On far-flung Pluto, it may be raining moon dust. Models suggest that Pluto's small moons are even now sprinkling dust on its equator, which could explain why Pluto's middle is darker than its poles. A NASA spacecraft headed for Pluto's neighbourhood should be able to check out the claim when it arrives next year.
Pluto and its moons lie in the Kuiper belt, a region beyond the orbit of Neptune filled with mostly small, icy worlds. While Pluto is only about half the size of Mercury, it boasts five known moons. The largest, Charon, is half Pluto's size. The other four – Nix, Hydra, Kerberos and Styx – are much smaller. All five appear so similar that astronomers think a large object smashed into Pluto early in its history, ejecting debris that coalesced into moons.