So, what do you think about Pluto being demoted?

This is the question I've been asked by no fewer than a half-dozen friends and family. Presumably, they think I must have an opinion on the subject since I'm probably the biggest space-nut they know. Well, it just so happens that I have been following the debate, and I have formed an opinion.

This issue has been hashed and rehashed for weeks in the media so I will not recount all of the arguments for or against retaining Pluto as a planet. My opinion on the matter is not very surprising or revolutionary, but it is ultimately pragmatic.

When asked how I feel about the IAU's decision to redefine the word planet, I turn the question around ask the person what they think a planet is. The responses vary only by a little, but it seems that everyone already has a sense of what a planet is. This speaks directly to the difficulty that the IAU will have in trying to get their new definition accepted. The word is already in common usage, and is already known by nearly everyone on the face of the Earth. While the new technical definition of planet may serve to resolve some issues amongst astronomers, everyone else will be left scratching their head.

"Ok, large body which orbits the Sun. Check."
"Um, does not orbit another larger nearby body. That would be a moon. Ok, check."
"Has managed to clear out its orbit? Huh? What the heck does that mean?"

And there is the problem. That last little criteria (which I'm sure was thrown in there to exclude Ceres, Vesta, 2003 UB313 (a.k.a. Xena) and other similar bodies) seems like an ad hoc kludge, or worse still an arbitrary requirement meant to exclude someone or something from an exclusive group. (By the way, does anyone really believe that Mercury cleared its orbit all by itself? Or for that matter, I think the orbits of Venus, Earth, and Mars are still quite littered with debris. Don't believe me? Have a look at this or this.)

So, we return to the heart of the matter. What is a planet? Well, since the word is already in common usage, I think it very unlikely that any effort to redefine it is going to be successful. The word has already accumulated too much cultural baggage and the family of planets around our sun has been established for more than a century now as consisting of: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Instead of trying to redefine this word and then having to go through all of the trouble of reeducating the public (which they will most likely resist), why not just let the word, and all of its current connotations, stand as is. Astronomers could then choose some new terminology which will allow them to converse without having to resort to using the non-technical term planet.

I personally like the idea put forth by Dr. deGrasse Tyson of the Hayden Planetarium. He advocates the usage of terms which describe the various families of bodies which inhabit the solar system. The gas giants would include Jupiter, Saturn, Uranus, and Neptune; the terrestrials would include Mercury, Venus, Earth, Mars; the asteroids would, of course, include Ceres and Vesta; Kuiper belt objects would then encompass Pluto, Charon, 2003 UB313 (Xena), Sedna, and other similar bodies yet to be discovered; and finally Oort cloud bodies. Each of these families possess distinct traits which would, by association, establish some common knowledge about the basic properties of the bodies within each category.

I think this kind of taxonomy is more likely to be widely accepted than the attempt to redefine planets based on the new IAU criteria. Humans are good at categorization if they can be shown how the categories are different. The more distinct the categories are, the easier it is to keep them straight. Ultimately, that is why the new definition of planet will not stick. As far as most people are concerned, the distinction of planet from not planet under the new definition is pretty slim, especially when considering that the accepted planets are not all that similar to begin with.