[Space] GURPS Handbook of the Planets

[Agemegos]

Originally Posted by Nemoricus

The limitations of the GURPS Space generator are probably more significant in the long run. And while I'm quite interested in discussing them, would they take this thread off-topic too much? If so, I'll be happy to take that to PMs.

There is a pervasive family of problems that comes about from using die-rolls and table look-ups, and thus producing discrete values. Thus for example the ratios of the semi-major axes of successive orbits, and atmospheric mass for another example, take on a limited number of discrete values. It would be almost unimaginably difficult to do things any other way for manual generation. But with a computer doing everything it is quicker and easier as well as better to use continuous variables. When I modified a version for my own use I replaced many of the 3d6 and 2d6 look-ups with a continuous bell-curve transformation, and some of the d6 lookups with a linear transformation.

The way the GURPS Space star system generation sequence works, in which world types are chosen first, and by a method that varies according to distance from the star by discrete steps, produces a subtle trend for planets to get smaller with increasing distance from the star. If I were starting from scratch I'd generate the world's mass and composition first (x, y, and z amounts of metal, stone, and ice, depending on the ambient temperature) and derive size, and mass, then MMWR, then atmosphere type, except in the instance of a user-defined planet. I think it worked out the way it did so that the algorithms explained in Ch. 4 could be re-used in Ch. 5.

There are some problems with the climate table. It's possible to get a planet with oceans, and the atmospheric composition that goes with oceans, when by my reckoning it ought to be frozen solid to the equator. The hydrographic percentage of a planet really ought to be diminished to account for sea-beds that are filled with ice either in polar ice-caps or in the dark-side ice-caps of tide-locked worlds.

The system doesn't take account of how the boiling point of water varies with atmospheric pressure. In fact, the boiling point of water on the surface is one of the values I'd like a planet generator to tell me. And if it is lower than ambient temperature the planet's type ought to become "dry greenhouse planet".

Stephen Dole's seminal Habitable Planets for Man considered day-length as a limitation on habitability. If the day is short that the rotation is fast, which means a strong Coriolis effect and strong winds. If the day length is long then diurnal temperature variation should approach the tide-locked solution—in any case very long hot days and very long cold nights are likely to make a planet unattractive to settlers and problematical for agriculture. I'd like the generator to calculate the diurnal temperature variation and an index of the violence of weather (depending on temperature, rotation rate, diameter, and atmosphere pressure) and take it into account in the habitability rating.

From what I can make out from what astronomers are reporting, epistellar and eccentric gas giants are a lot less common than the GURPS Space generator makes them. A total of no more than about 4% of systems have these interesting arrangements of gas giants. Also, I suspect that the "no gas giants" arrangement isn't very common either.

I believe that the generation sequence as it stands gives planetary orbits eccentricities that are a lot too large. Even in "conventional gas giants" systems they tend to be ten times larger than are typical in this solar system. As an Easter egg in my generator I calculated and reported aphelion and perihelion equilibrium temperatures: the results would have dramatic effects on annual climatic variation, even affecting habitability. The generator as it stands considers no consequences either of eccentricity or of obliquity (axial tilt)— assigns large values to both and treats them as decorations.

I'd like to handle sulphur moons by calculating the geothermal flux, taking into account tidal kneading. That could generalise geological activity in general, with exponential decay of the other components.

There are significant issues surrounding the different proportions of visible and invisible light that different stars put out. I think the system uses visual luminosity to determine temperature and everything, where it ought to use bolometric. My private-use variant uses bolometric luminosities.

On the other hand the visual insolation of a planet is an important factor in primary ecological production. Therefore it goes to determine agricultural productivity, and ought to be a factor in habitability. Also, visual insolation will determine how fast primitive photosynthesis on the planet will have been able to generate oxygen, and it ought to be an important factor in the timing of the transition from Ocean to Garden world type.

The generator as it stands provides that the Ocean-to-Garden transition will occur at an average age of 3.6 billion years, and I think that that is a bit too late. The oxygen catastrophe on Earth came at more like 2.2 billion.

There are several issues having to do with the habitability of tide-locked worlds. It seems to me that a tide-locked world with an equable average temperature ought to have much diminished Habitability because the sunny face is too hot, the shady face is covered with snow and ice, and the transition zone is in permanent twilight where the light is rather too dim for plants. I suspect that the habitability of normal and warmer tide-locked worlds ought to be reduced, and that of the cooler ones ought to be increased (or left the same) to account for the fact that ecosystems and agriculture will be most productive on such worlds when the sunny face rather than the twilight zone is a comfortable temperature. In any case a modifier to habitability for tide-locked and orbitally-resonant worlds is called for.

[Nemoricus]

The way the GURPS Space star system generation sequence works, in which world types are chosen first, and by a method that varies according to distance from the star by discrete steps, produces a subtle trend for planets to get smaller with increasing distance from the star.

For terrestrial worlds, there is an implicit relationship between the distance from a star and the size of the planet, and it arises from the square root of the blackbody temperature being used to determine the diameter/surface gravity (Step 6 in Basic World Generation). I've tried to come up with some explanation for why this was chosen, but I haven't been able to find one. Perhaps it's meant to represent the increasing sparseness of material as one moves out in a system? That seems a bit thin to me....

The hydrographic percentage of a planet really ought to be diminished to account for sea-beds that are filled with ice either in polar ice-caps or in the dark-side ice-caps of tide-locked worlds.

Additionally, the presence of frozen water decreases the absorption factor of a planet due to the increased reflectiveness of water ice as opposed to liquid water. A planet with substantial amounts of ice could end up frozen over surprisingly close to the star. In fact, it's been hypothesized that our own Earth might have gone through such a phase.

The system doesn't take account of how the boiling point of water varies with atmospheric pressure.

As far as calculating the vapor pressure of water goes, the Antoine equation seems to provide a good answer. Just plug in the temperature at the surface and see if the vapor pressure exceeds the atmospheric pressure. To convert from mm Hg to atmospheres, just divide the former by 760. If the surface temperature exceeds about 647 K, which is the critical temperature of water, then no amount of pressure will keep water liquid.

Vaporization of water has significant effects that will complicate the analysis, though. Water vapor can form clouds, which would increase the albedo of the planet, reducing its temperature, which in turn reduces the evaporation of water. Given this, it seems likely that significant amounts of water will regulate the temperature, preventing it from getting too extreme either way.

Of course, then there's the matter of ice that was brought up earlier....

I'd like the generator to calculate the diurnal temperature variation and an index of the violence of weather (depending on temperature, rotation rate, diameter, and atmosphere pressure) and take it into account in the habitability rating.

No disagreement there.

From what I can make out from what astronomers are reporting, epistellar and eccentric gas giants are a lot less common than the GURPS Space generator makes them. A total of no more than about 4% of systems have these interesting arrangements of gas giants. Also, I suspect that the "no gas giants" arrangement isn't very common either.

The range for conventional gas giants should probably be increased, then. Though, what are the proportions of gas giants that both exist inside the snow line and have reasonably circular orbits?

I believe that the generation sequence as it stands gives planetary orbits eccentricities that are a lot too large. Even in "conventional gas giants" systems they tend to be ten times larger than are typical in this solar system.

The situation is worse than it seems. Even in the conventional gas giant model, the maximum eccentricity, 0.2, can lead to worlds with orbits that actually cross each other. There are no rules or suggestions for what should be done in this case. A reasonable suggestion is that the more massive body would eject the smaller one, trap it in a resonance, or capture it as a moon, with the first being the most likely and the last the least. If the two are similar in mass, then pick one at random to survive.

For conventional gas giant systems, though, the distribution should probably be a little more biased to the lower eccentricities.

High obliquity should have dramatic effects on seasons, and should probably have results akin to that of a tidally locked body.

I'd like to handle sulphur moons by calculating the geothermal flux, taking into account tidal kneading. That could generalise geological activity in general, with exponential decay of the other components.

I agree, though simply considering the tidal forces might suffice in place of considering the flux, too. Also, if there's a single major moon around a gas giant, we're very unlikely to see Ionian levels of volcanic activity, since it lacks the orbital perturbations that the other Galilean moons provide.

I think the system uses visual luminosity to determine temperature and everything, where it ought to use bolometric.

So far as I can tell, the luminosities given in the table are bolometric. My review wasn't comprehensive, though, so I might have missed something.

On the other hand the visual insolation of a planet is an important factor in primary ecological production. Therefore it goes to determine agricultural productivity, and ought to be a factor in habitability. Also, visual insolation will determine how fast primitive photosynthesis on the planet will have been able to generate oxygen, and it ought to be an important factor in the timing of the transition from Ocean to Garden world type.

I'm still looking for a simple expression that relates blackbody temperature to the power in visible wavelengths.

Also, the temperature would have interesting effects on the color of plants on a garden world. Around a low luminosity M-type star, they might be black so that they can absorb as much light as possible, while those around F and A stars might be bluish. The exact details are probably out of scope for this topic, but noting the peak wavelengths could be useful for people looking to add some flavor to their worlds.

The generator as it stands provides that the Ocean-to-Garden transition will occur at an average age of 3.6 billion years, and I think that that is a bit too late. The oxygen catastrophe on Earth came at more like 2.2 billion.

Decreasing the increment from 500 million years to 300 million years seems like it would solve that problem.

Speaking of which, it seems highly unlikely that life would emerge on worlds with high degrees of tectonic and volcanic activity. At the extreme ranges, it would imply that the surface could still be molten, and so worlds in that state shouldn't be garden worlds. It's also questionable whether they could be oceanic, too.

That implies that we need at least one more type for standard and large worlds. Hephaestean worlds, perhaps?

There are several issues having to do with the habitability of tide-locked worlds. It seems to me that a tide-locked world with an equable average temperature ought to have much diminished Habitability because the sunny face is too hot, the shady face is covered with snow and ice, and the transition zone is in permanent twilight where the light is rather too dim for plants.

You'd probably also get rather nasty winds at the terminator, given the rather extreme temperature gradient that exists. Which, in turn, makes the planet even less attractive for habitation.

There are several important factors that would also impact a planet's evolution and habitability that are not brought up in the book. One of the biggest is that magnetic fields are completely omitted, which not only protects an atmosphere from degradation, it reduces the danger to life on the surface.

Moons of gas giants, especially large ones like Jupiter, have particularly high radiation fluxes, which makes visiting them tricky for space probes and potentially lethal for manned missions. Even if the moons are resource rich, the added hazard might outweigh that factor.

In multiple star systems, if the companion star(s) are close enough, then the luminosity of all stars are important in determining the temperature of a planet, and variation in their distances would lead to significant cycles.

I'm working on a second post to include my thoughts on gas giants, since that's an area that needs more attention and this one took some time to write.

In the mean time, here's some interesting features of the system that I've noticed:

1. Star systems can actually have as many as five stars, if my interpretation of the rules in Step 19 is valid. If a star system starts as a trinary system, and both companion stars end up in stable, distant orbits, each of them could have a companion of their own. It's not going to be too common, but it can happen, and the results could be a spectacular sky for any planets around them.

2. Under Terrestrial Planets in Step 24, a sufficiently distant Large planet could have as many as three(!) major moons due to its +1 modifier for size. Rules as written, this could result in all three being of Standard size! While it is conceivable that a Large planet could have a single Standard moon, particularly if it were of a relatively low mass for that size category, more than that starts to strain credibility. A progressive penalty should be applied to the size of terrestrial moons to avoid this situation.

3. The Large (Hadean) world type is absent. I do not believe that this represent any problem with the system, however. It's quite possible that any world of that type, at that sort of distance, would be far more likely to grow into a gas giant than to remain terrestrial. This is somewhat speculative, though.

[Agemegos]

Originally Posted by Nemoricus

For terrestrial worlds, there is an implicit relationship between the distance from a star and the size of the planet, and it arises from the square root of the blackbody temperature being used to determine the diameter/surface gravity (Step 6 in Basic World Generation). I've tried to come up with some explanation for why this was chosen, but I haven't been able to find one.

It allowed world type (i.e. atmosphere to be determined first, instead of being calculated from mass, diameter, and temperature). This in turn allowed Chapter 5 to re-use the algorithms of Chapter 4.

Additionally, the presence of frozen water decreases the absorption factor of a planet due to the increased reflectiveness of water ice as opposed to liquid water.

Yes, and the evolution of land plants creates another feedback loop in which (depending on the extent of arid zones) comparatively reflective bare rock gets covered with comparatively dark vegetation, altering the planet's albedo.

This gets us into the area of global temperature stabilisation through the carbonate-silicate cycle, which is another thing that I would like to see represented in a generator. In fact, it's so important that I'm going to spend the morning hacking it in to my private-use version.

As far as calculating the vapor pressure of water goes, the Antoine equation seems to provide a good answer.

So it's a matter of inverting the Antoine equation to get boiling point as a function of temperature. I'll look into it.

Vaporization of water has significant effects that will complicate the analysis, though. Water vapor can form clouds

Not while it is vapour. Only when it condenses again. Which requires locally low temperatures. We don't find that the warm parts of the world are cloudy and the cold parts clear. The whole cloudiness thing is way to complicated, and I'm happy with an assumption that cloudiness will depend only on hydrographic cover and a good-enough hack.

Given this, it seems likely that significant amounts of water will regulate the temperature, preventing it from getting too extreme either way.

Water vapour is also a greenhouse gas. And high temperatures cause clouds to evaporate just as they do other water.

what are the proportions of gas giants that both exist inside the snow line and have reasonably circular orbits?

Reply hazy. Ask again when the Terrestrial Planet Finder has been operating for a few years.

The situation is worse than it seems. Even in the conventional gas giant model, the maximum eccentricity, 0.2, can lead to worlds with orbits that actually cross each other. There are no rules or suggestions for what should be done in this case.

If you are generating a system rather than a history, just apply the orbital spacing factor to successive perihelions and aphelions rather than the semi-major axes. E.g. when working outwards, multiply the aphelion distance you have by the orbital spacing factor to get the perihelion distance of the next object, then generate that object's eccentricity, calculate its semi-major axis and then its aphelioin distance, and loop.

For conventional gas giant systems, though, the distribution should probably be a little more biased to the lower eccentricities.

Much more.

I agree, though simply considering the tidal forces might suffice in place of considering the flux, too.

You really need to consider the eccentricity of the orbit, too. If Io's orbit were not eccentric it would experience no tidal kneading despite extreme tidal strain.

Also, if there's a single major moon around a gas giant, we're very unlikely to see Ionian levels of volcanic activity, since it lacks the orbital perturbations that the other Galilean moons provide.

Yes, if you want to do this you have to take into account that tidal forces quickly circularise and equatorialise the orbits of gas giant moons unless there are continual perturbations.

So far as I can tell, the luminosities given in the table are bolometric.

It doesn't say, and they looked like visual when I did a quick check. I treat them as bolometric when doing my calculation of visual illumination at the surface, though. That's because they are determining temperature and I want to get the relationship between temperature and illumination right.

In my private variant I take visual and bolometric luminosities from an astronomical catalogue, so the issue doesn't arise.

I'm still looking for a simple expression that relates blackbody temperature to the power in visible wavelengths.

Essentially you want to integrate Planck's law.

I came across an on-line calculator once.

Also, the temperature would have interesting effects on the color of plants on a garden world. Around a low luminosity M-type star, they might be black so that they can absorb as much light as possible….

It seems to me that they ought to be as black as possible everywhere. The colour of plants is probably dictated by the bounds of chemical possibility accessible to evolution, not by the illumination spectrum.

Decreasing the increment from 500 million years to 300 million years seems like it would solve that problem.

Yes, but not address the issue of the pace of the oxygen transition depending on aeric primary ecological production, which is to say on surface illumination and specifically on surface illumination in the band of wavelengths useful to photosynthesis.

Speaking of which, it seems highly unlikely that life would emerge on worlds with high degrees of tectonic and volcanic activity. At the extreme ranges, it would imply that the surface could still be molten, and so worlds in that state shouldn't be garden worlds. It's also questionable whether they could be oceanic, too.

That implies that we need at least one more type for standard and large worlds. Hephaestean worlds, perhaps?

Possibly. "Hadean" is unfortunately the scientific term for that condition.

You'd probably also get rather nasty winds at the terminator, given the rather extreme temperature gradient that exists. Which, in turn, makes the planet even less attractive for habitation.

Modelling work on this subject is interesting but limited. See Heng, K., Menou, K. and Phillipps, P.J., 2010, Atmospheric circulation of tidally-locked exoplanets: a suite of benchmark tests for dynamical solvers, Merlis, T.M. and Schneider, T., 2010 Atmospheric dynamics of Earth-like tidally locked aquaplanets, and Showman, A.P, Cho, J. Y.-K., Menou, K., 2009, Atmospheric Circulation of Exoplanets.

It turns out that the rotation of the world, though not apparent from the [lack of] motion of the sun, still has important effects on teh climate.

In multiple star systems, if the companion star(s) are close enough, then the luminosity of all stars are important in determining the temperature of a planet, and variation in their distances would lead to significant cycles.

When I've looked in detail at plausible scenarios I've found that it doesn't actually vary very much.

1. Star systems can actually have as many as five stars, if my interpretation of the rules in Step 19 is valid. If a star system starts as a trinary system, and both companion stars end up in stable, distant orbits, each of them could have a companion of their own. It's not going to be too common, but it can happen, and the results could be a spectacular sky for any planets around them.

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.

The Large (Hadean) world type is absent.

The world types are actually atmospheric categories. "Large" doesn't mean large, it means "having an atmosphere that includes helium but not hydrogen". And "hadean" (in this context, not as planetary scientists use it) means "so cold that the compounds that would normally make up the atmosphere at this temperature and escape velocity have frozen". Helium doesn't freeze, and it won't even liquefy under plausible conditions. In short, if a world is Large (as GURPS Space uses the term) it can't be hadean (as GURPS Space uses the term): no matter how cold you make it its helium atmosphere will remain and it'll be Large Ice.

[Nemoricus]

Originally Posted by Brett

It allowed world type (i.e. atmosphere to be determined first, instead of being calculated from mass, diameter, and temperature). This in turn allowed Chapter 5 to re-use the algorithms of Chapter 4.

The realization that the planetary sizes are based on which gases they are able to retain in their atmosphere clarified much of the logic involved in that calculation and which planetary types exist.

This gets us into the area of global temperature stabilisation through the carbonate-silicate cycle, which is another thing that I would like to see represented in a generator. In fact, it's so important that I'm going to spend the morning hacking it in to my private-use version.

I had not heard of this before, and the ramifications are so profound that I wish that I had before. It explains so much about the differences between Venus and Earth....

If you are generating a system rather than a history, just apply the orbital spacing factor to successive perihelions and aphelions rather than the semi-major axes.

While slightly more complex than the rules as written, it quite neatly removes any need to adjudicate any borderline cases. I like it quite a bit.

Yes, if you want to do this you have to take into account that tidal forces quickly circularise and equatorialise the orbits of gas giant moons unless there are continual perturbations.

Until I find a good way of simplifying the timescale for circularization, I think it would be simplest to say that gas giant moons of the inner and major groups are in circular, equatorial orbits. Outer moonlets can be rolled for.

Terrestial major moons, on the other hand, would be circular and have an orbital plane similar to the planet's own, if they're close enough to the star. I can't find my reference again, but apparently solar gravity dominates the Earth/Moon system, so while the two are gravitationally bounded, the plane of the moon's orbit is preferentially pulled into the ecliptic plane. Minor moonlets would fall into the equatorial plane, since they orbit closer.

It doesn't say, and they looked like visual when I did a quick check. I treat them as bolometric when doing my calculation of visual illumination at the surface, though. That's because they are determining temperature and I want to get the relationship between temperature and illumination right.

Which catalogue are you using as a reference for the magnitudes?

Yes, but not address the issue of the pace of the oxygen transition depending on aeric primary ecological production, which is to say on surface illumination and specifically on surface illumination in the band of wavelengths useful to photosynthesis.

The time could probably be scaled by the inverse of the power in those wavelengths and the hydrographic coverage. There's probably a threshold below which oxygen transitions won't occur at all, though, and a point of diminishing returns in the higher powers.

The former is probably more significant for our purposes, given the prevalence of M stars. The planet might be close to the star to support liquid water, but if the peak wavelength is substantially below the lowest photosynthetic band, life as we know it is quite unlikely. There could very well be a good number of chemotrophic microorganisms, but without the energy aerobic respiration provides, multicellular organisms and especially highly mobile ones probably won't be present.

Unless, of course, there are other mechanisms that provide either high energy or significant oxygenation.

It turns out that the rotation of the world, though not apparent from the [lack of] motion of the sun, still has important effects on teh climate.

Huh. Those are quite interesting.

Now, onto gas giants. Not nearly as much attention is payed to them as the terrestrial bodies, and in all fairness, that's a perfectly reasonable tack for the writers to take. The harsh, unforgiving nature of their atmospheres and the lack of any solid ground means that they're more in the nature of scenery rather than places to visit.

However, such a view belies the rich variety that they have, which is quite comparable to that of terrestrial bodies. The mass of a gas giant aside, the temperature has striking effects on its appearance. Looking at our own solar system, Jupiter has a rich, roiling atmosphere, dominated by bands whites, reds, and oranges, while Saturn is more yellowish in hue, with subtler bands. However, when the sun is obscured by Saturn's rings in the northern hemisphere's winter, the atmosphere turns blue as the gases that normally form clouds freeze out.

Going further out, Uranus is a fairly featureless bluish-green sphere, whereas Neptune is blue, with visible clouds and storms. The color is due to the methane in the atmospheres of both, while the difference in activity is due to the temperatures of each. Neptune is warmer despite its greater distance from the sun, possibly due to whatever event led to Uranus being tilted so sharply robbing it of much of its heat.

Going inward, the results are more speculative as no examples in our solar system exist, but the Sudarsky classification scheme gives some idea of what they might look like.

Given the dependence on temperature, the blackbody temperature and the residual heat of gravitational contraction should be looked at. The higher of the two would likely dominate, though as yet I'm not sure how the gravitational heat would be determined. A function of the mass and age, perhaps? Once this is determined, the general appearance of the gas giant could be based on what compounds would dominate its clouds, and on the trace compounds that color it further.

The treatment of the moons of gas giants is fairly simplistic in the current system, and takes no account of the mass of the body when generating them. In our solar system, the total mass of the moons tends to amount to 1/10000 the mass of the gas giant, with the overwhelming majority of it concentrated in one to four moons. However, it's possible that a truly massive gas giant could have many more major moons than that, depending on its distance from its star.

Since the current system for major moon orbital radii is kind of silly, generating orbital tracks for the major moons would make more sense. The simplest solution is to generate them in the same way those for planets are generated, with 3 planetary diameters for the inner bound. For the outer bound, I recall a rule of thumb that if a satellite orbits its primary at least 9 times for every time the primary goes around the star, it's stable. This works out to:

R = cubic root of ((Y^2) * M) * .765

Where R is the outer orbit limit in Earth diameters, Y is the planet's year in days, and M is the planet's mass in Earth masses.

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.

Working out the tides in a particularly large system would be an interesting exercise....

[Agemegos]

Originally Posted by Nemoricus

I had not heard of this before, and the ramifications are so profound that I wish that I had before. It explains so much about the differences between Venus and Earth....

It's also vital to the explanation of how the Earth has remained at a habitable temperature almost continuously through a span of time in which the Sun has brightened by 30%.

Until I find a good way of simplifying the timescale for circularization, I think it would be simplest to say that gas giant moons of the inner and major groups are in circular, equatorial orbits. Outer moonlets can be rolled for.

I think that's the right tack to take. The tidal effects on the moons of rapidly-rotating gas giants are intense, and unless there is a steady perturbation to maintain disequilibrium the orbits circularise, equatorialise, and space themselves out in shorter times than a lot of other things we are treating as instantaneous, such as Jeans escape. It can be only thousands or myriads of years.

Terrestial major moons, on the other hand, would be circular and have an orbital plane similar to the planet's own, if they're close enough to the star. I can't find my reference again, but apparently solar gravity dominates the Earth/Moon system, so while the two are gravitationally bounded, the plane of the moon's orbit is preferentially pulled into the ecliptic plane. Minor moonlets would fall into the equatorial plane, since they orbit closer.

That agrees with my understanding. The Moon is torn between tidal forces from Earth's bulge and rotation that tend to draw it into the equatorial plane, and tidal forces from the Sun that tend to draw it into the ecliptic plane. These effects are of comparable strength; if Earth were appreciably closer to the Sun (e.g. if the Sun were a smaller dimmer star) the Moon's orbit would lie in the ecliptic, if it were further away the Moon's orbit would lie in in the celestial equator.

Which catalogue are you using as a reference for the magnitudes?

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.

The time could probably be scaled by the inverse of the power in those wavelengths and the hydrographic coverage.

That's what I did, using visual luminosity

There's probably a threshold below which oxygen transitions won't occur at all, though, and a point of diminishing returns in the higher powers.

Yes, it turns out that IR isn't much use because the quantum of energy is too small to re-form a C-O bond. And UV is absorbed by atmospheric processes. Besides which the stars with UV-rich light burn out too quickly to allow the development of an oxygen atmosphere anyway. That's what limited the probabilities for planets of early stars in Dole's work. I figure that V-band luminosity is close enough to what I need, and it's certainly easily available.

This imposes a severe limitation on the likelihood of habitable planets orbiting late stars. No-one else seems to have considered it, but I consider that it is a decisive blow against anything cooler than M3, and comes close to counteracting the long lifespan of Ks and the earlier Ms.

Now, onto gas giants. Not nearly as much attention is payed to them as the terrestrial bodies, and in all fairness, that's a perfectly reasonable tack for the writers to take. The harsh, unforgiving nature of their atmospheres and the lack of any solid ground means that they're more in the nature of scenery rather than places to visit.

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.

Given the dependence on temperature, the blackbody temperature and the residual heat of gravitational contraction should be looked at. The higher of the two would likely dominate, though as yet I'm not sure how the gravitational heat would be determined. A function of the mass and age, perhaps?

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.

The treatment of the moons of gas giants is fairly simplistic in the current system, and takes no account of the mass of the body when generating them. In our solar system, the total mass of the moons tends to amount to 1/10000 the mass of the gas giant, with the overwhelming majority of it concentrated in one to four moons. However, it's possible that a truly massive gas giant could have many more major moons than that, depending on its distance from its star.

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.

Since the current system for major moon orbital radii is kind of silly, generating orbital tracks for the major moons would make more sense. The simplest solution is to generate them in the same way those for planets are generated,

That's what I do.

with 3 planetary diameters for the inner bound.

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

For the outer bound, I recall a rule of thumb that if a satellite orbits its primary at least 9 times for every time the primary goes around the star, it's stable.

Having a computer, you might as well use the Hill Sphere radius.

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.

Seems fair to me.

[Flyndaran]

As to low light photosynthesis
http://www.asu.edu/feature/includes/.../photosyn.html
TLDR: Green Sulfur Bacteria found surviving on the super dim light given off by deep sea vents.
I can't find any mention on exactly how dim that is, but I can't imagine it being even noticeable to human eyes.

[Agemegos]

Originally Posted by Flyndaran

As to low light photosynthesis
http://www.asu.edu/feature/includes/.../photosyn.html
TLDR: Green Sulfur Bacteria found surviving on the super dim light given off by deep sea vents.
I can't find any mention on exactly how dim that is, but I can't imagine it being even noticeable to human eyes.

Those don't excrete oxygen but sulphur. Which means that they can't effect the oxygen catastrophe.

The issue of dim visible light is not that it make photosynthesis impossible but that it makes it slow, thus making the excretion of oxygen slow, thus making he oxygen catastrophe late.

Dim stars put out a large proportion of their radiation as IR, which is effective at heating planets but not very effective at driving photosynthesis. That leaves the illumination on their planets dim even when total insolation is normal. We expect them to have their oxygen catastrophes late and to support only low-productivity agriculture.

[Flyndaran]

I'm sorry. I didn't realize the photosynthesis issue was intimately connected to the oxygen catastrophe event topic. I thought it was more about the possibility of life in general.
I like my alien worlds alien. :)

[Agemegos]

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.

[Flyndaran]

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.

I agree that it seems to take far too long, most of the time, when playing with the rules.