If you ask most astronomers what they think about the decision to demote Pluto from a full planet to a dwarf planet (and my discovery Eris along with it, don’t forget!) they will usually tell you that it is not important, that it is just semantics, and that the debate is overblown. And then they will proceed to tell you for the next three hours why they are right and everyone else is wrong.
I’m not going to do that. I am not going to tell you whether or not I think Pluto and Eris should be planets or not be planets. I am not going to say what I think about the term dwarf planet or the newest term Plutoid. I have pretty strong opinions about all of that, and could easily fill those promised three hours with them (because, of course, I am certainly right and everyone else is certainly wrong….).
Instead I’m going to address what I think are the bigger questions: Does it matter? Is it all just semantics? What’s in a name, anyway? Is it, in fact, true that a rose by any other name would indeed smell as sweet?
Let’s start by forgetting that the word “planet” ever existed and instead look at the solar system with fresh eyes. In almost every case that I can think of, the first thing that a scientist does when examining a new set of objects or animals or behaviors or phenomena is classification. Classification sits at the root of any scientific tree. Without classification everything is an individual with individual explanations and theories. Classification allows us to start to make sense of the universe around us.
Moving to the animal world, it is easy to think of some useful classifications. How about animals that walk versus animals that fly versus animals that swim? This system is a fine one to start with. But wait! What if someone comes along tomorrow and says that he prefers to classify animals as those that are herbivores versus those that are carnivores versus those that are omnivores? No problem! Depending on what aspect of the animal kingdom you are studying your classification scheme may differ. Animals with wings versus animals with fur versus animals with scales. Big animals versus small animals. Mammals versus reptiles versus birds. The possibilities are endless! Which one is right? You would never ask such a foolish question. You might ask the question of which ones are useful or which ones are meaningful, but never which one is right. If you are a scientist studying reproduction, you might decide that the most important category for you is egg-laying versus child-bearing, while your neighbor in the lab down the hall, who studies vocalization, might think the most important category is one based on which types of sounds the animals make. These are all good categorization schemes.
(One way to make a categorization truly bad, in my book, is to make up the rules for the categories and then not follow them. Imagine first deciding to split the animals into mammals, birds, and reptiles. And then declaring that dogs and cats belong with the bird family. The categorization system is OK, but the actual categorization is faulty. I’ll get back to this point next week when I talk about dwarf planets and Plutoids.)
Let’s move back to the solar system now and keep trying to forget the word “planet.” If you were someone who studies the solar system and you were ask to classify the objects in it, there are many different possibilities you might come up with. If you are interested in composition you might select rocky things versus gaseous things versus icy things. If you are interested in atmosphere you might select objects with thick atmosphere versus thin atmospheres versus no atmosphere. If you are interested in magnetic fields you could classify those with and those without.
As an astronomer who looks at the solar system through telescopes, I have in my mind a classification system that goes something like: objects that are so big and close that they are easily resolved with any telescopes, objects which are smaller, but resolved with the biggest telescopes on earth or the Hubble Space Telescope, and objects which are so small or far away that they appear only as points of light no matter what. Every time I sit down to consider a new astronomical project I make explicit use of this classification system.
The list is endless.
Which classification system is correct?
As with the animal classifications, this question is absurd and no one would ask it. Many different good and useful classification schemes can and should exist.
When the International Astronomical Union voted on the definition of the word “planet” it was not doing classification. The classification systems already existed. It was merely voting on which pre-existing system got to use the magical word “planet.”
Although there are an infinite number of classification systems one could devise, only two were seriously debated for the word “planet.” The first classification system that was discussed was objects that are round versus objects that are not round. While at first this seems a silly and arbitrary distinction, in one sense you could call the round versus not-round category as the “geologically interesting” versus “not-geologically interesting” divide (this statement will be disputed by the myriads of planetary scientists who study the geology of non-round objects, but I think that even they would, at least, understand the point I am trying to make here!). An object becomes round when it gets big enough that it begins to crush itself from its own gravity. This self-crushing can drive many interesting geological processes, thus the general feeling that round things have interesting geology, non-round things do not. By any reasonable estimate there are hundreds of such things in the solar system. No astronomer would (or should, at least) ever dispute that this is a useful classification scheme.
The second classification scheme that was discussed was large solitary objects versus collections of small objects. The large solitary objects are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The collections of small objects include the asteroids, mostly between Mars and Jupiter, the Kuiper belt objects, mostly outside of Neptune, and other miscellaneous interlopers, like comets. For those concerned with the formation and architecture of planetary systems this classification divides the objects in the solar system into the different groups which require different explanations (a better system would be to subdivide the large objects into two sub-categories -- rocky objects and gaseous objects – which require separate theories of formation). . No astronomer would (or should, at least) ever dispute that this is a useful classification scheme.
All of the important science of categorization is now done, and done correctly. All that is left to decide is who now gets to use the magical word “planet.” There is absolutely no scientific argument that anyone could possibly make to prefer one over the other. That would be akin to asking which one is correct. The answer is that they are both correct, and both useful.
Even most astronomers have missed this point. Some astronomers continue to attack and defend the planet definition on scientific grounds. They tend to try to obscure what they are really doing, which is trying to argue that one of the two classification schemes is better and the other flawed. Astronomers making such arguments are either being disingenuous or are simply not very thoughtful. Or perhaps both. There is even a conference being held this summer to discuss the “scientifically correct” definition of “planet” which is about the most nonsensical conference topic I can image.
So what is the world to do, and, again, does it matter?
I would argue that it matters critically. While astronomers (and I’ll include you astrologers in here, too) have an almost infinite number of ways of classifying the solar system, the vast majority of the public really only thinks about one. There are planets and then there are everything else.
I then ask a simple question: if the public is to have just one definition with which to try to understand the solar system, which is the best one to use? Which best captures the richness and complexity of the solar system? Which tells them the most about universe around them with just a simple word?
I have my own prejudice on what the right answer is here, but it is simply that: a prejudice. You can have a different one. Then, when it comes time to do the science, we can all revert to whatever classification is most useful for the problems we want to address.