[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.