Mike Brown's Planets: classification

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Free the dwarf planets!

Most people will probably think of tomorrow as the 5 year anniversary of the demotion of former-planet Pluto. That seems fair; the Pluto demotion got all of the news, caused all of the fights, and promoted all of the discussion. But now that tempers have cooled and the world has come to terms with a new more scientific eight-planet solar system, it is time to remember the other important thing that happened on that day five years ago. On August 24^th 2006 the International Astronomical Union (IAU) defined a new class of objects in the solar system: the dwarf planets.

As you will recall, the IAU declared that planets are the objects which go around the sun and gravitationally dominate their orbits. In our solar system, the eight planets are unique in that behavior. But there are other much smaller bodies out there – Pluto being the most famous – that look like planets (simply meaning that they are round) but are not dominant. Pluto and many of these other objects all circle the sun in similar orbits in what is called the Kuiper belt. These objects are the dwarf planets.

At the time this new class of dwarf planets was proposed, the IAU also declared that three dwarf planets were then known: Ceres (the largest asteroid), Eris (the newly discovered largest Kuiper belt object which precipitated all of this mess), and Pluto. In the entire five years since then, the IAU has declared two other objects to be dwarf planets: Makemake and Haumea.

A reasonable person might think that this means that there are five known objects in the solar system which fit the IAU definition of dwarf planet, but this reasonable person would be nowhere close to correct. By my best estimate there are possibly 390 known dwarf planets in the solar system (don’t worry, I’ll explain below).

What is going on here?

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So is Pluto a planet after all?

The news last week that Eris might actually be a tiny bit smaller than Pluto led to the inevitable question: doesn’t this mean that Pluto should be a planet, after all? [update: the final analysis suggests that to the best of our ability to measure Pluto and Eris are the same size. No way to know which is bigger or smaller.]

The simple obvious answer to this question is no. Pluto was not demoted in 2006 simply because it was no longer the largest known object beyond Neptune, but because it was one of many many such small objects beyond Neptune. The fact that it might still be the largest gives it some bragging rights at the next dwarf planet convention, but – just like we never considered Eris a planet when we thought it larger than Pluto – being the largest known thing beyond Neptune doesn’t get you an invitation to the planet ball.

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Planetary Placemats

This morning Lilah started asking about Christmas. With her fourth birthday now more than a month behind her it seems to the natural thing to start contemplating. As a warm up, she started trying to remember presents from last Christmas.

“Daddy! Daddy! You gave me those baby scissors!” she exclaimed, running over to her little table sitting in the middle of the kitchen and pulling out a pair of 4-inch miniature yet quite sharp scissors. She uses these most every day, though we almost lost those on a plane ride to Seattle this summer when we unthinking brought them in carry-on. I was both relieved and flabbergasted when the security inspector pulled them out, looked at them, and decided that they were OK to bring aboard.

“But Daddy, where is my mat?”

Mat? Mat? What mat? I thought and thought until I remember, with a little shock, that for my own amusement, I got Lilah a plastic “Nine Planets Placement” for Christmas last year that had nice photos and facts of all nine (ahem) planets. It had gotten shoved under a pile of other placemats in a drawer, but I dug down, pulled it out, and Lilah cheered.

“The planets!” Lilah exclaimed.

“Ugh” I thought.

With the third-year anniversary of the demotion of Pluto having just occurred, I’ve been thinking a lot about planets again (or perhaps I should just say “still”). But rather than worrying about planet classification anymore, which I think is on pretty solid ground these days, I’ve been wondering about the people who simply can’t give up on the concept that Pluto simply has to be a planet. Why are they so attached to the 18^th largest object in the solar system when they probably can’t even name all of the 17 larger things? (try this at home: can you without looking it up?)

Lilah’s placemat drove home a likely part of the problem. Most people have absolutely no idea what the solar system actually looks like. They see pictures of planets of placemats, on lunch boxes, on walls at school, but none get the scale of the solar system even remotely correct. Why? First: it’s boring. The solar system is mostly empty space. How much empty space? If you were to draw a top-down view of the solar system from the center out to the edge of the Kuiper belt, it would be 99.999999% (that’s 8 nines, if you’re counting) empty. And 99% of that non-empty fraction is taken by the sun. Making a placemat with that much empty space is pretty dull (though presumably you would save on printing costs). I would show you here what it would look like, except that you would need to view it on a monitor with 12,000 pixels across (about 10 times your typical laptop screen). The sun would occupy only one pixel in the center. You’d see nothing else. If you had grown up with a picture of the real solar system on your placemat, you would be forgiven for thinking the number of planets was precisely zero.

We need a better placemat. What’s the solution? There is no choice except to dispense with trying to depict both the distances between planets and the sizes of planets on the same scale. You can do a little better if you shove the sun almost out of the frame, keep the relative distances between the planets correct, and exaggerate the sizes of planets by a factor of about 8000.

(The Kuiper belt with Eris and Pluto and the rest is really there, way off on the right side. Try squinting.)

This solution still does not make for a great placemat. It’s still mostly empty space, and most things are too small to see well. If Jupiter is going to fit on your placemat at all (and let’s not even talk about the sun), Mercury is going to be so small that it will look like just a tiny dot (and, again, let’s not even talk about Pluto, which is half again smaller). If you had grown up with this placemat you would probably have a lot of respect for Jupiter and Saturn and wonder why everyone made such a big deal about the rest of them.

As a placemat maker, there is one other step you can take while still maintaining scientifically integrity. You can keep the planets in the right order, but give up on showing their true distances from each other. Shoving the planets together a bit more allows them all to be somewhat bigger. Now you can even make out Ceres, the largest asteroid. The band of tiny Kuiper belt objects begins to be visible.

“Alas!” cries the honorable maker of placemats. “How am I to put any artwork on tiny disks that size? What of the canyons on Mars? The scarps of Mercury? The mottled face of Pluto?”

There is a solution, of course. Forgo almost everything. You’ve already had to throw away the correct relative spacings between planets to make the placemat more interesting. Now also throw away the correct relative sizes! Make Jupiter and Saturn significantly smaller, make the tiny tiny terrestrial planets significantly bigger. Grossly exaggerate the size of puny Pluto. This is the perfect solution. This is Lilah’s placemat.

I find this solution perfectly awful.

My objection here is not the inclusion of Pluto as a planet (that’s just anachronistically cute, sort of like ‘here be dragons’ on an old map), my objection is that everything about the solar system is so wrong that of course people are going to be generally confused. How could astronomers possibly vote to get rid of Pluto when there it is, as big as Mercury, nearly as big as the earth itself?

Just how bad is it? If you take Jupiter to be the right size and scale everything from there, Mercury, Venus, Earth, and Mars should be 6,4,4, and 5 times smaller, respectively. The smallest ones, Mercury and Mars, are the most exaggerated.

The giant planets are a bit odd. Saturn is actually 80% too small, presumably because its rings take up too much space to be aesthetically pleasing. Uranus and Neptune are 1.2 and 1.5 times too big, respectively.

And Pluto? It remains the runt even in this solar system, where its size is exaggerated by a factor of 10.

If you grew up with a placemat like this, or a wall poster in your third grade classroom, or a lunch box you carried every day, I now understand why you feel Pluto still deserves to be a planet. It’s because you and I are talking about entirely different solar systems. Even I would agree to Pluto’s special place in the solar system of Lilah’s placemat. Sadly, that solar system and the real solar system have little in common.

I do have a better solution for the placemat makers out there. It keeps the relative sizes of planets correct and keeps their ordering correct, but, like all of the ones above, it has to dispense with the relative spacing between planets. The trick, though, is to pile the planets on top of each other, and to not even show all of the monster Jupiter. You can pack much more into the frame, like this.

There is room on this placemat to put real depictions of the planets. And you can even see many of the dwarf planets out in the Kuiper belt. If you look carefully, you can see the elliptical Haumea and you might even be able to identify a few other of your favorites.

Imagine a world in which this was the image that children – that adults! – had of the solar system. Would we even be having conversations about Pluto’s planethood? It seems pretty unlikely to me. Rather, we would talk of the great difference between giant planets and terrestrial planets, we would talk of the band of asteroids, and we would talk of the ever-increasing number of tiny icy objects out there on the very edge of the solar system. In short, we would talk science rather than definitions. But, occasionally, we would remember the old solar system of our youth and talk nostalgically to our children, and say “when I was your age, Pluto was still a planet” and then, when our child looked up quizzically, we would look down in the corner of the placemat, and try to point out the former planet amongst those many many objects and realize that we had absolutely no idea which tiny point it even was.

Don't try to blame it on Rio

Three years ago, in Prague, astronomers had perhaps the most contentious gathering in modern astronomical history. Usually the International Astronomical Union meeting is nothing but a once-every-three year's chance for astronomers to advertise their latest discovery or newest idea while spending some time in a nice international destination, having dinners with old friends and catching up on their celestial gossip. On the final day of each meeting, in a session attended by almost no one, resolutions are passed, usually all but unanimously, on such pressing topics as the precise definition, to the millisecond, of Barycentric Dynamical Time (I have no idea what this actually even means, but, presumably, it matters critically to someone).

The meeting three years ago was different. The usually placid astronomers had spent their time in Prague arguing and bickering day and night about Pluto and about planets and about how to reconcile the two. While several of the typically unintelligible resolutions were indeed to be voted on this last day, the final two resolutions would be all about Pluto. The usually sparsely attended final session was full of surly astronomers itching for a fight.

Everyone by now knows the outcome. In an overwhelming vote, astronomers agreed to tighten the definition of the word “planet” to mean, essentially, the large dominant things in the solar system. Which Pluto is not. That was the end of Pluto as a planet. That was the end, too, of Eris (the slightly-larger-than-Pluto iceball that I discovered that had precipitated the mess) as a planet.

The solar system makes more sense now, and, in three years most people have come to terms with the new solar system. But not everyone (as an example, read the inevitable comments soon to follow this post….).

Three years ago, when the vote put Pluto into its proper place, some vowed to carry the fight forward. “Onward to Rio!” they cried.

It’s three years later, and it is time, once again, for the Internation Astronomical Union meeting, this time in Rio de Janeiro The meeting. It starts tomorrow and I am currently staring out across Copacabana beach watching the winter waves pound the shore (Winter? Only sort of. The beach is packed and the clothes are skimpy) waiting for the real fireworks to begin.

But will there be any? There is an entire weeklong program called “Icy Bodies in the Solar System” with talks about the Kuiper belt, comets, icy satellites, dwarf planets, and even one talk about Pluto. But nowhere is there slated to be any official discussion about Pluto. Will it happen anyway? Will the partisan defenders of Pluto try to storm the meeting in protest to finally have their day in court?

It would be the right forum, for sure. Because of the special program, many of the astronomers who think deeply about planets and the outer solar system are here. Why not ask?

I predict that by the end of the week, the topic of Pluto and planets will come up, at best, only outside of the actual meeting over a few glasses of caipirhina. I suspect no one will press the fight about Pluto because even the partisans are reluctantly admitting to themselves that the fight is over and planets have won.

After the vote three years ago the Pluto partisans tried every trick and argument they could come up with to convince people that the IAU vote had been ill conceived or procedurally wrong, or poorly attended or anything else. All of that could have been fixed by now and a new vote could be taken. Except that Pluto would lose again. And new excuses would be needed (how about: the moon was full and astronomers became lunatics!).

Will they actually give up? I suspect not. It’s easy enough to keep the controversy alive in the media long after most serious scientists have moved on to better things. But I think the fighting will all be guerilla style these days.

But don’t give up hope! Perhaps something will unexpectedly spill into the open and Rio can turn into a place as fun as Prague. Stay tuned….

For entertainment, and should anyone care, I am (sigh) tweeting the IAU meeting. You can follow me at, appropriately enough, http://twitter.com/plutokiller

What is a dwarf planet?

Now that the IAU has officially declared the fifth dwarf planet (in order of size: Eris, Pluto, Makemake, Haumea, Ceres), we are likely in for a dry spell on new dwarf planets. The preliminary searches of the sky are all but complete, and (as far as I know) no one has any new objects the size of Haumea hiding in their back pockets. We'll probably be at five official dwarf planets for a while.

Now is a good time, then, to remind ourselves what a dwarf planet really is.

When the final vote on the definition of "planet" was made, and the eight dominant bodies in the solar system were declared (quite rationally) a class separate from the others, a new class of objects was defined. The "dwarf planets" are all of those objects which are not one of the eight dominant bodies (Mercury through Neptune) yet still, at least in one way, resemble a planet. The best description I can come up with is that a dwarf planet is something that looks like a planet, but is not a planet. The official definition is that dwarf planets are bodies in the solar system which are large enough to become round due to their own gravitational attraction.

Why do astronomers care about round? If you place a boulder in space it will just stay whatever irregular shape it is. If you add more boulders to it you can still have an irregular pile. But if you add enough boulders to the pile they will eventually pull themselves into a round shape. This transition from irregularly shaped to round objects is important in the solar system, and, in some ways, marks the transition from an object which is geologically dead and one which might have interesting processes worthy of study.

[Haumea is, of course, not round, but that is only because it is spinning so fast. If you stopped it spinning it would become a sphere. That still counts.]

So how many dwarf planets are there? Five, of course. The IAU says so.

But let's ask the more scientifically interesting question: how many (non-planet) objects in the solar system are large enough to be round due to their own gravitational pull?

Still five, right?

Well, no. Here is where the IAU and reality part ways.

There are many more objects that precisely fit the definition of dwarf planet but that the IAU chosen not to recognize. But if the category of dwarf planet is important, then it is the reality that is important, not the official list. So let's examine reality.

So how many dwarf planets are there? Ceres is still the only asteroid that is known to be round. After that it gets complicated. All of the rest of the new dwarf planets are in the distant region of the Kuiper belt, where we can't actually see them well enough to know for sure if they are round or not.

While we can't see most of the objects in the Kuiper belt well enough to determine whether they are round or not, we can estimate how big an object has to be before it becomes round and therefore how many objects in the Kuiper belt are likely round. In the asteroid belt Ceres, with a diameter of 900 km, is the only object large enough to be round, so somewhere around 900 km is a good cutoff for rocky bodies like asteroids. Kuiper belt objects have a lot of ice in their interiors, though. Ice is not as hard as rock, so it less easily withstands the force of gravity, and it takes less force to make an ice ball round.

The best estimate for how big an icy body needs to be to become round comes from looking at icy satellites of the giant planets. The smallest body that is generally round is Saturn's satellite Mimas, which has a diameter of about 400 km. Several satellites which have diameters around 200 km are not round. So somewhere between 200 and 400 km an icy body becomes round. Objects with more ice will become round at smaller sizes while those with less rock might be bigger. We will take 400 km as a reasonable lower limit and assume that anything larger than 400 km in the Kuiper belt is round, and thus a dwarf planet. We might be a bit off in one direction or another, but 400 km seems like a good estimate.

How many objects larger than 400 km are there in the Kuiper belt? We can't answer this question precisely, because we don't know the sizes of more than a handful of Kuiper belt objects, but, again, we can make a reasonable guess. If we assume that the typical small Kuiper belt object reflects 10% of the sunlight that hits its surface we know how bright a 400 km object would be in the Kuiper belt. As of now, about 50 objects this size or larger are known in the Kuiper belt (including, of course, Eris, Pluto, Makemake, and Haumea). Our best estimate is that a complete survey of the Kuiper belt would double this number, so there are roughly 100 dwarf planets in the Kuiper belt, of which 50 are currently known.

The new dwarf planets in the solar system are very different from the previous 8 planets. Most are so small that they are smaller across than the distance from Los Angeles to San Francisco. They are so small that about 30,000 of them could fit inside the earth.

Does it matter how many dwarf planets we say there are?

I think the answer is "yes." If you believe that there are only 4 dwarf planets in the Kuiper belt then you place an oversized importance on those 4 objects and you get an exceedingly warped picture of what the outer solar is like. The important thing about the Kuiper belt is that beyond Neptune there are many many many objects with hundreds being large enough to be round. The four "IAU Dwarf Planets" in the outer solar system are all fascinating objects -- hey! I discovered 3 of them, I must think there are at least a little interest -- but it would be a gross exaggeration to think of them as the only objects, or even the only important objects, in the fascinating region of space beyond Neptune.

The great planet debate wasn't

Last week, in Baltimore, at the conclusion of a conference about planets and definitions, two astronomers faced off in what was termed the Great Planet Debate.

I missed the conference, and thus missed the debate, but, nonetheless, courtesy of a press release supplied by one of the participants, I can already declare a winner by default.

As I have said earlier, there is important science in classification, and that science is really not much of a subject of debate. Everyone can agree which objects in the solar system are dynamically dominant. Everyone can agree which are round. Everyone can agree which are rock or gas or ice. The only debate is about which of the many different important classification schemes should get to use that magical word “planet” to describe its members. And that debate is merely aesthetic, not scientific. So the “Great Planet Debate” is merely a debate about aesthetics, which I guess is OK, but, in my opinion, unlikely to be terribly Great.

But, according to the press release, the astronomer who was arguing against the current 8 planet definition wants, instead, to use a definition that says that anything round is a planet, and thus there should be 13 planets.

STOP!

Suddenly there could be a scientific debate here, and this astronomer should be crushed. Everything round is a planet and there are thirteen round things? Where did that come from?

The planets would be the familiar Mercury through Pluto, for nine. Ceres, the largest asteroid, makes ten. Charon’s moon makes eleven, and my two discoveries, Eris and Makemake, make 12 and 13.

Regardless of your opinion of whether or not this is a fitting definition of the world planet, this is bad classification, and thus bad science.

So how many round things are there?

We don’t actually know the answer to that, since most of the objects in the Kuiper belt are so far away that we can’t see their shapes. Pluto and Charon have been measured to be round, so they count. Eris is assumed to be round because it is more massive than Pluto. Makemake has a poorly measured size and no known mass (it has no moon, which is the only way to measure a mass), but it is big, so probably massive, so probably round.

So what about other objects in the Kuiper belt? We can’t see them well enough to determine whether they are round or not, but we can estimate how big an object has to be before it becomes round and therefore how many objects in the Kuiper belt are likely round. In the asteroid belt Ceres, with a diameter of 900 km, is the only object large enough to be round, so somewhere around 900 km is a good cutoff for rocky bodies like asteroids. Kuiper belt objects have a lot of ice in their interiors, though. Ice is not as hard as rock, so it less easily withstands the force of gravity, and it takes less force to make an ice ball round. The best estimate for how big an icy body needs to be to become round comes from looking at icy satellites of the giant planets. The smallest body that is generally round is Saturn's satellite Mimas, which has a diameter of about 400 km. Several satellites which have diameters around 200 km are not round. So somewhere between 200 and 400 km an icy body becomes round. Objects with more ice will become round at smaller sizes while those with less rock might be bigger. We will take 400 km as a reasonable lower limit and assume that anything larger than 400 km in the Kuiper belt is round.

How many objects larger than 400 km are there in the Kuiper belt? We can't answer this question precisely, because we don't know the sizes of more than a handful of Kuiper belt objects, but, again, we can make a reasonable guess. If we assume that the typical small Kuiper belt object reflects 10% of the sunlight that hits its surface we know how bright a 400 km object would be in the Kuiper belt. Currently there are about 60 objects this size or larger in the Kuiper belt (including, of course, Eris and Pluto and Makemake), and one (Sedna) in the region beyond the Kuiper belt.

We have not yet completed our survey of the Kuiper belt. Our best estimate is that a complete survey of the Kuiper belt would double this number. For now, the number of known objects in the solar system which are likely to be round is about 70, with the number increasing as the survey of the Kuiper belt is completed.

Beyond the Kuiper belt there may be even more dwarf planets than in the Kuiper belt. Our best guess is that the region where Sedna resides could contain another ~2000 round objects.

So the victory in the Great Planet Debate goes, by default, to the 8 planet side. Whether or not you like the aesthetics of the 8 planet side, you have to disqualify the everything-round-is-a-planet side for thoroughly mangling the science of their own classification scheme. This is not to say that an 8 dynamically dominant planet definition is better than a ~70 round planet definition, but there can be no debate that an 8 planet definition is vastly superior to a 13 planet definition based on bad scientific classification.

How can this fundamental mistake have been made? Surely if you believe in the utmost importance of things being round, you would at least try to understand what was round and what was not, right? My speculation (some would say “paranoid speculation”) is that this was done on purpose. There is no doubt that the astronomer arguing the everything-round definition knows that there are many other round things. So why would he pretend there were not? Because, I suspect, he knows that arguing for 13 planets sounds more palatable than arguing for 70 planets. Arguing for 13 planets makes it seem like stingy astronomers are just being mean to the 4 being excluded. Arguing for 70 makes you seem a bit of an extremist.

There are good aesthetic arguments that can be made for the 70 planet everything-round definition. Make them! Argue them! Have a lively aesthetic debate! But don’t start by getting the science wrong. Particularly if it is being done on purpose.

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”

Scientific

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.

Cultural

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).

Procedural

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.

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