Oort Cloud

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Vast spherical swarm ~1012-1013 long-period comets orbiting the Solar System with semimajor axes between 1,000 and 50,000 AU, typically with low orbital eccentricity. The Oort cloud, while roughly spherical at the largest radius, is wedge-shaped where it merges with the outer planet region in the vicinity of the Kuiper belt of comets. The cloud is divided into different regions of dynamical stability: the Kuiper Belt (35-50 AU; affected by planetary perturbations), a dynamically inert region (50-2000 AU; not affected by gravity of planets or stars), the inner Oort cloud (2,000-15,000 AU; affected by galactic tidal forces), and the outer Oort cloud (15,000-100,000 AU; affected by stellar perturbations). The Oort cloud has never been observed rather its existence is inferred from analysis of the orbits of comets, which come in from the cloud. Recent discoveries of Kuiper Belt objects have confirmed its existence.

Some or all content above used with permission from J. H. Wittke.