Planets

[stoutfellow]

In - was it 1972? somewhen around there - my Scout troop went to summer camp at Camp Hual-Cu-Cuish, in northern San Diego County. (This was unusual; usually we spent summer camp at Camp Mataguay, in the southern part of the county.) It was an enjoyable and productive camp for me; I earned three merit badges that week, in Basketweaving, Swimming, and Astronomy. (The Basketweaving was a gimme; the camp store sold these little kits, and I think almost my whole troop got that badge.)

At the first meeting for the Astronomy badge, I noticed a Question of the Day on the blackboard: "Define the Universe. Give two examples." Being ornery, I decided to take them up on it. I cobbled together some sort of description broad enough to cover the Ptolemaic and Copernican models, and used those as my examples. The next day, I handed it in to the instructor. He glanced over it, scowled, called me a smartass, and gave me (as the prize) a free bottle of Coke. The QotD the next day was... more serious, and very hard.

I was reminded of this by the recent kerfuffle over the IAU's decision to redefine the word "planet", in such a way as to include the current crop, together with the asteroid Ceres, Pluto's moon Charon, and the Kuiper Belt Object temporarily known as "Xena". It occurred to me to wonder what the point was - why the concept "planet" was useful enough to fret about. Since pompe knows more about these things than I do, and since he had just posted about the controversy, I took the opportunity to ask him that question. His answer (scroll down to the comments) is, I think, cogent, and leads to a different and quite plausible definition.

Anyone have any other ideas on the subject?

Comments

[countrycousin.livejournal.com]

I understand your question to be why is it important to have a category labeled "planet".

Clearly there are a variety of properties that can be observed; some bodies have some; some have others.

It seems to me that the answer to your question is, what are we going to ask school kids to learn about as common knowledge that corresponds with what we now call "planet". That doesn't seem to be the question pompe answered; he seemed to answer what he felt ought to be the characteristics shown by bodies important enough to be called "planet". I must say, I find it hard to argue with him.

The other side of the coin: one characteristic used by the IAU was that the body be large enough so that gravity dominates its shape; it makes it round. That seems an important characteristic, and one that pompe doesn't mention. I think the ones that he does mention wind up being more restrictive. Since we can detect much smaller bodies in our vicinity it seems a reasonable criterion.

But I wonder how much of the final decision was reluctance to let the public off the hook at all? We've made them learn nine; by golly, we're not going to let them get away with learning less. ;<)

[pompe.livejournal.com]

I think "round" is a bad characteristic. Or to put it like this, all planets probably are round but there are a lot of things smaller than planets which also are round. Too small (and too many) to be dynamically important but too big to not be round.

Take Saturn's moon Enceladus as an example, it is spherical and yet smaller than Ceres - only half the diameter.

Bode's Law?

[p-o-u-n-c-e-r.livejournal.com]

I'm still wondering if Bode's Law plays into the definition in any useful fashion. Pompe's notion of "clearing the zone" seems relevant -- but assumes more or less "round" orbits, much as Countryboy points out the IAU insists on "round" masses. If we had a body the mass of Mars, in the shape of Clark's monolith (1x4x9) in the orbit of Halley's Comet -- well, it'd be interesting but it wouldn't be a planet.

Re: Bode's Law?

[pompe.livejournal.com]

An object in an elliptic orbit would still clear out its zone decently. Might even do so better than a body in a circular orbit. Mercury's orbit is pretty elliptic, as an example.

However, there are - perhaps rare - resonant solutions which would leave two objects in about the same orbital space. I'd still argue they would clear out their orbit, though.