Theia is a hypothetical planet that, according to the giant impact theory of the Moon's formation, collided with the Earth over four billion years ago. This impact would have destroyed Theia and thrown massive quantities of ejecta into orbit around Earth, from which the Moon coalesced in a matter of months or days. Earth also would have gained significant amounts of angular momentum from such a collision, as well as increasing its total mass to its current level.
According to this theory, Theia would have been approximately Mars-sized and struck Earth at a glancing angle. Little else can be conjectured about it. Theia, in Greek mythology, was a Greek Titan who, appropriately, gave birth to the Moon goddess Selene.
