Last week I argued that how you categorize things matters, but there is almost never only a single good way to make categories.
In the case of the solar system, the debate has mostly been over whether to use a geological classification to distinguish planets from non-planets (if you’re big enough to be round you’re a planet [with caveats] and if you’re not you’re not) or to use a dynamical (or, as I prefer to call it, population) classification (if you’re big enough to be mostly solitary you’re a planet, if you’re part of a larger population you’re not). Much of the debate and complaint has been quibbling about the detailed wording of these classification systems. Precisely how round would you have to be? What about the fact that Neptune’s orbit is crossed by Pluto (and many other Kuiper belt objects)? Isn’t the wording of [either definition] somewhat flawed?
T o which I simply want to say: any astronomer who claims that either of these classification schemes is inherently bad or makes no inherent sense needs to turn in the eyepiece to his telescope.
In the face of many equally good classification schemes, how, then, is a scientist to decide where to put the word “planet”?
No scientific solution exists, because the question is not a scientific one.
What to do?
One rational approach, it appears to me, would be to let an international body which officially sanctions the naming of things in space (also a very non-scientific endeavor) make the decision. The appropriate body would be the International Astronomical Union (IAU), and, in fact, that’s the body that voted to accept the dynamical/population definition mentioned above that leaves us with eight planets.
I support that decision.
I would also have supported a decision by the IAU that the hundreds of round things out there are planets (the geological definition).
I have opinions, of course. I think that the dynamical definition does a more appropriate job of educating the public about the large important bodies in the solar system and their relationship to the vast populations of smaller bodies (the asteroids, the comets, the Kuiper belt objects). And I think that culturally a smaller number (eight) of mental waypoints is more helpful than a larger number (say, 100). So I would (and did) argue strongly against the geological definition. But I could have worked with it.
Because the IAU did, in fact, vote, and we do, in fact, have the word “planet” assigned to the category that includes the largest eight bodies in the solar system, I generally consider the debate about Pluto (and about Eris) closed. I wish we could all move on to understand how these new discoveries past the edge of the planets are changing our view of what the solar system is all about.
But we can’t. The debates still go on. And if they are going to go on, I would like to propose the following:
Ground rules for debating the definition of “planet”
- Discussing different possible ways to categorize bodies in the solar system is interesting. The geological and the dynamical/population categories get all of the attention, but there are many many other interesting ways of looking at the solar system. Scientific contribution welcome!
- Claiming that there are scientific reasons why any one of them deserves to be the category of “planet” should disqualify you from further discussion on the grounds that you are conflating the job of science and the job of culture.
- Proposing a classification system that is purported to be for scientific reasons but which is scientifically inconsistent (see original IAU proposal that gave 12 planets, as an example) should cause you to go back to school for remedial science training.
- Discussing the cultural (educational, emotional, etc.) pros and cons of the word “planet” being applied to different classification systems is perhaps the most important debate that needs to happen. Once the categories are defined, this debate is purely cultural; the scientists have made all of the scientific contributions they can at this point. They’re still allowed to speak (I guess….) but it is not clear whether their words should be afforded any more weight than someone else with intelligent comments to make.
- Discussing the larger question of whether or not the word “planet” need be applied to any scientific classification system is also a discussion worth happening (we could, for example, cut the scientists out entirely and culturally say that there are 9 planets just because we say so, and not worry about the science, much like we do with the definition of “continent” here on earth).
- Many interesting questions to ask. Who decides how such a word is to be used? If it is to be scientific is there any good reason why the IAU should not be making such decisions? If it is not to be scientific, does anyone make those decisions or does society just gradually adopt practices?
- Misleading statements about the previous vote should also be disallowed. Yes, the whole IAU procedure was a bit mucked up, but the results would likely have been the same no matter who was in the room at the time. Surveys done after the IAU vote – yes there were some! – showed that astronomers by a large number thought that the 8 planets definition was a good one. So complaining about the IAU vote gets you the label of “misinformed about how most astronomers think.”
I’m sure I’ve left out some good ones, or overstated some that are not so good. Comments/debate welcome. Over the coming weeks, I will flesh these out more, and propose my own answers to some of these.