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Defining Planethood
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| steeljaw354 | Date: Monday, 15.08.2016, 13:04 | Message # 196 |
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| 1) Orbits one or more stars or stellar remnants
2) Is in hydrostatic equilibrium
3) Doesn't undergo fusion currently or in the past
4) Doesn't orbit another object that applies to this set of rules. Or else it's a moon.
I have revamped my set of rules, is it better? Why do some people even include the orbital clearing? Is an excuse to avoid a large number?
Edited by steeljaw354 - Monday, 15.08.2016, 13:07 |
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| midtskogen | Date: Monday, 15.08.2016, 17:31 | Message # 197 |
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| Quote Watsisname (  ) There could be a useful approach in that the accretion phase of planet growth doesn't last very long relative to the age of the system -- but then again that also depends on the mass of the star. What if we're dealing with high mass stars? So... it's more complicated than we might like. It works well for our system and we think other sun-like and low-mass systems.
Proto-planets could stop being proto-planet once it reaches some level of maturity regardless of what's left in its orbit. I'm not sure. I think what I don't like about "cleared its neighbourhood" in the current defintion isn't really the whether it's a completely worthy planet or not, but that those planets which don't qualify are to be a single group called dwarf planets. In our solar system they're all dwarfs and will never reach full planethood, but some in other solar systems might not even be "dwarfs" at all. By this definition our Neptune, Saturn and perhaps Jupiter were at some point "dwarf planets", which is a poor name, but it might be useful to distinguish between planets that will likely reach planethood and those which never will.
NIL DIFFICILE VOLENTI
Edited by midtskogen - Monday, 15.08.2016, 17:32 |
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| steeljaw354 | Date: Monday, 15.08.2016, 17:44 | Message # 198 |
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| Why are these objects not planets in your view? Does the orbit/neighborhood actually matter? It's the physical characteristics of the object we should be using to define it. Moons aside.
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| Watsisname | Date: Wednesday, 17.08.2016, 07:20 | Message # 199 |
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| Quote midtskogen ( ) Proto-planets could stop being proto-planet once it reaches some level of maturity regardless of what's left in its orbit. I'm not sure. I think what I don't like about "cleared its neighbourhood" in the current defintion isn't really the whether it's a completely worthy planet or not, but that those planets which don't qualify are to be a single group called dwarf planets. In our solar system they're all dwarfs and will never reach full planethood, but some in other solar systems might not even be "dwarfs" at all. By this definition our Neptune, Saturn and perhaps Jupiter were at some point "dwarf planets", which is a poor name, but it might be useful to distinguish between planets that will likely reach planethood and those which never will.
Yeah, I get that. Soter's "orbit-clearing" discriminant implies still-growing gas giants are still called dwarf planets, even if they're several Earth masses. Which is kind of silly. But I think the idea is that this criterion is meant for mature systems, not young ones where accretion is still an important process and the objects are evolving quickly. I don't think we would seriously call a nascent Jovian a "dwarf planet".
Margot's Pi is interesting in this case -- it would call the still-growing Jovian a planet once it surpassed the orbit-clearing mass. Personally, that's what I like about Margot's Pi. It doesn't even consider how much stuff remains in the orbit. It doesn't care if the orbit is 'clear' or not. It brings us directly to the accretion-scattering dynamics in young systems which result in Soter's discriminant working in mature systems. But by the same token, it also doesn't account for binary planets, or Earth-Theia situations.
Quote steeljaw354 ( ) Why are these objects not planets in your view? Does the orbit/neighborhood actually matter? It's the physical characteristics of the object we should be using to define it. Moons aside.
Physical characteristics are important, but I agree with the astronomical community in thinking that the dynamics are important as well. Think of the graphic I showed in the Q&A thread for Margot's Pi: the orbit-clearing threshold depends not just on the mass of the object, but also how far from its star it is orbiting! So location does matter.  We say that the 'clearing' of the orbit is important because it's a result of the processes that build a planet from a bunch of rocks.
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| steeljaw354 | Date: Wednesday, 17.08.2016, 13:58 | Message # 200 |
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| Location doesn't matter, if a planet is jupiter sized and hasn't cleared it's orbit, people will say it's a dwarf planet, it makes 0 sense. Classification by location makes no sense, moons aside. So what if it's orbit is to far from it's star? It's still a planet regardless of it's orbital location or if it's cleared or not.
Edited by steeljaw354 - Wednesday, 17.08.2016, 14:04 |
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| spacer | Date: Wednesday, 17.08.2016, 14:04 | Message # 201 |
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| Quote steeljaw354 ( ) Classification by location makes no sense
wats just showed it is make sense. because the location is matter. if an object is father from the star, it will also move slower.
and the location will also related to the past. what was in that location billions of years ago. where the planets formed etc.
but wats will explain better
"we began as wanderers, and we are wanderers still"
-carl sagan
-space engine photographer
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| steeljaw354 | Date: Wednesday, 17.08.2016, 14:05 | Message # 202 |
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| Even if the object is jupiter sized and it's orbit isn't clear?
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| spacer | Date: Wednesday, 17.08.2016, 14:12 | Message # 203 |
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| a jupiter sized planet orbit will be probably clear by its own gravity.
"we began as wanderers, and we are wanderers still"
-carl sagan
-space engine photographer
Edited by spacer - Wednesday, 17.08.2016, 14:14 |
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| steeljaw354 | Date: Wednesday, 17.08.2016, 14:20 | Message # 204 |
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| Not if you have a really massive star like something like Rigel, and massive stars like that usually have large disks. The more massive the star, the more massive the disk, how big does "orbital clearing" have to be? Why does a planet need to clear it's orbit?
How do you expect something jupiter sized to clear something like that?
Edited by steeljaw354 - Wednesday, 17.08.2016, 14:20 |
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| Watsisname | Date: Wednesday, 17.08.2016, 17:00 | Message # 205 |
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| Quote steeljaw354 ( ) The more massive the star, the more massive the disk, how big does "orbital clearing" have to be?
It doesn't have to clear the whole disk. Just its orbital space, defined with respect to its Hill Sphere. Margot uses 2sqrt(3) of the Hill Sphere radius as the minimum range for a planet to clear. The motivation for that choice is that this is the size of the feeding zone of the planet, and this is consistent with simulations for a wide range of possible systems.
How effective the body is at clearing that space depends on the transference of orbital energy by scattering, or the direct removal by accretion. Here's the formula:
This shows that for a star the mass of Rigel, 1 Jupiter mass is not enough to clear an orbit at more than 100 AU. That's not because the disk is too massive. It's because the orbit is too far out (where they are slow), and a star that massive doesn't live long enough. This is why there is a disk there at that distance and not planets. The definition you think makes no sense proves the image that you show.
Massive stars like the A's and B's can form planets, but they are closer in. There are also strong winds and evaporation of the disk by the intense radiation. Forming planets around high mass stars is hard, and they must form very quickly or not at all.
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| steeljaw354 | Date: Wednesday, 17.08.2016, 17:05 | Message # 206 |
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| Classifying planets by location makes no sense. They should be classified by physical attributes, not by location. Would you think that this is a better definition? Math shouldn't be used to determine if the object is a planet or not, mine is simple, no math or arbitrary boundaries. Planemos are entirely different discussion as are moons also. Yes it might include alot of objects, but mnemonics don't really matter in science. The number of these objects by the rules below is probably around 30, but so what?
1) Must orbit a star or stellar remnant.
2) Must maintain hydrostatic equilibrium.
3) Must not be massive enough for fusion in the present, future or past.
Edited by steeljaw354 - Wednesday, 17.08.2016, 17:14 |
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| Watsisname | Date: Wednesday, 17.08.2016, 18:30 | Message # 207 |
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| Your definition isn't bad. I told you that with the last one. It's a lot better than some of the others you've suggested. Personally, I prefer a system that captures the dynamics of planet formation, and explains how a few dominant bodies are separated from a lot of non-dominating ones. Which depends on location, distasteful as you find that for whatever reason. :)
I'm sorry you don't like math, but math is a tool which helps us understand nature. Without math we wouldn't have Space Engine.
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| steeljaw354 | Date: Wednesday, 17.08.2016, 18:39 | Message # 208 |
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| It's not that I don't like math, it's that it shouldn't be included in a planet definition. If you cloned mercury and put where every planet is, Mercury-Pluto. And you erased all moons. All of the mercuries would be planets except for the one that is in Pluto's place by your standards. Exact same objects classified differently makes no sense what so ever.
Classifying objects by orbital distance, whether or not the orbit is clear isn't a good system to use. Again if Neptune and Jupiter had cleared there orbits there would be no objects in their orbits that isn't the case. The IAU definition says nothing about orbital domination.
Quote is massive enough to be rounded by its own gravity,
is not massive enough to cause thermonuclear fusion, and
has cleared its neighbouring region of planetesimals
Edited by steeljaw354 - Wednesday, 17.08.2016, 18:44 |
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| Watsisname | Date: Thursday, 18.08.2016, 02:07 | Message # 209 |
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| Quote steeljaw354 ( ) It's not that I don't like math, it's that it shouldn't be included in a planet definition.
You don't have to include the math. You can recognize without math that the 8 currently defined planets in our system are very unique in their orbital space, whereas objects like Ceres and Pluto and Eris are not that unique -- they share the region with a lot of similar objects.
The math (easy math -- dividing one number by another number) shows that this difference isn't small. It's huge. Too huge for us to simply ignore.
The harder math shows why that happens. It captures the dynamics of planet formation. That's why it's so useful. That's why this definition works. And, much as you may not like it, this dynamics is orbital-distance dependent. Orbits are faster at smaller distances, so it takes a less massive object to dominate its orbit at smaller distances. This crazy-sounding location-dependent definition comes directly from nature. To dismiss it is to ignore nature.
Quote steeljaw354 ( ) The IAU definition says nothing about orbital domination.
Orbital domination and orbital clearing are exactly the same idea expressed in different language. One naturally leads to the other.
Quote steeljaw354 ( ) If you cloned mercury and put where every planet is, Mercury-Pluto. And you erased all moons. All of the mercuries would be planets except for the one that is in Pluto's place by your standards.
The planet formation process does not allow an isolated Mercury to form in Pluto's place. Precisely because it isn't massive enough to clear that orbit in solar system timescales. Either the disk at that location must form a more massive planet, or it remains a disk. The definition which you think makes no sense, works.
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| spacer | Date: Thursday, 18.08.2016, 02:32 | Message # 210 |
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| Watsisname, that what make mercury a planet if understand right.
if it was in pluto place. his orbit probably wont be clear and then mercury would not be a planet?
or you mean that there is no chance for a mercury sized object to even form there?
"we began as wanderers, and we are wanderers still"
-carl sagan
-space engine photographer
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