I’m writing one-page space rules for D&D. I’ve given myself some extra challenges: it must be all-editions; it must be lavishly illustrated; and it must allow procedural generation of solar systems and alien encounters.

So far I’ve detailed the space-travel rules and the alien races. Now it’s time to go planetside: let’s build some solar systems. Obviously, Spelljammer is a primary inspiration here, but I’m also drawing from Star Wars, John Carter of Mars, Jack Vance’s Dying Earth stories, and OD&D’s dungeonbuilding advice.

Star color from Jack Vance: When PCs enter a dungeon room, they can get a lot of information right away (there’s a treasure chest, there’s a monster, there’s a door). What do PCs learn when they enter a star system? I thought I’d use a star’s color as an index of its potential value. From Krypton to Vance’s Dying Earth stories to Dark Sun, red suns represent planets on the verge of extinction. By analogy, young blue stars might host virgin worlds and prehistoric creatures. This isn’t good planetary science, but it’s passable space opera.

Planetary conditions from Star Wars: Spelljammer has a conceit where every planet has an element type: water, fire, earth, and air planets. This is neat and very D&D, but I prefer the familiar Star Wars model: every planet has a prevailing terrain type, largely determined by its temperature. There are cold planets (Hoth), temperate planets (Coruscant), and hot planets (Tatooine), plus a few exotics like Bespin.

Treasure and monster placement from OD&D: In keeping with my “starmap as a dungeon” idea, I’ve used some of the monster/treasure placement guidelines from 0e/1e’s random dungeon generation guidelines. Strategic Review #3 suggests that, for every 20 dungeon rooms, 12 are empty, 3 contain monsters and treasure, 2 contain monsters only, 1 room contains a “special”, 1 a trick/trap, and one room has just treasure. I’ll make my planetary defenses and rewards approximately match this pattern: about 12 out of 20 star systems will be barren, and there will be proportionally similar quantities of monster-, trap-, and treasure-laden planets. Monstrous inhabitants and hazards are determined by the color of the star and the Encounters rules, and treasure by planet type (one in six planets is a “treasure world”).

“Extras” from Spelljammer: Spelljammer has seven page of star-system-building rules, of which the best part is the “extras” table, which assigns moons, rings, colonies, and other exotic trappings to planets. I’ll add such a table.

Here’s are my charts for star color, planet temperature, and moons.

Star Type: Star color dictates tech level. Roll d20.

1 Blue star. Primeval plants/beasts.

2 White star. Stone age tribes/exotic monsters.

3 Yellow star. Civilized planet. Technology supersedes D&D norm (for instance, laser swords, d10 damage). Powerful machine-aided magic. Fleets and armies.

4 Orange star. Declining world. Post-apocalyptic barbarism (like the PCs’ home world). Monsters in ancient dungeons.

5-6 Red star. Dying planet, littered with forgotten dungeons.

7 Exotic system. Asteroid field, planar portal, black hole, supernova, nebula, double or triple star system. No planets.

8 Green, purple, or black star: Sentient star with sinister power (for instance, casts Suggestion 1/day on everyone in system, or implants visitors with slaad). Roll d6 on this chart for civilization level of inhabited planets.

9-19 Any color star (roll d6 for color only): only barren planets.

20 1d4 habitable planets: roll d6 for the star’s color and d6 for each planet’s civilization level (they don’t have to match). Planet type: roll d6

1 ice world: mix of tundra/ice/mountain.

2 temperate world: forest/plain/hill mix.

3 hot world: jungle/desert/mountain/lava mix.

4 extreme world: too hot/cold for unprotected visitors.

5 exotic world: gas giant; ring world; asteroids; artificial, hollow, or living planet; fungal forest; mercury sea.

6: Treasure world. Around a hot star: hot world. 1d10x100 of gems/minerals on landing site: mining finds 10x more. White star: same, but cold world. Yellow star: temperate world. High-tech analogues of magic items. Otherwise: hot world. 2x times normal treasure in forgotten dungeons. Moon type: For main planet, and one barren planet, roll d12.

1-6: No moons.

7: Planetary ring.

8: d4 barren moons.

9: habitable moon: roll d6 on Star Color for tech level.

10: Colony/mine (roll d8+3 on Encounters for owner)

11-12: roll d8+2 twice on this table.

Here is the complete one-pager, ready for space adventure.

Here it is as a PDF.