The Pluto system in 2012.

In the not-too-distant past, Pluto was thought to be an odd bird in the outer reaches of the solar system because it has a moon, Charon. But Pluto now has plenty of company.

Pluto is now known to have many moons. And many of the largest objects discovered in the Kuiper Belt have one or more moons. The puzzle for planetary scientists is that, as a whole, the hundreds of objects now known to inhabit the Kuiper belt beyond the orbit of Neptune have only about an 11 percent chance of possessing their own satellites. That many of the largest objects now known have satellites means that different processes are at work for the large and small bodies.

At Pluto, the favored theory is that all the moons are relics of a collision between Pluto and another large Kuiper Belt object billions of years ago.