Why isn't Pluto a planet?

"For many years, it was accepted as fact that our solar system had nine planets. However, one important meeting of astronomers in 2006 reduced that number to eight. They decided that Pluto was not a planet, a declaration that outraged some people. Readers will learn more about distant Pluto, such as how it was accidentally discovered. Brilliant photographs accompany the fascinating facts"--Provided by publisher.