Greetings from Magrathea

The Drake Equation Revisited: Part I, Frank Drake

Two disclaimers. First, take all of this with a grain of salt. Or many grains. A whole tablespoon, in fact. I'm not a geologist, so all of this is suspect. Second, as I mentioned in the previous two columns, world building can get as technical or "artistic" as you want. You can say, "it's all like earth," and bypass pretty much everything below. Or you can banish science to the aether and say that the planet exists because you as the GM made it so. I hope I've made my case that neither of these options are always the best case.

Enough with that, then. What makes a planet "suitable for life?" A few things:

1. Temperature. This is partly determined by distance from the star (does it fall within the Goldilocks zone), albedo (what percentage of light/heat is reflected back to space), and atmosphere (what percentage of heat is retained on the planet). Venus, for example, is farther away from the sun, but the high amount of carbon dioxide retains heat. As you will see below, there's a bunch of factors that play into this, to the point where casual gamers will just throw up their hands and roll dice. As we know from climate change science, this is tough, and most gamers don't have the computational models to generate accurate results, although if you are interested, there are a lot of examples of Daisyworld to run.
2. Gravity. This is based on the size and composition of the planet ó larger and more dense planets have a higher gravity. A good example of an outlier is Saturn, with despite being about ten times the size of Earth, has a gravitational pull only slightly more than Earth (10.44 meters per second squared versus 9.87 meters per second squared on Big Blue). A low gravity means that atmosphere is thinner or nonexistent, but there's more than that. Higher gravity means that organisms need to be more compact and closer to the ground in order to deal with the increased weight. A lower gravity means that organisms can grow larger ó less weight means less muscle mass is needed to move. Higher gravity affects erosion ó deeper canyons, but smaller hills and mountains.
3. Water. What percentage of water is on the planet? We're assuming that water exists on the planet; most life requires water (both as a medium and as part of cellular respiration). A few organisms don't need water (cite), but anything interesting requires either a lot more science and planning than I have (some odd silicon-based life form or alternate energy structure), or water. Water is going to factor into some of the decisions above (it is going to absorb and release heat slower than land, so it's is going to stabilize temperatures). If a planet has a high temperature, then there won't be as much liquid water; most of it will evaporate into the atmosphere. Likewise, if temperature is low, more of the water is going to be locked as ice into polar caps.
4. Tilt and Orbit. Earth's orbit gives us night and day and seasons, which is pretty important. If the planet doesn't have the proper orbit, then one side will bake and the other side will be too cold to be habitable. Other objects influence a planet's orbit; in our case, the large moon is the main driver. Other planets or other stars in a binary star system may also serve that purpose. Some say that any real life requires the moon, although this is under contention. A planet that is tidally locked doesn't prevent life from existing, though. A planet that is far enough from the sun may have one side tolerable, with the other end of the planet being too hot or cold for life. Alternatively, life could exist on the border.
5. Radiation. Earth has the Van Allen belts, which not only produces fancy lights mistaken for angels or UFOs, but also protects life from radiation. Increased radiation is going to make life more unpredictable and weed out weaker, less-hardy plants and animals. At the same time, lower radiation allows for more complex organisms. Protection from radiation is going to be handled by the planet's core; a metallic core means a higher magnetic field, which means more protection.
6. Atmosphere. Again, Earth-typical has oxygen and carbon dioxide, which is important for photosynthesis and cellular respiration. Nitrogen is there mostly for filler and fertilizer. A high amount of oxygen is going to be good to animals (up to a point); lower oxygen means shortness of breath and less muscle activity. Of course, animals and plants get used to it. Hint ó that is going to be a theme here. High levels of oxygen is going affect flammability. You will also have particulates in the atmosphere, which is going to affect visibility, ability to breathe, and temperature. Particulates can be natural (volcanoes, geysers, or fire) or artificial (factories or combustion).

So, what does all this mean? Well, here's two examples.

Jotunn: this is a high gravity world on the outer edge of the star's habitable zone for life. Jotunn has a massive, iron-core moon that stabilizes its orbit, but it still has a low orbit; a "day" is 40 hours, and a season is three years. Because of the high gravity, there's a thick atmosphere, mainly oxygen and nitrogen. The level of oxygen is higher than Earth. This insulates the planet, but it still is cold, with an average summer daytime temperature on the equator of 40 degrees Fahrenheit. The planet is rather flat, with few mountains and crevices (because of the lack of normal rivers). Life has evolved to be small and low to the ground, with thick layers of fat and skin to protect from the cold. The high oxygen plus high gravity has changed life to focus on muscle instead of cardiovascular.

Kaos: this is an Earth-style planet, except slightly larger and with a smaller molten core. Because of this, the gravity is slightly lower than Earth, and there is little protection from radiation. To make things easy, temperature and percentage of water is similar to Earth as well. Life here is sparse and often infertile, with major mutations causing wild swings in behavior and physiology. The day-night schedule is similar to Earth (slightly less, because of the lower mass of the planet). Because of the less gravity, organisms grow large. Simple organisms do well here.

I quote Patrick Tomilson epilogue to Building Worlds in a Hostile Universe: "My intention in writing this was not to scratch every other type of planet and solar system off your list of potentials. Instead, I wanted to convey just how improbable our planet, and therefore our type of life, may be. If you need inspiration, look around our planet's forgotten corners. We find organisms living in complete darkness, under crushing pressures, scalding temperatures, in pools of acid, without oxygen, eating rock and metal, photosynthesizing radiation, and generally carrying on in a fashion that drives biologists into alcohol dependency."

From a practical purpose, there's going to be two types of games where this is used. First is a single world. It can be science-fiction or odd fantasy, like Pern or Sharon Shinn's Samaria series. Second is the Serenity-style game, where you have a bunch of worlds and players hopping from one to another. Creating a world with any level of integrity is a lot of work, and creating dozens of them is, well, daunting. But, it's fun, and from a gaming perspective, there's an evolutionary slant to this as well. Create a world with a high oxygen content, where fires ignite quickly. Create a world where gravity is so low, human-powered flight is much more realistic. See what the players do.

Speaking of evolution, next month, I was going to focus more on evolution and ecology. And I want to end with my shameless plug of my story, Virtual Campus. Until next month.