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Hill sphereThe spherical region around a planet (or satellite) within which that body dominates the motion of particles. Once the core of a planet exceeds a mass of about 1023 kg, it gravitationally attracts gas from the Hill sphere and is able to form an atmosphere in this way. Jupiter's Hill sphere, for example, had a volume about 1000 times greater than that of the Earth.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006