Theia is a hypothesized ancient planet in the early Solar System which, according to the giant-impact hypothesis, collided with the Proto-Earth (also known as "Gaia") around 4.5 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. Theia is hypothesized to have been about the size of Mars and likely formed at the L4 or L5 Lagrange points of the Earth's orbit, although some hypotheses debatably suggested it may have formed in the Outer Solar System and later migrated into the Earth's orbit, and might have provided much of Earth's water.