http://en.wikipedia.org/wiki/Pluto_%28mythology%29

"In 1905, American astronomer Percival Lowell suggested the existence of Pluto, despite never having seen it. Lowell was studying some unusual nuances in the orbits of Neptune and Uranus and theorized that only the gravity of an unknown planet would be causing them.

In 1930, after Lowell's death, American astronomer Clyde Tombaugh observed Pluto. By tradition, the discovering astronomer of a new space object gets naming rights. But it was an 11-year-old British girl's idea to name the planet Pluto. Venetia Burney told her grandfather that the name fit the new planet because it stayed hidden for so long, and the Roman god Pluto could disappear at will. Venetia's grandfather wrote to Tombaugh and offered the suggestion..."
http://www.cliffsnotes.com/cliffsnotes/sciences/how-did-the-planet-pluto-get-its-name-i-know-it-s-named-after-the-mythical-god-of-the-underworld-but-why