Of planets and plutons

The furore over the status of Pluto – planet, “pluton”, or whatever they’ve decided to refer to it as now - is really missing the point. I can’t believe that the International Astronomical Union is wasting it’s time discussing the exact descriptive boundaries of a noun which has always been scientifically useless.

Ok, that’s a little extreme – back when ancient astronomers were restricted to observing the heavens with the naked eye, it made sense to subdivide all the points of light into those which appeared fixed relative to each other (the stars) and those which moved about (the planets). The planets move (we know now) because they’re part of our solar system, so are much closer to us than the stars are. Therefore, if we’re going by the original sense of the word, any visible body which orbits the Sun is a planet. If the Greeks had been able to observe Pluto, they would unquestionably have said that it was a planet.

We now know a lot more about those little points in the sky – enough to know that they are not at all alike. Any category that encompasses everything from little balls of silicate to gas giants is not really telling you a hell of a lot about its members beyond very broad generalities - it’s kind of like saying you can say something meaningful about life on Earth by referring to everything as a “self-replicating life-form’. For example, one thing all of these bodies do have in common is that they all coalesced out of the same gas cloud. So did Pluto, but then so did and all the other Kuiper Belt objects, and all the asteroids... forming a continuum of objects all the way from dust particles to (as our current census of extra-solar planets illustrates) virtual stars.

Let’s face it, this argument is purely aesthetic and, to a certain extent, nostalgic. People want to give the ‘classical planets’ some sort of elevated status, and are performing all sorts of descriptive gymnastics so we can include Pluto (whose discovery, thanks to one committed astronomer, was probably a few decades before its time). As a scientist, I don’t care whether it’s a planet or not, I want to know things like:

• What’s its composition? Silicates? Ice? Gases?
  
• Is it differentiated? Beyond a certain size, internal heating is enough to cause melting, causing heavier elements to separate out and migrate to the core, as iron has in our own planet. If you want to have a lower size limit, this would be my personal candidate; anything which has a clearly delineated crust, mantle and core is in. Perhaps that’s a little too difficult to definitively establish though.
  
• Does it have an atmosphere? What’s it made of? Is it just a leftover or actively maintained?
  
• Is it geologically active (well, it had to get in there somewhere didn’t it)?
  

Those are some of the interesting questions. Note that “is it large enough for its gravity to squash it into a sphere?” is not among them. Channelling my inner Trekkie, I think that eventually we’re going to have to come up with a slightly more useful classification system (not that this is in any way ideal, but conceptually it’s a good notion), especially as we discover more extrasolar planets. In that context, I suppose the proposal to classify transNeptunian objects with irregular orbits as ‘plutons’ is a vague stumble in the right direction, although I’m not a great fan of them appropriating a perfectly good geological word (pluton being the name for a large, subsurface volcanic intrusion) for their new planetary subclass.

Most commentators seem to agree with me over the silliness of the debate and are also less than impressed by the new classifications. Or both. In the long run, though, whatever Pluto is defined as, I’m still going to be waiting eagerly for the data from New Horizons, which will tell us more about what it’s like.