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Double planet

A double planet is a set of two planets of comparable mass orbiting one another. There is some debate on where to draw the line between a double planet and a system consisting of a planet and moon. In most cases, the moon is of very small mass relative to its host planet. However, there are some examples of moon/planet mass ratios much closer than average: particularly, the Earth and its Moon, and Pluto with its moon Charon. A commonly accepted cutoff point is when the common point that the two objects orbit around (the barycenter) is not located inside either body, in which case Pluto and Charon count as a double planet and Earth does not (the issue of whether Pluto is a planet at all or is instead simply a large Kuiper belt object is a separate matter). There is no "official" definition, however; the term "double planet" is an informal one. Hypothetically, the moon/planet mass ratios between two bodies could vary anywhere between zero through one, where two planets of exactly the same mass orbit each other.