"Double planet" is an informal term used to describe a planet with a moon that may be large enough to be considered a planet in its own right; one definition re

"Double planet" is an informal term used to describe a planet with a moon that may be large enough to be considered a planet in its own right; one definition requires that the objects orbit a centre of gravity that is above their surfaces. The formal term is "binary system". http://en.wikipedia.org/wiki/Double_planet http://en.wikipedia.org/wiki/Double_planet