Pluto, Discovery, and Classification in Astronomy

Abstract

The debate over the demotion of Pluto to a “dwarf planet” in Prague at the IAU General Assembly in 2006 showed that classification is far from a boring subject, and in fact opens a window on a little-known aspect of the history of astronomy over the last 400 years. How does an astronomer know when he or she has discovered a new class of astronomical object? Who decides if a dwarf planet, or a quasar or a pulsar, is a new class of object? To put it more broadly, how does an astronomer know if he or she has discovered something new, especially to the extent that it is declared a totally new class of object? This chapter begins with the Pluto debate, but quickly broadens out to the discovery of other new classes of objects over the last 400 years of telescopic astronomy. Classification is an open-ended process. Even as the vote was taken in Prague, NASA’s New Horizons spacecraft was speeding toward the edge of the Solar System, some three billion miles away, to explore the nature of the object so much in dispute. The discoveries made there revitalized the debate over its classification status, as did the arrival of NASA’s Dawn spacecraft at another newly designated dwarf planet, the asteroid Ceres.