Planet Pluto

Pluto is a dwarf planet in the outer Solar System. It was counted as the ninth planet from its discovery in 1930 until 2006. It takes its name from Pluto, the Roman name for Hades, the god of the Underworld, and its moons carry names from the same myths.

The Name in Myth

Pluto is the Latin form of the Greek Plouton, a name for Hades, the ruler of the dead. Homer makes Hades a son of Cronus and Rhea and the third of three brothers, with Zeus and Poseidon: when the world was divided in three, Poseidon received the sea, Zeus the broad sky and Hades the murky darkness (Iliad 15.187–193). Hesiod calls him “mighty Hades, who dwells in houses under the earth” (Theogony 455).

The Greeks connected the name Plouton to ploutos, wealth. Plato has Socrates say that he was called Pluto as the giver of wealth, because wealth comes up from below out of the earth (Cratylus 403a). Apollodorus tells that Pluto fell in love with Persephone and, with the help of Zeus, carried her off in secret (Library 1.5.1). More about the god is on the page for Hades.

The Planet

Pluto was discovered in 1930 by the American astronomer Clyde Tombaugh at the Lowell Observatory in Arizona. The name was suggested by Venetia Burney, an eleven-year-old schoolgirl in Oxford, England, who thought the god of the Underworld suited a distant, dark and cold world.

Pluto orbits the Sun once in about 248 years, at an average distance of about 39 times the distance of the Earth. Its orbit is elongated and tilted, and for part of each orbit it comes closer to the Sun than Neptune. It is far smaller than the eight planets: its diameter is about 2,380 kilometres, less than that of Earth’s Moon. Its surface is mostly frozen nitrogen, methane and water ice. Pluto is not in perpetual darkness: it receives sunlight, but at that distance the Sun is only a very bright star in its sky.

Reclassification

In 2006 the International Astronomical Union defined a planet as a body that orbits the Sun, is massive enough to be round, and has cleared the neighbourhood around its orbit. Pluto, which shares its region with many other icy bodies of the Kuiper Belt, does not meet the last condition, and it was reclassified as a dwarf planet. In July 2015 the New Horizons spacecraft flew past Pluto and sent back the first close images of its surface.

The Moons of Pluto

Pluto has five known moons, all named from Greek myth with a link to Hades and his realm:

Charon, discovered in 1978 and named for the ferryman of the dead. Virgil describes him as the dreadful keeper of the waters, who poles the boat of the dead across the river (Aeneid 6.298–304).
Nix, named for Nyx, the goddess of night.
Hydra, named for the many-headed serpent of Lerna, which Hesiod lists among the offspring of Echidna, killed by Heracles with the help of Iolaus (Theogony 313–318).
Kerberos, named for Cerberus, the hound of Hades, whom Hesiod calls the fifty-headed dog of Hades with a voice of bronze (Theogony 311–312).
Styx, named for the river of the Underworld, and for the goddess who in Hesiod is the eldest daughter of Oceanus and lives apart from the gods (Theogony 776–778).