A thoroughly sporadic column from astronomer Mike Brown on space and science, planets and dwarf planets, the sun, the moon, the stars, and the joys and frustrations of search, discovery, and life. With a family in tow. Or towing. Or perhaps in mutual orbit.

Ground rules for debating the definiton of "planet"

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.

What's in a name?

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.

Plutoid fever

Almost two years ago, during the same contentious meeting in which Pluto was demoted from a full-fledged planet to a “dwarf planet,” a few other votes were taken, but mostly forgotten. One of the forgotten votes that was actually approved was that Pluto was to be declared the” prototype of a new class of objects”. OK. Done. What exactly that means is a bit hard to say. As far as I could tell it was an attempt to be nice to Pluto after the indignity of its demotion. Who would vote “no” to that?
The next vote that was taken was about what to call this new class of objects. The proposal, if I remember correctly, was to call them “Plutonian objects.” The proposal was voted down by a very small margin. Why, again? Hard to say. People were in funny moods.
The class of objects, then, remained unnamed, with a promise – a threat – that a committee would come up with something and there would then be no vote.
The committee has spoken! After the close vote on “Plutonian objects”, the committee deliberated for almost two years and settled on “Plutoids” and now it is settled. A “Plutoid” is a dwarf planet (meaning it must be large enough to be round) that is beyond Neptune.
But wait! There’s more! The committee did more than promised! They added one more twist to the rule. While originally all dwarf planets beyond Neptune were to be part of this new category, the committee decided to restrict the definition to the brightest of the dwarf planets. For now the only ones that count are Pluto itself, as well as three of my babies: Eris, 2005 FY9, and 2003 EL61.
I have been asked: will there be controversy? Will there be bickering? Will people fight and contend?
I suspect the answer is, in fact, that there will mostly be nothing.
The class of objects was supposed to get a name, now it has a name. The name seems pretty non-controversial, if also a bit clunky.
The one thing that almost no one will even notice is the part that I find the most odd, though, which is the restriction that the object be a particular brightness. Not a particular size: a particular brightness.
That makes for some funny situations. If you take Pluto and cover it with dirt it would no longer be a Plutoid. Or take something much smaller and cover it was snow instead of rocks and it might be a Plutoid. Or, may favorite example, if you take Eris, which is currently the intrinsically brightest object, bring it closer to the sun (where it will be in 290 years), melt some of the ice on the surface, and exposure some of the darker substrate, it might just get dark enough to no longer be a Plutoid. Now you see it; now you don’t.
[a clarification, from Daniel Fischer]
But, OK, it’s a definition. And I can at least understand the committee’s feeling that they wanted to put a concrete brightness limit instead of a harder to determine roundness limit.
What does anyone else think?
There is still a small but extremely vocal group of astronomers who remain incensed about Pluto’s demotion. They will use this as a soapbox to repeat their initial complaints about Pluto.
Other astronomers are likely to yawn. Plutoids? Sure, why not. Most astronomers have moved well beyond the Pluto-debate and the semantics associated with it. If Pluto is happy being a Plutoid then it is probably OK with the rest of us.


mike brown, and his planets, are on vacation for the next 2 weeks.