Early this week, the spacecraft New Horizons began its flyby of Pluto, sending a wealth of information back to Earth about Pluto and its moons. It’s an exciting time for astronomers and those intrigued by the dark dwarf planet.
Pluto has special significance not only because it is the only planet in our solar system to have its status as a planet stripped and downgraded to a dwarf planet, but also because along with its largest satellite Charon, it is our solar system’s only binary planet system. In addition to Charon, Pluto has four additional satellites that have intriguing characteristics. Three of these—Styx, Nix, and Hydra—are in what astronomers call an orbital resonance, which means their orbits are related by a ratio of two whole numbers, making the orbits predictable and relatively stable. However, at least Nix and Hydra have an elongated shape and feel the gravity of both Pluto and Charon, such that their motion is extremely complicated. Astronomers refer to this as chaotic rotation, meaning that to an observer on its surface, the Sun can rise and set from random, different directions and the length of a day can also vary greatly. It is also possible that Styx and Kerberos are also in chaotic rotation; however this hasn’t been confirmed yet.
How a planet gets its name
Of course, Pluto is also interesting linguistically. How does a planet get its name? Since 1919, the International Astronomical Union (IAU) has chosen names for planets and celestial bodies as they are discovered. They have a number of guidelines for suitable names.
Keeping with tradition, the planet (as it was classified when it was discovered in 1930) was named after the Roman god of the underworld, Pluto. The name was suggested by Venetia Burney, a schoolgirl from Oxford who had offered it to her grandfather who passed the suggestion on to an astronomy professor, or so the story goes. When it was put to vote by the IAU, Pluto won unanimously. It was also a nice nod to the astronomer Percival Lowell, as his initials are the first two letters of the name Pluto.
Naming Pluto’s moons
Pluto’s largest satellite or moon, Charon, was discovered in 1978 by James Christy and named after the Greek mythological ferryman of the dead. Charon was suggested by Christy who chose the name for its mythological relevance as well as the similarity to his wife’s name ‘Charlene.’ In fact, it’s this charming story that explains why there are two pronunciations of Charon: one with an initial hard ‘k’ and the other with a soft ‘ch’ sound. Apparently Christy and his colleagues pronounce it with a soft ‘ch’ in reference to Christy’s wife, while the hard ‘k’ pronunciation is used by everyone else. Charon, like Pluto, has a contested classification. Although it was first discovered as a moon or satellite of Pluto, as part of a binary system, it could be declared as a dwarf planet in its own right.
Pluto’s additional moons (or true moons if you consider Pluto and Charon as a binary system) were all discovered in the last ten years: Styx (2012), Nix (2005), Kerberos (2011), and Hydra (2005). Each of these is also named in keeping with underworld mythological conventions. Styx is the river one would cross during death. Nix is another spelling for Nyx, the Greek goddess of the night and darkness; the alternate spelling for Nyx was used as there was already an asteroid named Nyx. Kerberos is actually the Greek spelling of Cerberus, the underworld’s three-headed dog. The Greek spelling was chosen as Cerberus was already the name of an asteroid. Hydra is a multi-headed serpent creature that was guardian to the subterranean entrance to the underworld.
While the mythologies for which Pluto and its satellites are named captivate our imaginations, we continue to discover how the celestial bodies themselves are equally if not more captivating in reality. We can only wait and see what New Horizons uncovers about the mysterious dwarf planet and its orbiting minions.
Image Credit: “New Horizons 3-billion-mile Journey to Pluto Reaches Historic #plutoflyby!” by NASA’s Marshall Space Flight Center. CC BY NC 2.0 via Flickr.