Artwork of the young Earth-Moon system. The Earth had recently formed when it was struck by a protoplanet ??called Theia ??roughly three times the size of Mars. Debris from the impact went into orbit where it quickly coalesced to form the Moon, probably within just a few thousand years. Initially the Earth and Moon were much closer together than they are now, and spinning more quickly. In this depiction, debris from the collision continues to rain down on both worlds.