PLANETS & MOONS A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z                    HOME ABOUT CATEGORIES SITE MAP COPYRIGHT ADVERTISE CONTACT entire Web this site double planet 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. Related category    • PLANETS AND MOONS Also on this site: Encyclopedia of Alternative Energy & Sustainable Living Encyclopedia of History Transport Concepts & Designs (partner site) BACK TO TOP