Well, if the International Astronomical Union can't decide what a planet is (and even if it does, it's likely to change as more objects are discovered), my question is what purpose does it serve to astronomy?

Having a definition at all, or periodically reviewing it in the light of advancing knowledge?

It has absolutely nothing to do with advancing knowledge. It's to do with keeping the numbers down, while trying to keep what people consider planets to be planets.

There have been arguments about whether Pluto should be a planet ever since it was discovered.

True, but because of our advancing knowledge, some of those arguments have become untenable.

The only advancing knowledge is of the number and nature of things in orbit around the sun.

The classification of these is purely arbitrary.

Yes. And?

And it serves as much as a useful purpose as arguing whether Tottenham Hotspur are a 'big club'.

I mean, all classifications are purely arbitrary. The argument here is about what form the classification should take: whether it should consist of a fairly short list of universally applicable criteria, or a more case-by-case Wittgensteinian jumble, or something (frankly) weird and parochial like yours, according to which Ceres is a planet but anything orbiting another star can't be, even if it's exactly like, I dunno, Vulcan or Gallifrey or something.

I can see arguments for the first two, but yours doesn't look thought through, to be honest.

Stumpy Pepys wrote:

QUOTE:
And it serves as much as a useful purpose as arguing whether Tottenham Hotspur are a 'big club'.

Then why are you arguing with the IAU about it?

"The only advancing knowledge is of the number and nature of things in orbit around the sun."

ie planets and planet-like objects. Seems a good basis on which to adjust your definition of planets.

Planet means wanderer. As I'm sure you all know. The romance aspect is important. well, to me. I can't see why you can't have a general definition of 'planet' that applies universally and also classify objects in our particular solar system with regard to history, etc.

By the original definition there are only 5 planets, plus the moon. Objects that can be seen with naked eye that "wander" across the night sky. Maybe we should stick to that. We have named the days of the week (plus the Sun itself) after them, after all.

GY and WE -- you're forgetting about the point I originally made, around the cultural meaning of planets.

You can't embark on a wholly objective definition of a planet because Earth, Venus and so on have to be planets and you've got to build your arbitrary definition to accomodate that.

Indeed, one of the objections to the current definition of a planet is that it's possible to use it to exclude Earth.

So if a planet is essentially a cultural construct, then my or Rogin's definition makes more sense.

But, Stumps, wait a minute. The only reason you think Earth's a planet is that the definition was once tidied up to include it, when the Copernican model achieved acceptance. The Earth doesn't "wander": that is, move against a background of fixed stars. Not from here.

Doesn't that show that there's a role, historically, for science-driven neatening-up of definitions? As well as all that messy historical contingency?

It only makes sense if it's not encumbered by cultural baggage.

According to one of the tenets of the current definition of a planet, it must have cleared the neighbourhood around its own orbit. As the earth is arguably amidst an asteroid belt, then it's possible that the earth isn't a planet.