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PostPosted: Fri Mar 17, 2017 12:27 pm 
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Our general understanding is that Jupiter's disruptive influence prevented the formation of a genuine planet where the asteroid belt is now, right? Instead of a real planet, we got Ceres, Vesta, and a bunch of rocks.

I'm wondering how to think about that in terms of neighboring orbits and how close is too close, even for an asteroid belt to form. Is there a guideline based on the GG's mass and eccentricity that I could follow? How much space do they take up?


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PostPosted: Fri Mar 17, 2017 1:47 pm 
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According to the Grand Tack model that is because Jupiter formed at about 3.5 AU, migrated inwards to about 1.5 AU because of interactions with the disk, then captured Saturn in an orbital resonance that caused the pair of them to migrate outwards together. The asteroid gap got thinned because Jupiter migrated through it twice, scattering material first outwards then inwards, which (a) left little material for the formation of Mars, (b) left a gap with a scattering of junk where you would expect the second planet out from Earth (i.e. at 1.72 to 2.52 AU), and (c) produced the observed chemical structure of the asteroid belt.

A lot probably depends on how many gas giants you have. If there's only one it will migrate inward until perhaps it stops. If there are two the inner one can migrate inward then tack outward….

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Last edited by Agemegos on Fri Mar 17, 2017 1:48 pm, edited 1 time in total.

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PostPosted: Fri Mar 17, 2017 2:13 pm 
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When I was looking into this for exo systems, I read that our asteroid belt orbits are corralled by Jupiter, I simplified the distances for exo systems so that the asteroid belt is inside the orbit of the largest GG in the system between 1:2 and 1:4 resonances. Because I was adding asteroid belts after placing the planets in a system, I would often have a planet in the belt, if it were small enough I left it there, if it were larger I took a decision on whether to omit it or the asteroid belt.

Given things like the Grand Tack model and the number of hot jupiters we're seeing in exo systems it's pretty obvious that while planets form at certain distances there is a lot of orbital migration. I placed the asteroid belt in exo systems with the thinking that the largest GG would act just as Jupiter does in our system and "bullies" the other planets and bodies around it.

ETA: Orbits for planets are pretty much randomly determined, I've tweaked the 2300 method. Any attempt I can make with my feeble excel skills at modelling planetary migration is going to be laughable so I'm happy with keeping it random, seems like I get a fair few hot Jupiters that way. I seem to get a decent number of GG in the HZ too which often leads to garden worlds orbiting GG. I don't know how realistic that is but the image of watching Jupiter rise as you drink your morning cup of joe is something I really like! (I think this also comes from the snowballing threshold so I've been tweaking this to see what I get).


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PostPosted: Fri Mar 17, 2017 2:56 pm 
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Huh. So Jupiter is kind of like Godzilla in Tokyo, or basically my cats when I'm trying to organize pretty much anything on a tabletop.

Now I'm down the rabbit hole reading about the grand tack and the nice model and planetary migration. There goes my day...

And Hiro, whenever EDG or anyone explains why habitable moons around GG aren't likely, I always try really hard to forget what I just read. Where would we be without Yavin 4?


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PostPosted: Fri Mar 17, 2017 3:43 pm 
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I've not read much about garden worlds orbiting gas giants, I'm sure there are papers to read in Thrash's thread but most of the time they're over my head! I think there are a couple of issues, the biggest of which is radiation from the giant. I've tried reading up on this to understand the implications for the planets but didn't get too far. Looking at Jupiter and Saturn, it's my understanding that there's quite a difference in the radiation the two giants emit and their magnetic fields but I was unable to draw any conclusions that I could easily apply to an RPG. It just ain't simple. There's also the tide locking, moons are almost always tidelocked. This is kinda handy when the giant is orbiting an M class star as the giant will likely be tidelocked too so the moons "day" becomes a function of its distance/orbit from the giant. Calling a lot of these worlds habitable stretches the use of the word, it ain't gonna be earth like but it may work in the context of a big big hand wave.


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PostPosted: Fri Mar 17, 2017 7:16 pm 
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IIRC it's down to resonances. Our asteroid belt is mostly between the 4:1 and 2:1 resonance with Jupiter, with stragglers beyond that. The 5:1 and 3:2 resonances would mark the inner and outer boundaries of where asteroids could be in stable orbits. Beyond the 3:2 resonance asteroids would tend to be unstable and get thrown out of the system.

That's *really* simplified though. In reality Saturn affects the belt too (limiting the eccentricities and inclinations)..

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PostPosted: Fri Mar 17, 2017 8:33 pm 
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The 3:2 resonance with Jupiter is occupied by the Hildas, which are in eccentric orbits that reach their aphelions at about Jupiter's distance from the Sun, but when it is safely out of the way, and are always a perihelion when they are overtaking Jupiter, about as far from the Sun as the outer edge of the main belt. It is very illuminating to watch an animation of the orbits of the known asteroids in a frame of reference co-rotating with Jupiter and to reflect that all the objects are in elliptical orbits about the Sun.

The Greeks and Trojans seem to swirl around the L4 and L5 points in orbits shaped like beans or tadpoles. The Hildas seem to stream on a triangular course with angles ar L4, L5, and L3. I sometimes ponder life on a planet in a Hilda-type orbit, but I'm afraid that the eccentricity would be severe on the climate. The other possibility is an orbital cycler serving belter civilisations inthe leading and trailing Trojans.

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PostPosted: Fri Mar 17, 2017 11:00 pm 
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Matt Wilson wrote:
Huh. So Jupiter is kind of like Godzilla in Tokyo, or basically my cats when I'm trying to organize pretty much anything on a tabletop.

Now I'm down the rabbit hole reading about the grand tack and the nice model and planetary migration. There goes my day...

And Hiro, whenever EDG or anyone explains why habitable moons around GG aren't likely, I always try really hard to forget what I just read. Where would we be without Yavin 4?


It also comes down to how hard you want your SF and how simple you want your life as a GM/world builder.

The harder you go, the less likely your players will ever step out of a vacc suit.

Yeah, the rabbit hole is deep and convoluted in this one, I got to the point of simply stopping to add more detail to the world gen I'm using, it'll either never be finished or the players will get bored with the amount of precautions they have to take for that "long walk down the road to the chemist". Space is big.


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PostPosted: Sun Mar 19, 2017 1:26 pm 
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Quote:
It also comes down to how hard you want your SF and how simple you want your life as a GM/world builder.


It's weird for me, what does and doesn't trip up my suspension of disbelief. I think though that it comes down to a matter of understanding what's possible and then deciding what rules to break. Plus as I mentioned in another thread, I'm recently unemployed, and thinking about this stuff is a good way to keep my mind busy.


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PostPosted: Sun Mar 19, 2017 5:44 pm 
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My experience of keeping busy while unemployed has the downside of spending more energy gaming than looking for work... I've been in the same boat since the start of the year. Good luck with finding work.

Back to the important stuff ;)

As I delved into the sys gen process and granted, this is from my poorly educated POV, I found that there is an amazing amount of stuff that we just don't know that greatly outnumbers what we do know. What I took from that was that if I applied my own set of logic to the problem, (AKA hand waves and imagination) it was all good - if I liked what I was producing, it stayed and as science finds out more, I can choose to incorporate it or not.

It comes down to, what do I want the setting to be?


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