Pluto and the ‘Planet Problem’: Folk Concepts and Natural Kinds in Astronomy

The Pluto controversy provides fertile new ground to revisit the traditional philosophical problem of natural kinds and scientific change. Here I show that further insight into the Pluto case is gained by drawing out some of the striking analogies with what is termed the “species problem” in the philosophy of biology. I argue that the taxon ‘planet’ can still be considered a natural kind term despite the fact that a) its meaning and extension have changed over time, b) there are multiple scientifically compelling definitions of planet, and c) many of those definitions include historical and/or relational properties.