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the pluto files

21 march 2011

I wanted to read Neil DeGrasse Tyson's Pluto Files when it first appeared early in 2009, but the public library copy was always checked out (I think it still is, or maybe it's just lost). I resigned myself to catching up, as I so often do on topical subjects, long after the national conversation had ceased. But when I saw the book in a remainder catalog, I was excited and had to get it. And I'm glad I finally did. It's an essay padded out to 159 pages with copious lists of stuff, quotations, cartoons, and throwaway jokes, but it raises profound issues of epistemology.

Tyson himself, the most famous self-lampooning scientist in America, might sidestep charges of philosophical seriousness. But The Pluto Files is quite hip to them, and despite its fondness for the one-liner, it begs serious questions, not always allusively.

To sum up a nine-days' wonder from a few years back: During the 1990s and 2000s, it became more and more scientifically inaccurate to think of Pluto, the ninth planet, as a planet at all. When Tyson's team at the American Museum of Natural History decided to open a new Hall of the Universe in 2000 without a scale model of Pluto among the planets, eyebrows went up. And then all hell broke loose, as various news media and scientific organizations mounted aggressive campaigns for and against the little snowball's planetary status.

Meanwhile, as Tyson documents in detail, science educators and their young charges were bewildered. How could Pluto stop being a planet? Hadn't we all memorized it? Often, children's letters to Tyson were significantly more lucid than those of adults, even the adult professionals who had the keeping of planetary definition. Madeline Trost of Plantation, FL quite reasonably asked:

Do poeple live on Pluto? If there are poeple who live there they won't exist. Why can't Pluto be a planet? . . . Some poeple like Pluto. If it doen't exist then they don't have a favorite Planet. (123)
Trost's first point cuts to the heart of a nominalist/realist debate implicit in the Pluto kerfuffle. If we don't have a name for something, is it really there? Her second point is even more poignant. If we lose a cultural tradition – if Pluto is no more a planet than Venus is an evening star – then have our innocence and our comfort received a blow? non-fatal, perhaps, but diminishing all the same?

Because people really were upset. Pluto had been discovered in 1930 by an American, Clyde Tombaugh, who went to his grave in the 1990s insisting that he'd discovered an actual planet, not a trans-Neptunian ice object. Pluto seemed on the cusp of classification. Like planets, it orbits the sun and is round. Unlike the Moon or a baseball, it doesn't circle another planet while doing so. What more do you want?

Well, you could want Pluto to be less icy than it is: if it were as close to the Sun as Mercury, it would melt and break apart. You could want it to have cleared its orbital path of debris, as all other planets have done. Instead, Pluto inhabits an area that has come to be called the Kuiper Belt, full of other plutonic bodies, at least one of which (the dwarf planet Eris) is larger than Pluto itself. So it's a poser: if Pluto is a planet, so is Eris, and so is the asteroid Ceres, and so are any number of other rocks. But if it's not, then what is it doing on so many schoolroom science posters?

The curiosity of it is, almost nobody participating in the debate, including, I bet, many of the astrophysicists, had ever actually seen Pluto. I've certainly never seen Pluto. I have seen Uranus – through binoculars, if you know where to look, it's an interesting sight, a planetary disk where no star appears to the naked eye. I've seen Neptune through a telescope. Pluto, on the other hand, doesn't look like a blessed thing even through a strong telescope, with the result that few people since Clyde Tombaugh have ever bothered to look at it.

Pluto exists for us at the end of a chain of inferences that belies our sensory perceptions. Why does anyone care about it? You might answer that plutonium atoms and influenza viruses are also beyond our sensory perceptions, but we rightly care a lot about them; still, they can kill us, something that Pluto has shown no signs of doing.

As Tyson insists throughout The Pluto Files, labelling has little to do with reality. His is not just a preference for nominalism. Pluto was floating around in orbit long before there were human beings to call it a planet or not; just as important, the notion of "planet" itself was developed in ignorance of Pluto. Nor is "planet" a natural category. Tyson shows that "planet" originally meant anything that apparently moves (regularly) across the sky, which for some pre-Copernican cultures included the Sun and Moon. After Copernicus, the idea that Earth is a planet caught on. Still later, Ceres was added, and then removed after so many Ceres-like asteroids were discovered that no mnemonic could teach schoolkids their names.

If I call a hare a rabbit, I'm confusing things that exhibit functional genetic differences. Calling Pluto a planet, or a dwarf planet, or just an "object," has a different significance. There are lots and lots of things floating in space. Some of them involve nuclear fusion at their core, and they are really stars. Others involve more or less of an atmosphere, and might sustain life; we might call them "class M planets" if we're Star Trek fans, and that label describes a functional category. Others are gas giants, and though they're somewhat less interesting because they're hard to land a LEM on, it seems useful to create a functional category for them too.

But "planet" is a vernacular word that lumps a bunch of dissimilar things that orbit stars. Take a cypress, a magnolia, an elm, a pine, an aspen, a sequoia, a date palm, a fig, a banana, a bay laurel, and a bonsai juniper. They're all "trees," even though they share no common ancestor more recent than hundreds of millions of years ago, they range from a foot tall to hundreds of feet tall, some grow in pots and some in national parks, and some are botanically grasses. But try telling someone that a palm is not a tree. It has a trunk, branches, leaves, you can sit under it, and it bears fruit. Tree, stupid.

And so has gone the debate over Pluto. Folk taxonomy is difficult to revise – even when it's a folk taxonomy taken on faith by millions of people who have never even seen the individual taxonomic item they're arguing about. More important, even for a significant minority of scientists, a name becomes a reality. We're more used to this kind of reasoning-from-the-label in the humanities. An entire sub-industry of literary studies is devoted to "the sublime," asking endlessly whether such-or-so text truly belongs to "the sublime" or not, and apparently long ago having forgotten that there is nothing whatsoever, real or artificial, that is naturally "sublime": the term is a pure label.

So it is with "planet." As always, one of Tyson's infant correspondents figured out the issue more quickly, and had an approach to its resolution as convincing as that of many of Pluto's adult defenders. Will Galmot, age seven, was the first museum visitor to realize that the AMNH had omitted Pluto from the planets. He wrote Tyson in March 2000, saying

You are missing planet Pluto. Please make a model of it. This is what it looks like. It is a planet. (110)
And on an enclosed piece of paper, Galmot drew a blue orb and labeled it Pluto. He still makes the best argument in favor of the little sphere's planethood.

Tyson, Neil DeGrasse. The Pluto Files: The rise and fall of America's favorite planet. New York: Norton, 2009.