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Old 11-16-2018, 10:08 PM   #1
scc
 
Join Date: Mar 2013
Default [Space] Dealing With Planetary Eccentricity

So I'm generating a system at the moment (With and Eccentric Gas Giant arrangement) and I notice something problematic, namely my planets are going to keep crashing into one another because their orbits are so eccentric, to wit:
Inner Limit of 0.135 AU
Planet 1: 0.05 to 0.26
Planet 2: 0.17 to 0.25
Planet 3: 0.18 to 0.54
Planet 4: 0.46 to 0.70
Planet 5: 0.30 to 1.68
Planet 6: 0.95 to 2.23
Planet 7: 1.36 to 4.07
Then there's an asteroid belt at 4.61 AU before
Planet 9: 7.45 to 10.07
Before a Forbidden Zone beginning at 11.67 AU

Now I think the first to planets might be possible and I can simply reduce their eccentricity, the whole thing really calls out for a proper fix and/or errata, what should this be?
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Old 11-16-2018, 10:19 PM   #2
David Johnston2
 
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Default Re: [Space] Dealing With Planetary Eccentricity

Quote:
Originally Posted by scc View Post
So I'm generating a system at the moment (With and Eccentric Gas Giant arrangement) and I notice something problematic, namely my planets are going to keep crashing into one another because their orbits are so eccentric, to wit:
Inner Limit of 0.135 AU
Planet 1: 0.05 to 0.26
Planet 2: 0.17 to 0.25
Planet 3: 0.18 to 0.54
Planet 4: 0.46 to 0.70
Planet 5: 0.30 to 1.68
Planet 6: 0.95 to 2.23
Planet 7: 1.36 to 4.07
Then there's an asteroid belt at 4.61 AU before
Planet 9: 7.45 to 10.07
Before a Forbidden Zone beginning at 11.67 AU

Now I think the first to planets might be possible and I can simply reduce their eccentricity, the whole thing really calls out for a proper fix and/or errata, what should this be?
Here's the thing about such an arrangement:

No. They won't.

Think about Pluto and the way it's orbit crosses inside that of Uranus. And yet they won't collide, if for no other than they aren't orbiting in the same plane. Pluto's at an angle to the rest of the solar system.
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Old 11-16-2018, 10:30 PM   #3
AlexanderHowl
 
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Default Re: [Space] Dealing With Planetary Eccentricity

Pluto crosses Neptune, not Uranus. As for eccentricities, the rules states that two orbits cannot be closed than 0.15 AU.

Last edited by AlexanderHowl; 11-16-2018 at 10:37 PM.
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Old 11-16-2018, 11:28 PM   #4
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Default Re: [Space] Dealing With Planetary Eccentricity

@Howl, The rule refers to semimajor axes, not the orbital paths. If it applied to orbital paths, our own solar system wouldn't be "rules legal". Again, Pluto and Neptune have orbits that cross paths, but their 2:3 resonance prevents them from colliding.

@scc, Those are some pretty eccentric looking orbits. Can you post just what eccentricities you rolled? It looks like (from inner to outter): 0.677, 0.19, 0.5, 0.207, 0.697, 0.403, 0.499, and 0.15. That does kind of track with the spread of Jovian exoplanet eccentricities, so I guess I'd question how old this solar system is, first. If it is young, it might not be stable and one or more of those will get ejected. If it isn't young, I'd consider either picking planets to eject and clearing those orbits or reducing the eccentricities of most of them. If you look at multi-exoplanet systems, they tend to have lower eccentricity orbits, with maybe one or two in the 0.2ish range.

As for planetary orbits crossing each other, this doesn't result in a collision, generally, because those planets are in a resonance that ensures they are never at that intersecting point at the same time. Otherwise, the planets would have already collided.

Also, Jon Zeigler (Author of GURPS Space) has been putting together a revised solar system generator based on more modern exoplanetology - something that was nascent when Space was written. It can be found here.
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Last edited by Humabout; 11-16-2018 at 11:47 PM.
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Old 11-16-2018, 11:37 PM   #5
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Default Re: [Space] Dealing With Planetary Eccentricity

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As for planetary orbits crossing each other, this doesn't result in a collision, generally, because those planets are in a resonance that ensures they are never at that intersecting point at the same time. Otherwise, the planets would have already collided.
That doesn't really work for a 7 planet long chain of crossing orbits, specifically when the last planet is a 700 Me Gas Giant.

Last edited by scc; 11-16-2018 at 11:43 PM.
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Old 11-16-2018, 11:53 PM   #6
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Default Re: [Space] Dealing With Planetary Eccentricity

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That doesn't really work for a 7 planet long chain of crossing orbits, specifically when the last planet is a 700 Me Gas Giant.
While I'm not arguing that that many crossed orbital paths is necessarily stable, you should remember that space is big. Moreover, just because orbital paths cross does not mean that the planets will try to cross paths at the same time. The only way such orbits are stable is if there is a resonance that prevents them from colliding. Otherwise, one is likely to just get ejected from the system. It's not like you can make the argument that because two roads intersect, then every car on those roads will collide with each other.
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Old 11-17-2018, 11:02 AM   #7
Vaevictis Asmadi
 
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Default Re: [Space] Dealing With Planetary Eccentricity

Resonance doesn't occur automatically with crossed orbits in Space. You have to get the orbital periods exactly right for resonance.

The fix I have seen suggested, which I always use, is to divide all eccentricities by 10.
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Old 11-17-2018, 12:12 PM   #8
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Default Re: [Space] Dealing With Planetary Eccentricity

Well, what GURPS Space generally ignores is the evolution of the system. One of two things generally happens: either gravitational interactions will eject one or more bodies (more likely) or it will tug (read: cauae bodies to migrate into) the bodies into an orbital resonance. The latter case is stable, but as you mention, requires specific orbits.

As for dividing eccentricities, his are realistic for exoplanets, but they shouldn't all be that high. I'd suggest fixing the biggest gas giant and then looking at orbital periods. Whichever are close to a common resonance ratio (2:3, etc.), shift their semi-major axis so the period matches, and consider ejecting its neighbors. Either all of that or check out Jon's newer version that does include a system evolution step...
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Old 11-18-2018, 11:56 AM   #9
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Default Re: [Space] Dealing With Planetary Eccentricity

Oh man, I hope that when Jon finishes Architect of Worlds, he's able and willing to post "conversion notes" to go with the book, to convert its results into GURPS values. He's basically doing what I've been trying for years to do with the Space system, except he's a professional with more research.
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