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PostPosted: Sat Mar 17, 2018 12:46 pm 
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So, once upon a time I hacked Omer's python program to create a star system generator.

Yeah, there's not much left of the original at this point besides the def for dice rolls. :)

Anyway, it pulls from 2300 and Gurps Space, a couple other sourcebooks I found on the webs, and posts and comments here as well.

Right now the process is something like:

  1. pull star data to calculate snow line, inner and outer limits, BBT for stuff in orbit.
  2. figure out a starting point for planets. Either a big gas giant or the inner/outer edge.
  3. place successive worlds using resonances -- I think I stole this idea from Brett? 1.41, 1.59, etc
  4. roll up diameters, and then figure out density, etc.
  5. determine various world characteristics based on BBT and zone -- icy cores and lower density for outer, atm types, etc. I have some mods for tide-locked worlds to strip water and atmosphere down a bit -- with Gurps Space as a guideline.

Couple things I want to do differently.

1) thinking about instead of diameter, generating world mass. If I set up the ranges and odds correctly, then I can allow for a very slim chance of a gas giant in the inner system. Is there a minimum mass for an inner system GG to survive? The nice thing about computers is I can create dice-unfriendly ranges. To allow for a reasonable chance of garden worlds, I might add some mass cheats based on local BBT. Of course in the outer zone worlds are going to be either significant multiples of earth mass, or tiny fractions (moons). But inward from the snow line anything goes, really.

1a) If I want to think about orbital resonances, I assume I need to factor in eccentricity, yeah? Multiply 1.59 by the 1.02 eccentricity or similar?

2) Not entirely happy with figuring out atmospheres. Right now it's a GURPS-space thing, tied to gravity, but influenced by size. Mars-size worlds are guaranteed not to have more than a trace atm, at least in the inner system. And I wonder if that's so likely. No chance of a Mars with magnetic field and volcanism to keep a few hundred millibars around?


Of course my hacker skills aren't nearly good enough to deal with multi-star systems. Usually what I do is run the program, then go and, for example, delete worlds in Alpha Centauri A/B further out than ~3AU. And then there are systems with close-orbiting pairs, like, um... G208-044, I think? I'd have to do that manually.

The other tricky one is subgiant stars. Like Mu Herculis and Delta Pavonis. They're both presumably a lot hotter than they used to be. Not a big deal for barren rocks, but worlds in the hab and middle zone might look a little different.

Plus there's the A-type stars like Vega and Sirius. I assume that whatever's orbiting them probably hasn't even cooled yet.

Anyway, if anyone has suggestions, I'll share the file if I can make some productive changes.


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PostPosted: Sat Mar 17, 2018 4:12 pm 
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Matt Wilson wrote:
determine various world characteristics based on BBT and zone -- icy cores and lower density for outer, atm types, etc. ...
Is there a minimum mass for an inner system GG to survive?

Zones are pretty much not a thing in current models of planetary evolution. They do affect where planetesimals form, but then the whole system plays gravity billiards and gets thoroughly stirred up. Gas giants form in the outer zone but (as demonstrated by hot jupiters) then migrate considerable distances on relatively short time scales, both inward and outward, based on the residual density patterns in the proto-planetary nebula.

You should probably peruse this review paper: Lecture notes on the formation and early evolution of planetary systems

Quote:
If I want to think about orbital resonances, I assume I need to factor in eccentricity, yeah? Multiply 1.59 by the 1.02 eccentricity or similar?

Technically, no. Resonances are based on orbital period, which is strictly a function of semi-major axis. Really extreme eccentricities may raise the possibility of close encounters and mutual scattering, but (as demonstrated by Neptune and the plutino population, or Jupiter and the Hilda asteroids) even orbit crossing isn't necessarily a show-stopper with the right resonances.

Quote:
Mars-size worlds are guaranteed not to have more than a trace atm, at least in the inner system. And I wonder if that's so likely. No chance of a Mars with magnetic field and volcanism to keep a few hundred millibars around?

Magnetic fields are important for non-thermal atmospheric loss (e.g., molecular sputtering from the stellar wind), but the minimum loss rate is set by thermal processes. These are a result of planetary mass, equilibrium temperature, and atmospheric composition (molecular mass).

EDG's tutorial is here: Atmospheric retention for beginners.


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PostPosted: Mon Mar 19, 2018 1:31 pm 
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Rather than Mass, I suggest Gravity. Most interstellar travellers could care less about Mass or Density, but how much they will weigh is critical to any op. While Mass means more during world creation, Gravity is the characterstic needed most for game play.

Also, From MGT traveller, you can get the Mainworld's Temperature. THAT to me is your starting point in the system. Place the Mainworld first and then go in and out based on resonances (I like that a lot and will steal it myself).

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PostPosted: Wed Mar 21, 2018 3:29 pm 
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Cyborg IM1 wrote:
Rather than Mass, I suggest Gravity. Most interstellar travellers could care less about Mass or Density, but how much they will weigh is critical to any op. While Mass means more during world creation, Gravity is the characterstic needed most for game play.


I like mass mostly because wikipedia lists real exoplanets by mass. It makes it relatively easy to plug them into the results and extrapolate.

If it were a by-hand method I'd definitely prefer the stars without number approach of "can I breathe the air or not".

On a related note, I would be the complainiest traveller ever when it came to higher gravity planets. "Bart's World? Noooooo, it's 1.2 gees. I'm staying on the ship, wahhhh."


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