A band of astronomers at the IAU meeting in Prague are revolting against the proposed IAU definition.


Astronomers, who are normally mild mannered types, are revolting against the IAU proposal that would eventually add hundreds of planets to the solar system. A counter-proposal has been offered, though it is not clear (at least to me) how such a counter-proposal officially makes its way to the floor.

In my earlier discussion of the different options I suggested that there were three potential options available to astronomers who feel a need to define planets. Two of these options are strictly scientific and one option is strictly cultural. The two scientific definitions leave us with either 8 or 53 (currently) planets, while the cultural definition gives 10, with room to (very slowly) grow. The IAU proposed the more radical of the scientific definitions, that anything round should be called a planet, and that the number of planets in the solar system will very quickly reach the hundreds.

I have been critical of the IAU proposal for several reasons. Most importantly, though, I disagree philosophically. There is no doubt that the concept of roundness due to self gravity -- hydrostatic equilibrium, to astronomers -- is an important and fundamental property of the large objects in the solar system and that there is a natural (though fuzzy) dividing line between large spherically shaped and small irregularly shaped objects. But there is no good reason that the word "planet" needs to be hijacked to suddenly become the word which describes this difference. If astronomers need a word to describe an important concept and if no word is available they should make up a word. When Quaoar was discovered in 2002, we used the word "planetoid" to describe just such an object that is smaller than a planet but still spherical. I have no particular attachment to this word, and would happily use any other that seemed reasonable (but, please, not "Pluton" which is a word already in use by geologists [and I teach intro geology at Caltech!] to describe something else entirely!).

The new proposal  from the revolting astronomers takes the unpopular but much more justified stance that Pluto really and truly should be demoted. If this proposal is accepted, people all over, from school kids to astronomers, will feel like part of their landscape has been ripped away from them, but that is no reason to not accept the scientific reasonableness of this proposal. I suspect that any sentient being arriving into the solar system for the first time would very quickly classify the four giant planets in one group, the four terrestrial planets in another group, the asteroids in a third group, and Kuiper belt objects in a fourth group, and have a few leftovers. It seems impossible to fathom that the biggest one or two or even 43 objects from the Kuiper belt group would be somehow removed and placed into any of the other groups. They would quickly come to the conclusion that there are 8 major bodies orbiting the sun. (Well, maybe they would only say four: Jupiter, Saturn, Uranus, Neptune. But I'm not yet ready to lead the fight to demote Mercury, Venus, Earth, and Mars.....).

But what about Pluto? Pluto will always have special status as the first object discovered in the Kuiper belt, the first Kuiper belt object visited by a spacecraft, and the brightest and most easily studied of the Kuiper belt objects. Admitting that Pluto is not, in fact, a major body of the solar system in no way diminishes its scientific importance.

It's true, though, that admitting that Pluto should not really be considered a planet does diminish its cultural importance, and this is where this definition of the word planet gets into trouble.

Is there a way out? Perhaps. There is one other reasonable option available to astronomers, and that is to realize that the word "planet" need not have a scientific definition. Consider it this way: if the word planet is suddenly redefined to mean either 8 or 53, how will it affect astronomy? Not one tiny bit whatsoever. Astronomers like me will continue to go to telescopes and study these objects to learn where they came from and what they are made out of whether they are called "planets", "Kuiper belt objects", or "batholiths." For astronomers, this argument is purely semantic. Who is affected, then? I would argue that it is the public, it is our culture, that would be affected, and, in fact, this is why this is the one astronomical argument, out of the many many many that are out there, that anyone actually seems to care about. In light of this realization, perhaps it makes sense to have a cultural definition of the word planet, rather than a scientific definition.

By "cultural definition" what I mean is "what people mean when they say the word planet." As far as I can tell, most non-astronomers are content to keep Pluto as a planet, and, when you discover something new in the outer solar system, the first question they ask is "Is it bigger than Pluto?" When you admit that, well, no, it is 1/2 the size of Pluto (2002 discovery of Quaoar) or maybe 3/4 the size of Pluto (2004 discovery of Sedna) they look a little disappointed and say "well, ok, so I guess it's not a planet, huh?" But when you say "YES! It IS bigger than Pluto" (2005 discovery of 2003 UB313) they say "Hurrah! The 10th planet has been found!" Or something like that. To me that clearly shows that culture believes that Pluto should remain a planet and that only objects larger than Pluto should be called planets.

This purely culturally based definition is simple and concise. The major flaw in the minds of most astronomers is that there is no science there. Absolutely true. If you feel the need for a scientific definition even though the definition has no affect on science, the answer is clearly that there are eight planets (and, indeed, I have signed on as a supporter of the 8 planet counter-proposal to the IAU). There are other words describing the landscape around us that are equally unscientific, however, and work just fine. The word "continent" is the obvious example. No geologist would ever attempt a scientific definition of the word, and no one in the public seems to mind. Astronomers would be wise to imitate their ground-dwelling colleagues here and not try too hard to rearrange what we call the things that we think of as being in our back yard.