Just a short note, to call your attention to an interesting result in recent astrophysics that’s quite relevant to the Architect of Worlds project.

It’s well known that we’ve discovered thousands of exoplanets in the last couple of decades. Now the state of the art is approaching the point where we can get clues about the environment on those planets. For example, one recent result (here’s an article in Scientific American) is the first indication we have of the kind of atmosphere that exists around an Earth-sized rocky exoplanet. In this case, the planet is in its primary star’s habitable zone, and it’s more than large enough to retain a significant atmosphere against thermal or Jeans escape. Yet there doesn’t seem to be much if any atmosphere there.

I’ve long since worked out a model for Jeans escape; that much was in the world-design system I wrote for GURPS Space back in the day. (If you’ve used that system, you may recall a “minimum molecular weight retained” or MMWR calculation. That’s specifically relevant to Jeans escape.)

The problem is that thermal loss isn’t the only way a planet’s atmosphere can get stripped off. If the primary star is prone to flares and has an energetic stellar wind, that will do the trick too. This is specifically relevant to red dwarfs, like the star LHS 3844 which has the planet mentioned above. Red dwarf stars punch well over their weight in the stellar-wind and flare department, especially early in their lives. Thus, any rocky planet close enough to be in the liquid-water zone will probably get a serious sandblasting early on.

This is kind of a new area in astrophysics, and there are a lot of competing models out there. Some scientists are predicting that Earth-sized planets should be able to retain their atmospheres in at least some cases, others are much less sanguine. The LHS 3844 result certainly seems to support the pessimistic case.

For Architect of Worlds, I’ve been thinking in terms of assigning each planet a “volatiles budget” from its formation and early years, modified by things like the planet’s MMWR, whether it formed inside the snow line, whether there’s a dominant gas giant to fling comets in-system, and so on. A big random factor as well, since it looks as if this feature is strongly subject to chance. Then we would reduce that volatiles budget to reflect non-thermal processes of atmospheric escape, photodissasociation of water molecules, and so on. (Hmm. Maybe have a separate budget just for water, since that goes through some significantly different processes and might not be correlated with atmospheric volatiles.)

The devil’s in the details, of course, and for all my tinkering I have yet to come up with a model that satisfies me (or even fits all the cases we know about). This might actually be the biggest obstacle to getting the third section of the design sequence hammered out.