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 greater 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 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.
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