Planet pr0n

Bad Astronomy
The entire universe in blog form
Aug. 13 2008 9:00 AM

Planet pr0n

Before we get to the pr0n, I want to remind you of a prize fight with Tyson.

Is Pluto a planet?OK, so not Mike Tyson, but Neil DeGrasse Tyson, astronomer and chief dude at the Hayden Planetarium. He'll be duking it out -- verbally -- with planetary astronomer Mark Sykes, and the topic will be how to define a planet, and it's all part of a bigger conference called The Great Planet Debate to talk about the current state of planetary science, and hopefully to hammer out a better definition of the word "planet".

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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The debate takes place Thursday, August 14th at 4:30 p.m. EDT and will be streamed live over the web (and recorded for watching later). Other parts of the conference will be streamed and archived as well, so take a look around the site to see what leading scientists are thinking.

I wish I could see it, but seeing as how I'm on a boat floating near the Equator, I'm at something of a disadvantage.

I'm pretty sure the debate between Tyson and Sykes will wind up inevitably be drawn into the topic of Pluto -- even if they start it off about planets in general, you just know it'll go there. It's a big part of what Neil is known for. He says it's not a planet, and Sykes, I suspect, will hammer away at the silly and arbitrary "planet" definition created out of whole cloth by the International Astronomical Union a couple of years ago. We know enough about Pluto to know whether it's a planet or not, but the problem is we don't have a definition for what a planet is. Well, we do, but it's silly.

I recently had lunch with Alan Stern, planetary scientist and Principle Investigator for the New Horizons Pluto mission, and our mutual friend and colleague astronomer Dan Durda. Alan and I agree on many things about planetary definitions. For example, under the current rules, for an object to be a planet it has to be able to gravitationally sweep clean its neighborhood. That makes the Earth a planet: any asteroid or whatever that has an orbit similar to Earth's will, over time, either hit us or get slingshot away by our gravity.

But if you take Earth and put it way out in the solar system, way past Neptune, the volume of space is so much larger that an Earth-sized body cannot sweep up all the material. In that case, we could literally move Earth out a few billion kilometers, and according to the new rules it would no longer be a planet.

And that's just dumb.

However, Alan and I had a short (but fun) discussion about the boundaries of such definitions. He says, for example, that a better definition is one where a planet is an object whose surface is substantially modified by gravity (note that this is already in the IAU rules). So, for example, an object made of iron and 500 km across has enough gravity that it will form something close to a sphere. But an object made of ice (which is softer than iron) will do that when it's substantially smaller; maybe 300 km across. So you can have two objects, both the same size, and one will be a planet and one won't. I think that's a bit silly, but as Alan and Dan said, at the boundaries you have problems.

I've been chewing that over, and now I think that's my very point. A definition is all about boundaries. How do you divide one thing from another? And with planets -- even with stars -- that boundary line is pretty fuzzy. Certainly, when you find a ball 12,000 km across, it's a planet. But what if it's 400 km across, and only kinda spherical? What do you call it?

Maybe, in the end, planets are like pornography (you were wondering when I'd get to that, weren't you?): you know 'em when you see 'em. But that's not very satisfying, is it? And if it's bad for a Supreme Court to say that, it's certainly no good for science. But I think that may be all we have, because making a definition for what a planet is will always be arbitrary, and the boundaries forever blurry.

On the other hand, after further conversation with Alan, I'm mulling over his point: the universe provides us with an incredible diversity of objects. Classifying them, labeling them, is a good way to be able to do a zoological analysis on them, if you will. When you look for similarities and differences across populations you start to see what makes things the way they are. Heck, I even gave talks on this when I was back at Sonoma State University: how would know the difference between mammals and reptiles if you didn't label them? And note that you recognize both as being subsets of animals. You may find weird things (platypuses) but they can help you understand the classifications better, divide up the different kinds of animals. What works for biology works for astronomy, too.

A major point I make is that given this, you have to be careful. By labeling something, that puts it in a box in your head. You might miss some interesting feature if you only examine the object in the box. That happened to me years ago with brown dwarfs, when I thought of them as failed stars, and missed the fact that they can have weather like planets do.

Also, it's hard to deny that the public plays a role in this. They pay the bills, and they want to know if Pluto, Ceres, Quaoar, and Eris are planets or not. I don't think our science should be driven by public opinion -- if they want Pluto to be a planet, that may be too bad if it doesn't fit the bill. But it would be kinda nice to give them an answer to that, one that can be easily understood. I'm willing to go along with this, provisionally, as long as all the dark dusty corners of the definition are examined. And we have to mind our borders.

So Alan has swayed me. Sortof. But I very much look forward to talking about this with him. That discussion really was a lot of fun, and there is a lot of territory yet to cover.

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