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Message started by Mal on 11/11/08 at 16:20:35

Title: Belts between Uranus and Neptune
Post by Mal on 11/11/08 at 16:20:35

I've been looking at belts near jovians that are relatively close to their star, but what about ones that are further away?

Looking at Neptune, the interesting thing is that its 3:2 distance is at 22.93 AU, but its Inner Reach distance (from the JUS paper) is at 27.36 AU - so there's a wider band (about 4.5 AU) between the two distances where a rocky planet might be stable. So what would happen here?

I took the fullsystem.gsim, hacked out everything except Uranus and Neptune, and then created:

100 massless point asteroids between 22 and 23.9 AU (starts just beyond the edge of Uranus' Outer Reach).
100 massless point asteroids between 24 and 27 AU
100 massless point asteroids between 27.5 and 29 AU

And then ran the simulation at a 12.1363 day timestep (which seems OK, since the slowest orbital period here is 84 years). For the output data file, I recorded a,e, and i at steps of 1 year.

Again, I've attached the gsim file if you want to play with it yourselves. Results follow... (I'll let it run for 10,000 years and post the results).

Title: Re: Belts between Uranus and Neptune
Post by Mal on 11/11/08 at 17:03:17

And here's the results.

You can see that the two inner belts of asteroids are largely intact, but the outer belt (within Neptune's Inner Reach) is severely disrupted (you can't see it too well here, but if you watch the graph being built you can see that some of the asteroids have really weird paths through sma-ecc space). It's kinda hard to tell which resonances correspond to the spikes... some may be Neptune's inner resonances, others may be Uranus' outer resonances. Toward the outer edge of the middle belt, you can see that things bulge up a bit, which means that it's getting a bit unstable there. (Neptune itself is at 30 AU).

But either way, the belt within Neptune's Reach (the furthest belt from the sun) is really screwed up. While it looks like a belt would be as stable between the 3:2 resonance and the Inner Reach as it would be within 3:2, I'm still not sure if that means that any rocky planet trying to form in this region would actually be able to coalesce into a single body or would just remain as a belt of planetoids. For now - until I learn otherwise - I'm going to assume that it'd become an asteroid belt rather than be ejected from the system completely.

Title: Re: Belts between Uranus and Neptune
Post by Mal on 11/11/08 at 19:20:32

Out of curiosity I did the simulation again but without Uranus there. The results are interesting:

First, without Uranus there it's suddenly obvious which peaks are due to resonances with Neptune: there's the 3:2 at 22.9 AU, the 4:3 at 24.8 AU, (unknown small peak around 25.5 AU), I think the third big peak is the 5:4 at 25.89 AU, and the slightly mangled looking peak around 26.6 AU is the 6:5. It also looks like Neptune may have (briefly?) caught itself a companion, given the squiggle around 0.17 eccentricity at 30 AU.

That means the other peaks on the previous graph must be the Uranian 4:5 at 22.28 AU, the Uranian 3:4 at 23.26, and the Uranian 2:3 at 25.16 AU.

Generally the graph looks similar though, but the graph without Uranus is somewhat more orderly. Again though, it looks like a belt would be stable in the "middle zone" between the 3:2 and the Inner Reach, though you can see that the resonances are getting somewhat more crowded there - maybe not enough to totally disrupt the belt (like in the Inner Zone) but probably enough to preclude planet formation.

Title: Re: Belts between Uranus and Neptune
Post by frankuitaalst on 11/11/08 at 22:17:54

Mal , looks as if you're preparing a doctoral scription  :)
Nice pictures and interpretation !
It would interest me to have the above simulation but starting with low timestep . Can you provide it ?  
Things really start quick when I open it .

Title: Re: Belts between Uranus and Neptune
Post by Mal on 11/11/08 at 22:52:19


frankuitaalst wrote:
Mal , looks as if you're preparing a doctoral scription  :)


Sometimes I wonder myself ;)


Quote:
It would interest me to have the above simulation but starting with low timestep . Can you provide it ?  
Things really start quick when I open it .


OK, when you first open it, pause time.
Then open it again and it'll stay paused. You can then adjust the timestep down to whatever you like, and then unpause it (save it first at the slower timestep and it'll remember that when you next open it again).

That should do it.

Title: Re: Belts between Uranus and Neptune
Post by Mal on 11/11/08 at 23:07:29

Just to see what things are like in a normal planet-forming environment, I made a belt of 200 massless point asteroids between 7.5 and 22 AU and ran it with a 3 day timestep (at 12 days, the timestep would be too high for the asteroids closest to the sun and we'd get false results).

As you can see, things calm down very rapidly within 22 AU - the only peak is at the 2:1 resonance at 18.93 AU. So I think the peaks further out than 3:2 in the previous graph would probably preclude planet formation... though the orbits are pretty stable between 4:1 and 2:1 too. I know that the asteroid belt is there because Jupiter's gravitational influence in that region is such that the collisional velocities favour destruction over accretion, so maybe the apparent orbital stability between 4:1 and 2:1 is a bit misleading here. I guess that closer to the star, the collisional velocities favour accretion instead.

So we know that material closer to the sun than Jupiter's 4:1 can accrete into planets (or we wouldn't be here). So maybe we can say that the planet-forming zone is closer to the sun than the 4:1 resonance, the asteroid-belt forming zone is between 4:1 and the jovian's Inner Reach, and the ejection zone is beyond the Inner Reach?

Title: Re: Belts between Uranus and Neptune
Post by frankuitaalst on 11/11/08 at 23:36:38

[quote author=Mal link=1226449237/0#4 date=1226472739OK, when you first open it, pause time.
Then open it again and it'll stay paused. You can then adjust the timestep down to whatever you like, and then unpause it (save it first at the slower timestep and it'll remember that when you next open it again).

That should do it. [/quote]
Ok , that's understood . But I wonder if there's a method to alter the timestep without having opened the file in Gsim ( by texteditor ) . Or are the velocities in the file timesteprelated ? Tony ?

Title: Re: Belts between Uranus and Neptune
Post by Mal on 11/12/08 at 14:22:04


frankuitaalst wrote:
Ok , that's understood . But I wonder if there's a method to alter the timestep without having opened the file in Gsim ( by texteditor ) . Or are the velocities in the file timesteprelated ? Tony ?


As far as I know, if you start off with it paused then changing the timestep and starting it up won't make the results any different than if you'd edited the timestep beforehand. Maybe this one is more for Tony to answer though?

Title: Re: Belts between Uranus and Neptune
Post by frankuitaalst on 11/12/08 at 22:49:19

Fine ! nice found , problem solved .  It's the first time I pause the program first and then open the new file .
It works , time enough to set the timetep . Thanks .

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