The Answer:

We are always getting questions about the planet Pluto. It is the smallest and most mysterious in our solar system partly because its the only planet that hasn't been visited by a spacecraft.

Because it's so far from the Sun and always in perpetual darkness, the name "Pluto" was borrowed from the Roman god of the underworld, who also happens to be the son of Saturn and brother of Jupiter and Neptune. (In Greek mythology, Pluto is known by the name Hades.)

Here's some more information about Pluto.

—The Fact Monster