This is an image of Pluto with its moon Charon. Click on image for full size version (36K GIF) Image courtesy of NASA

After the discovery of Neptune in 1846, mathematics suggested that there still might be a ninth planet. Scientists set out to discover it, and it was finally identified in 1930 by Clyde Tombaugh after a careful search of the sky.

Because Pluto is so small, it is also very dim in the sky. At 39 Astronomical Units from the sun, and with 248 years to complete its orbit around the sun, Pluto also moves very slowly. So it was many years before the 9th planet could be identified by its motion.

Pluto is named after the Roman god of the underworld. It has one moon named Charon. Some people say that Pluto isn't a planet at all. They say it's really a satellite that escaped Neptune's gravitational pull.