The moon once had a smaller sibling, according to astronomers from the University of California, Santa Cruz. The researchers used computer models to simulate the size, orbit, and ultimate destruction of the second moon that they believe once shared the night skies above our Earth. But what became of our second moon? Here, a brief guide to the new theory:

Where did the second moon come from?
Astronomers believe the second moon was created the same way as our current moon: A Mars-sized planet crashed into Earth, kicking up an enormous ring of debris that orbited our world. Over time, this debris coalesced into our existing large moon, as well as a second, smaller moon. The two satellites co-existed peacefully in the sky for some 80 million years, looking like siblings. They were the same color and shape, though our moon was three times larger than the other.

What happened to our second moon?
About 4.5 billion years ago — before there was even life on Earth — the second moon hit our existing moon in a dramatic, slow-motion collision occurring over several hours. Because the smaller moon was more solid than the still-molten surface of our larger moon, the collision resulted in a "splat" that scattered wrecked pieces of the smaller moon all over the larger moon's far side. That helps to explain why the two sides of our moon are so different.

What's different about the two sides of the moon?
The side facing Earth is relatively smooth and flat, whereas the far side of the moon, which never faces the Earth, is rugged, mountainous, and pockmarked with craters. Astronomers speculate that the force of the impact of the smaller moon was responsible for creating much of the far side's rocky, debris-strewn landscape.

Are scientists certain this happened?
No. Like any new theory, this one is speculative, and some astronomers are already finding holes in the theory. It doesn't explain, for example, why the ragged landscape of the moon's far side has a much higher concentration of aluminum than the near side. (Both moons would have been made of the same material.) A NASA lunar mission scheduled for launch this September, the Gravity Recovery and Interior Laboratory, or GRAIL, may help to confirm (or debunk) this new theory.

Sources: Associated Press, International Business Times, Nat. Geo., Scientific American