Sunday, August 27, 2006


Everyone realizes, of course, that the question of whether Pluto is a "planet" is not a well-defined scientific question, since "planet" is not a well-defined concept in the sense of proton and electron or even species (defined in terms of fertile interbreeding) and star (defined in terms of nuclear fission). It's a term of convenience that we like using to group together the "major" objects circling a star, inherently having fuzzy boundaries.

That being said, I thought Pluto should be excluded from the club, or more precisely that my sense of how the term "planet" is most conveniently used suggests excluding it. In addition to all the differences between it and both the rocky inner planets and the gas giants, the fact that a whole bunch of other objects, not easily distinguished from Pluto, may be out there, suggests to me that we would have too many planets from a convenience standpoint if we didn't give Pluto the old heave-ho. And calling the asteroid Ceres a planet, as the earlier definition would have done, seemed a real violation - again, just of useful and familiar language categories although not of any scientific principle (one can define "planet" however one likes).

But I see a serious flaw in the latest approach taken by the scientific panel, even though I agreed with the result. The problem was how they did it, which brings to mind a judge getting the "right" result in a case of first impression by creating new law that is bound to function poorly in future cases. They emphasized the idea of "dominance," meaning that the would-be planet must be much bigger than anything in its neighborhood.

The problem is that local dominance really doesn't capture what makes us think of some objects as planets. If the Moon were bigger, would we think the Earth wasn't a planet? If Jupiter were a double planet, would we want to boot it from the club? For that matter, does Charon's size have anything to do with the reasons for thinking that maybe we don't want to call Pluto a planet?

If some inexorable law of solar system formation dictated that anything we really wanted to call a planet would fortuitously have the characteristic of local dominance, the spurious use of this special factor to get the "right" result would be harmless error (resorting to legal jargon once again). But I see no reason to think this must be so, even if it generally tends to be. True, in our solar system the inner 8 planets are locally dominant. The Earth is by far the closest to being an outlier (and of course wouldn't be classified as locally dominant if we made the definition more demanding), and this is thought to reflect an extraordinary event - collision with a Mars-sized planetoid in the early years of our solar system. But again this bit of history only goes to show that strange things can and will happen across a range of solar systems (which we are getting ever better at detecting in outer space).

Use of the local dominance factor may have anomalous results as we learn more about the outer reaches of our solar system. E.g., suppose there are 20 locally dominant iceballs out there with eccentric orbits, barely Pluto's size, and one more that's 50 times the size of Earth, less eccentric in its orbit, and closer in, only it's a double planet. Would it be the only non-planet of the bunch? Plus, with all the other solar systems we may detect, the definition is unlikely to consistently give us what we "want."

Given the term "planet's" peculiar linguistic usage and status, it's probably a mistake for scientists to try to come up with a tight definition, even though this ordinarily is the way they do business. They've designed a rule (as in "the speed limit is 55 mph") when what they need is a standard ("unreasonably fast under the circumstances").


Stuart Levine said...

Dan--Stars are the product of nuclear fusion, not fission.

michael a. livingston said...

I think what bothers we most is the way the decision was made. It seems to me that Pluto either is, or isn't, a planet. Democracy is great, but I just don't see this as something that can be changed by majority vote, even if is by professional astronomers. Suppose, for example, that the status of Jupiter were in question and the vote was held in Rome instead of Prague. Would the voters be influenced by the large statues of Jupiter still on display in the city? It reminds me of the state legislature that tried to change pi from 3.14 to 3.00, by legislation. Of course, my grandfather never accepted that there was anything smaller than a molecule, and he was a lot smarter than I am.