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] 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]
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