Tuesday, July 28, 2009

So Is Pluto a Planet or Not?

John Derbyshire alerts us to the fact that the International Astronomical Union will be meeting in August 2009 for the first time since it demoted Pluto from the status of "planet", and that the issue of Pluto's planethood may come up yet again.

But he makes a couple of mistakes in his rush to condemn creeping New Ageism. First, he says: "The deciding factor was the opinion, held by a majority of delegates, that merely being massive enough to form a sphere under your own gravity is not enough. (If it were, we'd count the Moon as a planet.)" Not true, of course; the Moon has never been counted a planet. There is the important requirement that the body in question must orbit the Sun, which eliminates the Moon from consideration.

Second, he quotes Mark Sykes of the Planetary Science Institute in Tucson, Arizona as saying:

We are in the midst of a conceptual revolution. We are shaking off the last vestiges of the mythological view of planets as special objects in the sky — and the idea that there has to be a small number of them because they're special.

OK. But Derb's comment to this is where he goes wrong: "Just beneath the surface of those words you can spot the egalitarian sensibilites of our age. Nothing is special! Everything is equal to everything else!" That's a serious reach, and it's surprising coming from a man who understands and appreciates science and the scientific method as much as he does.

It's important in science to classify things. But classification is complicated by two factors: first, many things fall naturally into multiple categories (e.g., my telephone is both "a piece of office equipment" and "a communication device", but there are instances of each class that do not belong in the other); second, the dividing line between categories can be vague (is a futon a "bed" or a "couch"?). That doesn't mean that everything is equal to everything else, though. It's plausible to call a futon a bed or a couch, or both, but no one would call a futon a type of car.

Biologists have been frustrated by this problem for ages. The definition of "species" has historically been "a class of living things which can interbreed to produce viable offspring". On the surface, this seems simple enough. Are animals X and Y in the same species? Breed them: if their offspring is viable, then the answer is "yes". Unfortunately, Nature has a few tricks up Her sleeve. There are cases where three classes of animals exists, X, Y and Z, where X can breed with Y and Y with Z, but not X with Z. By our definition, then, X and Y are the same species; so are Y and Z. But X and Z are not. The definition made an implicit assumption, transitivity, which does not hold. And this causes the scheme to break down.

But it does not, as Derb surely understands, mean that biologists say that any two living things are in the same species, or that the concept of "species" is meaningless. It's more complicated than we thought a century ago, but in a way similar to the way our knowledge of gravity is more complicated than we though a century ago. Newtonian gravitation still works in most cases, but you need to keep in mind the Einsteinian correction in certain cases. When we find that an old classification system does not work because of new facts, we have to find a new one. (I'll return to this point in a bit, though.)

In the case of Pluto, there are several problems with its planethood. On the case for the defense, clearly it does orbit the Sun and has sufficient gravity to be spherical. But the prosecution is forced to point out that the same is true of other known bodies in our Solar System, such as the asteroid Ceres. Not wanting to add additional planets, astronomers looking to keep Pluto in the list of planets sought other criteria that were true of Pluto but not other bodies. This proved difficult. Eccentricity doesn't work: Pluto's orbit is more eccentric than Ceres'. Nor does orbital inclication: Pluto's orbit is highly inclined to the ecliptic. Pluto has moons of its own, and that proved briefly interesting, but some asteroids were found to have moons, too.

In the end, the IAU was left with two choices: either demote Pluto, or promote lots of other bodies. The least distruptive option was the former, and that's what they did.

Much as it may bother Derb, what Mark Sykes had to say is simply true: the Solar System is pretty complicated, and coming up with a pure classification system for every body in it is hard. The size of bodies is a smooth curve up from millimetric grains to centimetric pebbles, to meter-scale boulders, to kilometer-scale mountains, and on up to really large asteroids that have enough gravity to become spherical (a dividing line, by the way, which is also imprecise), continuing up to large moons and the smaller planets. Up to Earth size, all the solid bodies in the Solar System fall nicely into a curve. It's the four gas giants (Jupiter, Saturn, Uranus and Nepture) which are the exceptions. (But are they? Maybe they have rocky planetlike cores and just have huge gas envelopes because of how they were formed.)

The fact is that if we classified Pluto as a planet, we'd be doing it for purely historical reasons. Now that doesn't make it wrong, of course. We can still talk about species A and species B and biologists won't immediately start bringing up the problem with species classification (unless it happens to matter in that particular case). But if we're doing real science, we have to accept that Pluto isn't particularly exceptional. If we want to call it a planet, let's just give it an asterisk and say: It's traditional to call it a planet, but from here on it to be a planet you have to satisfy all these criteria. Tradition does have value, of course. Just as we didn't drop Newtonian gravitation from high school curricula after Einstein came along, it's not absolutely necessary to demote Pluto in the same of purity.

But doing so isn't creeping postmodernism, either. It's an effort to promote the classification project that is at the heart of science.

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