One fateful day about 4.5 billion years ago , a Mars-sized body called Theia collided with proto-Earth , turning both into a molten mess of rock and metal. Once the debris had coalesced, two distinct objects remained locked in an orbit – Earth and its Moon. But where did Theia come from? Right here in the warmer, cozier confines of the inner Solar System, it appears. In fact, Theia and proto-Earth may have been neighbors! More specifically, Theia may have originated even closer to the Sun than