Atmospheric scientists, forensic scientists, life scientists, neuroscientists, ocean scientists, plant scientists, and almost all other scientists know what lies at the heart of their respective fields, but planetary scientists do not. The last generally accepted definition of a planet, a wanderer, comes from the ancient Greeks. Although the need for an updated definition has existed since the discovery of the asteroid belt two centuries ago, recent discoveries of objects in the Kuiper belt and in orbit around other stars have reminded planetary scientists of this unsatisfactory situation.