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PostPosted: Sun Mar 19, 2017 1:10 pm 
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I've been messing around with the system-gen stuff in GURPS Space (4e). Someone here (Mr. Aga. M. Egos, maybe?) mentioned developing some detailed stuff around that a while back. There's a lot in there that I like, but a few things I'm wondering about:

1) Gas Giants in the inner system. I've seen something elsewhere that suggests smaller GG (~0.3 Mj) would not survive migration intact and might end up as something else like a panthalassic water world.

2) Number of Worlds. GS assumes that the inner limit for tenable orbits is something like 0.1 * luminosity, and the outer limit is 40 * stellar mass. And that all the space in between is going to be full of worlds. So far in my experience that leads to systems with HEAPS of planets. I recall some sample systems from EDG that sported very few worlds, and I'm wondering if a random roll for # of worlds makes more sense. Maybe start at the first GG past the snow line and roll randomly both inward and outward. Thoughts?

3) Snow line. GS assumes the snow line is 4.85 times luminosity, but I've seen it closer to 2.77 in other places. Assuming that the GURPS number is modern-day, and 2.77 is a historical number reflecting stellar temperatures during planetary formation.

4) Moons. GS has an odd rule about moons based on absolute distance from the primary. As in "don't roll for moons if the world is closer than 0.5 AU to the star" -- I don't recall the exact words, but something like that. Wouldn't it be more accurate to base it on tidal effects? 0.5 AU from Proxima Centauri is a lot different than 0.5 AU from Vega.

5) World types. GS divides worlds into size categories, sort of like what First In does (and 2300), based on MMWR -- but unlike 2300 you roll the size first and then determine diameter/density that fit. A "standard" world is the right size to hold oxygen and water vapor, for example. But in order for the world size you rolled to fit those parameters, you can end up with some weird extremes, like a Mars-sized "garden" world that's extremely dense, or a low-density world with a huge diameter. My not-scientist assumption is that bigger worlds are more likely to be denser than smaller worlds. T/F? Oh, and the other thing is that it assumes anything that is smaller than a "standard world" (i.e. a MMWR > 18) in the inner system will only have a trace atmosphere. Too limiting?

5b) not really a question, but a little funny to me that they describe MMWR using size. A "small" world close to a star might be larger than a "standard" world in the middle zone.

6) Presence of GG. <=10 on 3d6 means no GG in the system. That seems shockingly low to me. ???

Sorry about the question bombardment. Unemployment leads to restless minds.


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PostPosted: Sun Mar 19, 2017 10:34 pm 
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Matt Wilson wrote:
1) Gas Giants in the inner system. I've seen something elsewhere that suggests smaller GG (~0.3 Mj) would not survive migration intact and might end up as something else like a panthalassic water world.


Where did you see that?

Remember the GSpace was published in 2006 and written in 2005, when the Nice model was brand new. At that time hot Jupiters and eccentric Jupiters were more of a surprising discovery than evidence of planetary migration.

Quote:
2) Number of Worlds. GS assumes that the inner limit for tenable orbits is something like 0.1 * luminosity, and the outer limit is 40 * stellar mass. And that all the space in between is going to be full of worlds. So far in my experience that leads to systems with HEAPS of planets. I recall some sample systems from EDG that sported very few worlds, and I'm wondering if a random roll for # of worlds makes more sense. Maybe start at the first GG past the snow line and roll randomly both inward and outward. Thoughts?


GSpace does start with the first gas giant and work both inwards and outwards.

Quote:
3) Snow line. GS assumes the snow line is 4.85 times luminosity, but I've seen it closer to 2.77 in other places. Assuming that the GURPS number is modern-day, and 2.77 is a historical number reflecting stellar temperatures during planetary formation.


More likely the GSpace number was put where it was to produce Jupiter. Remember that GSpace was designed before the Grand Tack Hypothesis appeared to explain why Jupiter is so far out, and I think before we discovered that Ceres probably has an icy mantle. But maybe that's the ice line for some other compound than water. Maybe nitrogen or methane.

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4) Moons. GS has an odd rule about moons based on absolute distance from the primary. As in "don't roll for moons if the world is closer than 0.5 AU to the star" -- I don't recall the exact words, but something like that. Wouldn't it be more accurate to base it on tidal effects?


Yes, I think it would.

Quote:
5) World types. GS divides worlds into size categories, sort of like what First In does (and 2300), based on MMWR -- but unlike 2300 you roll the size first and then determine diameter/density that fit.


The GSpace and First In system design sequences were designed by the same author, Jon Zeigler.

Quote:
A "standard" world is the right size to hold oxygen and water vapor, for example. But in order for the world size you rolled to fit those parameters, you can end up with some weird extremes, like a Mars-sized "garden" world that's extremely dense, or a low-density world with a huge diameter.


Yep. This is an artifact of supporting the "what do you want" Basic world-building sequence, and it produces a gradation of planetary size with distance from the star that has no physical basis. It's one of my main reasons for wanting to replace the GSpace world design sequence with my own.

Quote:
My not-scientist assumption is that bigger worlds are more likely to be denser than smaller worlds. T/F?


Maybe, a bit. Gas giants self-compress very readily because of the gas laws. Their diameter varies with mass in a most unexpected way. Above a certain point adding to their mass makes them denser but not larger. As for rocky planets they do self-compress to some extent: high pressures in their interiors produce denser phases of solid materials (including ice IX and ice XI on the bottoms of the oceans of large thalassics). Large planets have high gravity so they reach high pressures at lower depths in their lithospheres than smaller planets, and given that they have more depth to play with that means much larger interiors above the pressure necessary for any given phase change.

On the other hand, variations of composition dominate over self-compression effects. Mercury is denser than Venus and much denser than Mars. Uranus and Neptune are both denser than Saturn, and Neptune is denser than Jupiter.

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Oh, and the other thing is that it assumes anything that is smaller than a "standard world" (i.e. a MMWR > 18) in the inner system will only have a trace atmosphere. Too limiting?


MMR > 18 is a "small" world, right? With an atmosphere of nitrogen and carbon dioxide? And you're talking about the Small Rock type with blackbody temperature above 140K? Like Mars?

I suppose that perhaps they ought to have a chance of retaining a significant atmosphere. There's certainly plenty of nitrogen in Earth's orbit.

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5b) not really a question, but a little funny to me that they describe MMWR using size. A "small" world close to a star might be larger than a "standard" world in the middle zone.

I think this was a bad choice. It leads to confusion.

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6) Presence of GG. <=10 on 3d6 means no GG in the system. That seems shockingly low to me. ???


And also to me. GSpace makes hot and epistellar gas giants way more common than is consistent with the astronomical evidence, too.

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PostPosted: Tue Mar 21, 2017 12:47 pm 
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Quote:
This is an artifact of supporting the "what do you want" Basic world-building sequence, and it produces a gradation of planetary size with distance from the star that has no physical basis.


Can you expand on that a bit? I understand MMWR and how the farther out you get, the easier it is for a world to hold on to an atmosphere. Is there something else in your comment here suggesting that worlds are more/less likely to be a certain physical size at a certain distance?

Quote:
It's one of my main reasons for wanting to replace the GSpace world design sequence with my own.


Curious to know what you'd change/keep. Something more like 2300 where you determine diameter/density first and then find out what that gets you?


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PostPosted: Wed Apr 05, 2017 2:08 am 
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Matt Wilson wrote:
Quote:
This is an artifact of supporting the "what do you want" Basic world-building sequence, and it produces a gradation of planetary size with distance from the star that has no physical basis.


Can you expand on that a bit? I understand MMWR and how the farther out you get, the easier it is for a world to hold on to an atmosphere. Is there something else in your comment here suggesting that worlds are more/less likely to be a certain physical size at a certain distance?


GURPS Space has two world-building sequences, Basic and Advanced. The Basic Sequence (steps 1 to 14, on pp.73–98) starts with "world type" (a combination of atmospheric type and surface material/conditions in fifteen categories not counting gas giants) and progresses through atmospheric composition and mass, ocean coverage, surface temperature, density/mass/gravity, resources and habitability, settlement type, tech level, population, unity, government/society/mode of production (muddled together, because GURPS does that), control rating (law level), income and trade, bases and installations. It is designed so that you can roll for each step or make a choice of what is possible given results so far. The Advanced Sequence (steps 15–39) starts with the number of stars in the system, then progresses through their masses, the system's age, the stars' luminosity classes and spectral types, surface temperature, and luminosity, orbits of the companion stars (in the case of a multiple system); then for each star the existence and arrangement of its gas giants, the orbits of it "first gas giant" (the one defining the arrangement of its system) and any designed planet to be inserted, other orbits, what planets are in them, what moons those planets have, and then for each planet and moon, what type it is. And at that point the advanced system recapitulates the basic system, Steps 26 to 29 referring to Steps 3, 4, 5, and 6. Steps 30 and 31 introduce day lengths and tidal braking. Steps 32 to 39 refer to Steps 7 to 14. Tidal locking and spin-orbit resonance are dealt with as afterthoughts.

The Basic Sequence starts with world type because that's the fundamental question that restricts everything else when you are using it as a design system. Then it proceeds through a bunch of other design choices (that can be rolled) to determine a bunch of other things — surface temperature, atmospheric composition and column mass, ocean cover, surface temperature — that go together to determine its albedo and greenhouse effect, which gives you its black body temperature, which tells you (in step 21) where it has to go in a system, when you come to designing a system. That is really slick. I'm in awe. It was so slick that I could implement the whole thing, including letting the user design a planet and inserting it into the right place in the system, as an Excel workbook with no macros.

The problem arises when the Advanced Sequence "re-uses code" from steps 3 to 6 as steps 26 to 29. It saves word-count, explanation, and complexity, and it is what makes the system so slick; but it introduces a subtle bias. In the Advanced Sequence you place the orbits, then you calculate the black-body temperatures at those orbits and then you roll on a table of world types that are possible at those BB temperatures. That means that the ratio of tiny:small:standard:large worlds remains the same right across each band of temperatures. But those aren't world sizes, that are categories of minimum molecular weight retained. The planets in each category get smaller as you go further out from the star. What ought to happen is that 'tiny' worlds (i.e. ones that won't retain nitrogen) get less common (because the largest of them become 'small' at low temperature). But instead of getting less common they get smaller. And since the largest 'large' planets don't start getting replaced by small gas giants you get a phenomenon of smaller planets at low temperatures than at high ones, which is not physical. The distribution of masses ought to remain the same, and the world types ought to slide over that distribution.

Quote:
Quote:
It's one of my main reasons for wanting to replace the GSpace world design sequence with my own.


Curious to know what you'd change/keep. Something more like 2300 where you determine diameter/density first and then find out what that gets you?


Well, I'm still in doubt about how to place the gas giants. I'd prefer not to have to place them by a separate preliminary process and squeeze the terrestrials in afterwards, and I feel very unconfident about having to simulate migrational history. But assuming that that problem is solved I'd like to go to each orbit in turn and determine randomly its initial endowment of iron and siderophilic material, of stone and lithophilic material, of chalcophiles, of ices, and of gasses. Calculate mass and radius, then determine black body temperature when the star enters the main sequence. Figure what boils off and remove it. Generate an initial rotational rate. Then generate moons to the same stage. Calculate tidal forces, move moons accordingly (including removing moons that spiral in to the Roche limit and ones that spiral out to the Hill Sphere radius, and adjust the planet's rotation rate. I hope to be able to do that well enough with a single step, but there is a problem with the geostationary orbit radius moving out as the planet's rotation slows. I might have to do that by a series of steps. Any way, once I have game-date orbits and rotation rates, turn the star up to its current luminosity, calculate the energy budgets (insolation + tidal kneading + radiogenic geothermal + energy of crystallisation for the cooling metallic core + outgoing thermal radiation) and the blackbody temperature. Then re-calculate Jeans escape. Then work through the volatiles in order of increasing boiling point to work out the albedo and greenhouse effect of after each one comes into the atmosphere, and whether that produces a temperature and pressure high enough to melt and then to boil the next. When I get to water, if CO2 is sufficient to melt it and there is enough geothermal heat flux to suppose vulcanism, reduce CO2 to a point where a carbonate-silicate cycle stabilises climate or zero, whichever is lower. Calculate the surface temperature and pressure. If water boils, assume runaway greenhousing (i.e. return all the CO2 to the air. Figure out how long the planet has had liquid water at the surface (in face of solar brightening) and the flux of light suitable to drive photosynthesis, and figure out whether it has passed its oxygen catastrophe. Estimate ocean and ice cover and vegetation, calculate albedo, recalculate climate….

It's an algorithm to be executed by a computer, obviously.

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My SFRPG setting, Flat Black

© My posts to this board are copyright under the Berne Convention. They may be quoted on the board with appropriate attribution. They may not be reproduced beyond the board except with explicit permission from me.


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PostPosted: Thu Apr 06, 2017 1:23 pm 
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Thanks for taking the time to share all this!

Quote:
What ought to happen is that 'tiny' worlds (i.e. ones that won't retain nitrogen) get less common (because the largest of them become 'small' at low temperature). But instead of getting less common they get smaller. And since the largest 'large' planets don't start getting replaced by small gas giants you get a phenomenon of smaller planets at low temperatures than at high ones, which is not physical.


Ah, right. I noticed that too. It's also awkward in the other direction, how a "tiny" world in a really hot inner orbit ain't all that tiny. The basic worldgen is pretty slick though, if you're just doing the equivalent of Traveller mainworlds.

Quote:
I'd prefer not to have to place them by a separate preliminary process and squeeze the terrestrials in afterwards, and I feel very unconfident about having to simulate migrational history.


I didn't know how to do that very well in my GURPS Space python app either. I did do something that let me start from the snow-line-ish GG and work in both directions, but I'm not sure it added a lot more detail than if I just worked inward or outward from either end.

Quote:
It's an algorithm to be executed by a computer, obviously.


Ha, even the code for what you described would be some work. But it would be fun as hell to play with. I hope it gets done. Maybe get Omer or Simon to write the code and I'll back your kickstarter :)


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PostPosted: Thu Apr 06, 2017 11:32 pm 
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Matt Wilson wrote:
Quote:
I'd prefer not to have to place them by a separate preliminary process and squeeze the terrestrials in afterwards, and I feel very unconfident about having to simulate migrational history.


I didn't know how to do that very well in my GURPS Space python app either. I did do something that let me start from the snow-line-ish GG and work in both directions, but I'm not sure it added a lot more detail than if I just worked inward or outward from either end.

That's how the "advanced" sequence in GURPS Space 4th edition works as written. You place the first gas giant in Step 21. And then in Step 22 you start at the first gas giant (if any) and work both outwards towards the outer radius limit and inwards toward the inner radius limit (or the edge of a forbidden zone. Or if there is no gas giant, you start a random distance inwards from the outer limit and work inwards.

My concern is that whether the system has one gas giant or two might make a big difference to system architecture. If there is only one it will have migrated inwards only, and stopped when the gas disk cleared. And with that the case you're likely to have a gas giant anywhere from the snow line inwards (a hot Jupiter, perhaps, or perhaps it has gone altogether or turned into something weird like a gaseous iron dwarf), with a depleted zone from its orbit to the snow line and a resonant train of terrestrials starwards of it. Whereas with two gas giants you might get a grand tack, which leave you with two gas giants in a resonance with a resonant train of ice giants outside the outer one, a depleted zone inside the inner one, and then a resonant train of terrestrials heading inwards. I'd really like to be able to step outwards from the apastron of one orbit to the periastron of the next, generate the contents of that orbit and move on, setting system-architecture flags when and it I get something that implies something about the architecture of the system. E.g. if a gas giant pops up in the inner system I deduce it must have migrated in, and therefore that I have a depleted zone out at least to the ice line.

I have to read some papers about system architecture.

_________________
— Brett Evill

My SFRPG setting, Flat Black

© My posts to this board are copyright under the Berne Convention. They may be quoted on the board with appropriate attribution. They may not be reproduced beyond the board except with explicit permission from me.


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