In their new book, Christopher Knight and Alan Butler pose the question Who Built the Moon?, suggesting that it may not be a naturally formed object.
Previous theories on the moon's formation have suggested that a chunk of the Earth was pulled out by the sun's gravity, that the moon was a wandering planet caught in our orbit, and that the moon and Earth formed together at the same time. Most recently, the "Big Whack" theory has come into vogue, positing that a roving planetoid smacked into the Earth, and the resulting debris created the moon.