Beyond Pluto: The Discovery of the 10th Planet

Dr. Michael E. Brown

Division of Geological and Planetary Sciences

California Institute of Technology

For the past seven years we've been scanning the skies for planets beyond Pluto. In 2005, after a search of about half of the sky and the discovery of dozens of objects almost the size of Pluto, we finally found 2003 UB313, the first object larger than Pluto, and the first that might be called a new planet. In addition to a new avalanche of scientific questions, this discovery drives home the need to answer a question that astronomers have been unwilling to answer for years: "What is a planet?" I'll discuss the story of the discovery and try to give a perspective on why the question of planethood is difficult and what the real answer should be.