Pluto-Charon is a true binary planet:
the barycenter of the system lies between Pluto and Charon.
With a radius of roughly 600 km, Charon is about half the size of Pluto (1200 km) and has about 12% the mass of Pluto.
Both Pluto and Charon are half rock and half ice.
The four circumbinary satellites -- Styx, Nix, Kerberos, and Hydra -- are only 10--40 km across. They are probably mostly ice. Their orbits are packed as closely as possible for a stable system.
Our numerical simulations show that the
four satellites grow out of the debris left over from the giant collision that made the Pluto-Charon binary.
