Moons like the Earth's - which are formed in catastrophic collisions - are extremely rare in the Universe, a study by US astronomers suggests. The Moon was created when an object as big as the planet Mars smacked into the Earth billions of years ago. The impact hurled debris into orbit, some of which eventually consolidated to form our Moon. The Astrophysical Journal reports that just 5-10% of planetary systems in the Universe have moons created this way.