To study any subject scientifically one needs a detached attitude which is obviously harder when one's own interests or emotions are involved. ~George Orwell
I figured we wouldn't be hearing much on the subject of defining the term “planet” for some time, once the sound and fury of reaction to the IAU decision died down. Oh, there was a petition, and Owen Gingerich was going to have his own conference to create a definition, presumably based on the one he originally helped create for the IAU. Mr. Gingrich was very upset because he had chaired the committee charged with coming up with a draft definition, which was no great shakes, but at the International Astronomical Union (IAU) session to decide the final definition, a new proposal was put forth and voted in.
Just to catch you up, the original draft definition read like this: A planet is an object that has enough mass to be round, is in orbit around a star, and is itself not a star or a satellite of another planet. There were also sub-categories in the Gingerich proposal, including “plutons”, a term that was already taken by the Geology crowd. It also promoted Charon to a planet, which promised to be confusing at the very least.
The final “approved" definition ran this way: 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. This verbiage was deemed to have dropped the planetary roll call to 8, although a technical reading of the “clearing the neighborhood” bit could have disqualified Earth and Jupiter, and possibly Mars and Neptune, as well.
I call that a problematic definition, only because I'm using polite words here.
After all the fuss, everything seems to have quieted down rather abruptly. The Gingerich conference hasn't generated any news that I've seen, so I'm not even sure it's going to happen. Meanwhile, the IAU meets again in 2009, when it's presumed they will take up the issue again to try to regain some sense of authority in astronomical matters.
Robert Roy Britt over at Space.com has written rather boldly that there will never be a solid astronomical definition of the term “planet.” I find that disquieting in some ways, but he may have a point. Basically, there's a lot of stuff wandering around in the universe. There are stars, comets, large round gaseous things that have never ignited that are called brown dwarfs or planets (based vaguely on size), rocky round things that are either called planets or moons (depending on what they orbit), smaller round rocky things called dwarf planets (currently), ice round things called ice dwarfs (currently), and lumpy littler things called variously asteroids, Kuiper Belt objects, near-Earth objects, or moons (depending on where they happen to be). As Geoff Marcy says in the article, “Categorizing a thing does not magically add insight.”
True as far as it goes, but categorizing things does force us to attempt to pin down the properties of that thing. That's where the definition of an object comes in. By putting something into the “comet” category, say, we pretty much identify it as a member of a class of things that have a lot of ice and appear to be fairly loosely put together, which put off tails as they approach the Sun because the ices begin to melt and spew out of the comet.
But even comets are problematic. We've had a close look at a few of these interesting things, including crashing a probe into one. One of the Deep Impact scientists put it baldly when he pointed out that, so far, no two comets have been the same. So even something as well-known as a comet is still an unknown quantity in many ways.
Do we need a new definition of comets? Probably not, but we might ultimately need some subdivisions, such as rocky comets, sandy comets, and ice-predominant comets.
So what's wrong with subdivisions of planets? Not a thing, as I pointed out in earlier articles (see links below). What's wrong is creating ill-defined boundaries that tell us nothing about the objects. Lumping Pluto and Ceres into the same group (dwarf planets) is like lumping a dump truck and a boat together. The truck and the boat are both “vehicles” that carry passengers and cargo, but you need a finer grading to actually be able to tell what the major properties of both are. For example, dump trucks don't float worth a darn. That's important if you're trying to decide which one to take to the lake.
That's what is wrong with the IAU's definition. What was wrong with Gingerich's approach was that the overall category of “planet” was that it danced around the question, creating all sorts of ill-defined subcategories that still didn't tell you what would float and what wouldn't.
The exotic objects we're finding orbiting distant stars are just adding to the confusion.
I'm not enough of a scientist to come up with a definitive answer to the question of describing what constitutes a planet or what subcategories make sense. I think, though, that somehow the subdivisions should “add up” to the whole. That is, things like dwarf planets, ice dwarf, asteroids, and Kuiper Belt objects have a place in the evolution of the Solar System. Any set of definitions should not only separate these and other objects into groups, but they should also show how they are interrelated. Nothing I've seen from Mr. Gingerich or the IAU or anyone else seems to address this.
It's too easy to fob off the problem by saying we'll never come up with a good definition or, worse, saying we don't need a good definition. Science is not about taking the easy way out. It's time for astronomers to get their heads on straight and deal with the issue.
To borrow from Bette Davis, Science ain't for sissies.
Some earlier explorations into the subject: