fission hypothesis

The fission hypothesis, in the context of astronomy, refers to theories about the formation of celestial bodies, particularly the Moon. One early theory, proposed by Sir George Darwin, suggested that the Moon was formed when the Earth spun so rapidly that chunks of material flew off, eventually condensing into the Moon. This "fission theory" was compelling because it explained the similar composition of Earth's mantle and the Moon. However, it was largely dismissed because scientists doubted Earth could spin fast enough to eject material, and no evidence of such rapid spinning has been found.1

Another theory posits that the Moon and Earth formed together from a primordial accretion disk, but this fails to explain the current angular momentum of the Moon around Earth. Capture theories suggest the Moon formed elsewhere and was later trapped by Earth's gravity, but the mechanics of such a capture are problematic. The most widely accepted theory today is the giant-impact hypothesis, where a Mars-sized body called Theia collided with the proto-Earth, ejecting debris that coalesced into the Moon.12