Earlier today, the International Astronomical Union (IAU) voted on names for the two most recently discovered moons of Pluto. The larger of the two, formerly designated P4 will now be known as Kerberos, while P5 has been named Styx. The decision was a slight departure from the results predicted by a public poll, because those chosen names were already being used to describe other bodies. While the results of that poll (Vulcan and Cerberus) were widely reported as definitive, it is the IAU which ultimately decides on how astronomical bodies shall be named. They ensure that there are no duplicates, and that names make sense in international languages.
The moons were discovered by the Hubble Space Telescope in 2011 and 2012, although the astronomers responsible took their time verifying what they’d seen before making the news public. Kerberos and Styx have reasonably circular orbits which lie in the same plane as the other three satellites. Kerberos has an estimated diameter of 13 to 34 kilometers, and Styx is thought to be irregular in shape and is 10 to 25 kilometers across.
The name Vulcan is not specifically used by any particular object, but was used in the 19th century as the name of a hypothetical planet between Mercury and the Sun which astronomers expected to find. The word Vulcanoid continues to be used to describe any asteroid which orbits in that region. Cerberus is in use by an object: an asteroid discovered in 1971. The IAU Working Group for Planetary System Nomenclature and the IAU Committee on Small Body Nomenclature decided to deal with Cerberus by using an alternative spelling: Kerberos, the three-headed dog guarding the entrance to the underworld. No such concession was made for Vulcan, however, since it doesn’t fit with the mythological naming conventions for the Pluto system – everything else has a name connected to ancient Roman beliefs regarding the underworld. Instead, they went for the 1st runner up in the public poll: Styx, named for the river separating the underworld from the realm of the living.