Pluto-charon system

The pluto-charon system is a binary planet system consisting of the dwarf planet Pluto and the moon of Pluto Charon. It is named "Pluto-charon system" because charon isn't exactly a moon of pluto. Charon is so massive compared to pluto that its gravity has made the barycenter of its orbit outside pluto's surface, making it a binary system rather than just being a moon.