimated to contain 1,000 billion comet nuclei at a distance of between 1,000 and 100,000 AU. This is far beyond the orbit of Neptune (30 AU) with the end region nearly a quarter of the way to the nearest star system, Alpha Centauri.