[Space] GURPS Handbook of the Planets

[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.