What do you think Constitutes a Planet?

[Aegis]

I would have to say that a planet would need to be spherical, orbit a star, and have a substantial amount of atmosphere for there to be a good contrast between empty space and the atmosphere itself.

With that being said, I'd have to say that Pluto is not a planet, as it does not meet all of my prerequisites.

 

[Cuivienen]

How about linking it to a certain amount of atmospheric density?

EDIT: that is the definition of a planet.

I believe Mercury has essentially no atmosphere as it's so close to the Sun. All of its atmosphere has been stripped away by solar wind. However, Mercury has to be a planet.

 

[warpus]

Yeah, the requirement of an atmosphere is a bad one.

 

[Aegis]

If Mercury was not so close to the sun it would have an atmosphere, so I think it's fine.

Perhaps the requirement should be that a planet is capable of producing an atmosphere on it's own? That way, Mercury would fit.

If Mercury was a little further out into the solar system, would it have an atmosphere? Yes.

If Pluto was a lot closer into the solar system, would it have an atmosphere? No.

Which is why I draw a distinction between the two.

 

[nc-1701]

Why does Mercury need to be a planet?

 

[warpus]

If Mercury was not so close to the sun it would have an atmosphere, so I think it's fine.

Perhaps the requirement should be that a planet is capable of producing an atmosphere on it's own? That way, Mercury would fit.

No, planets do not produce their own atmospheres.

 

[Aegis]

Actually, yes they do. From my understanding, atmosphere in planets like Earth and Mars is produced from gas that comes from within the planet and is held in by the planet's gravity. The colder the planet, the thicker the atmosphere. The hotter the planet, the thinner the atmosphere is. That's why planets like Jupiter and Uranus have very thick atmospheres, whereas Earth's is much thinner, and in the case of Mercury, nonexistent.

I also know that Jupiter and Uranus still have their original atmospheres, but Earth does not, however I do not know why. Perhaps it has something to do with the strength of gravity or the proximity to the sun. *Shrugs*

 

[Kal'thzar]

However, Mercury has to be a planet.

Why? We're changing the definition.

 

[Sidhe]

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.

Well OK but it aint messing with my head.

FYI Interferometers, which filter out the light of stars so that we can theoretcally see the light of planets may even detect life, I mean plant life though, it's lovely green signature may be quite prevalent.

I heard there were plans to build one in Earths orbit. Or even Launch one around jupiter. Who knows?

Last time I checked I think we'd found about 100 or so planets around other Systems usually by using the wobble of stars to infer there presence. Essentially a star moving towards and away from us will give away the presence of a planet. By using the red and blue shift we can infer any planets masses. This only works for Gaseous giants though as the boiling atmosphere of suns hides very small gravitaitonal shifts in stars.

 

[IglooDame]

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."

Agreed, Pluto stays a planet for pretty much the same reason that Europe and Asia are each considered continents - textbook modification prevention (TMP).

 

[Aegis]

Aren't Europe and Asia on separate tectonic plates, though?

 

[Perfection]

Yeah, Mercury is between Pluto and Mars in size; there is no way Pluto should be a planet and not Mercury.

My definition: a planet should be
a) round
b) orbiting a star directly, no other object (so no moons)
c) not part of or sharing an orbit with a larger field of objects. By this I mean that since there is a belt of asteroids, and a belt of KBOs (A Kuiper Belt, if you will) that nothing in there should be called a (major) planet. We can call everyhting in these fields (eg Pluto, Ceres) minor planets, round or not.

By and large that definition is good, but I have a few criticisms:
1. I'd submit that my definition based on gravitional effects is better then a field presence test for the following reasons:
A. With bodies like Sedna, you are left holding the bag waiting for another object to come along and show that it is indeed not a planet. Do we call it a planet now and rescind it later if more similar orbited objects are found?
B. What about small (Ceres-sized) rogue bodies? How should that impact the definition?
2. It neglects the large-size controversy

An object

Don't forget non-stellar!

that orbits a star, is round, has a regular orbit that varies in distance no more than say 25%,

I disagree on this orbital characteristic being a determining trait. We know that this is commonly violated by Jupiter-mass exrasolar bodies. Certainly these should be considered planets.

is atleast 2000km in diameter,

Why 2Mm? Seems arbitrary to me

and it should probably have an atmosphere.

How about linking it to a certain amount of atmospheric density?

I would have to say that a planet would need to be spherical, orbit a star, and have a substantial amount of atmosphere for there to be a good contrast between empty space and the atmosphere itself.

I disagree, A 14 Earth-mass object has been discovered that has little chance of having any atmophere (too close to the parent star), I'd submit that it should be considered a planet.

And of course, we have the problem that Pluto actually does have an atmophere (albeit a thin one)

If Pluto was a lot closer into the solar system, would it have an atmosphere? No.

Actually, if it was closer it actually would have a much larger atmophere because of the sublimation of frozen gasses on its surface (Pluto's current atmophere has seasonal variation because of this process)

Why does Mercury need to be a planet?

Because it holds practically all the traits associated with planethood, and the few traits it doesn't hold aren't held by other objects (certain extra-solar bodies) that should clearly be planets, and are held by things that IMO shouldn't be considered planets (Pluto).

Actually, yes they do. From my understanding, atmosphere in planets like Earth and Mars is produced from gas that comes from within the planet and is held in by the planet's gravity. The colder the planet, the thicker the atmosphere. The hotter the planet, the thinner the atmosphere is. That's why planets like Jupiter and Uranus have very thick atmospheres, whereas Earth's is much thinner, and in the case of Mercury, nonexistent.

I also know that Jupiter and Uranus still have their original atmospheres, but Earth does not, however I do not know why. Perhaps it has something to do with the strength of gravity or the proximity to the sun. *Shrugs*

Ability to hold and retain atmopere as well as its existance is a very complex issue, I don't think basing a definitino off these characteristics would be easy or beneficial.

 

[Eran of Arcadia]

When I said "round", that was referring to gravitational effects; any object big enough to pull itself into a sphere (or close, anyways).

 

[IglooDame]

Aren't Europe and Asia on separate tectonic plates, though?

Even if they are (I don't know offhand), so is India, and it's only a subcontinent.

 

[Bozo Erectus]

Anything thats round, and not 'potato shaped'.

When I said "round", that was referring to gravitational effects; any object big enough to pull itself into a sphere (or close, anyways).

Great minds think alike. Yours and mine, too

 

[Perfection]

When I said "round", that was referring to gravitational effects; any object big enough to pull itself into a sphere (or close, anyways).

But not enough. A planet should have significant gravitational influence on its orbital locus. It should clear away Debris and affect the concentration of minor bodies signigicantly.

Aren't Europe and Asia on separate tectonic plates, though?

They aren't (mostly), IIRC Middle East and India are on seperate tectonic plates and Siberia is on the North American. The Rest of Aisia and all of Europe are on a single plate.

Great minds think alike. Yours and mine, too

Bozo, Roundness in and of itself is not that good a metric, I say so for the following reasons:
1. Making roundess the sole measure would more than double the current amount of planets and reverse the ruling that Ceres is not a planet.
2. What makes a body round is not completly gravity centered. A larger much heavier rock can be irregular when a smaller chunck of ice exhibits very smooth gravitional rounding.

 

[Eukaryote]

A sphere formation, of rock or gas, that orbits around a star, that's at least 2000km in diamiter, that's not a moon or part of an astoriod belt. Pluto is simply a big astoriod ya know, it's in an astoriod belt, it's not a planet. If it is then there's humdreds of planets in our solar system.

 

[mdwh]

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.

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.

In what sense do you mean "sorting"?

Scientists are still interested in small bodies like Pluto and Sedna, whether or not they are called planets, and all such objects we discovered will be identified with some name. Primary sorting/classification will be by the star they orbit, not by whether they are a planet or not. An obvious secondary sorting would be whether they are gas giants or not.

After all, we could have the same debate of "What counts as an asteroid?", in the sense of should we give a name/number to every small rock or pebble we find, or do we draw the line somewhere?

No one cares about questions like "What counts as a continent?" or "What counts as an island", or "What counts as a mountain?" I'd say the only reason this is such an issue is because the question of how many planets there are is something that is ingrained in popular culture. It's one of those general knowledge questions. The discovery of a new planet has always been seen as a big thing, yet now we have the problem that, if Pluto is seen as a planet, then actually we're discovering loads of them.

But that's not a problem to concern scientists, and it won't be a problem when it comes to other solar systems - the general public aren't going to care about how many planets there are around one of many stars.

Indeed, we could also ask "What counts as a moon?" - we seem to keep discovering new moons of Jupiter, but does every small bit of rock count? But no one cares about that, because "How many moons does Jupiter have?" is something that far fewer people know or care about. (On a related note, doesn't Earth sometimes acquire additonal "moons"?)

 

[mdwh]

Some interesting reading at http://en.wikipedia.org/wiki/Definition_of_planet - in particular: "The International Astronomical Union, or IAU, which is the body responsible for resolving issues of astronomical nomenclature, has stated that it intends to release its final decision on the matter in September 2006."

 

[Eran of Arcadia]

OK, fine Perfection, maybe your gravitational limit is better than mine. Happy?