In this video from NASA, learn about the history of defining a planet, and examine how classification changes based on new knowledge. Our understanding of the solar system has evolved since the ancient Greeks, who first used the word planetes to distinguish the objects that moved against the backdrop of stars in the night sky. The discovery of Pluto and Eris rekindled the debate over what a planet is, and in 2006, the International Astronomical Union (IAU) issued definitions for planets and dwarf planets. However, the debate continues among scientists all over the world. This resource is part of the NASA Planetary Sciences Collection.