Pluto's Moons Get Named
Styx and Kerberos join Nix, Hydra and Charon among Pluto's named moons. NASA, ESA and M. Showalter

Pluto’s two tiniest moons -- P4, discovered in 2011, and P5, discovered in 2012 -- now have official names. Styx, formerly P5, and Kerberos, formerly P4, were voted on by the public and approved on Tuesday by the International Astronomical Union.

Styx and Kerberos join Pluto’s three other moons Charon, Nix and Hydra, all named after figures from Greek and Roman mythology. Pluto is the god of the Underworld in Greek mythology, Charon is the ferryman of the dead, Nix is named after Nyx the Greek goddess of the night; and Hydra is a many-headed water beast that lived in a lake that held an entrance to the Underworld. Styx is the river Charon crosses, and Kerberos, better known as Cerberus, is a three-headed dog that guards the entrance to the Underworld. Nix's name changed to avoid any confusion with the asteroid 3908 Nyx.

Charon, the largest of Pluto’s moons, is roughly half the size of the dwarf planet. Dr. Mark Showalter, senior research scientist at the SETI Institute, led the discovery of Pluto’s new moons using images collected by the Hubble Space Telescope. Their names came out of a public poll set up in February, which drew more than 500,000 votes.

But the top vote-getter, Vulcan, didn't make the cut. Actor William Shatner's submission, in honor of Mr. Spock’s home planet, named for the God of Volcanoes in Roman mythology, was ultimately rejected because it was not associated with the Underworld. Styx and Kerberos, meanwhile, made it through two separate IAU committees, Working Group for Planetary System Nomenclature and the Committee on Small Body Nomenclature, before their approval.

Other popular names included Persephone, Orpheus, Hypnos and Eurydice. Persephone is the Queen of the Underworld while Hypnos is the physical manifestation of sleep whose mother was Nyx. Eurydice was the wife of Orpheus, whose story is set in the Underworld.

Eurydice died on their wedding day and Orpheus, a famed musician, played the saddest song ever heard. He was urged to go to the Underworld to rescue her. Orpheus eventually won over Hades and Persephone and even Kerberos succumbed to his songs. Eurydice was allowed to leave the Underworld with Orpheus as long as he did not look back. Right before returning to the upper world, Orpheus looked back, thus dooming Eurydice to the Underworld.

Because of Pluto’s small size, the number of moons the planet has amassed is a bit of a mystery, but NASA’s New Horizons mission will reach Pluto in 2015 and should offer some new insight on the distant planet, notes SETI. Styx lies between Nix and Charon, while Kerberos’ orbit lies between Nix and Hydra.