double planet

Pluto and Charon

A double planet is two objects of comparable size and of planetary mass orbiting one another. There is controversy over where to draw the line between a double planet and a system consisting of a planet and a moon. In most cases, satellites are of much small mass than their primary. However, there are two known examples of moon/planet mass ratios much greater than average: those of Earth and the Moon, and of Pluto and Charon.


A commonly accepted criterion of a double planet is when the center of gravity of the two objects is not located inside either body. By this rule, Pluto and Charon count as a double planet while the Earth-Moon system does not. The question of whether Pluto is a planet at all, or should instead be regarded as a large, wayward Kuiper Belt object is a separate matter.