Still working through my data pull from the HIPPARCOS data set. I haven’t found any more planetary systems that the draft Architect of Worlds model simply won’t fit, although the famous Gliese 667 C system came close.

One thing I have discovered is that my assumption about red dwarf stars seems to have been premature. A little further research tells me that the photosynthesis problem isn’t an absolute deal-breaker. The problem isn’t that photosynthesis is impossible under red-dwarf starlight, it’s that an early photosynthetic organism would have to adapt to long periods of visible-light scarcity, punctuated by the nasty stellar flares young red dwarfs tend to generate. One might imagine mats or colonies of photosynthetic microbes that drift to the surface of a planet’s ocean to take in the sunlight, then submerge to ride it out when flare weather sets in. Eventually, most red dwarf stars seem to settle in and stop producing major flares, so if their planets can give rise to life at all, evolution to complex biospheres seems at least possible.

So, rather than forbid red dwarfs from having garden worlds at all, I’ve decided to impose a penalty, requiring them to take a lot longer to develop complex biospheres. Even so, since red dwarfs burn so steadily over many billions of years, an ocean planet has plenty of time to work on the problem. Red dwarfs that are at least as old as Sol, certainly the ones that are a few billion years older, are possible candidates.

I worked out a set of criteria to determine whether I should work out a red dwarf star’s planetary system at all: at least as old as Sol, bright enough that the habitable zone falls out where the inner planets are likely to orbit, and with metallicity high enough to permit terrestrial planets at least one-quarter as massive as Earth. I’d say maybe one out of three red dwarfs in the solar neighborhood have fit the criteria well enough for me to break out the calculator, spreadsheet, and dice.

Now another facet of the new model comes into play. The draft model often generates systems of planets whose orbits are more tightly packed than one would expect, just looking at our own system. Which in turn significantly increases the probability that at least one planet will sit in the liquid-water habitable zone. In fact, sometimes I’m getting two planets in the zone in the same system. That’s not a result that the GURPS Space 4/e model would have produced very often, if ever.

The upshot is that although any given red dwarf is unlikely to host a garden world, there are so many red dwarfs that I’m getting a significant number of them. Lots of “eyeball planets” out there, it seems; possibly as many as the more Earth-like worlds with reasonable day-night cycles.

So far, I’ve worked out planetary systems to about 25 light-years from Sol, including all the K-class and hotter stars, now also including all the red dwarfs that seem to be plausible hosts for garden worlds. 168 lines in the HIPPARCOS database, although a handful of those aren’t actual stars, and 16 stars that have complex biospheres present. Looks like roughly one out of ten stars is giving me at least one garden world. More than I expected, actually, but it’s a result I can live with.