Names Selected by the Public in a Vote for New Pluto Moons Approved by the IAU


Hubble image of Pluto system.

The IAU is pleased to announce that today it has officially recognised the names Kerberos and Styx for the fourth and fifth moons of Pluto respectively (formerly known as P4 and P5). These names were backed by voters in a recently held popular contest, aimed at allowing the public to suggest names for the two recently discovered moons of the most famous dwarf planet in the Solar System.

The new moons were discovered in 2011 and 2012, during observations of the Pluto system made with the NASA/ESA Hubble Space Telescope Wide Field Camera 3, and increasing the number of known Pluto moons to five. Kerberos lies between the orbits of Nix and Hydra, two bigger moons discovered by Hubble in 2005, and Styx lies between Charon, the innermost and biggest moon, and Nix. Both have circular orbits assumed to be in the plane of the other satellites in the system. Kerberos has an estimated diameter of 13 to 34 kilometres, and Styx is thought to be irregular in shape and is 10 to 25 kilometres across.

The IAU acts as the arbiter of the naming process of celestial bodies, and is advised and supported by astronomers active in different fields. On discovery, astronomical objects receive unambiguous and official catalogue designations. When common names are assigned, the IAU rules ensure that the names work across different languages and cultures in order to support collaborative worldwide research and avoid confusion.

After the discovery, the leader of the research team, Mark Showalter (SETI Institute), decided to call for a public vote to suggest names for the two objects. To be consistent with the names of the other Pluto satellites, the names had to be picked from classical mythology, in particular with reference to the underworld -- the realm where the souls of the deceased go in the afterlife. The contest concluded with the proposed names Vulcan, Cerberus and Styx ranking first, second and third respectively. Showalter submitted Vulcan and Cerberus to the IAU where the Working Group for Planetary System Nomenclature (WGPSN: and the Committee on Small Body Nomenclature (WGSBN: discussed the names for approval.

However, the name Vulcan had already been used for a hypothetical planet between Mercury and the Sun. Although this planet was found not to exist, the term "vulcanoid" remains attached to any asteroid existing inside the orbit of Mercury, and the name Vulcan could not be accepted for one of Pluto's satellites (also, Vulcan does not fit into the underworld mythological scheme). Instead the third most popular name was chosen -- Styx, the name of the goddess who ruled over the underworld river, also called the Styx.

After a final deliberation, the IAU Working Group for Planetary System Nomenclature and the IAU Committee on Small Body Nomenclature, in charge of naming dwarf planets and their systems, agreed to change Cerberus to Kerberos -- the Greek spelling of the word, to avoid confusion with an asteroid called 1865 Cerberus. According to mythology, Cerberus -- or Kerberos in Greek -- was a many-headed dog that guarded the entrance to the underworld.

The IAU wholeheartedly welcomes the public's interest in recent discoveries, and continues to stress the importance of having a unified naming procedure following certain rules, such as involving the IAU as early as possible, and making the process open and free to all. Read more about the naming of astronomical objects here:

The process of possibly giving public names to exoplanets (see iau1301:, and more generally to yet-to-be discovered Solar System planets and to planetary satellites, is currently under review by the new IAU Executive Committee Task Group Public Naming of Planets and Planetary Satellites.

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