What do you think Constitutes a Planet?

[EdwardTking]

If I were to come up with a definition here's the criteria I'd use:
1. Substeller: Incapable of fusion
2. Significant mass: Enough to have a significant impact on the trajectory of small bodies in its orbital locus (Clears away belts, has trojans, etc)
3. Stellar orbit: Orbits a star(s), brown dwarf(ves), or stellar remnant(s) directly (may allow exception for ejected bodies).

Well I agree with your criteria 1. As I regard the moon as a planet,
I don't agree with your definition 3, but it is not unreasonable.

My big problem is with your 2.

"Enough to have a significant impact on the trajectory of small bodies in its orbital locus (Clears away belts, has trojans, etc)"

Firstly, I can not see how that can be defined without an arbitary cut-off.

Secondly, we may be able to detect or even see bodies conforming to
definitions 1 and 3 orbiting distant stars; but how can we know that
it has an impact on small bodies which may or may not be there.
We simply won't know because they'd be too small to see.

Furthermore a body may be in an unstable eccentric orbit with nothing else in
that obit, but that may reflect the fact that larger planets cleared that orbit before the body was diverted by a collision or near approach into that orbit.

And the maths gets quite horrible with multiple stellar star systems.

 

[Perfection]

Well I agree with your criteria 1. As I regard the moon as a planet,
I don't agree with your definition 3, but it is not unreasonable.

My big problem is with your 2.

"Enough to have a significant impact on the trajectory of small bodies in its orbital locus (Clears away belts, has trojans, etc)"

Firstly, I can not see how that can be defined without an arbitary cut-off.

Well perhaps so, but as of now the solar system as we see it falls nicely into the two catagories of not having a significant impact, or having one.

Secondly, we may be able to detect or even see bodies conforming to
definitions 1 and 3 orbiting distant stars; but how can we know that
it has an impact on small bodies which may or may not be there.
We simply won't know because they'd be too small to see.

It's pretty easy to calculate the impact it would have on mass in nearby orbits.

Furthermore a body may be in an unstable eccentric orbit with nothing else in
that obit, but that may reflect the fact that larger planets cleared that orbit before the body was diverted by a collision or near approach into that orbit.

And the maths gets quite horrible with multiple stellar star systems.

I'm no sure how these are problems in my system, please explain more.

 

[deo]

Ok, here is my definition

1. Must orbit a star ( or many stars)
2. Must have a mass low enough to not be capable of fusion
3. Must have a gravity enough strong to be able to have an Atmosphere even if it dosnt have one.
4. Must have a core with a high enough temperature so that it can form some sort of a magma. (IIRC asteroids cant do that)

 

[Perfection]

3. Must have a gravity enough strong to be able to have an Atmosphere even if it dosnt have one.

That really gets into nasty territory because atmospheres are highly complex creatures. Factors such as replenishment rate (via vulcanism among other processes), temperature, eccentricity of orbit (many worlds like comets and Pluto have atmopheres that last for awhile then freeze), composition, and time (non-replenished atmopheres tend to evaporate of the long haul) all have major impacts on atmopheres besides gravity. It would be very difficult to try to control these other factors to come up with a gravity-only system.

4. Must have a core with a high enough temperature so that it can form some sort of a magma. (IIRC asteroids cant do that)

Well Asteroids have had magma in the past and some planets won't have magma in their future (cores cool and solidify). So I'm douting the efficacy of such a system.

Plus I doubt that Gas Giants have magma. Thier cores are probobly completely solid from the intense pressure (though I'm not sure about that).

 

[deo]

I'm just an amateur , well but IMO there can not be a definition that defines exactly a planet, just like there is no exact definition for a human.

 

[Perfection]

I'm just an amateur , well but IMO there can not be a definition that defines exactly a planet, just like there is no exact definition for a human.

I dunno, my definition scheme is pretty good IMNSHO

 

[Bluemofia]

I dunno, my definition scheme is pretty good IMNSHO

Not so humble opinion eh?

But, I can't complain. Can't think of any planets that don't fit.

Interestingly by this definition, Earth and Orpheus during the pre-collision era were not planets. That's why I posit that my orbital impact definition is better than a simplistic "what's in its orbit?" test.

Well, if they are close, they will be bound to disturb each other's orbits some time or another, and then one will be either sent towards the sun, out into outer space, or into the other.

If they orbit at opposite ends of the same orbit, well, then that would be an exception.

BTW, I don't consider proto-planets to be planets until the asteroid mass bombardment phase is over, so that the system has relativly stable orbits.

 

[Gogf]

Once again, I must point out that such a definition would more than double the number of of planets and would turn Ceres (the largest asteroid) into a planet.

Such a definition is not feasible because of this.

I personally don't see what's wrong with this, but if the scientific community objects, I guess that's a good enough reasons not to use it.

I guess yours sounds fine then, but what would be a "significant effect"?

 

[Cheezy the Wiz]

Well it seems that most of the support is for Perfection's criteria, and I have to admit, it is a good list. I think we should implement the stages of 'planetness' into this, though.
So we'll use Perfection's criteria for that of a Class I ( thats what we'll call it for now, I'm all for a classier name, though), that would be the big eight.
Are we is agreement for the most part about this?
If we are, then let's move on to the Class II department. What should be the criteria that separates a Class II from the Class III that is sure to follow?
Or, if we only have two classes, perhaps we ought to call them Major and Minor planets, or something to that extent. Whatever we decide, K.I.S.S.

I'm glad we are making progress on this.

 

[Sidhe]

Perfections criteria are fine by me, provided we keep Pluto I don't have a problem with it being non definable under our criteria but all future planets coming under the general terms of Perfections. I don't think there can be agreement on whether Pluto should remain a planet between us or the scientific community though, it really depends on how sentimental you are

Although really I don't have a problem for now with the current definition I just looked up, I was going to ask but google is quicker.

IAU Definition of a "Planet'':
The International Astronomical Union position statement.

We define "planet'' as follows.

A "planet'' is an object that has a mass between that of Pluto and the Deuterium-burning threshold* and that forms in orbit around an object that can generate energy by nuclear reactions.

* Deuterium Burning Limit: Objects less massive than 13 Jupiter masses never burn Deuterium nor generate significant energy from any nuclear reactions. Fortuitously, this Deuterium-burning limit at 13 Mjup resides near the upper-end of the observed planet mass distribution. Thus 13 Jupiter masses constitutes an arbitrary but doubley motivated limit.

Corollary #1: Observations may enhance but not authenticate planet status.

Corollary #2: Sharp parameter boundaries for the domain of "planets" can be neither physically nor empirically justified at this time. Objects which have masses and formation histories near the perimeter of the adopted parameter domain constitute part of a natural continuum.

Upper Mass Limit for Planets:
The observed mass distribution of companions to nearby stars motivates the upper mass limit. Without the empirical mass distribution, no upper mass limit would be compelling.

The mass distribution exhibits a clear paucity above 5 to 10 Mjup. Less than 1% of stars harbor companions having more than 10 Mjup within 10 AU . The mass distribution rises steeply toward smaller masses, down to the detection limit near 1 Mjup. This mass distribution empirically motivates an upper boundary for planetary masses at about 10 Mjup.

Thus the empirically-based upper mass limit for planets resides at 10 MJUP and coincides conveniently with the boundary of nuclear Deuterium burning at 13 Mjup, making this proposed upper mass limit for planets analogous to the substellar boundary for H-burning.

Nicely grey, probably not satisfactory for the more precision minded.

 

[Perfection]

Not so humble opinion eh?

But, I can't complain. Can't think of any planets that don't fit.



Well, if they are close, they will be bound to disturb each other's orbits some time or another, and then one will be either sent towards the sun, out into outer space, or into the other.

Of course, but the question becomes what to call them during this time. I don't like "wait and see" approaches

If they orbit at opposite ends of the same orbit, well, then that would be an exception.

Nah, they'd still interact.

BTW, I don't consider proto-planets to be planets until the asteroid mass bombardment phase is over, so that the system has relativly stable orbits.

Well, I don't know how long it took Orpheus to smash into Earth. It may have been after the bulk of the condensation into planets.

I personally don't see what's wrong with this, but if the scientific community objects, I guess that's a good enough reasons not to use it.

Well definitions should fit well with current usage, and my definition works well with current modern usage (having only one alteration from common use). Yours shatters common notions

I guess yours sounds fine then, but what would be a "significant effect"?

Oh, making swaths of space unstable for the orbits of small bodies, having small bodies in resonance orbits, having trojans, and the like.

Well it seems that most of the support is for Perfection's criteria, and I have to admit, it is a good list. I think we should implement the stages of 'planetness' into this, though.
So we'll use Perfection's criteria for that of a Class I ( thats what we'll call it for now, I'm all for a classier name, though), that would be the big eight.
Are we is agreement for the most part about this?
If we are, then let's move on to the Class II department. What should be the criteria that separates a Class II from the Class III that is sure to follow?
Or, if we only have two classes, perhaps we ought to call them Major and Minor planets, or something to that extent. Whatever we decide, K.I.S.S.

I'm glad we are making progress on this.

Nah, the class idea is silly and confusing. K.I.S.S., don't put in a whole bunch of classes when it can easily be avoided. There's all sorts of good words one could come up with for smaller bodies that are more descriptive and don't involve ambiguity in the term "planet".

Perfections criteria are fine by me, provided we keep Pluto I don't have a problem with it being non definable under our criteria but all future planets coming under the general terms of Perfections. I don't think there can be agreement on whether Pluto should remain a planet between us or the scientific community though, it really depends on how sentimental you are

Hey, I love Pluto, I really do! New Horizons probe is gonna kick ass when it gets there (and I'll be watching it really closely). But, come on, it's just a classification! Let Pluto go and be with its friends and not be tied down by overgrown hunks of rock and gas!

Nicely grey, probably not satisfactory for the more precision minded.

As well as based on an arbitrary historical cut-off.

 

[Cheezy the Wiz]

We aren't done with this yet folks!

 

[Tulkas12]

Can't believe I missed htis thread. Well I think this is going to be a problem for some time to come. I suggest a size and orbit rule. Size will generally edit out the sphere issue as anything of signifigant size will have its gravity center fluch out a near sphere (in planetary terms please). The orbit thing is catchy as well, and I'm betting that even if we came up with something fairly solid we shouldn't be surprised when our conventions fail. Its a vast solar system and a considerably mosr vast galaxy, and well the universe. . .well just trying to visualize it is impossible so that syas alot.

Again size and orbit, of course its arbitrary but than again what isn't when you write up rules.

 

[Perfection]

Can't believe I missed htis thread. Well I think this is going to be a problem for some time to come. I suggest a size and orbit rule. Size will generally edit out the sphere issue as anything of signifigant size will have its gravity center fluch out a near sphere (in planetary terms please). The orbit thing is catchy as well, and I'm betting that even if we came up with something fairly solid we shouldn't be surprised when our conventions fail. Its a vast solar system and a considerably mosr vast galaxy, and well the universe. . .well just trying to visualize it is impossible so that syas alot.

Again size and orbit, of course its arbitrary but than again what isn't when you write up rules.

Your ideas seem somewhat in-line with my definition scheme. Tel me what you think of them.

 

[Tulkas12]

Your ideas seem somewhat in-line with my definition scheme. Tel me what you think of them.

I like it, it just seems that it would avoid alot of the error we will undoubtedly encounter. We will encounter things that will bend this too probably, but as for a
set criteria I'm in agreement with you.

 

[Bluemofia]

Of course, but the question becomes what to call them during this time. I don't like "wait and see" approaches

Planetesmals or Proto-Planets. I'm not picky.

Then again, when do you consider a body a planet when the system is forming?

Nah, they'd still interact.

How? They are at opposite ends of their orbits, obviously past the limit for their gravity to become a significant force on each other, with their star becomming more dominant.

Well, I don't know how long it took Orpheus to smash into Earth. It may have been after the bulk of the condensation into planets.

There was still mass bombardment during that phase. Thus, I still call them planetesmals.

 

[Perfection]

Planetesmals or Proto-Planets. I'm not picky.

Well the problem is it doesn't only apply to developing systems, changing systems too could cause the planetness of something to be questioned in simple orbit location systems.

Then again, when do you consider a body a planet when the system is forming?

You go potential to alter the situtation.

How? They are at opposite ends of their orbits, obviously past the limit for their gravity to become a significant force on each other, with their star becomming more dominant.

Uh, because it is significant, I mean the planets have thousands and millions of years for those perterbations to take effect.

There was still mass bombardment during that phase. Thus, I still call them planetesmals.

There is still mass bombardment today...

 

[Eran of Arcadia]

They call the thing that smacked into earth and created the moon, Orpheus?

I had a dream the other night that I went back in time to see it happen. Only in my dream it was called Omega, and some aliens had colonized it. I had a heck of a time convincing them to leave, because they were about to collide with my future home planet.

 

[El_Machinae]

How about:

"Big enough that a person couldn't jump off of it"?

 

[Bluemofia]

Well the problem is it doesn't only apply to developing systems, changing systems too could cause the planetness of something to be questioned in simple orbit location systems.

Hmm, but then the only way something like that happens is an extreme close (and probably rare) approaches to another star, so exceptions would probably be made based on that.

You go potential to alter the situtation.

Potential? This is up to much speculation of what could potentially have formed a planet, such as an object in the asteroid belt could have potentially formed a planet, except there was Jupiter there.

Uh, because it is significant, I mean the planets have thousands and millions of years for those perterbations to take effect.

Fine.

There is still mass bombardment today...

Not enough to have the surfaces of planets to have molten surfaces due to asteroid bombardment.