Theia is a hypothesized ancient planet in the early Solar System. She collided with the young Earth around 4.5 billion years ago, with some of the resulting ejected debris gathering to form the Moon luna Such a collision, with the two planets' cores and mantles fusing, could explain why Earth's core is larger than expected for a body its size. Collision simulations support the idea that the large low-shear-velocity provinces in the lower mantle