Pluto is a dwarf planet located in the Kuiper belt. It was previously considered to be the ninth planet of the Solar System, but its status as a planet was questioned in 1992 after numerous similar sized objects were found in the Kuiper belt. The discovery of Eris in 2005, a dwarf-planet that is 27% bigger than Pluto, forced the International Astronomical Union (IAU) to redefine the word "planet" during its twenty-sixth General Assembly. The IAU defined a planet as a celestial body that has an adequate mass to assume an almost round shape while rotating around the Sun, and has "cleared the neighborhood around its orbit." As a result, the IAU reclassified Pluto as a dwarf planet.
Moons of Pluto
Pluto has five moons: Charon, Styx, Nix, Kerberos, and Hyrda. Charon is the largest of Pluto's moons and was discovered by US astronomer James Christy on June 22, 1978, roughly fifty years after the dwarf planet's discovery. Two more moons, Nix and Hydra, were identified on May 15, 2005, by members of the Pluto Companion Search Team while preparing for the New Horizons mission. Kerberos was first identified on July 20, 2011, while Styx was discovered on July 7, 2012.
Charon, which is Pluto’s innermost and biggest moon, is approximately 597 miles from the surface of Pluto and is nearly half its size. The moon is primarily gray, but its northern pole has a reddish tint. A large percentage of Charon's surface is covered by ice, but the reddish part of its north pole is composed of tholins, which are ethane, methane, or carbon dioxide molecules that are sometimes mixed with nitrogen or water. Charon’s northern pole is reddened by continuous exposure to ultraviolet light from the Sun. Charon orbits Pluto every 6.3 days, and the two planetary bodies are gravitationally locked, meaning the same face is maintained during orbit. Although the surface of Charon appears icy and rigid, more than 50% of its interior is composed of rock. Pluto and Charon have their barycenter about 600 miles above Pluto's surface.
Hydra, Nix, Kerberos, and Styx are 34 miles, 26 miles, 7 miles, and 4 miles wide along their longest axis, respectively. These moons are smaller than Charon and irregularly shaped, and rotate at two to four times the distance of Charon, ranging from 26,532 miles (Styx) to 40,264 miles (Hydra). Styx has an elongated shape, and Nix is an oddly shaped celestial body. Hydra has an uneven surface that includes some craters, and it measures roughly 34 miles by 25 miles. Hydra orbits Pluto every 39 days, while Nix orbits the dwarf planet every 25 days. Kerberos has the most mysterious appearance, as its surface appears lumpy. It takes Kerberos 32 days to orbit Pluto, and it orbits between Hydra and Nix, exerting a strong gravitational pull on the other moons despite its small size.
Origin of Pluto's Moons
Pluto’s satellite system is believed to have been created by a large collision between Pluto and another celestial body. The impact produced materials that coalesced, forming various satellites that orbit Pluto. The almost circular shape of these moons suggests that they were created through the collision.