For many years, children learned in school that there are nine planets in our solar system. The ninth planet, Pluto, was discovered in 1930. However, in 2006, Pluto was demoted. Why did this happen after nearly 80 years? What sets a planet apart from an asteroid, comet, or other celestial body? What exactly is a dwarf planet—Pluto's new designation? Readers learn the answers to these questions and more through engaging text, augmented by informative fact boxes and full-color photographs.