What is the biggest a moon can be in relation to its mother planet?

The Earth’s moon is 27 per cent of its size, but how far this size ratio can be pushed depends on your very definition of a ‘moon’.

23rd October 2017
What is the biggest a moon can be in relation to its mother planet? © Getty

Asked by: Edward Seymour, Hove

A ‘moon’ is an astronomical body that orbits a planet; the definition doesn’t involve size. So, a ‘moon’ could be a small rock or it could be as large as its ‘parent’. However, similar-sized objects orbiting each other are normally called ‘double’ (for example, Pluto-Charon is often considered a ‘double dwarf planet’). But the distinction between ‘double’ and ‘parent-moon’ systems is not officially defined. Some astronomers define a ‘parent-moon’ system as one that has the point about which both objects orbit (the barycenter) inside the larger object, but this distinction is quite arbitrary because it depends on both size and separation.

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