A binary planet system is a theorised arrangement where the mass of two closely orbiting objects, are similar.

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The criteria which distinguish between a ‘binary planet’ (or ‘double planet’) and a ‘planet-moon system’ (or ‘planet-satellite system’) are, unfortunately, not well defined. An obvious criterion is the ratio of the two objects’ masses. A value close to 1 implies the objects are very similar in mass, but how close to 1 does the ratio have to be for the system to be a double planet rather than a planet and moon?

The Earth-Moon system has a ratio of only 0.01230, while the Pluto-Charon system has a ratio of 0.122. Neither is particularly close to 1, and so neither is regarded as a binary planet.

Other criteria have been suggested, such as the position of the ‘barycentre’ about which the two objects orbit, the strength of the gravitational force of the parent star on the pair, or how the objects were formed in the first place.

The Pluto-Charon system could be described as a double planet because both objects orbit around a point above their surfaces. This is not true of the Earth-Moon system, as the barycentre is about 1,700km below Earth’s surface. But this, and other definitions, are not wholly adequate – and none are fully adopted by astronomers.

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Asked by: Derryck Morton, Poughill, Devon

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