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. 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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