What is the leading theory for the Moon's formation?

The leading theory for the Moon’s formation is the giant-impact hypothesis. It proposes that a Mars-sized protoplanet, often called Theia, struck the young Earth roughly 4.5 billion years ago, ejecting material that eventually gathered into the Moon.

The theory helps explain several important lunar features. The Moon has a relatively small iron core compared with Earth, consistent with its formation largely from rocky mantle material. Earth and the Moon also have very similar isotopic compositions, suggesting a close shared origin rather than the Moon being a captured unrelated body.

Early versions of the hypothesis imagined Theia hitting Earth and debris forming a disk around it. Modern models explore more energetic impacts and complex mixing between the two bodies, because the remarkable chemical similarity between Earth and Moon is difficult to explain with a simple collision.

Accretion, fission, and capture were historically proposed alternatives. They can explain parts of the evidence, but none currently fits the full set of lunar characteristics as well as the giant-impact model.