PlutoCharon

Pluto–Charon is a binary system in the outer Solar System consisting of the dwarf planet Pluto and its largest moon, Charon. The two bodies orbit a common barycenter that lies outside Pluto, making the pair a double-dwarf planet in some classifications. They are tidally locked to each other, so each always shows the same face to the other.

Pluto is about 2,376 km in diameter, while Charon is about 1,212 km. Pluto’s mass is roughly 1.31×10^22 kg and Charon’s is about 1.52×10^21 kg, giving a mass ratio around 8:1. The average distance between the two is about 19,600 km, and they complete an orbit around their mutual center of mass every 6.387 days. The system is also accompanied by four smaller moons—Styx, Nix, Kerberos, and Hydra—that orbit Pluto at greater distances.

Discovered: Pluto was identified as a planet in 1930 by Clyde Tombaugh. Charon was discovered in 1978 by James W. Christy. The name Charon, from Greek mythology, was chosen after a naming contest; the pair have been described as a binary system since.

Formation and exploration: The Pluto–Charon system is thought to have formed from a giant impact early in the Solar System’s history, a scenario that also explains the presence of additional moons. The system was explored in detail by NASA’s New Horizons spacecraft during a 2015 flyby, revealing diverse terrains on both bodies and advancing studies of icy worlds and binary planetary formation.