Saturday, August 26, 2006

Dwarfing Pluto

First Law of Scientific Progress: The advance of science can be measured by the rate at which exceptions to previously held laws accumulate.
Corollaries:
1. Exceptions always outnumber rules.
2. There are always exceptions to established exceptons.
3. By the time one masters the exceptions, no one recalls the rules to which they apply.
~ quoted by Arthur Bloch, Murphy's Law, Book Three

The International Astronomical Union has spoken or, more properly, mumbled its long awaited definition of a planet. In the grand tradition of rules-making bodies, they ended up satisfying no one with a definition that had but one purpose: To exclude Pluto as a planet. Sort of.

What exactly did they come up with as a definition? As I noted in Devaluing Pluto, the original proposal would have made Ceres, Pluto, and “Xena” planets. It would also have added an arcane bit of reasoning that would make Charon a planet, too, because it didn't orbit around Pluto (both orbit a common center). Pluto would have been a planet, a member of a double planet system and a “pluton” (which turned out to be a term already staked out in geology, indicating a certain lack of research on the part of the committee). Then along came Julio Fernandez with his proposed amendments that included such rather vague statements as “by far the largest”, which served only to heat the debate further. If you want the gory details you can look here and here, but the bottom line of what they ended up with is this:
  • A planet must orbit the sun;
  • it must be massive enough for its gravity to force it to take on a nearly round shape; and
  • it must have cleared the neighborhood of its orbit of other objects.
Okay, so what disqualifies Pluto? It's in orbit around the sun, it's round, but it hasn't cleared its orbit of one particular object: Neptune. Pluto is not a planet because its orbit crosses that of Neptune. If you don't see what that has to do with the three rules above, join the club. It would seem that Neptune had not cleared its neighborhood very well, if Pluto is running around in it. Should we drop Neptune from the list?

After all that discussion, we're still left with terms like “nearly round” and “neighborhood”. As one astronomer pointed out, neither the Earth nor Jupiter have exactly cleared out their neighborhoods, what with all those near-Earth-objects people keep telling us are going to send us to the happy-dinosaur-hunting-ground. As for Jupiter, it's 50,000 trojan asteroids certainly make it hard to call it's neighborhood cleared.

Just to add a little more confusion to the picture, the IAU added two new classes of objects. “Dwarf planets” include Pluto, Ceres, and the zillion objects to be discovered in the Kuiper Belt. “Small solar system bodies” cover everything else orbiting the sun (seems like they could have said “everything else” and left it at that). Charon gets classified as a satellite, even though Pluto orbits Charon as much as Charon orbits Pluto, thus setting up an argument for another day.

For yet another dimension of silliness in all of this, this article, among others, makes it sound like the New Horizons mission headed for Pluto is somehow now in doubt. This is absurd. Pluto is still out there and worth studying as a Kuiper Belt object and the most distant object visited by a human probe. Besides that, there the on-again, off-again, on-again mission to the asteroids. Now that they are in the “everything else” category, are they somehow not worth studying? Good lord, don't anyone tell Michael “who needs science missions” Griffin. He'll drop funding for the asteroid mission again and insist that New Horizons be destroyed in space.

Michael Brown, discoverer of 2003 UB313, which he has called Xena, is ecstatic about the new definitions, because it opens up a whole new discussion on naming conventions for dwarf planets. He seems very concerned that there weren't enough Roman gods to use for naming trans-Neptunian objects, forgetting that Pluto was named for the Disney character; only later did they relate it to the god of the underworld. If we're calling them “dwarf planets”, there are loads of names available, like Sleepy, Sneezy, Dopey, Gimli, Tatoo, etc. Of course, what he really wants is for a naming convention that allows his rock to be called Xena. I presume Mr. Brown is counting on a visit from Lucy Lawless to thank him for the honor.

Many scientists are less than enthusiastic about this nonsense. They're using terms like “farce” and “absurd”. A “dwarf planet” is a planet. The name says nothing about the object and lumps highly dissimilar objects together. The definition is so vague that a black hole orbiting a star that it is devouring could be considered a planet.

What would have been so difficult about calling Pluto a “Kuiper Belt object” or even a “trans-Neptunian object” and leaving it at that? Better yet, the IAU could have waited until New Horizons got to Pluto and sent us back some real information on what it actually is. As I said, Ceres and Pluto are radically different objects; lumping them together is bad science. Pluto may be, as one scientist put it, more kin to a comet than to any of the eight planets. It's orbit and composition (so far as we can determine) come closer to that description than anything else.

The ultimate irony is that, aside from edits to textbooks, nothing really changes. Pluto is still out there and fascinating. New Horizons is still on its way to take a look at it (assuming Mr. Griffin doesn't dismantle the science team). And, once the fuss dies down, most people won't really care, and astronomers will go on with their researches and observations, calling Pluto whatever they want to.

And next for the IAU? I don't know; maybe they could work on clarifying the tie-breakers for the NFL playoffs.

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