The International Astronomical Union announced yesterday that the names Kerberos and Styx have officially been recognized for the fourth and fifth moons of Pluto, previously referred to as P4 and P5.
This Hubble image shows five moons orbiting Pluto (NASA / ESA / M. Showalter, SETI Institute)
P4 and P5 were discovered in 2011 and 2012, during observations of the Pluto system made with the NASA/ESA Hubble Space Telescope.
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 8 to 21 miles (13 – 34 km), and Styx is thought to be irregular in shape and is 6 to 16 miles (10 – 25 km) across.
After the discovery, Dr Mark Showalter of SETI Institute, the leader of the research team, decided to call for a public vote to suggest names for P4 and P5.
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.
Dr Showalter submitted Vulcan and Cerberus to the International Astronomical Union (IAU) where working groups 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. 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, IAU’s working groups 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.
