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[GUEST POST] James L. Sutter on Building Worlds: Using Astronomy to Create Interesting Settings

James L. Sutter is the author of the novel Death’s Heretic and co-creator of the Pathfinder Roleplaying Game campaign setting. His short stories have appeared in such publications as Escape Pod, Starship Sofa, Apex Magazine, and the #1 Amazon bestseller Machine of Death, and his anthology Before They Were Giants pairs the first published stories of SF luminaries with new interviews and writing advice from the authors themselves. In addition, James has written numerous roleplaying game supplements and is the Fiction Editor for Paizo Publishing. For more information, check out or follow him on Twitter at @jameslsutter.

Building Worlds: Using Astronomy to Create Interesting Settings

Far too often, I run into authors who assume that science should be left to science fiction, on the grounds that fantasy is inherently science’s opposite. In my opinion, nothing could be further from the truth. Even if your world is filled with boundless magic, there are still natural laws governing ninety-nine percent of your characters’ daily activities. And by injecting a little science into your setting, you can create far more interesting worlds than if you attempt to totally revamp reality from the ground up. Truth, as they say, is still stranger than fiction.

Whether you’re creating science fiction or fantasy, one of the easiest ways to inject some strangeness into your setting is to take a page from astronomy. As one of the designers for the Pathfinder Roleplaying Game campaign setting, and in particular the one in charge of designing that setting’s solar system, I’ve found the tactic extremely useful, and several of the examples presented below ended up in the resulting sourcebook, Distant Worlds. By tweaking your planet even slightly off Earth-standard, you can radically alter your world, and the resulting changes may lead you and the cultures who reside there in directions you had never expected. When creating a new planet, here are a few factors to consider.

  • Size: When designing a planet, most authors and game designers tend to go big, on the premise that it’s better to have too much canvas than too little. But if your planet is larger than Earth, is the gravitational pull stronger? How does that shape the residents? Perhaps your dwarves are from a high-g world, while your elves are tall and thin because they live in orbital space stations. There are also more logistical concerns–on a big world, the horizon’s going to be farther away, so your besieged castle may have a lot longer to prepare once they see the enemy approaching. And if distances increase, how does that affect your demographics? Is there so much empty space that territorial wars are inefficient, or many of your nations never make contact at all? (Remember, it was relatively recently that our own nations expanded enough to interact and start filling in their maps.)
  • Composition: What’s your planet made of? If it’s hollow and full of prehistoric megafauna, how does that affect the gravity? How do volcanoes work if the core isn’t molten? If you go the classic gaming route and have huge networks of caverns running throughout your world, why haven’t they all been flooded? And don’t forget to think about magnetic fields–if your planet’s composition is different than ours, your compasses may not point north anymore. In that case, what would they point to? A city? An artifact? God?
  • Movement: How fast a planet rotates determines how long its days are, and how long it takes to make one revolution around the sun determines the length of a year. Both may have huge effects on a planet’s cultures, especially if there’s an axial tilt. (Think about the difference between a winter in northern Alaska–the Land of the Midnight Sun–versus a winter in the Bahamas.) This also plays into how your characters reckon time.
  • Moons: I’ve yet to meet an SF artist who didn’t constantly put multiple moons in the sky. This is great in that it automatically tells the viewers they’re on another world, but are you prepared for the physical consequences? Earth’s moon is responsible for its tides. If your world has an enormous or particularly close moon, or several different ones, do you have enormous and erratic shifts in your tides, with giant tsunamis a regular occurrence? If you go the opposite route and have no moon at all, what stirs the seas and keeps them from becoming giant stagnant puddles? How does your lunar model affect your magic system–when does a werewolf change if the world has three moons? And of course, this doesn’t even begin to address the question of how things change if those moons are themselves inhabited, or reachable via magic or science…
Stranger Worlds Than These

Most of the above factors presume that you’re taking a relatively Earth-like world and tweaking it slightly. Presented below are a few more extreme astronomical phenomena and considerations to play with.

  • Tidal Lock: This is when a celestial body rotates at a speed such that the same side always faces another celestial body (much like our own moon and its fabled “dark” side). If your world is tidally locked with the sun, one side might boil while the other froze (though last I checked, recent research suggested that the temperature differences might not be so extreme as to prevent life on either). What sort of cultures would evolve on each side, or on the line of the terminator? Would they ever come into contact, and if so, what would happen?
  • Tidal Heating: This is when the pull of another world’s gravitational field warps a planet or moon’s shape, with the resulting internal friction creating massive amounts of heat. You want a Mordor-style world of horrible volcanoes? Maybe it’s not inherently evil, just tidally heated. (Why ya gotta be so judgemental, anyway? Horrible wastelands didn’t choose to be that way…)
  • Eccentric Orbit: The more elliptical your planet’s orbit gets, the more uneven its seasons become. Your world might slingshot around the sun and then move out into the darkness for a thousand years, causing all sorts of weird evolution to compensate, or it might simply have disproportionately cold or long winters. (If George R. R. Martin can build a fantasy world off an astronomical phenomenon, so can you!)
  • Orbital Interactions: Earth passes through several clouds of dust and debris every year–those are the annual meteor showers like the Leonids, Persieds, etc. Your world might have similar fields left behind by comets or other events, and they could range from brilliant rains of star-fire to extinction-level events. And while bringing your planets too close together may get you smacked by your friendly neighborhood physicist, Anne McCaffery’s Pern books used the idea of two planets passing near each other to great effect.
  • Atmospheric Conditions: One of the factoids that first got me interested in astronomy was a college professor who explained how Venus’s atmosphere is so thick that the wind is like a brick wall slowly scraping across the landscape. How would life evolve on a world like that, or one with exceptionally thin air? What about one with no atmosphere at all, just a crust of ice over darkened seas? (Europa, I’m looking at you.)
  • Transient Objects: Not all worlds are here to stay. Our own sun is always capturing comets, asteroids, and other objects that wander too close to the gravity well. Maybe one is a generation ship, a world that travels through the blackness and only stops in a system long enough to refuel or use the sun’s gravity to change course. How would such an object’s residents view the cultures of the systems they passed through?
  • Dying Sun: No discussion of astronomy in fantasy would be complete without a nod to Jack Vance, Leigh Brackett, and the other greats who’ve used this concept to such effect. How does the death of a system’s central star affect its cultures? Do societies fall into anarchy as the lessened light cripples agriculture? Do the residents try to escape before the red-giant sun devours their world?

These are only a few seeds to help authors, gamers, and other creators take inspiration from the stars and planets. What astronomical phenomena have you used in your own work, or seen used well by others?

5 Comments on [GUEST POST] James L. Sutter on Building Worlds: Using Astronomy to Create Interesting Settings

  1. Very interesting stuff here.  The first book that comes to mind (at least for me) that builds on an astronomy/culture link is Vernor Vinge’s Deepness in the Sky.  I have always wanted to do an elliptical orbit planet with a winter and summer culture story. 

  2. Tell us more about Distant Worlds… 

  3. Distant Worlds sounds interesting. Given the context of this post, James, do I take it that Distant Worlds is going to be more real-world astrophysics than, say, Spellljammer was?

  4. Thanks Michaele!

    Paul: Distant Worlds comes out in February, and is a detailed guide to the solar system around Golarion (the setting for the Pathfinder Roleplaying Game). While there are some rules in there–new monsters, notes on dealing with vacuum and differing levels of gravity, etc.–it mostly focuses on notable locations and cultures for the various worlds. It’s still very much a fantasy sourcebook–it uses all the normal sword-and-sorcery rules for Pathfinder–but it’s got a lot of science fiction influence in there. This blog post really came from a lot of my work there, so Distant Worlds has everything from classic Red and Green Planet worlds to inhabited gas giants (and their rings and moons), an atmosphereless planet inhabited by liches, asteroid belts, generation ships, tidally heated and locked worlds, eccentric and retrograde orbits, etc. There are also a number of analogues for worlds in our own solar system, so if you want your players to see what Mercury would be like inhabited by solar-powered robots, or visit the oceans of Europa, this book should help. 

    That said, I’m a fantasy author rather than a physicist, so while I’ve tried to bring more scientific *flavor* into the game, I haven’t by any means designed realistic simulations of astrophysics. That would require at least its own book (and perhaps a whole new game system)…


  5. A useful, and easy to use, note regarding size and composition:

    Surface gravity is directly proportional to the the radius and the density.  No fancy math required, double the radius double the gravity, double the density double the gravity.

    (Practical limits for density are between 0.2, (ice), and 2, (nickel/iron), times Earth’s.)

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