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.

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.
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.

Your Saturday Newspaper

I don’t know about other newspapers, but every week my local -- the Los Angeles Times -- devotes about a half a page to a few science stories. I love these, not just for learning a little bit about the universe around us, but also for getting a quick glimpse into the life of some scientist somewhere finally getting his or her paragraph of fame. This week: estrogen may ease psychosis; mummified fetuses from King Tut’s tomb are going to have their DNA tested, a hidden tribe of gorillas was found, virus can get sick from other viruses, and Antarctica used to have moss. Having a paragraph or two of your science appearing in your daily newspaper is both exciting – “my research is interesting to the world” – and depressing – “I spent two years on this project and all that makes the newspaper is that schizophrenic women should take estrogen.”
The route from doing research to that Saturday paragraph is indeed a long one, and one of the important steps after the research is all completed is publication of the results in the right scientific journal. Scientific journals are not all the same. Some are trade journals that specialize in a specific field (I publish much of my research in, not surprisingly, “The Astronomical Journal”) and accept most of the papers submitted to them (after a sometimes lengthy review and revision process). Others are more general with the implicit promise that the papers published there are more interesting, more important, and will get more notice. And, of course, it is much harder to get a paper published in one of these. Reporters know which journals are those top exclusive general ones, and so, when looking for stories for that Saturday column, they peruse those journals (and read press releases, presumably) and never bother with the trade journals.
The two top general journals in which everyone seems to want to get papers published are Nature and Science. If you start looking at those Saturday columns you will be amazed by how many of the stories come from papers published there.
Interestingly, though, along with publishing important ground-breaking papers appears to come the requirement that a larger than usual fraction of the conclusions published in these journals turn out to be incorrect. This leads to the semi-joking line that you often hear amongst astronomers: “Just because it is published in Nature doesn’t necessarily mean that it is wrong.” But it also leads to the real ambivalence that some feel for results published in those journals. People sometimes consider them to be flash and hype with no real substance and turn their noses up at the papers published inside.
I’ve been known to fall into this camp myself.
So it might be amusing to know that, tomorrow, my hope is to have a new paper ready for submission to Science. Why would I do this when I then turn around and scoff at others? Hypocrisy, I think is the answer. Or, as my friend Caltech oceanographer Jess Adkins likes to say: Nature and Science are the People Magazine of science. And like People, no one wants to admit to reading it, but everyone wants to be in it.
And so I submit.
The process is an interesting one. By tomorrow, I should have the full manuscript describing all of the results and with a few figures demonstrating important points and a slew of references to previous work all written in the precise format demanded by Science. I’ll log on to their web site and do all of the submission there. And then I’ll wait.
I think the rest of the process goes something like this (I may have some of the precise details mixed up here, but you’ll get the general idea). The paper will first go to an editor in the field of astronomy/planetary science who will decide whether or not there is even a chance that the results are interesting enough to warrant publication in Science. If the editor doesn’t think so, I will get a rejection notice within days. If the editor likes it, the paper will go to an editorial staff meeting with all of the other editors where it can again be voted up or voted down. I can again get that rejection notice (this one would be in, perhaps, two weeks).
If the editorial board likes it, the editor will send it out for peer review. The manuscript will get mailed to (typically) two experts in the field who will offer their detailed advice on the technical merits of the paper and whether or not it warrants publication in Science.
As is often said: peer review is a highly flawed system, but it beats all of the alternatives. It is easy to imagine some of the problems that could arise at this stage. A few that I have run across: reviewers who are competitor, reviewers who just don’t like you, reviewers who aren’t knowledgeable enough about your field. With only two reviewers for most manuscript, the process can be thoroughly random. The same paper sent to twenty different reviewers would get twenty different reviews. But, sadly, it beats all of the alternatives.
With luck, the two reviewers will like the paper and, with even more luck, suggest ways to improve the paper. They may point in flaws in the paper and how those flaws can be fixed. Or they may point in flaws in the paper and say they are not fixable.
If the reviewers don’t like the paper sufficiently that is the end of the line and you are rejected. If they potentially like it but with reservations you are given a chance to respond to their suggestions or complaints and modify the paper accordingly and then they get to review it again.
Finally, again, with luck, the editor send you that email saying the paper is accepted, and you sigh from relief. Or you get the rejection email, and you decide what to do next: another general journal? Reformat to go to a trade journal? Sulk in irritation for a while? I have done all of these and more.
The whole process can take a long time. If I submit the paper tomorrow, there is a chance that it might appear in your newspaper in, perhaps, January. Keep your eyes peeled to that Saturday section. But don’t look for an article on dwarf planets or on the Kuiper belt or on the early solar system. I’m taking a break from those this summer to pursue research on what I think of as my hobby field, Titan. Titan is a fascinating world with methane lakes at the north pole, dark dunes at the equator, a thick atmosphere that is almost like the Earth’s (it’s just missing a minor component [oxygen] that we like so much), and a Los Angeles-like haze that makes the surface hard to see. It’s my favorite body inside the Kuiper belt, and, when I get a little tired of studying the little points of light that make up the Kuiper belt, I move in to the relative warmth of the Saturn system and see what’s new on Titan. During my break this summer I think I discovered something pretty interesting. Interesting enough to be published in Science, even. But we’ll all have to wait to see if I can navigate that laborious publication paper and make it to that one paragraph in your Saturday newspaper.