Moomaw speaks

Bruce Moomaw's a freelance space science journalist who's a pretty sharp cookie. We've had some interesting discussions over the years, and if there's one thing I've learned about him, it's that he's passionate about space. We don't always agree, but he pretty much knows his stuff.

He went on a tear today on a mailing list I'm on, and I thought he outlined the problems with the new IAU definition of "planet" so well that I asked if I could repost here. He agreed, so now you good folks get to bathe in his starry-eyed wisdom:

So now we have a defintion which is fuzzy on THREE grounds: it retains the uncertainty about what is meant by something being "nearly" rounded by gravity; and it now adds to that uncertainty:

(1) Just how wide is the "neighborhood" that a planet is supposed to have gravitationally cleared out around itself?

(2) Just how much bigger must an object be than the other objects in its "neighborhood" (such as asteroids or Centaurs flying past it) for it to be considered the only "planet-sized object" in that neighborhood?

Christ. As Leo Durocher said during the Mets' nadir in the early 1960s, can't ANYONE play this game? Quite apart from the reaction of the general public to this game of Revolving Planets (and you need only look at the humor columns in the newapapers to see that), the astonishing fuzziness in this defintion raises genuine, honest-to-God questions about the scientific competence of planetary astronomers.

I see that several of the Johns Hopkins astronomers have homed in on the same points raised by Alan Stern, Ken Arromdee and (belatedly) myself. Is Neptune sufficiently bigger than Pluto (which crosses its orbit) that you can't say that the latter is also a planet? What about Sedna, which is a good deal smaller than Pluto but which (as Ken points out) is so far the only known object within a huge range of orbital distances from the Sun? If we find KBOs bigger than Mercury (a very real possibility), are we going to insist with a straight face that they are not "planets"? And what about the huge gas giants now known to be in very eccentric orbits around many other stars? It's likely that a few of them cross orbits, having settled into resonant orbital-period relationships that protect them from ever colliding (just like Neptune and Pluto's 3:2 resonance). Are we going to solemnly
declare that two such worlds are not planets because they're in the same "neighborhood"?

What the hell was the IAU THINKING? We need a pure and simple size-linked definition, and we need one with a sharp clear edge -- none of this nonsense about something being "sufficiently" gravitationally rounded. Such a definition could utilize mass, maximum or minimum diameter, or average diameter -- with the latter by far the easiest to measure. After that's established, it really doesn't matter that much to me if that average diameter is 1000 km, 2000 km, or 4000 km. But we need to get out of this silly mess as fast and neatly as possible before (to repeat) planetologists make themselves into public laughingstocks at a time when their funding is already in danger -- and this is the only way to do it.

Basically, my read is that they worked up this new, Rube Goldbergian criterion for planetary status solely to avoid taking the obvious route an setting an arbitrary size limit. They wanted to base it on measurable scientific factors rather than a human-created, arbitrary dividing line. Unfortunately, what they came up with was a messy slight-of-hand pushed by the anti-Pluto radicals that opens the door to far more problems than it can ever hope to solve. I think Bruce expresses those shortcomings quite effectively.

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