Charon

Charon is the largest natural satellite of Pluto. It is so large compared to Pluto, that its barycentre lies outside both objects. This is a common definition of a binary object, leading some astronomers to consider them a double dwarf planet.

From the discovery of Nix and Hydra, it was possible to estimate Charon's mass as roughly 10% that of Pluto. This implies it's approximately 45% ice, thought mostly to be water ice.