What do you think Constitutes a Planet?

I was inspired by Godwynn's planet size relation thread to post this one. It seems that we are at a disagreement as to what ought to be considered a planet, and what ought to just be an asteroid.

We need to come up with criteria with which we can evaluate whether any given object in the Solar System or abroad ought be called a planet. The first subject should be Pluto, since that is the most obvious problem, and then we can advance to other solar objects.
Let's try to refrain from lots of links and whatnot, and just come up with the criteria on our own.
Let's keep this a friendly discussion in here, too. It's just a title for a rock, there'll be no need to get ugly in here.

Alright, let's get started. What characteristics ought a an object possess to be considered a planet?

Anything thats round, and not 'potato shaped'.

I'm fairly liberal although it seems I'm alone in this view here, I tend to think that although Pluto is not really a planet as we know it, it should be the exception that proves the rule. Planetary definition in future definitely needs a rule book, and I find no reason to cast out planets on the basis of this rule book. Perfection and The Last Conformist disagree. We had this discussion On another thread it is relevant, and I will try to find it. I guess it counts as a link but might help put the discussion in context.

EDIT: My mistake Bozo is also fairly liberal

http://forums.civfanatics.com/showthread.php?t=175145&highlight=Pluto

something that chuck norris cant lift.

Something thats bigger than an asteroid and revolves around nothing except for a star.

Bozo Erectus said:

Anything thats round, and not 'potato shaped'.

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So you'd be in favor of including Ceres and a number of KBO's, along with some satellites?

Dawgphood001 said:

Something thats bigger than an asteroid and revolves around nothing except for a star.

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Same as above +
In a relativly fixed orbit.

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Whatever the definition is, if we include Pluto, we're also going to have to include 'planets' such as Sedna, Xena, and Quaoar.

IMO the most sensible thing to do would be to remove Pluto from the list - it is simply nothing like the 8 planets.

Pick a definition involving Pluto, and add "discovered before XXX", how's that?

Classical planets FTW.

warpus said:

Whatever the definition is, if we include Pluto, we're also going to have to include 'planets' such as Sedna, Xena, and Quaoar.

IMO the most sensible thing to do would be to remove Pluto from the list - it is simply nothing like the 8 planets.

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Why? Are you incapable of discretion when it comes to arbitrary values of what is a planet, can you not discriminate, or do all rules have to be absolute?

Agree with Erik, let's go along with an after, it only makes one planet non planetary but planetary in distinct terms, it will not mess with my mind to think it so and yet not so. Where's the beef here?

warpus said:

Whatever the definition is, if we include Pluto, we're also going to have to include 'planets' such as Sedna, Xena, and Quaoar.

IMO the most sensible thing to do would be to remove Pluto from the list - it is simply nothing like the 8 planets.

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Why? I refer you to my answer?

Anything that has something orbiting it.. and isnt a sun/blackhole.. various other things...

ainwood said:

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Was that directed at me? If it was/ while pluto does rotate around the sun it appears to rotate around something else (if we go by it's odd orbit patterns alone)

Of course not Elta it's the Chuck Norris thing

Sidhe said:

Of course not Elta it's the Chuck Norris thing

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Man ..I am slow
What? I thought that was funny
a legitimate zing in my book
...I hope this isn't spam

A planet ought to be something which orbits a star, and has a gracitational pull stronger than the materials it is made of. That's a fancy way of saying that it's round. If it is round, that means that it was formed in a certain way, at the same time as the star is orbits. The rest (asteroids and whatnot) are just left over from the formation of the Solar System, and really aren't cohesive units in themselves. Size is not an issue, as the smaller objects are ruled out by the 'round clause' ( you won't find a small spherical object).
That means that, by this definition, Pluto remains a planet, since it orbits the Sun and is round. However, it is no longer the ninth planet, as there are other objects classified as asteroids in the Asteroid Belt that are now eligible as planets. The notorious round moons, like our moon, that are much larger than Pluto, remain moons, since they orbit a planet first, and not the Sun directly.

Cheezy the Wiz said:

A planet ought to be something which orbits a star, and has a gracitational pull stronger than the materials it is made of. That's a fancy way of saying that it's round. If it is round, that means that it was formed in a certain way, at the same time as the star is orbits. The rest (asteroids and whatnot) are just left over from the formation of the Solar System, and really aren't cohesive units in themselves. Size is not an issue, as the smaller objects are ruled out by the 'round clause' ( you won't find a small spherical object).
That means that, by this definition, Pluto remains a planet, since it orbits the Sun and is round. However, it is no longer the ninth planet, as there are other objects classified as asteroids in the Asteroid Belt that are now eligible as planets. The notorious round moons, like our moon, that are much larger than Pluto, remain moons, since they orbit a planet first, and not the Sun directly.

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"New" planets:

Ceres (only asteroid with any significant level of rounding)
Sedna
Quaoar
Xena

Cuivienen said:

"New" planets:

Ceres (only asteroid with any significant level of rounding)
Sedna
Quaoar
Xena

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I don't mean a "significant level" of rounding, as you put it, but an imperrfect sphere, in the Earth is. From a distance it is a sphere, but it's surface is imperfect ( mountains, valleys, etc). A planet must have sufficient gravity to force itself into one of these faux-spheres. I'm afraid Ceres must remain a pretty rock in the sky. I've never heard of the other three.

Erik Mesoy said:

Pick a definition involving Pluto, and add "discovered before XXX", how's that?

Classical planets FTW.

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But what about extrasolar planets, orbiting other suns, that we haven't discovered yet, but someday will?

Sidhe said:

Why? Are you incapable of discretion when it comes to arbitrary values of what is a planet, can you not discriminate, or do all rules have to be absolute?

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It just makes sense.

If we have an absolute set of rules of what is a planet and what isn't - then we'll have a much easier time sorting extrasolar objects we discover.. and let me tell you, we're going to be discovering a lot more of these things in the future.

Cheezy the Wiz said:

That means that, by this definition, Pluto remains a planet

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Pluto & Charon technically orbit eachother - and their centre of gravity orbits the sun. AFAIK it's not inside Pluto. I guess that'd disqualify Pluto then

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I have no beef against Pluto, but I don't really understand why people are so fascinated with it. It was classified as a planet years before we really understood what else was out there - and most scientists would agree that it was classified incorrectly.

It doesn't really matter if we call it a Planet or not.. but it makes sense to straighten out the definition of the word 'Planet' so that we have an easier time classifying all the extra-solar planets we're going to discover.. and if we include Pluto, then we'll probably end up with not 9, but 15 planets in our solar system.. if not more.

warpus said:

Pluto & Charon technically orbit eachother - and their centre of gravity orbits the sun. AFAIK it's not inside Pluto. I guess that'd disqualify Pluto then I have no beef against Pluto, but I don't really understand why people are so fascinated with it. It was classified as a planet years before we really understood what else was out there - and most scientists would agree that it was classified incorrectly.

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Well one reason I think it ought to be classified as a planet is that it is already, and it just be confusing to have to forget about it and say "NOPE NOPE YOU'RE WRONG THERE"S 8 PLANETS YOU'RE DUMB."

I think the case with Charon and Pluto is that their orbit of eachother is more evident. You see, by Newton's Third Law, every action ( a planet's gravitational pull, for instance) has an equal and opposite reaction force ( the moon pulling back on the planet). Most of the time, this reverse pull is too small to be "seen," since the mass of a moon is most often much smaller than that of the planet it orbits; on Earth we see this pull as the ocean tides.
However, in Pluto's case, Charon is much closer to Pluto's mass, and thus it's pull is enough that we can physically see it, as the center of orbit is outside of Pluto. However, it is Charon that still orbits Pluto, they don't really orbit eachother, like, say, sister suns do.

If you wanted to get REAL technical, none of the planets are planets then, since they all follow elliptical orbits, the 'centers' of which ( if you understand ellipses and calculus) are outisde of the surface of the Sun, so we orbit a nothing point very close to the Sun. It is this phenomenon that compells scientists to believe there is some sort of 'dark matter' that we cannot see, to account for the rest of the mass of the universe. It gets kinda complicated after that.