I read this definition somewhere:I dunno if it's standard or official in any way, but it makes sense to me.

But that would make many moons and some asteroids planets too...

P.S. Welcome to the board.

[Edited to make a correction: no moons, only asteroids. I suppose the "orbits a star" bit in that definition was meant to exclude moons.]

It wouldn't cover moons because they don't directly orbit stars. But it would cover asteroids.

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Depends. In a geocentric reference frame, the moon circles the earth, but in a heliocentric reference frame, the path of the moon around the sun is never even concave, away from the sun. In other words, the curvature of its path is always towards the sun.

Let's not get back onto that again!

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Into a heliocentric reference frame?

Papers from the IAU:

Definition of a planet. Unfortunately this phrase: Doesn't help much for the Sedna debate.

And here is the IAU's position paper on Pluto. Per an IAU page dated Aug 22/02:

This whole thing got me to thinking. We need a new classification system. Mercury, Earth, Venus, and Mars are completely different types of objects than Jupiter, Saturn, Neptune, and Uranus. I have no idea about the nature Pluto's make up but I'm going to make an uneducated guess and say that Sedna is a similar type of object. As it stands right now big balls of rock and even bigger balls of gas fall under the classification of "planet". With these big gas planets it's not even known if they really have a surface other than really, really, compressed gas (or am I wrong). Then we have all these big moons including "Luna" which is larger than Pluto.

Maybe this is how we need to do it:

Big Balls of Rock

Mercury
Venus
Earth/Moon
Mars

Big Balls of Gas

Jupiter
Saturn
Neptune
Uranus

Big Cold Balls of Cold Stuff

Pluto/Charon
Sedna

Of course they need cooler names than that, but you get my point.

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So it all comes down to what's "Big"?

At least for the 'rock' and 'cold stuff'; there aren't any 'Not-so-Big' Balls of gas to have to worry about how to classify them.

More generally, you seem to be heading towards a classification scheme that's just size and bulk composition/temperature rather than, say, how and/or where they were formed, or the extent to which they are merely the largest members of many objects otherwise very similar.

Another approach might be to leave things with whatever labels they have today, and not worry too much about future, popular labels ... at the same time invent a formal classification scheme that's used by the 'in' group. Much like there's an 'echidna' or 'spiny anteater', .... and 'Tachyglossus aculeatus'

We do indeed need a clear definition of what is a planet and what is either a double planet or planet/moon-system.
The second one seems easy to resolve for me: If the common gravitational center is still within the more massive of the two bodys (as with Terra/Luna), it's a planet/moon-system, if it lies outside the body, a double planet.
For a body to have planetary status, of course it needs to revolve directly around Sol.
I'd also add as a criterion that
a.) it was formed from planetesimals in its orbit when our solar system emerged (*) and
b.) is not member of a group of similar objects (asteroid belt, Oort cloud, Kuiper belt)

(*)I'd call any body Sol captures after formation of the solar system a "captured Sol satellite".

As for Pluto: No planet until maybe a future Pluto probe confirms it is not an errant Kuiper belt object or the like.

Two problems with that. The center of mass of the Sun/Jupiter system is outside the Sun. And, eventually, as the moon moves away from the Earth, the center of mass will be outside the Earth--which means that the moon would become a planet just by moving out, not even twice its current distance.

So would that mean the asteriods between Mars and Jup are planets? Argh, this is confusing!! #-o

No atmosphere though

Comets and Centaurs like 2060 Chiron (which is also classified as comet) do have some kind of atmospheres at least in some part of their orbit. But Pluto too have likely a temporary atmosphere.

Here is a more official statement from Working Group on Extrasolar Planets(WGESP) of the Int. Astronomical Union.

Looks like they need our help.

I still like the idea of picking a minimum diameter for an object that orbits a star and does not orbit another planet larger than itself. Lesser diameter objects would be minor planets. Here you would just pick a diameter like Mercury (Pluto gets "Grand fathered").

Nevertheless, for the sake of character, an atmosphere is worth considering. The term "planet" has some wiggle room in any definition allowing such ideas. The term "planet" (Greek for "wanderer") is not a unit of measure and is a common public word that deserves some "wandering" room. Oddly enough, if scrutinized, the mere fact that a planet must be constrained to orbit a star makes it no longer a true "wanderer". The term "planet" also has a home feel to it. A place you could actually go to and not be concerned about falling off, for example. If we embrace the term with social grace, then why not an atmosphere? This would embrace the "homey" feel.

Therefore...
Atmospheric objects at least as large as Pluto, or non-atmospheric objects at least as large as Mercury, are planets. A planet becomes a moon if it orbits a larger planet (regardless of atmosphere).

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"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

The most logical criterion - as has been suggested - would be to consider any Sun-orbiting body that is spherical due to its own gravity to be a planet, but then that would put the number of planets in the Solar System well into double digits.

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I suspect the double digit number of planets would loose that "warm, fuzzy feel" to the word "planet". We are accustomed to "planets" that look like the 9 in our system.

Also, how spherical is "spherical"? Spherical as Earth only? What is the largest non-spherical object possible? Could a large cold object impact "softly" another cold Pluto-sized planet giving you a, somewhat, "two-scoop" looking planet? [Admittedly, I did eat way too much ice cream last nite. ]


[added the italics to clarify]

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"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

I think gravity would be too strong for that. If not, then it's obviously not a planet.

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And, that is? :-?

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