Pluto's Planethood: What Now?
Two leading scientific experts debate whether eight is enough.
- By airspacemag.com
- Air & Space magazine, September 2006
(Page 4 of 5)
I disagree that it’s difficult to teach. I have been having a lot of discussions about this, and I find [the IAU definition] not only easy but delightful to teach. I feel like I finally get to teach people the structure of the solar system—how planets form, why there are belts, why there are individual solitary bodies. I love the question, "What about the Near Earth Asteroids?" You can talk about dynamically unstable orbits. What about Trojan asteroids? You can talk about how Jupiter has captured the Trojans at a Lagrangian point. The quibbles are where the science gets interesting. Any definition that is so clear that there’s no room for discussion, is, I don’t think, a very interesting definition.
Stern: It’s not true that I’m unhappy with dynamics as a criterion for planet classification. I co-wrote an article after the 2000 IAU meeting with Hal Levison, who’s a dynamicist, where we coined the terms uber-planets and unter-planets in a jocular way, because we didn’t want to use pejorative terms like "major" and "minor." I do think from a dynamical standpoint, there are planets that really matter in the architecture of the solar system, and those that don’t. They’re both planets. Just as you can have wet and dry planets, or life-bearing and non-life-bearing planets, you can have dynamically important planets and dynamically unimportant planets.
I would make a term for "planetary bodies." And anything that’s large enough to be in hydrostatic equilibrium would qualify. Then, if it were orbiting a star, I would call it a planet. If it were orbiting another planet, I would call it a satellite that’s a planetary body. And if it’s ejected from the solar system— which we expect has happened although we haven’t detected objects like that—I would call it a "free floater" or "rogue planet." But they’re all planets.
With stars, we don’t classify whether something’s a star based on whether it’s in a cluster or it dominates a region of the galaxy. It’s a star because of its intrinsic properties. In astronomy objects are typically defined by what they are, not where they are or what they’re near.
Brown: I’d be much happier with Alan’s physical definition than the one the IAU initially proposed. I thought their initial proposal was philosophically flawed in that you had things that were planets only if they were in orbit around a star. But if you put it in orbit around another planet, it suddenly becomes a satellite. If you really go with the physical criteria, you don’t care where it’s located.
A&S: What’s the right number of planets? The draft IAU definition would have opened the door to dozens of planets, or even a couple hundred, in our solar system. Would that bother you? Is it important to have a number that can fit on ten fingers, or in an easy rhyme?
Brown: The ability to memorize the planets is not important. So if the number is 50 and kids couldn’t memorize them, it’s actually not the important criterion here. I don’t understand the complaint that this is unteachable. To any level of sophistication, you can explain the difference between—Alan has described it perfectly himself—the large, important bodies in the solar system and the ones that aren’t important. The problem is the way it happened at the IAU meeting. The majority wanted a dynamical definition, but were not given the time to come up with a precisely worded one. Most of the people in the dynamical camp really did not want the word "dwarf planet," but that was forced through by the pro-Pluto camp. So you’re left with this ridiculous baggage of dwarf planets not being planets.
Stern: I’ve heard a lot of my colleagues say "we just can’t have a lot of planets." I’m not of that opinion. I think nature gives us whatever it gives us. There used to be a countable number of stars. Now there’s an uncountable number of stars. We don’t limit the number of stars to make it easier on ourselves or fit that old notion of Arabic astronomy. If it’s really 50 or 500, to many of us it doesn’t matter. But there are clearly people on both sides, even in the scientific community.