Okay, for the last few weeks I’ve been logging a play-through of the Phil Eklund games Bios: Genesis and Bios: Megafauna, in a demonstration of how those games can be used to support worldbuilding for science fiction. A quick way to review those posts would be to check out the Worldbuilding by Simulation category and look at all the posts since the beginning of June 2018.

Now it’s time to do some math, and design the star system and main habitable planet compatible with the results of the Bios games. I’ll be using the current draft design sequences from my Architect of Worlds project. In particular, the current draft of the star system design sequence can be found at Architect of Worlds: Designing Star Systems. The design sequence for designing planets hasn’t been published yet, and I need to do a fairly extensive revision pass before that happens, but its current draft should be sufficient for this purpose.

I begin by coming up with a pair of names for the habitable planet (Toswao) and its primary star (Karjann). I have absolutely no constructed language work to back those up, and probably won’t go that far for a single story. Those names simply emerged from the back of my mind under the stimulus of a random-name generator; I think they look and sound pleasant, so there we go.

Primary Star

Looking back on the Bios: Megafauna game, I recall that Toswao has spent most of its history with very warm climate, well above Earth’s present average temperature. That suggests a primary star that’s a touch more massive than Sol, and therefore probably more luminous.

Meanwhile, we also know that Toswao is quite a bit younger than Earth. With adjustments, the Bios: Genesis game covered about 3.75 billion years from planetary formation to the end of the Proterozoic period. The Bios: Megafauna game covered about 240 million years from there to the first appearance of a tool-and-language-using species. Add that up and we get 3.99 billion years, which I’m comfortable rounding off to 4.0 billion. Evolution moved fast here! That doesn’t necessarily indicate anything about Karjann, but in my mind the notion of a somewhat more energetic primary star also fits a faster pace of development. So I decide to non-randomly select a primary star mass of 1.04 solar masses.

With a dice roll, I find that Karjann is a solo star – no need to generate details for any companions. I set the star system’s age at exactly 4.0 billion years, and randomly generate the system’s metallicity, ending up with a value so close to 1.0 that I decide to round that off as well. Working through the design sequence, I end up with the following parameters:

Karjann

• Mass: 1.04 solar masses
• Main Sequence Lifespan: 8.6 billion years
• Current Age: 4.0 billion years
• Metallicity: 1.0
• Current Effective Temperature: 5800 K
• Current Luminosity: 1.23 sols
• Radius: 0.0051 AU (767,000 km)
• Spectral Class: G2V

Karjann turns out to be quite similar to Sol, a cheerful yellow star about halfway through its stable lifespan, a touch hotter and noticeably brighter.

Planetary System

Before beginning planetary system design, I need to figure out where the habitable world (Toswao) is going to be placed. Here, I have a few clues.

The final state of the Bios: Megafauna game suggested that the planet’s atmosphere had 34% free oxygen. This is pretty high, equivalent to the highest level ever seen in Earth’s atmosphere, back in the Cretaceous era. Some research tells me that such a high free oxygen level has to be supported by very high levels of carbon dioxide, several times the current value in Earth’s atmosphere. So I pin the current CO2 level as about six times Earth’s pre-industrial level, or about 1800 parts per million.

The final Bios: Megafauna state also suggests a planetary albedo of 0.8, but that isn’t at all plausible. The most reflective water-vapor clouds have about that albedo, so a long-term planetary albedo that high means that the entire planet is covered with the brightest possible cloud canopy. Unlikely over a long period, and how is anything surviving with direct sunlight cut off from the photosynthetic base of the food chain? Still, a planet with more hydrographic surface than Earth is likely to have more cloud cover, and therefore a higher overall albedo. I’ll set the planet’s albedo to 0.5, which is probably still very high, but not utterly implausible.

That albedo also suggests a lot more water vapor in the atmosphere than Earth currently sees. I’ll tentatively assume double the amount.

At low concentrations, greenhouse gases appear to affect the planetary average temperature in a logarithmic fashion: every time you double the amount of a greenhouse gas, the temperature goes up by a fixed amount. This question is hideously complex, and climate scientists don’t have any simple models for it, but for CO2 the effect seems to be about 3 K of increase for every doubling of the concentration in the atmosphere. Assuming that water vapor behaves similarly, the greenhouse effect on Toswao appears to be about 11 K more aggressive than on pre-industrial Earth. That gives us a total greenhouse effect of about 44 K.

In Bios: Megafauna, planetary climate is marked on a scale which varies up and down during the game. Next to the bottom of the scale is a space marked “Ice Age,” which I tentatively interpret as a planetary average temperature of 280 K, equivalent to the middle of the last glacial age. The top space on the scale is marked “Runaway Greenhouse,” which I’ll tentatively take as a planetary average temperature of about 350 K, high enough (assuming standard atmospheric pressure) for the equatorial oceans to start boiling. There are twelve spaces between these two points on the scale, so a rough guess of about 6 K per space makes sense. At the end of my play-through, the climate was in the higher of the two spaces in the “Cool” climate band, two spaces above the “Ice Age” point. That suggests a planetary average temperature of about 292 K, a bit warmer than present-day Earth.

If the actual surface temperature is about 292 K, then a greenhouse effect of 44 K suggests an albedo-adjusted blackbody temperature of about 248 K. The relevant formula is:

Here, A is the planetary albedo, L is the primary star’s luminosity in sols, and R is the planet’s orbital radius in AU. Plugging in values and solving for R, we get an orbital radius of about 0.99 AU, surprisingly quite close to the value for Earth.

Okay, now that I know where to place Toswao, I can lay out the whole planetary system. In particular, I determine that the primary gas giant (the Jupiter-analogue) engaged in moderate inward migration, but then got caught up in a “Grand Tack” event which pulled it back outward to its present position. This depleted the population of planetesimals in the inner system, leading to smaller planets, more widely spaced. Here’s the basic table of planets:

Radius	Planet Type	Planet Mass
0.41 AU	Terrestrial Planet	1.05
0.68 AU	Leftover Oligarch	0.15
0.99 AU	Terrestrial Planet (Toswao)	1.18
1.68 AU	Terrestrial Planet	0.65
2.95 AU	Terrestrial Planet	1.12
4.27 AU	Large Gas Giant	350
6.83 AU	Large Gas Giant	400

I didn’t meddle too much with the random dice rolls here, aside from ensuring that a Terrestrial Planet would appear at the right orbital radius to become Toswao. I did have two results that I wanted to ensure, given the outcome of the Bios: Genesis game.

First, I needed there to be a Mars-analogue close to Toswao, so that at least some microbial life would make the journey very early in the system’s development. The dice gave me a Leftover Oligarch in the next inward orbit, so I was happy with that. That planet was probably relatively cool and moist in the first hundred million years or so after formation, but while Karjann has heated up over the eons, the small planet has been baked dry and is now more barren than Mars.

Second, I wanted to make sure there was no “late heavy bombardment” (LHB), since that event didn’t take place in the Bios: Genesis game. The best theory we have about the LHB, assuming it happened at all, is that our gas giant planets went through a period of orbital instability that also disrupted the Kuiper Belt. Here, the outermost gas giant is still well within the “slow-accretion line” that represents the nominal start of the system’s Kuiper Belt. Hence the belt has never been disrupted, and is probably much more full and dense than ours. An analogue might be the Tau Ceti system, whose Kuiper Belt appears to be at least ten times as dense as Sol’s.

So here we go, before I head off to work this morning. The Karjann system has a solo star and seven planets, including the Earthlike world Toswao. The inner planets are likely to be a super-hot Venus type, and a very hot, dry Mars-like. Beyond Toswao we have two cold worlds unlike anything in the Sol system, probably with lots of ice, the outermost likely to have a methane-ammonia atmosphere if the temperatures are right. Then two Jovians, which apparently gathered up all the mass that might otherwise have formed ice giants on the fringes of the system. Finally, a dense Kuiper Belt that probably indicates lots of comets. No asteroid belt, although there are likely to be extensive collections of junk in the Trojan points of the two gas giants.

Next time, I’ll focus on Toswao and its major natural satellite, and work out the details of its physical environment.