Charon: Pluto's dwarf planet partner

A grey sphere peppered with craters and a distinct rusty red surface at the north pole.

Charon is the largest moon of the dwarf planet Pluto, but calling it a moon is really a misnomer; Pluto and Charon actually form a binary system. Whereas a moon usually orbits a planet, both Pluto and Charon orbit a point in space between them — their common center of mass. The other four moons in the system — Styx, Nix, Kerberos and Hydra — orbit both Pluto and Charon.

The center of mass is located between Pluto and Charon because there's not a huge difference in their diameters and masses, so they balance out somewhat. Pluto has a mass of 1.3 x 10²² kilograms and a diameter of 1,477 miles (2,377 kilometers), whereas Charon has a mass of 1.6 x 10²¹ kg and a diameter of 753 miles (1,212 km).