[Space] GURPS Handbook of the Planets

[RyanW]

Originally Posted by Brett

Yes, and such complicated systems indeed exist, are indeed rare, and indeed turn out to consist of trees of binary orbits. Consider e.g. Castor, a hexary system.

As far as GURPS Space in concerned, Castor is best treated as a trinary system with a distant third member having its own companion, and a separate binary system. Though they are gravitationally bound, they are beyond even distant separation in GURPS Space terms.

[RyanW]

Originally Posted by Brett

In the GURPS Space star system generation sequence the oxygen catastrophe changes the world type, the world's albedo, it's greenhouse factor, and its surface temperature. That's what makes its timing specially significant to the generator.

It does? I thought all the relevant tables listed "Standard Garden or Ocean" and gave them the same stats (except world type, of course). Am I missing something, or misremembering?

[Nemoricus]

Originally Posted by Brett

The Extended Hipparcos Catalogue (XHIP) by Andersen & Francis, which is up-to-date with positional data from the Hipparcos New Reduction, and has pretty much all other data from other published catalogues collated in with it. It is that absolute motherlode of data for purposes like this. You can download any extract you care to specify from Vizier.

*bookmarks that for later reference*

Yeah. My chief interest is in determining where in by universe the people live. For someone who was more interested in illustrating their universe or setting an AV game there gas giants would be of much more interest. Sudarsky tells you how to determint the colour given the temperature. Then there is size to consider, which turns out to vary with mass less than you would think.

My own interest is in generating the system as a whole, with the uninhabitable worlds being just as interesting as the habitable ones.

The original amount of energy it has can be determined using the formula for the gravitational binding energy of a sphere, and then you can use the specific heat of hydrogen to get its temperature. With a suitable value for its emittivity you can use the Stefan-Boltzmann Law for radiation to get its thermal radiation per unit area, radius gives you area, so you get to solve a fairly easy DE to get a temperature as a function of time.

That should do quite nicely, though it might fail for the less massive gas giants like Neptune and Uranus, where hydrogen isn't quite as dominant as it is in Jupiter and Saturn.

I read a paper once that modelled the transfer of material from the protostellar cloud to the accretion disks of forming gas giants, the formation of their regular moons, the migration of then moons and their destruction as the spiralled in to the Roche limit. The message I took away is that the ratio of the total mass of a gas giant's moon to the mass of the gas giant is pretty much a constant regardless of everything else.

I'm aware. I'm just saying that a big gas giant might have that mass spread across more major moons.

Or work out the Roche limit and multiply by one random orbital ratio.

The choice of that figure was to remain consistent with where GURPS Space places its major moons. However, I now see that with particularly dense gas giants and light moons, that might result in them being placed inside the Roche limit. Calculating the Roche limit for the least dense moons would work better.

Out of curiosity, what modifications have you made to your private version of the generator?

Originally Posted by RyanW

It does? I thought all the relevant tables listed "Standard Garden or Ocean" and gave them the same stats (except world type, of course). Am I missing something, or misremembering?

You're right. Garden and ocean worlds are equivalent except for the presence of oxygen in the air. However, the presence of living organisms should have effects on greenhouse factors, absorption factor, and similar.

[Agemegos]

Originally Posted by RyanW

It does? I thought all the relevant tables listed "Standard Garden or Ocean" and gave them the same stats (except world type, of course). Am I missing something, or misremembering?

No, that would have been me. It is only Atmosphere Composition that follows from that resolution. The effect of vegetation on albedo and of the sequestration of CO2 on the greenhouse factor are not reflected in the rules.

[Agemegos]

Originally Posted by Nemoricus

That should do quite nicely, though it might fail for the less massive gas giants like Neptune and Uranus, where hydrogen isn't quite as dominant as it is in Jupiter and Saturn.

My grasp of physical chemistry is weak, but perhaps it would be possible to use a weighted average of the the specific heats of the chief constituents.

I'm aware. I'm just saying that a big gas giant might have that mass spread across more major moons.

Aah! Good point.

Out of curiosity, what modifications have you made to your private version of the generator?

• I set it up to read stellar data out of a catalogue (based on XHIP, but with random infill of missing data) instead of generating the star at random.
  
• I altered the Gas Giant Arrangement Table to produce far fewer Eccentric and Epistellar results and somewhat fewer "no gas giant" results.
  
• I replaced use of the Orbital Spacing Table with a continuous random variable.
  
• I tweaked the Orbit Contents Table to get more Terrestrial Planet Standard and Terrestrial Planet Large. (I might have over-done that, since I am now getting rather more habitable planets than my revision of Dole led me to expect).
  
• I used the same placement of moons as is in the Handbook, but that's different from Space.
  
• I used a continuous random variable instead of a 3d6 roll for Atmospheric Mass.
  
• I replaced the method for resoving the Ocean/Garden ambiguity, which multiplies the age of the system by the level of visual illumination on the planet, and then compares that to a randomly determined expected time of oxygen catastrophe.
  
• I tweaked the threshold for Marginal Atmosphere to get fewer of those.
  
• I replaced the die roll for hydrographic coverage with a continuous random variable.
  
• I altered the definitions of the climates so that a planet that ought to be frozen to the equator is "frozen", and one that is warmer than 30 C at the poles is "infernal", and so as not to use "tropical" as a temperature category.
  
• I altered the determination of size, mass, and gravity to use a continuous random variable instead of the die roll for density.
  
• I coded a modifier to temperature for worlds with water oceans and active volcanoes or tectonics, to reflect stabilisation of the temperature at about 15 C by the carbonate-silicate cycle.
  
• I calculate the level of illumination with visible light, but that was an Easter egg in the Handbook already.
  
• I calculate the apparent periods and apparent sizes of the sun and the primary and any moons, but that was an Easter egg in the Handbook too.
  
• Instead of using Space's "habitability" score I determine factors for the gravity (based on data in Dole), atmosphere (with adjustsments for volcanism and tectonism), hydrographics (based on the proporition of any land surface like not to be arid), temperature (based on the proportion of the world's surface likely to be between 0C and 30C, the limits of agriculture), level of visible illumination, and day-length (following Dole, but I would like to improve this by calculating diurnal temperature variation and windiness and basing on those instead). Then I multiply those together to determine a habitability index on a scale from 0 to 100%. Earth comes out to 75.6%, chiefly limited by the presence of arid deserts and cold polar regions. Then I calculate a relative carrying capacity that depends on the square of diameter, the proportion of the surface not covered with water, and slightly different factors for the things that determin the habitability index. That's adjusted to make Earth come out to 1.00.
  
• I adjusted the "hydrographics" score of tide-locked worlds so that water on the night face doesn't count unless the night face temperature is above freezing point.

That's about it.

[ericthered]

Is there a reason the link on the OP no longer works?

[Agemegos]

Originally Posted by ericthered

Is there a reason the link on the OP no longer works?

Sorry about that. I updated the the file to beta 22 (because I had mended a bug mentioned above), which changed the file name. I've updated the link now.

[Agemegos]

Originally Posted by Brett

[list][*] I tweaked the Orbit Contents Table to get more Terrestrial Planet Standard and Terrestrial Planet Large. (I might have over-done that, since I am now getting rather more habitable planets than my revision of Dole led me to expect).

It turns out on close examination that this is in fact okay, and that I'm getting results in line with what I expected. I was confused before because I was getting bunches of habitable tide-locked worlds orbiting dim stars in the generator output. The estimates I based on revising Dole had those written off as completely uninhabitable.

[Whill]

Originally Posted by Nemoricus

Since the maximum mass a gas giant could have in orbit around it is .4 Earth masses and the Tiny size class is ludicrously permissive, I see no particular reason why you couldn't start at the innermost orbit and roll to determine the proportion of the gas giant's remaining mass budget the moon gets.

A gas giant can only have a moon system that totals no more than .4 Earth masses? If that is in the GURPS Space system, I missed it. Or is that in reality based on current science?

So does this mean I can't have even 1 Earth-massed moon around my gas giant? If these more massive major moons just can't form there, are there any plausible migration/capture scenarios that could account for them being there?

[Celti]

Brett, have you considered a Pyramid article on what you view the Space generation sequence should be? This leaves the canonical Space sequence as a "good-enough" option for those who don't care about little details like this, while you could put together a monstrously-complex sequence that does care — somewhat like The Deadly Spring did for bows. Also similar to TDS, you could include a new version of your spreadsheet for it, or even partner with someone around here with programming skill to write an actual program.