Astronomer Mike Brown

Nice lecture transcript of Dr. Mike Brown of "tenth planet" fame from Astrobiology.

He explains why, in his view, Pluto isn't a planet, and it is a somewhat compelling case, having to do with science teaching concepts, rather than definitions. He also does a nice job describing the debate over planets using an anology with the concept of continents in geology,and how in some respects the argument is a pretty silly one. No one argues about whether or not Europe is a continent (maybe it shouldn't be, but it is still listed as one).

"In astronomy and in most sciences, we don’t tend to focus on a precise definition for things. We focus on the concepts. The word “planet” now incorporates scientific concepts that are relatively new, but it was never meant to be a scientific definition. Given that there are many alternative ways to classify, not one of which means the word “planet,” it’s clear that the word “planet” is not a scientific word and need not be given a scientific definition.

Look at the word “continent” that geologists have – if you haven’t figured it out by now, the word continent actually has no scientific definition. There is no reason that Europe is a continent. Australia – just a big island. But why are they called that? Because culturally, Europe has always been a continent. I think they came up with the word so they grandfathered themselves in. India is a subcontinent -- does anybody know what a subcontinent really is? India -- that’s the answer. Indonesia is sometimes defined as a microcontinent. But geologists don’t sit around and argue, “Yes, Europe should be a continent.” “No it shouldn’t!” “Australia must go!” "

He illustrates his point further by utilizing a hypothetical alien visitor to our solar system. As you travelled closer and closer to our system, you would first notice the sun, our star, and then the four gas planets would become visible. The next objects you would notice would be the four terrestrial rocky planets, and you would probably make them a different category from the first four. As you got closer, you would then notice the swarm of objects (asteroids) between the two groups, and probably categorize them as something else, and as you got even closer, you would notice yet another group of objects, outside the four biggest ones, and make them another category. We call them Kuiper belt objects.

It's a compelling argument, which, of course, is still controversial, despite being "settled" to some degree. Personally, while I understand the argument and logic behind it, I still think that the alternate view (which Brown describes as round objects on which geologic processes take place), which gives us quite a number of planets, is also pretty compelling, particularly when you have smaller objects orbiting larger ones (like Pluto and Charon) in the "nonplanet" groups.