A new scientific truth does not triumph by convincing opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it. ~Max Planck
The Pluto planetary predicament has now reached the silly stage.
At New Mexico State University, a protest, consisting of about 50 people was organized to protest the IAU definition of a planet. What is supposed to make this protest somehow more meaningful is that Clyde Tombaugh, discoverer of Pluto, came to NMSU in 1955 and started what has become a highly-regarded astronomical research program. His wife, Patsy, and son, Al, were at the protest, where Ms. Tombaugh opined, “I'm disappointed all this has happened.”
So am I, madam, so am I.
I have no personal knowledge of the late Mr. Tombaugh beyond the usual information about how he came to identify the little dot on the photographic plates that revealed what was then a ninth planet. But, somehow, I don't think he would be getting so bent out of shape about all of this.
And to cap the silliness, the California legislature, evidently having solved all the other problems of the state, took a daring stand and passed a resolution condemning the “mean-spirited” IAU for kicking Pluto out of the planet club. Of course, the fact that Disneyland is in California has nothing to do with this.
Frankly, I've always thought California politicians came from Fantasyland and were lineal descendants of Goofy or Mickey Mouse. As to their governor ... no, no, this is about science, not politicians. Suffice it to say that the opinions of the California legislature count for diddly-squat in this discussion, other than to point out just how emotional this debate has become.
I'm not going to defend the IAU, because they had a proposal, which wasn't very good, modified it in some manner which is still unclear to something that was no better and possibly worse, then used a vote of a small minority of its members set a standard definition.
This isn't science, as I have already written.
Most definitions in science are based on the properties of the thing being defined. The scientific definition of an atom, for instance, has changed over time as our knowledge of the atom has increased. No one got bent out of shape when it was discovered that the nucleus contained neutrons and protons instead of being some solid lump. No one considers that electrons orbit the nucleus like little [ahem] planets any longer, but there was no protest when this model came into being.
The concept of a planet, though, is different. It's different because there is no real definition of a planet and never has been.
“Planet” means “wanderer.” When the ancients looked up into the sky, they saw the Sun, the Moon, and the stars. The stars were “permanent”, that is, from season to season, they kept their relative positions in the sky. The pole star stayed the pole star; Orion's belt didn't shift up or down or spread out. But, they couldn't miss the fact that there were a few objects up there that looked like stars but didn't behave like them. Eventually, these things were called “planets” and, with a few modifications as more objects were found, everything that circled the Sun and was round became a “planet.” Everything else was a “satellite” or an “asteroid”.
So, beyond being round and orbiting the sun, what is a planet? The answer is that we don't know. And extrasolar discoveries aren't helping. We're finding objects several times larger than Jupiter in strange, fast orbits or highly eccentric orbits. Are these planets or brown dwarfs (essentially failed stars)?
Well, then, how could we define a planet? Let's look at what we've got in our solar system that directly orbits the Sun. There are four rocky planets with solid cores in the inner part of the system. Then, we have the asteroids, which are probably left over rubble from the formation of the solar system. Beyond them, there are four gas giants, each a miniature solar system of its own, with satellites aplenty. Farther out, we have the Kuiper Belt, more leftovers, of unknown makeup, although we know that some comets, generally short-period ones, come from there. Even farther out, we have the Oort Cloud surrounding the solar system on all sides, also a source of comets, mostly ones that are very long period.
With all this diversity, how do we decide that something is a planet or not? What makes a large Kuiper Belt object not a planet? What is “large”? At one time, four asteroids were called planets. Evidently, as the nature of the asteroid belt became known and the true size and nature of the asteroids became better known, it was clear that these objects were somehow different.
There's no solution that satisfies everyone, because the issue is not about what a planet is. The issue is over whether Pluto should continue to be called the ninth planet. Moreover, it's become a purely emotional question, either because people just love Pluto or because some egos became bruised by the method the IAU used to set a standard.
Let's set all the emotions aside for the moment. What Pluto is, almost certainly, is a Kuiper Belt object, more like a comet than it is like any of the traditional planets. What we need to do is properly define asteroids and Kuiper Belt members. We've got a pretty good idea about the asteroids; New Horizons will help us to some extent with identifying what Pluto's characteristics are.
Astronomers need to stop concocting definitions to include or exclude Pluto or Xena (now officially known as Eris; sorry, Mr. Brown) or any other object. Instead, they should be looking at the characteristics of the solar system's constituent parts and classifying them accordingly. I'm sorry if some children will be disappointed if Pluto isn't a planet any more, but, if that's where the science leads, then that's where we should go.
We just may have to wait until a lot of people die off to get there.