What is a Planet?

Bob Glushko has a very interesting post on the problem of Pluto, over at Doc or Die:

For millennia we earthlings have had a notion of planet as a “wandering” celestial object, but because we only knew of planets in our own solar system, we could define “planet” by enumeration. Very few categories can be understood that way, that is, by making an exhaustive list of their members. But once we acknowledge the existence of planets outside our solar system, the set of planets becomes unbounded, and the lack of a definition becomes apparent. Then we can have arguments about the definitions, and hence biases of one kind or another get built into the categorization.

The popular reaction to this new way of understanding “planet” by extensional definition rather than by enumeration suggests that many people are living with the delusion that there is an objective reality in which categories and definitions are objective and unchanging. And it is scary to read that the IAC members gave serious discussion to the likely impact their new definition would have on elementary school science. I thought progress in science, scientific revolutions, creative destruction and all that meant that we should look forward and not worry about the “installed base” of people with a sixth-grade science education.