Pluto, or Star of the Underworld's King (冥王星, Meiōsei) in Japanese, is the farthest (or in the case of its declassification, it is not at all) planet away from the Sun in the Milky Way galaxy.
Sailor Moon series
In the Sailor Moon metaseries, Pluto was the home planet of and the planet that was protected by Sailor Pluto.
In the manga, Charon Castle was known to orbit the planet, and it was the castle of Princess Pluto
In reality, Pluto is actually not the ninth planet of the Solar System anymore. It was classified as a "dwarf planet" in 2006. This change was due to the new definition that included that a planet must have cleared the neighborhood around its orbit, which Pluto fails to achieve.
Pluto has only three moons: Charon, Nix, and Hydra. Charon is actually classified as the twin planet of Pluto.
Pluto is the ruling planet over the Zodiac symbol Scorpio (Setsuna Meiou was born under the sign of Scorpio.) Pluto is also associated with power, fearlessness, big business, and wealth.