Nature reports on a new way of looking at lunar formation that almost reads like a myth. The moon came to be when Earth collided with a near-identical sister planet:
The ‘giant impact’ hypothesis, first proposed in the 1970s, suggests that the Moon was formed from the debris scattered when a Mars-sized planet slammed into the early Earth some 4.5 billion years ago. This fits well with what we know about the Moon, including its mass and lack of any significant iron core.
But the theory also implies that the Moon is made up mostly of impactor material. Since lunar and Earth rocks have such similar compositions, this suggests that Earth and the planet that smacked into it resembled each other too. They would have needed to be sister planets, with a relationship much closer than that of any other planetary bodies we have studied in our Solar System.