Until recently, our Earth-Moon system was thought to be a rare configuration — most moons in the Solar System are proportionally very small compared to the planets they orbit, but our Moon is proportionally large at about one-quarter the size of Earth. Now, results from a new computer simulation conducted by physicists in Zurich and Colorado indicate that such a configuration may be more common than previously thought.
Our Moon is believed to have formed 100 million years after the initial formation of the Solar System when a Mars-sized planet collided with Earth, breaking the smaller planet apart, and forming a debris ring that eventually coalesced into what is now the Moon. According to the simulations, there is about a one in 12 chance of a similar scenario occurring in other systems.
While interesting, keep in mind that these are simulations, not observations, and they do not take into account all of the different variables. They simply tell us that formation of other planets with proportionally large moons is plausible.