For the past few decades, if you asked an astronomer how the moon was created, nearly all of them would tell you that a Mars-sized world nicknamed Theia crashed into the early Earth, sending a cloud of debris high into space where it coalesced into our silvery satellite. And there’s strong evidence to support this idea.

But dig deeper, and you’ll find nagging problems with the theory. Almost all models of the giant impact imply that the moon should still contain a lot of Theia. But the mo