Giant-Impact Hypothesis

Theia itself no longer exists as a separate body: the giant-impact hypothesis holds that it merged into early Earth, and the debris it threw into orbit went on to form the Moon.

The giant-impact hypothesis is the leading scientific explanation for the origin of Earth's Moon. It proposes that a Mars-sized protoplanet, often called Theia, collided with the young Earth roughly 4.5 billion years ago. Theia itself did not survive the collision as a separate body: the impact ejected a large volume of debris into orbit around Earth, and that debris subsequently coalesced into the Moon. The hypothesis has been the dominant model among planetary scientists since the 1980s in part because it accounts for otherwise puzzling features of the Earth-Moon system, including the Moon's relatively small iron core and its close similarity to Earth's own mantle in isotopic composition, though the exact details of the impact geometry remain an active subject of refinement.