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

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.