Science at Work -- Redefining a Planet

As astronomers find ever more objects in our own Solar System and numerous planets orbiting other stars, their idea of what defines a planet has changed. Several years ago, Pluto was demoted from full planet status to become what was called a dwarf planet. A dwarf planet was defined as an object that is

This definition almost immediately was attacked as too restrictive but it will stand until the International Astronomical Union meets in 2009. In the meantime, however, Pluto has been given some special status by denoting it and the other large-ish objects in the extreme outer Solar System as "Plutoids."