What is a planet? We've been asking that question at least since Greek astronomers came up with the word to describe the bright points of light that seemed to wander among fixed stars. Our solar system's planet count has soared as high as 15 before it was decided that some discoveries were different and should be called asteroids.
Many disagreed in 1930 when Pluto was added as our solar system's ninth planet. The debate flared again in 2005 when Eris -- about the same size as Pluto -- was found deep in a zone beyond Neptune called the Kuiper Belt. Was it the 10th planet? Or are Eris and Pluto examples of an intriguing, new kind of world?
The International Astronomical Union decided in 2006 that a new system of classification was needed to describe these new worlds, which are more developed than asteroids, but different than the known planets. Pluto, Eris and the asteroid Ceres became the first dwarf planets. Unlike planets, dwarf planets lack the gravitational muscle to sweep up or scatter objects near their orbits. They end up orbiting the sun in zones of similar objects such as the asteroid and Kuiper belts.
Our solar system's planet count now stands at eight. But the lively debate continues as we continue to explore and make new discoveries.