The most accepted explanation today for the origin of the Moon is the giant impact. About 4.5 billion years ago, a body the size of Mars (nicknamed Theia) is thought to have collided with the still-young Earth. Debris launched into orbit then clumped together and formed the Moon. The idea was proposed in the 70s and gained ground in the 80s, after more sophisticated computer simulations.
Why this is the winning hypothesis
Before Theia, three hypotheses competed:
- Co-formation. The Moon and Earth formed together from the same protoplanetary disc.
- Capture. The Moon was born elsewhere and was gravitationally captured by Earth.
- Fission. Earth was spinning so fast it ejected material that became the Moon.
Each had serious problems:
- Co-formation doesn't explain the lack of lunar iron. If the two bodies formed together, they should have similar composition, but the Moon has much less metallic core proportionally.
- Capture would require extremely improbable orbital conditions to slow down a large body and put it in stable orbit.
- Fission would require an absurdly fast initial Earth rotation (something like 2 hours per day), with no plausible mechanism to slow it later.