Asked by: Edward Seymour, Hove
The definition of a planet includes the proviso that it has ‘cleared its orbit’ of other large bodies. So, strictly speaking, two ‘planets’ in the same orbit would not be classed as planets. But it is possible for two planet-like bodies to share the same orbit around a central star without colliding: the second object would need to be positioned at a particular point in the first object’s gravitational field.
At this ‘Lagrange point’, the centrifugal and gravitational forces acting on the second object are such that it follows the same orbit as the first object. However, for large objects, such a precise orbital configuration is unlikely to remain stable for very long. Indeed, the Earth may have had a co-orbital companion early in its history, with which it soon collided, possibly forming the Moon.
There are, though, several examples of smaller bodies sharing a planet’s orbit, such as the Trojan asteroids, which follow Jupiter’s path around the Sun.
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