Pluto is named after the mythological god of the underworld who was able to render himself invisible. This excerpt from the book Pluto and Charon (by Alan Stern and Jacqueline Mitton) explains how Pluto was named:
Something deep-seated in human nature calls on us to name things. It's almost as if a thing isn't real, or whole, until we name it — and so, X had to have a name.
Suggestions flooded in: "Zeus," "Cronos," "Lowell," "Minerva." Widow Lowell first liked "Zeus," but later suggested "Percival," then "Lowell," and then, finally, "Constance," her own name.
Dozens of other well-meaning suggestions came pouring in as well. Then there were hundreds, then thousands. But when all was said and done, the moniker for the newly discovered X that the Lowell staff preferred was the one suggested* by 11-year-old Venetia Burney of Oxford, England: Pluto — Pluto, the Greek god of the Underworld; the brother of Jupiter, Neptune, and Juno; and third son of Saturn, who was able, when he wished, to render himself invisible.
Both the American Astronomical Society and the UK's Royal Astronomical Society adopted Pluto as the official name and as the official symbol for the new world. was Percival Lowell's monogram.
*The French astronomer P. Reynaud had suggested Pluto as the natural mythic name for Lowell's putative Planet X in 1919, but this was not remembered until 1930.
The Roman god of the underworld, also known as god of the dead, is Pluto. The journey to the underworld leads the dead to Styx, the River of the Dead. Here, they are required to pay a toll for the ferry. Charon, the ferryman, collected this toll, which was paid for with a coin buried with the dead. Next, the dead must cross the path of Cerberus, a three-headed dog that only allows the dead to pass by. The final challenge before reaching the underworld was to face the judges of the dead.
Pluto chose the position as ruler of the underworld after he and his brothers conquered and divided their father's rule of the world. In the beginning, Pluto was seen as a cruel god because he ignored the prayers and sacrifices made in his honor. His image softened with time and he was viewed by later cults as a kinder, more gentler god. Pluto eventually took a goddess, Proserpine, maiden of spring. He emerged from the underworld and swept her up in his chariot. Proserpine became goddess of the dead. Although her fate was in the hands of Zeus and she was only obliged to stay with Pluto for half of the year and return to her mother the other half, hence the change from winter to spring.
James Christy and Robert Harrington announced their discovery of Pluto's large moon on July 7, 1978. Christy proposed the name "Charon," after the mythological ferryman who carried souls across the river Acheron, one of the five mythical rivers that surrounded Pluto's underworld. Apart from the mythological connection for this name, Christy chose it because the first four letters also matched the name of his wife, Charlene.
According to International Astronomical Union rules, Pluto's moons are named for characters associated with the underworld of Greek and Roman mythology.
In 2006, the nine-member team that discovered Pluto's two smaller moons (initially designated P2 and P3) in June 2005 christened the pair Nyx and Hydra; because asteroid 3908 already bears the Greek name Nyx, the IAU changed Nyx to its Egyptian equivalent, Nix.
In mythology, Nix is the goddess of darkness and night, befitting a satellite orbiting distant Pluto, the god of the underworld. Nix is also the mother of Charon, relevant to the giant impact believed to have created Pluto's satellites, indicating Charon was borne of the material from which Nix formed.
Hydra is the terrifying monster with the body of a serpent and nine heads, befitting the outermost moon of Pluto, the ninth planet discovered in the solar system.
In addition, just as Pluto's name begins with the letters "P" and "L" to honor Percival Lowell, who motivated the search that led to its discovery, Nix and Hydra honor the search for new satellites and the New Horizons mission to Pluto by starting with the letters "N" and "H." The first letter of Hydra also honors the Hubble Space Telescope, which was used to detect the satellites.
The IAU approved the names Kerberos and Styx for Pluto's two smallest moons – discovered in 2011 and 2012, respectively – in 2013. Kerberos was named after the three-headed dog of Greek mythology; Styx for the mythological river that separates the world of the living from the realm of the dead.
The names were selected based on the results of an unprecedented Internet vote held in February 2012. The online ballot received almost 500,000 votes, including 30,000 write-in suggestions. Kerberos is the Greek form of the name Cerberus, which actually ranked second in the voting. Styx ranked third. The top vote-getter was "Vulcan," based on a suggestion from actor William Shatner of TV's "Star Trek" fame. Vulcan was the name of the home planet of Star Trek character Mr. Spock. The IAU gave serious consideration to this name, which happens to be shared by the Roman god of volcanoes. However, because the name has already been used in astronomy, and because the Roman god is not closely associated with Pluto, the proposal was rejected.