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Defining Planethood
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| steeljaw354 | Date: Thursday, 09.06.2016, 10:33 | Message # 151 |
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| Here is the image.
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| midtskogen | Date: Thursday, 09.06.2016, 10:43 | Message # 152 |
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| Rename a planet because an incorrect pronunciation in a single language is the source of jokes? Chances are high that Caelus could be something awkward in some other language.
NIL DIFFICILE VOLENTI
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| HarbingerDawn | Date: Thursday, 09.06.2016, 11:27 | Message # 153 |
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| Quote steeljaw354 (  ) And I also think uranus should be renamed to "Caelus" to avoid the issues with the name "uranus". It is pronounced "Say-less" instead of the rather awkward name that uranus has.
Uranus is not pronounced the way you think it is. How about instead of renaming it, just start pronouncing it correctly...
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| JackDole | Date: Thursday, 09.06.2016, 11:30 | Message # 154 |
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| Tartaros or Tartarus, would be a better name for Planet 9
The Tartaros is the deepest part of the underworld. And he is the God which rules there. Actually both are the same, the location and the God.
Pluto has nothing to say there.
Don't forget to look here.
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| spacer | Date: Thursday, 09.06.2016, 11:41 | Message # 155 |
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| JackDole, i like that. need some name that related to something like ghost like, dark and deep.
"we began as wanderers, and we are wanderers still"
-carl sagan
-space engine photographer
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| Mosfet | Date: Thursday, 09.06.2016, 14:54 | Message # 156 |
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| Quote steeljaw354 ( ) rather awkward name that uranus has.
This is me when someone whines about that name:
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| steeljaw354 | Date: Thursday, 09.06.2016, 19:10 | Message # 157 |
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| Or what about the name Nibiru?
Edited by steeljaw354 - Thursday, 09.06.2016, 19:12 |
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| Watsisname | Date: Friday, 10.06.2016, 02:09 | Message # 158 |
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| I just realized that the table in wikipedia and numerous other sources get the value for Soter's planetary discriminant wrong for Mars. According to Soter's paper, it's 5.1*103, not 1.8*105. So I edited the wikipedia page to fix that, and I also produced a new plot, which also adds a couple other KBO's:
The impact is still the same. There is a distinct bi-modal distribution, with members of each population contained within about 3 orders of magnitude, and the two populations separated by 4 orders of magnitude. So a natural question to ask is why is that? When we see such distributions in nature, there's usually an important physical mechanism behind them. In this case, simulations suggest it is a result of the dynamics of accretion and scattering during planet formation and early solar system evolution.
Some might argue that the gap between these two populations could be filled in other systems; that maybe our system is an outlier based on how giant planet migration played out. But simulations suggest the same kind of distribution should appear in general. There will be some differences in terms of the compactness of the system, e.g. around low mass stars, where timescale of removal of objects by collisions becomes important relative to removal by scattering.
Finally, while Soter's discriminant is a really powerful measure of planethood, it does have a flaw in that it requires knowledge of all the minor material in the orbital space. Of course, we have no such measures outside of our solar system. This is why other metrics were created which attempt to capture "how well" a planet accretes or scatters material from its orbital space, based purely on mass and orbital period which we can determine remotely. Those measures seem to be consistent with Soter's discriminant.
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| Watsisname | Date: Friday, 10.06.2016, 04:41 | Message # 159 |
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| One more post to answer an excellent question, which will lead to an important statement about dynamics. :)
Quote midtskogen ( ) Quote Watsisname
With over 40,000 orbits since the formation of the solar system, then statistically it would have passed through Jupiter's Hill sphere about a thousand times. Jupiter would have scattered it. Ok, but would all the other planets have scattered it if it went through their orbit?
Hill Sphere radius is proportional to planet's semi-major axis, and cube root of planet mass. Circumference of planet's orbit scales with semi-major axis. So the probability of a planet-crossing orbit intersecting the Hill Sphere does not depend on that planet's orbital distance, and it grows as the third root of planet mass. That's not a very strong dependence, so you shouldn't expect a huge difference among the giant planets.
Indeed, here are the numbers:
# of intersections for Jupiter: 978
Saturn: 654
Uranus: 350
Neptune: 370
All well over 100. So we expect scattering.
What about the much less massive inner planets?
Mars: 68
Earth: 143
Venus: 134
Mercury: 54
That's still more than 1. Maybe not so much more than 1 to be really convincing. However, if the object was a potential rival to Jupiter, then the inner planets would be passing through its Hill Sphere (a lot more frequently), and they would be scattered. The solar system would not look the way it does.
This result is important. The scattering process does not allow objects which are strongly mutually-interacting to exist in such a way that the can stay strongly interacting for very long. Why would there be object with a mass comparable to a solar system planet be in a hugely eccentric orbit with a period of 100,000 years, anyway? Because it got scattered.
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| midtskogen | Date: Friday, 10.06.2016, 07:29 | Message # 160 |
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| Quote Watsisname ( ) Soter's planetary discriminant
It's interesting to see that Earth is two and a half order of magnitudes more a planet than Mars, so Mars is much closer to being a non-planet (< 100) than to being a planet like Earth. So much for Mars as a second home for humans...
Quote Watsisname ( ) That's still more than 1. Maybe not so much more than 1 to be really convincing.
Well, we can probably safely assume that there aren't 100 such planet like objects in hugely eccentric orbits. It doesn't take much convincing to assume that our solar system indeed has settled. But I think the numbers show that it's reasonable that the process could take several hundred million years and the entire process of forming a proper planet a billion years. Which is a very long time even astronomically. It could also mean that the earliest forms of life on Earth appeared before Earth earned its planethood.
The formation of a solar system appears to start rapidly and logarithmically slow down. The star forms relatively fast, the planets need more time to form, and the orbits to settle takes even more time.
NIL DIFFICILE VOLENTI
Edited by midtskogen - Friday, 10.06.2016, 07:36 |
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| Watsisname | Date: Friday, 10.06.2016, 08:21 | Message # 161 |
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| Not as long as you think.  Remember your example begins with an object on a hugely eccentric orbit! In a forming system, material is in essentially circular orbits, not orbits with eccentricity close to 1. How do you get a massive object on such an eccentric orbit to begin with? By scattering it. (Scattering also tends to increase the inclination).
The math I was doing shows that even such a planet-crossing eccentric orbit would still involve scattering, and so we can safely rule out its existence. It could not exist on such an orbit for very long.
Quote midtskogen ( ) It's interesting to see that Earth is two and a half order of magnitudes more a planet than Mars, so Mars is much closer to being a non-planet (< 100) than to being a planet like Earth. So much for Mars as a second home for humans...
Being higher in the distribution does not make something "more a planet". It's not a planetary ranking system. It's a classification system with two distinct populations. Anything that is in the upper part of the bi-modal distribution is a planet. Anything that is in the lower part of the distribution is a dwarf planet. Why? Because dynamical processes naturally produce this clear separation between the two groups.
Suppose instead that Mars were between, say, 10 and 100. Then it would not clearly be a member of either population, and we would find that rather odd.
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| JackDole | Date: Friday, 10.06.2016, 09:04 | Message # 162 |
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| Quote Watsisname ( ) Soter's planetary discriminant
What value would have Planet 9 in this table? With about 10 Earth masses and a SemiMajorAxis of 600 AU (20 * Neptune).
Am I right in assuming that he would be a dwarf planet, according to this table?
Don't forget to look here.
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| Watsisname | Date: Friday, 10.06.2016, 11:10 | Message # 163 |
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| By Soter's discriminant? I have no idea. Take 10 Earth masses and divide it by the total mass within that region (besides itself). How much total mass is in that region?
This is why a scattering parameter is better for these cases -- we can compute based on parameters we can more easily observe. Let's try Margot's Π:
Taking m=10, M=1, a=600, and k=807 for a sun-like star, this evaluates to about 6.
6 is greater than 1 [citation needed], so this implies the object should be able to "clear its orbit", and would be defined as a planet by this metric.
However, all of the numbers associated with planet 9, including its very existence, are very speculative. So take this result with a mountain of salt:
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| midtskogen | Date: Friday, 10.06.2016, 15:07 | Message # 164 |
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| Quote Watsisname ( ) In a forming system, material is in essentially circular orbits
We may be somewhat spoiled with pretty much circular orbits. Wikipedia claims that many of the known planetary systems display much higher orbital eccentricity. There's evidence for planets with an eccentricity well above 0.5. Have these been scattered?
In our solar system we have one very dominant body, the sun, which may be somewhat unusual.
NIL DIFFICILE VOLENTI
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| Watsisname | Date: Saturday, 11.06.2016, 05:47 | Message # 165 |
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| To 'scatter' means to remove objects from the picture, dynamically speaking. So no, they were not scattered. High eccentricity planets are probably the result of secular interactions and Kozai mechanism. And the object in your example is e=0.998, which is an entirely different regime of eccentricity. More on that in a moment. First let's look at some data:
Of this sample of 850 planets with well characterized orbits, 289 of them have e<0.01. Then there is an immediate and huge dropoff in the distribution. The median is 0.07. The mean is 0.15. 7% have e>0.5. Only 2 have e>0.9. (One of those is HD 80606 b which is an insanely crazy place).
Let's take a moment to think about this. As with bi-modal distributions, you should expect dynamical processes to be responsible for the shape. In this case, there are a few. Most of the planets in the spike at e~0 are due to tidal circularization of close orbits. Most of the high eccentricity planets are due to the Kozai mechanism.
After discarding tidal circularization, the mean eccentricity is around 0.2. Not as nicely circular as our system, (it's about the same as Mercury's), but still "essentially circular". Simulations have no trouble with this.
Now for comparison, the 100,000 year Jupiter-crossing planet would have e=0.998. To put that in perspective, with a periapsis at Jupiter's orbit (5.2AU), it's semi-major axis would be 2150AU. In our solar system, the only viable pathway to this orbit is by scattering.
Such a resulting orbit is also enormously improbable -- the total orbital energy is very close to 0, meaning whichever interaction brought it to that orbit may as well have ejected it completely. So, this object does not exist, is improbable to ever exist, and if it did exist, would prove the rule anyway. Scattering is a powerful measure because it's a powerful process.
Edit: Added links to more info about Kozai, and one of the more eccentric planets known because it's awesome.
Edited by Watsisname - Saturday, 11.06.2016, 11:43 |
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