The dwarf planet Pluto is one of the largest members of the Kuiper belt, a collection of ice and rock debris revolving around the Sun beyond Neptune’s orbit. Pluto has five known moons. The largest, Charon, is half the size of Pluto. Charon’s period of revolution is exactly equal to the rotation period of Pluto itself; in other words, Charon is in synchronous orbit around Pluto. As a result, Charon is visible from only one hemisphere of Pluto.
Pluto’s other four moons—Hydra, Nix, Kerberos, and Styx—are much smaller than Charon. For every orbit completed by Charon, Hydra completes about one-sixth of an orbit, Kerberos about one-fifth, Nix about one-fourth, and Styx one-third. This means that the orbital periods of Hydra, Kerberos, Nix, and Styx are in a 6:5:4:3 ratio; that is, all five bodies pass one another periodically.
Charon was the first moon to be discovered, in 1978. The other moons were all discovered in the 21st century using the Hubble Space Telescope. The moons are named after characters in Greek mythology that are associated with the underworld. The moons are listed in order of their distance from Pluto, with the year of their discovery, orbital period in days, and diameter in kilometers.
| name | year discovered | distance from Pluto (km) | orbital period (days) | diameter (km) |
|---|---|---|---|---|
| Charon | 1978 | 17,536 | 6.387 | 1,208 |
| Styx | 2012 | 42,000 | 20.2 | 20–50 |
| Nix | 2005 | 48,708 | 24.86 | 88 |
| Kerberos | 2011 | 59,000 | 32.1 | 26–68 |
| Hydra | 2005 | 64,749 | 38.2 | 72 |