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.