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, while the cores of the two planets merged. Within weeks of the event, the debris formed a Saturn-like ring around the Earth. Later collisions in this ring led to the formation of 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.