Pluto has a fourth moon, scientists announced last week. They used images from the Hubble Space Telescope to find P4, the smallest of Pluto’s satellites, estimated to be between only 8 and 21 miles in diameter. It orbits between Nix and Hydra, which circle Pluto on the outside of Charon’s path.
P4, however, is just a temporary name for the new moon. And while there are many suggestions for a permanent name, I can say that it definitely won’t be Mickey, Donald or Goofy. That’s because there are strict naming conventions for astronomical bodies. When it comes to our solar system, we’re stuck with Greek mythology and each planet (or dwarf planet, as in this case) has its own set of stories to choose names from. Let’s look at the Pluto system:
Pluto: The dwarf planet—discovered on January 23, 1930 in Flagstaff, Arizona—is named for Pluto, the Greek god of the underworld. Perhaps better known as Hades, Pluto had two brothers, Zeus, ruler of the skies, and Poseidon, who held dominion over the seas. Pluto abducted his niece Persephone to be his wife and queen. But when Persephone went missing, her mother, Demeter, goddess of the harvest, became overcome with grief; the seasons ended and everything began to die. Zeus sent a messenger to retrieve to Persephone, but because she had eaten pomegranate seeds, she remained bound to Pluto for several months every year. Her yearly return to her mother brings the spring season.
Charon: Pluto’s largest moon was discovered in 1978 when astronomer Jim Christy, examining photographic plates with Pluto’s image, noticed an odd, periodic bulge. Charon, in mythology, was the ferryman who carried souls to the underworld. (The choice of Charon as the moon’s name, however, wasn’t entirely due to its connection to Pluto in mythology; Christy wanted to name the moon in honor of his wife, Charlene, and Charon was as close as he could get.)
Nix: One of two moons discovered in 2005 with Hubble images, Nix named for Nyx, the goddess of the night and Charon’s mother. (Nix is the Egyptian spelling; Nyx was already the name of an asteroid.) Nyx lived in Tartarus, the dungeon of the underworld, and in some stories she is said to be involved with dark doings, such as protecting spies during the Trojan War.
Hydra: The other of the two moons discovered in 2005, Hydra is named for a many-headed, serpent-like beast of ancient times. If one head was cut off, two grew in its place. The hydra guarded an entrance to the underworld near the city of Argos. Heracles (a.k.a. Hercules) slew the hydra in the second of his Twelve Labors.
Mark Showalter of SETI, the discoverer of P4, has said that the name of Cerberus comes up most often in discussions of potential names for the new moon. Cerberus would certainly fit in with the mythology of the Pluto system—he was the three-headed dog that guarded the gates to the underworld. The spelling of the name would have to be altered, though, as, like Nyx, Cerberus is already the name of an asteroid.
But are there other characters that would fit in? Major characters, such as Persephone, would be inappropriate for a moon so tiny, but there are plenty of options. Here are a few of my favorites:
Erberus: Husband (and brother—ew) to Nyx and father of Charon, he personified darkness.
Styx: The river that was the border between the living world and the underworld.
Hypnos: One of Nyx’s many sons, Hypnos was the personification of sleep. His twin brother was Thanatos, the personification of death.
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