Most of my effort over the last few days has been directed toward two tasks. First, continuing to test the Architect of Worlds model for planetary systems by generating collections of worlds for stars close to Sol. Second, using those results to motivate the first definitions for the next stage of the design sequence: determining the physical properties of an individual world.

The first is going as well as can be expected. So far, I’ve only found one star system that I flatly can’t model properly (the HR 8832 system, about 21 light-years from here, which is believed to have an even stranger collection of super-Earths and close-in gas giants than usual). Otherwise, I’m getting a very plausible set of planetary systems, a significant improvement over the results I would have gotten from the old GURPS Space 4/e design sequence.

As far as the second task goes, I’ve had something of a breakthrough: I’ve found a model I can live with to help the user decide whether a given planet is tide-locked to its primary star or not. It’s a horrible kludge – but the question of how long it takes a planet to tide-lock is very complex, and there’s no consensus in the literature about it. If a planet could be modeled as a uniform and perfectly elastic body, the math simplifies pretty well, but planets just aren’t like that. The equation I’ve come up with seems at least plausible, in the forty or so star systems for which I’ve generated data so far.

Right now, I’m wrestling with how to decide whether a given planet (or moon) has a substantial atmosphere or not, and whether it has oceans or not.

In GURPS Space 4/e, I kind of took a backwards approach – I had the user decide which of several categories a world fell into, and then he generated the world’s mass, density, and so on to fit. I think that was slightly more useful for the gaming context, but the math was kind of annoying (not least because SJG editorial policy forbade me from using SI units, so I tried scaling everything to Earth and the Sun, with weird outcomes). The math is a bit more straightforward doing it the other way – define a planet’s mass and density, then figure out what its surface environment will be like.

Of course, now I have to wrestle with questions like why Mars has almost no atmosphere despite being massive enough to retain molecular nitrogen and carbon dioxide (and it can’t just be because Mars has no magnetic field to speak of, because Venus doesn’t either, and it has a very thick atmosphere). Or, say, why Titan has a substantial atmosphere when the almost identical Ganymede has none.

Slowly, a classification scheme is emerging, but it will probably be a few more days before I’m happy with it.

Meanwhile, the upcoming week is going to be unusually busy at the office. I’m teaching one course, taking a second course, and facing impending deadlines on writing two more courses after that. Generally, my life is not quite that full! I may or may not have a lot of time to play with my worldbuilding over the next few days. We’ll see how things go.