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The Great Planet Debate Wasn't
By Mike Brown

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.

Planet Waves
Mike Brown.
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 eight-planet definition wants, instead, to use a definition that says that anything round is a planet, and thus there should be 13 planets.


Suddenly there could be a scientific debate here, and this astronomer should be crushed. Everything round is a planet and there are 13 round things? Where did that come from?

The planets would be the familiar Mercury through Pluto, for nine. Ceres, the largest asteroid, makes 10. Charon's moon makes 11, 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 word 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 eight-planet side. Whether or not you like the aesthetics of the eight-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 eight dynamically-dominant planet definition is better than a 70 round-planet definition, but there can be no debate that an eight-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 four 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.

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