I’ve never been very interested in the question of whether Pluto is a planet or not. But the only clear distinction that I know of is whether an object has sufficient gravity to collapse into more or less a sphere. By this definition, Pluto would be a planet, and so would a number of other objects. But is that all a planet is? If there were a really big comet that orbited the sun every ten thousand years, would we really want to call that a planet? Isn’t a planet just the lose term we use for the “wandering stars” in the sky? Does this have to be a big deal?
Tim DeBenedictis over at Space.com seems to think it is a big deal, Why Pluto Is a Planet, and Eris Is Too. He is unhappy about the 2006 IAU Resolution 5A that defined away Pluto’s planet status. And he’s not totally wrong. The resolution defines a planet as an object orbiting the sun that is large enough to be spherical and that has “cleared the neighborhood around its orbit.” DeBenedictis doesn’t even like — nor understand — the spherical requirement. But he’s right that the “cleared” aspect of the definition is vague.
But his alternative is just silly. He proposes that we simply define a planet as any object with a radius of 1,000 km orbiting the sun. This is convenient because it allows Pluto to slip in as a planet (Pluto has a radius of 1,200 km). It is arbitrary, but he rightly noted that all systems are arbitrary. But there is an obvious counter: not all definitions are equally arbitrary. Why not go with the “sphere” definition? I assume it is because DeBenedictis doesn’t want to allow Pluto in the family of planets at the cost of devaluing the title.
This is an attitude I see with a lot of people. Somehow, Pluto isn’t valuable if it isn’t a planet. But I would have thought that the Rosetta mission to Comet 67P/Churyumov–Gerasimenko would have dispelled this kind of thinking. All of the objects that we visit are ridiculously large. And they are all wondrously interesting. Call Pluto whatever you want, the New Horizons mission is so incredibly cool that I feel like a kid again — checking every day for news on it. And it is largely because Pluto is small and is strange that makes visiting it so exciting.
I would prefer to define planets the way that Potter Stewart defined obscenity, “I know it when I see it.” I think size is important. But I also think that orbit is important. Although I think Eris is kind of small to be considered a planet, I have a much bigger problem with its highly eccentric and absurdly inclined orbit. Pluto has the same problems, but to a lesser degree. And I doubt if we would even be having this conversation if Pluto hadn’t been so thoroughly sold to the public before we found out just how small it was.
It’s all kind of a mess. Pluto and Eris are very cool objects and they deserve their own categories. I’m tending to think that we call all the objects that are big enough to be spherical “planets.” Then we can subdivide them. Certainly there are fundamental differences between the solid inner planets and gas giant outer planets. So maybe we should have three categories: solid planets, gaseous planets, and weird planets. I really don’t care. But there are things that make Pluto distinctly different from Mercury, Venus, Earth, and Mars. But that’s not a bad thing; in fact, that’s a great thing.
What’s in a name? That which we call a rose
By any other name would smell as sweet…