Learn about the debate over the definition of a planet in this video from NOVA scienceNOW. Historically, there has been no scientific definition for a planet, leaving astronomers with the difficult task of properly classifying new discoveries in our solar system, such as Ceres and Eris. However, in August 2006, members of the International Astronomical Union (IAU) passed a resolution that defined a planet. Under the new definition, Pluto is not classified as a planet, but rather as a dwarf planet along with Ceres and Eris. This video is available in both English and Spanish audio, along with corresponding closed captions.