Theia (planet)

Theia (/ˈθiːə/) is a hypothesized ancient planet in the early Solar System which, according to the giant-impact hypothesis, collided with the early Earth around 4.5 billion years ago, with some of the resulting ejected debris coalescing to form the Moon.^[1][2] 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 may be remnants of Theia.^[3][4] Theia is hypothesized to have been about the size of Mars, and may have formed in the outer Solar System and provided much of Earth's water, though this is debated.^[5]