Pluto and its largest moon, Charon, are sometimes called a "binary system". This is because the barycentre of their orbits does not lie within them. The IAU has yet to formalise a definition for binary dwarf planets, and until it passes such a ruling, they classify Charon as a moon of Pluto. Pluto has four known smaller moons, Nix and Hydra, discovered in 2005, Kerberos, discovered in 2011, and Styx, discovered in 2012.