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Defining Planethood
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| steeljaw354 | Date: Saturday, 11.06.2016, 21:55 | Message # 166 |
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| Lets say we have a 1 earth mass black hole, orbiting the sun where earth is and it's orbit is clear, is it a planet by the IAU terms?
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| FaceDeer | Date: Sunday, 12.06.2016, 00:26 | Message # 167 |
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| Yes. You don't really need to ask, though, the definition is rather straightforward and you can see for yourself:
Quote A "planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and © has cleared the neighbourhood around its orbit. [1]
Criterion a: yes, it's orbiting the Sun.
Criterion b: yes, it's round due to self-gravity. Very much so.
Criterion c: yes, it has cleared its orbit.
It's unclear whether an Earth-sized black hole could even form, though, so this is a pretty hypothetical question. Black holes that small are hypothesized to have maybe been able to form during the big bang (primordial black hole) but there's no known process that could form them since then and there's no evidence that primordial black holes exist.
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| Watsisname | Date: Sunday, 12.06.2016, 01:04 | Message # 168 |
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| I would argue otherwise. While black holes are certainly very round (probably the most round things in the universe!), they are not round because of hydrostatic equilibrium.
Hydrostatic equilibrium means that for matter at all radii in a body, inward forces due to its gravity are balanced by outward pressure forces (and centrifugal forces if spinning). So it really only applies to distributions of matter. A black hole is not made of matter and there are no pressure forces. There is no balance of forces involved in producing the shape. If you put a particle near one, it just falls in.
So, black holes are not planets. They're distortions of the space-time. They have a very different physical nature and genesis. 
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| steeljaw354 | Date: Sunday, 12.06.2016, 01:17 | Message # 169 |
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| Then if this black hole is a planet by the IAU rule, that rule needs an overhaul, if a 1000km rule instead of orbital clearing would eliminate that black hole from planet status because an earth mass black hole is about peanut sized. Oh did I forget to mention that any "planet" that doesn't orbit sun, it is NOT a planet regardless of it's mass or size? That's a bias rule there. And a rule like this 1) Must be round, 2) Must orbit a star, 3) Must have a diameter of 1000km or higher, 4) Must not undergo nuclear fusion or deuterium fusion currently or in the past, (Black holes, white dwarfs, neutron stars are eliminated from planet status) No complicated math needed here. Just simple observation. Honestly, would you rather just use this instead of having to do math? I would.
Edited by steeljaw354 - Sunday, 12.06.2016, 01:26 |
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| Watsisname | Date: Sunday, 12.06.2016, 05:56 | Message # 170 |
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| Black holes are not planets by any established rule -- their shapes are not defined by hydrostatic equilibrium.
Planets that don't orbit stars are 'rogue' planets.
Using a 1000km limit is unsatisfactory because it is a doubly-biased choice of number and unit, and the objects have a smooth distribution of sizes on that scale. You are choosing 1000km because you believe it is easier, not because there is anything fundamentally different about a 999km object and a 1001km object.
Quote Honestly, would you rather just use this instead of having to do math? I would.
No, because they are not on equal footing even without considering the math. If you had two classification systems that had equally low bias in their definitions, but varying levels of mathematical difficulty, then sure, I would use the easier one.
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| FaceDeer | Date: Sunday, 12.06.2016, 06:13 | Message # 171 |
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| Actually, I'd argue that it doesn't matter why a black hole is round. The criterion is "has sufficient mass for its self-gravity to overcome rigid body forces", and an Earth-mass object does indeed have sufficient mass to overcome rigid body forces.
But I'd also argue that Steeljaw's objection is silly, for exactly the reason I mentioned in my first response - as far as we're aware there's no such thing as an Earth-mass black hole. The fact that the IAU's definition gives funny results for imaginary made-up things is hardly a black mark on the definition.
Steeljaw, there's a separate definition the IAU uses for exoplanets. It was devised before the official definition for Solar system planets was arrived at and only needs to deal with objects large enough to be detected with current or near-term foreseeable technologies so it doesn't need to be as sophisticated as the Solar system one. Presumably it will eventually get refined to more closely match the Solar system planet definition now that we have one since. It's quite possible to calculate orbit-clearing capabilities from just planetary mass and orbital period (things we can determine for many exoplanets).
And no, I would not rather use your 1000km criterion. I've explained why not before. It isn't based on any meaningful physical threshold, and there are already known cases (Sedna) that straddle the line so closely that it'd be impossible to definitively put them in one category or another. It's just an arbitrary number of no significance.
The formulae for rating a planet's orbit-clearing capability are not that complicated or hard to understand. It's not really a valid argument to just keep repeating that the math's "too complicated." This is astrophysics. You need to be able to do a little math sometimes for it to make sense.
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| midtskogen | Date: Sunday, 12.06.2016, 06:14 | Message # 172 |
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| Quote Watsisname (  ) Black holes are not planets by any established rule -- their shapes are not defined by hydrostatic equilibrium.
One can argue that their shape, in 3D space, isn't defined at all.
NIL DIFFICILE VOLENTI
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| Watsisname | Date: Sunday, 12.06.2016, 06:43 | Message # 173 |
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| The event horizon is well defined in 3D space -- it is a null surface. This is why the surface area of a black hole is invariant, even though the volume enclosed is not.
Further discussion of black hole shape and relativity-related stuff split here.
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| steeljaw354 | Date: Sunday, 12.06.2016, 14:42 | Message # 174 |
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| Would you like this instead?
1) Orbits the sun
2) Is in hydrostatic equilibrium
3) Is not man made
4) Doesn't undergo nuclear fusion or deuterium fusion currently or in the past
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| Watsisname | Date: Thursday, 16.06.2016, 06:31 | Message # 175 |
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| That's not bad. I think there are additional properties we can use that aren't accounted for here, but as a classification system this is not terribly biased or arbitrary. It would contain a heck of a lot of objects.
One potential issue is that the lower bound of "is in hydrostatic equilibrium" is somewhat fuzzy. This is one of the motivations for the discriminant or scattering ability -- it appears to be much less fuzzy.
The "not man made" criterion might be redundant, since I think it is safely covered by "is in hydrostatic equilibrim". At least until/if we build objects large enough to do that. But then maybe they are artificial planets?
"Orbits the sun" could be generalized to "orbits a star" if we want to refer to planetary systems in general, or we could say those are exoplanets. Similar idea for planets orbiting non-stellar objects, or freely wandering.
One other thing is it completely avoids the issue of binary planets vs. moons, Trojan bodies, and the like. How would you classify two objects of similar size/mass orbiting each other, orbiting the sun?
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| midtskogen | Date: Thursday, 16.06.2016, 11:23 | Message # 176 |
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| Quote Watsisname ( ) One other thing is it completely avoids the issue of binary planets vs. moons, Trojan bodies, and the like. How would you classify two objects of similar size/mass orbiting each other, orbiting the sun?
Perhaps:
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 share a centre of gravity within another object satisfying the above
That should exclude moons and allow double planets. Trojan planets would also be allowed, which to me makes sense.
NIL DIFFICILE VOLENTI
Edited by midtskogen - Thursday, 16.06.2016, 13:17 |
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| steeljaw354 | Date: Thursday, 16.06.2016, 19:27 | Message # 177 |
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| Thats a definition that I agree with, better than the IAU one. Systems like Earth-Luna aren't double planets but anything with the barycenter outside the surface should have both be planets.
Edited by steeljaw354 - Thursday, 16.06.2016, 23:04 |
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| Watsisname | Date: Friday, 17.06.2016, 01:43 | Message # 178 |
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| Harb and I talked about using location of barycenter as separator of moons and binary planets earlier back in page 2, and came to the conclusion that it's not a very good one -- it depends not just on the masses of the bodies, but also their densities and distance separating them.
Distinguishing by their ratio of masses may be better, but then it seems like an arbitrary decision as to what limit that should be.
So I think this one is a bit more tough. It would be nice if there is some natural separation between moons and things we would prefer to call binary plants. I'm not really aware of any though. Besides Pluto/Charon and maybe Earth/Luna, we don't know of a lot of examples.
Edit: Binary plants? An explosion of self-proliferating digital flora permeates my brain.
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| JackDole | Date: Friday, 17.06.2016, 02:26 | Message # 179 |
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| Quote Watsisname ( ) It would be nice if there is some natural separation between moons and things we would prefer to call binary plants.
How about this:
If the moon has enough mass and it is close enough to the planet to stabilize the axis of rotation of the planet, then it is a binary planet system.
As is the case with Earth-Moon.
Don't forget to look here.
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| midtskogen | Date: Friday, 17.06.2016, 05:04 | Message # 180 |
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| Quote Watsisname ( ) Harb and I talked about using location of barycenter as separator of moons and binary planets earlier back in page 2, and came to the conclusion that it's not a very good one -- it depends not just on the masses of the bodies, but also their densities and distance separating them.
The Sun-Jupiter example does of course not become a problem for a definition of binary planets, since the Sun is not a planet, failing both 1 and 3.
A centre of gravity inside or outside a body is of course pretty arbitrary but it's a physical borderline. That point could sometimes be inside, sometimes outside due to irregularities of the bodies or interactions with other bodies, but we could say "always outside" for those cases. A definition of what's inside and outside might also be somewhat arbitrary and hard to find a natural limit for (especially for planets with no proper surface), but we can live with that.
As for dependency on density, perhaps it's odd that a body would be a planet with one planet and not with another planet of equal mass but different density, but I don't see a major problem with it either.
Are there any non-pathological examples for when the above criteria fail?
NIL DIFFICILE VOLENTI
Edited by midtskogen - Friday, 17.06.2016, 06:31 |
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