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Forum » SpaceEngine » Science and Astronomy Discussions » Defining Planethood (What sets planets apart?)
Defining Planethood
HarbingerDawnDate: Saturday, 15.06.2013, 16:45 | Message # 16
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I don't know at what ratio the stability begins to fail

I used to know, but have since forgotten. I think that the example I listed would be stable, but I wanted a second opinion.

Quote (Watsisname)
I wonder what would happen to our understanding/definition of planethood if we were able to view other solar systems in arbitrary detail (resolving even asteroid size bodies). Do you think it would become more or less coherent with such a greater sample size?

I think that the advantages of my way of describing planethood would become more apparent when you consider a wider variety of examples. You start to encounter problems in trying to describe it mathematically, it gets very complicated. For example, in the scenario I brought up of the Earth-analog being trojan to the gas giant, would that Earth then be a planet under your criteria? From what I understand it would not be. Yet under my way of looking at it, it would indeed be a planet since it clearly would not belong in any other category.

This is why I advocate for a slightly less quantitative set of criteria for defining planethood. It's more flexible, and ultimately less problematic.





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Edited by HarbingerDawn - Saturday, 15.06.2013, 16:46
 
WatsisnameDate: Saturday, 15.06.2013, 16:59 | Message # 17
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Aha, found it. The L4 and L5 points are stable for mass ratio parameter μ<0.03852. So an Earth mass body could be stable as a Trojan body of a 6MJ giant. (Mass ratio 0.0005).

Edit:

Quote
For example, in the scenario I brought up of the Earth-analog being trojan to the gas giant, would that Earth then be a planet under your criteria? From what I understand it would not be.


Yes, you and Vladimir raised a very good counter-example. smile From my definition using mass ratios the Earth would clearly not be a planet, and possibly the Jovian might not be as well depending on what threshold we set the mass ratio (else, I believe, Ceres should be called a planet as well).

I'm not sure how I would choose to handle a case like this. From a standpoint of dynamics it's comparable to Jupiter, but if all the Trojan asteroids (which we obviously don't consider planets) were accreted together (and a lot more) to form a body which we'd ordinarily call a planet in its own right, but which exists in a stable configuration with the larger body. So is it a Trojan object of the Jovian, or is it a planet, or is it both? I'm tempted to think it should be called a Trojan planet. But then what decides the boundary between Trojan asteroid and Trojan planet? I don't think this is an easy question, and my definition of a planet would definitely require some modifications.







Edited by Watsisname - Saturday, 15.06.2013, 17:30
 
paradanDate: Monday, 17.06.2013, 14:49 | Message # 18
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Well gentlemen, I'm afraid that until Pluto get's its planet status back, Australia has been reduced to an island.
I imagine it's very upsetting for them.
 
HarbingerDawnDate: Monday, 17.06.2013, 15:21 | Message # 19
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I'm afraid that until Pluto get's its planet status back

It won't.

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Australia has been reduced to an island

It is, and that's not a reduction.





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Edited by HarbingerDawn - Monday, 17.06.2013, 15:22
 
smjjamesDate: Monday, 17.06.2013, 18:08 | Message # 20
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Australia has been reduced to an island.


It's an island continent AND it has it's own tectonic plate. However, it won't be an island forever, it'll collide into SE Asia.





 
HarbingerDawnDate: Monday, 17.06.2013, 19:26 | Message # 21
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But then what decides the boundary between Trojan asteroid and Trojan planet?

Whether there are many objects or few. Certainly an object which even might be considered a trojan planet would be either rare or unique in that planet's orbit. Also, if we were to disregard trojan planets are being true "planets", then should we likewise disregard binary planets?





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WatsisnameDate: Monday, 17.06.2013, 23:47 | Message # 22
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Whether there are many objects or few


Seems reasonable. It's almost like a logical extension of what formed the original definition of a planet I was using. If there is a planet-sized body there it presumably must have either accreted or scattered much of the other material (I'm not certain about the dynamics involved here, but I'm guessing this is true). And it's probably very unlikely that there would only be a single undifferentiated body there.

Quote
Also, if we were to disregard trojan planets are being true "planets", then should we likewise disregard binary planets?


I'm having a difficult time thinking of a good reason of dismissing one without also dismissing the other. From a perspective of dynamics they are very similar -- large bodies in stable configurations with one another, that have collectively cleared their region of similar-sized bodies.

Let me propose an extension of the definition I proposed earlier on, and see if it works or not. An object may be classified as a planet if:

1: It is massive enough to be differentiated (I'm thinking this is actually redundant given the latter requirements, but whatever.)
2: It has cleared out its orbit of comparable bodies (comparable in terms of size, mass, how do we set limit?), except for those in a stable orbital configuration with it (e.g. within its Hill Sphere or L4/L5 Lagrange points).
3: Alternatively to 2, the body is stably within the L4/L5 region of a larger planet, and has cleared/accreted other bodies in this region.

While on the note of binary planets there is also the matter of what separates a binary planet from a planet and moon. Location of barycenter seems like a good way to start, but is still arbitrary due to range of possible object densities.





 
HarbingerDawnDate: Tuesday, 18.06.2013, 00:09 | Message # 23
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While on the note of binary planets there is also the matter of what separates a binary planet from a planet and moon.

This must be defined by mass ratio. I use 1:20, but it's arbitrary, as any other ratio would be.

Quote (Watsisname)
Location of barycenter seems like a good way to start

No, location of the barycenter is a bad way to do it, since that depends more on distance than mass. For example, the Earth-Moon barycenter is inside Earth, yet the Sun-Jupiter barycenter is outside of either body. Should Sun-Jupiter then be considered a binary? Naturally the answer is no. Mass ratio removes the separation issue, and focuses purely on the physical relationship between the two bodies.

Quote (Watsisname)
It has cleared out its orbit of comparable bodies

This is still ambiguous. What if another large body is temporarily perturbed into the orbit of a previously-unimpeachable planet? Would both planets suddenly become not planets? And if so, what would they be?

Every system you (and the IAU) have proposed breaks down in situations like this, whereas my admittedly qualitative system continues to function. This highlights what I was trying to say at the beginning, which is that you simply must judge planethood on a situational basis according to a basic understanding of the objects in question and the environment in which they exist. Mathematical descriptions and definitions similar to the ones you have put forth continually fail given some case or another.

(Sorry if I happen to sound like a jerk, I have trouble modulating my tone, so I can come across sounding differently than I intend.)





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smjjamesDate: Tuesday, 18.06.2013, 00:34 | Message # 24
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It's a little out there, but I had a thought, what about dominating it's gravitational space? Of course though, we'd have to define what the gravitational space is, like the point where the objects gravitational pull is in equilibrium with it's sun.

I know it doesn't work 100%, but just throwing the idea out there for the sake of discussion.





 
HarbingerDawnDate: Tuesday, 18.06.2013, 02:55 | Message # 25
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smjjames, it sounds like you're talking about the same sort of thing that Watsisname has been this entire time. It's hard to tell though since you - by your own admission - haven't defined just what you're talking about. Try to be more clear and detailed when participating in discussions like this.




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Crashman1390Date: Tuesday, 18.06.2013, 03:59 | Message # 26
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What are you talking about?

I was told by someone that New horizons would reach Pluto in 2014.
Apparently he was an incompetent fool who didn't know what he was talking about.





[b]I was wrong, this forum still has a horrible community.[/b]
 
WatsisnameDate: Tuesday, 18.06.2013, 07:07 | Message # 27
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Once New Horizons passed Jupiter, NASA would be unable to change its arrival date at Pluto so drastically even if they wanted to. It simply doesn't have the delta-v for it. Not to mention what little delta-v it does have has better uses, such as debris avoidance (which they are very strongly considering, given what appears to be a significant debris hazard around Pluto). They'll also want some delta-v for setting up flyby's of any other suitable KBO's after Pluto.

Quote (HarbingerDawn)
No, location of the barycenter is a bad way to do it, since that depends more on distance than mass.

You're right, I should have given that a moment's more thought. Mass ratio would be most appropriate and has no undesired parameters, unlike the center of mass.

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What if another large body is temporarily perturbed into the orbit of a previously-unimpeachable planet? Would both planets suddenly become not planets? And if so, what would they be?

With the way I define planets, yes. smile There are a few different ways such an event could occur, each deserving some discussion:

It happens most frequently in young systems that have not yet settled down into a stable configuration. I don't consider such bodies to be planets; they're not yet dominating their orbital region. In our own solar system, Theia colliding with Earth is an example, and I would not consider Theia or the Earth to be planets at that point in time, but rather protoplanets.

Alternatively (and I understand this is what you're proposing), perturbations in mature solar systems may lead to chaotic situations where an object that was already unambiguously a planet may end up in an orbit that comes close to or even crosses that of another planet. (We actually can't completely rule out such a thing happening in our own solar system, far in the future). Or a rogue planet from interstellar space may come through the solar system (unlikely but not strictly impossible).

In that sort of situation, I would indeed revoke planetary status, until such time that the event has ended. Perhaps that sounds totally absurd, but this is how I think of planetary status -- not just representing the physical characteristics of an object, but dynamical, also. If a planet did get perturbed into an orbit of another planet, that's a really big deal, and one may be all but certain that there will be profound consequences for the system, ending with the ejection or destruction of planets, or formation of a new one. Therefore revoking planetary status until the system has settled down again makes sense to me.

I don't know what I'd call the participating objects during this sort of an event. Needs a good name, and rogue planet doesn't work because it means something else entirely.

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Every system you (and the IAU) have proposed breaks down in situations like this, whereas my admittedly qualitative system continues to function.


I don't disagree with you here, coming up with a rigorous and functional definition of a planet is extremely challenging (as evident from IAU's difficulty with the same subject). It may just be that as a scientist I like being quantitative wherever possible. And don't worry about your writing style, I think it's just fine and you're making extremely good points throughout. smile





 
HarbingerDawnDate: Tuesday, 18.06.2013, 21:08 | Message # 28
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(We actually can't completely rule out such a thing happening in our own solar system, far in the future)

It is actually thought to have happened in the past; the Nice model, which is the best model we have so far for how the Solar system developed, shows that the planets were all fully formed, and then Jupiter and Saturn fell into a 1:2 resonance which flung everything into chaos, culminating with Neptune and Uranus actually switching places, and all of the outer planets generally moving outward (though I'm not sure how that works). This caused Neptune to go flying into the then-dense Kuiper belt, and scattering its contents in all directions, leading to the peculiar configuration we see out there today and also causing the Late Heavy Bombardment.

Quote (Watsisname)
It happens most frequently in young systems that have not yet settled down into a stable configuration.

I would agree with not calling such objects planets in developing systems, but we already have another clear category to place those objects into (protoplanets). An interesting side question here is when does a protoplanet stop being a protoplanet? Should objects like Ceres and Vesta retain that classification forever, or are they reclassified when planet-forming is complete?

Quote (Watsisname)
In that sort of situation, I would indeed revoke planetary status, until such time that the event has ended. Perhaps that sounds totally absurd, but this is how I think of planetary status

It doesn't sound totally absurd, I get where you're coming from on this. I do, of course, disagree and would advocate for still calling them planets. To me it does seem wrong to try to change their classification under those circumstances. The only time I can think of (under my system and my way of thinking about this) that something which would otherwise be a planet should be reclassified is if it was clearly a member of a distinct population. For example, if there were a belt of 50 Earths orbiting some star, then I would propose not calling those planets, since they obviously form a separate population based on their orbital and physical characteristics.

I think that in order to revoke planet status from an object, there needs to be another obvious population/category to group it with, either one which is known or can be easily inferred.





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WatsisnameDate: Thursday, 20.06.2013, 23:37 | Message # 29
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(Super long post is super long).

The Nice model as I understand it is indeed the currently most favored model of late solar system formation/evolution, and it certainly does paint a picture of extreme chaos during that time period. The evolution of the solar system is a big interest of mine and I'd love to go into a little more depth here about it.

Quote
and all of the outer planets generally moving outward (though I'm not sure how that works).


Let's think about it from principles of physics. smile An object in a larger orbit has more angular momentum than one in a smaller orbit, so for a planet to migrate it must change its angular momentum. However, angular momentum is a conserved quantity. Therefore there must be an opposing change in angular momentum of something else. Generally this involves interactions between the planet and the nebular disk, but that had already been cleared out by this time.

So what happened? There was no gas, but there was still a dense disk of planetesimals in the outer solar system (about 34 Earth masses, apparently). Gradually objects near the inner part of the disk got kicked around by the outermost planet, and ended up being scattered by all the giants. This scattering is what allowed the transfer of angular momentum, shifting Jupiter slightly inward, while the rest of the outer planets migrated outward. Consequently Jupiter and Saturn crossed their 1:2 orbital resonance, which set the stage for even greater chaos.

The resonance pushed Uranus and Neptune into more eccentric orbits, leading to close encounters that may have caused them to switch places (it's not certain but it is hinted at by the models, and swapping their original places does seem to fit better with the expected conditions of the solar nebula). And as you noted, Neptune also went through the planetesimal disk, scattering a huge amount of material out of it and starting the Late Heavy Bombardment.

So yeah, a lot happened, but ultimately, it's the fact that Neptune (or Uranus, whichever was originally outermost) was still interacting with a debris disk in the outer solar system that starts it. Jupiter and Saturn going into the resonance was an intermediate step.

Quote
An interesting side question here is when does a protoplanet stop being a protoplanet? Should objects like Ceres and Vesta retain that classification forever, or are they reclassified when planet-forming is complete?


I think the term protoplanet should mean that the object is differentiated and undergoing accretional growth. So if it is destroyed, ejected from the system, accretes its neighbors to form a planet, or if further accretion comes to an end for some reason, then it is no longer a protoplanet. Ceres and Vesta may be called remnant/surviving protoplanets because they fall into the last category -- protoplanets that were neither destroyed or removed, but failed to become a planetary body because of the influence of Jupiter.

It should be noted that Ceres alone accounts for about a third of the total mass of the asteroid belt, so if everything in the belt today did somehow become a planet it wouldn't be very much larger than Ceres.

Another fun question is when does a planetesimal become a protoplanet, or when does a grain/clump become a planetesimal? I think the manner in which they grow is a crucial factor here, with grains/clumps growing primarily by electrostatic forces, then being called planetesimals when their self gravity becomes a significant contributor, and finally protoplanet when they become massive enough for differentiation to occur. These different regimes may be described in terms of mass or radius, but as is typical with this sort of thing there is no distinct boundary between them.

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It doesn't sound totally absurd, I get where you're coming from on this. I do, of course, disagree and would advocate for still calling them planets. To me it does seem wrong to try to change their classification under those circumstances.


That's perfectly fair. I believe either way we look at it is fine, and ultimately the objects in question don't care. And of course if such an event happened in our solar system right now then we'd have far greater concerns. biggrin
I do still believe our ways of defining a planet are strongly related to one another, with perhaps the main difference being I try to reduce the formulation into a list of principles that can then be applied elsewhere, whereas you begin the classification based upon what is observed. Your view certainly has the advantage if/when we are able to explore other solar systems in great detail, and it would be endlessly fascinating to see what would be learned from such data.

Edit: I should have proofread. Fixed some typos and incorrect words.







Edited by Watsisname - Friday, 21.06.2013, 09:56
 
WatsisnameDate: Monday, 24.06.2013, 22:04 | Message # 30
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What a funny coincidence, this month's (July) issue of National Geographic magazine's feature article is on exactly this topic -- the migration of the giant planets and the Late Heavy Bombardment. smile




 
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