| Pluto used to be classifyed as the 9th planet. The order of the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and then Neptune. All of the planets except for Earth and Uranus are named after Roman gods. Uranus is named after a Greek god. Because of their orbit, after a certain amount of years, Pluto and Neptune switch from the "9th" and 8th planet to the 8th and "9th".
In the book, Our Solar System by Seymour Simon, it's stated that a young American astronomer named Clyde Tombaugh discovered Pluto in February 1930. Pluto is made out of methane, water with rocks, and has an atmosphere of methane gas. It is less than two-thirds the size of Earth's moon.
Since Pluto is very small and unusually shaped, some scientist didn't think it really was a planet. After this conference between scientists, it was decided that Pluto is a dwarf planet.
I got all of this information from julvenile non-fiction books in the astronomy section.