Pluto and its largest moon Charon form a binary dwarf-planet system: their barycenter lies outside Pluto's surface, making both bodies mutually tidally locked as they orbit a common point every 6.4 days. Charon is roughly half Pluto's diameter, the largest moon-to-primary ratio in the Solar System. Four smaller moons — Styx, Nix, Kerberos, and Hydra — orbit the pair in near-resonant, chaotic paths shaped by the binary's gravity. New Horizons revealed Pluto's nitrogen-ice heart (Sputnik Planitia) and Charon's dark polar cap (Mordor Macula). The system's compact architecture is thought to result from a giant impact similar to the Earth-Moon formation event.