The giant-impact hypothesis is the leading scientific explanation for the origin of Earth's Moon, proposing that a Mars-sized protoplanet, often called Theia, collided with the young Earth roughly 4.5 billion years ago, ejecting a large volume of debris into orbit that subsequently coalesced into the Moon. The hypothesis accounts for several otherwise puzzling features of the Earth-Moon system, including the Moon's relatively small iron core and its close similarity to Earth's mantle in isotopic composition. It has been the dominant model among planetary scientists since the 1980s, though the exact details of the impact geometry remain an active subject of refinement.
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