Our moon is thought to have been created by a planet the size of Mars smashing into the proto-earth. The resulting debris is then thought to have formed the moon. It is possible something similar happened to form Pluto's moon Charon.
Nadya Grolova and colleagues from the University of Florida has been looking for evidence of this sort of collision in other solar systems. The technology to detect the planets directly doesn't exist but they are looking from the immense amounts of warm dust that such collisions would create, and the infra red radiation this dust would produce. They have looked at 400 stars that are only only about 30 million years old, which is when most of these collisions are predicted to occur. From their results they think that only 1 in 10-20 solar systems have these collisions and only some of the collisions will produce moons.
This is important because the moon is thought to have stabilised the orientation of the earth's axis thus making the earth's climate much more stable, and giving complex life enough time to develop.