Theia (planet) - Wikipedia
https://en.wikipedia.org/wiki/Theia_(planet)
Theia 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. 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 simulation…
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