Pluto is the ninth and last known planet of the Solar System. The American astronomer Clyde Tombaugh, working at the Percival Lowell Observatory in Flagstaff, Arizona discovered it in 1930. The existence of the planet had already been predicted by Percival Lowell, one of the most eminent astronomers of the United States, and by William H. Pickering. In 1929 Tombaugh began making a photographic survey of the part of the sky where the new planet was likely to be. By carefully comparing photographs taken several days apart, Tombarugh detected the planet by its movement against the background of "fixed" stars.

Named after the ancient ruling god of the underworld (Hades) in Greek and Roman mythology, Pluto is a mysterious, remote world, unimaginable cold and forbidding, from which the Sun would appear only as a brightish star among other countless stars in space. From Earth, Pluto looks like a faint star, showing no discernible disk, even with the most powerful telescopes.

It has an average distance from the Sun of nearly 6 million kilometers (3.67 million miles). This makes it the outermost of the known planets in the Solar System. But Pluto's orbit is more eccentric (that is, so elliptical, or oval-shaped), than other planetary orbits, and it moves inside the path of Neptune for part of its journey around the Sun. Traveling at an average speed of about 3 kilometers (2 miles) a second, Pluto takes over 248 Earth years to complete a single circuit of the Sun.