From lecture 3 in professor Charles Bailyn’s ASTR 160: Frontiers and Controversies in Astrophysics:
Bailyn first reminds us of the scientific method and that astronomy is an observational science. He then talks about classification and the six categories of objects in the Solar System:
So, here are the six categories that I would claim exist in the Solar System. And here’s my problem with the whole Pluto debate. The Pluto debate was basically about whether these guys are going to count as planets. But the thing is, “planets” is already a bad description, because it contains two quite different categories; namely, these inner terrestrial planets, and the outer Jovian planets. So, it seems to me that arguing whether category five should be part of some category that already contains two fundamentally different kinds of objects is kind of a strange argument to be having. Either we should split these two things off from each other, or, if we’re going to join these two kinds of the categories, fine, bring in anything you like. I don’t care, add the asteroids, too. And, in fact, in the original proposal, one of the asteroids qualified as well. And so, it doesn’t seem to me that this controversy was really paying justice to an appropriate classification of the things in the Solar System.