Theia (hypothetical planet)

Theia ( THEE-uh) is a hypothesized ancient planet in the early Solar System which, according to the giant-impact hypothesis, collided with the proto-Earth around 4.50 billion years ago, with some of the resulting ejected debris re-coalescing to form the Moon. Collision simulations support the idea that the two large low-shear-velocity provinces in the Earth's lower mantle may be remnants of Theia.