One of my favorite RPG books to just read through is GURPS (4E) Space. I have, due to whatever sickness I suffer from, read the sections about generating star systems and planets probably a dozen or so times over the years. It's a very nifty, nitty-gritty, crunchy experience of discovering what kind of star(s) your system has, what kinds of planets would most likely orbit it (them), how large their orbits would be, where the Habitable Zone for each system is likely to be, etc. It's pretty exhaustive.
It's also pretty exhausting.
I've gone through the process, I think, four times for no reason other than my own amusement and I think I've wound up with a star system full of dusty dead planetoids that is hard to imagine wanting to explore, let alone colonize, pretty much every time.
You can force the system to give you a Garden World, but that always feel vaguely... dishonest? Like a cheat code.
Somehow, despite the high-esteem to which I hold the system, the end result always leaves me feeling let down. "I cranked through all these tables for this?"
To give you a sense of the fun you've been missing out on, here's what it looks like:
It's like this for, like, 40 pages.
It's very cool. But it's very much a slog.
So, I'm always on the lookout for faster ways of generating star systems and planets and, well, there are actually a number of options for doing this quickly on the computer.
Donjon offers star system creation in three different flavors, in fact: Star Wars, Traveler, and Generic. The generic sci-fi one pumps out results pretty similar to what you would get from GURPS Space. And you get it instantly, rather than chugging through a bunch of tables and spinning the multiplication crank.
1) It's cool to know the exact radius and surface gravity of a planet. It's nice, I like it. But also who cares? It's like if every time I rolled for weather in Dolmenwood it told me: it's 78 degrees F and then told me separately what the humidity was. Just tell me if it's hot or mild!
2) It's actually too fast. While GURPS Space may be laborious, it also takes long enough that your keep thinking about how to implement whatever ice-rock feature you've just rolled up will fit into your story or game. So when a Star System generator spits out the bare stats of a planet, my mind goes blank. There's no spark.
It is fun to know that my habitable planet's gravity is 77% that of Earth as opposed to, say, 73%. Fun, but not useful.
I really don't want it to seem like I'm knocking these systems. I'm sure they work for other referees or world builders. They just don't hit the right notes for me personally.
There's writing advice that goes something like, "Write the stuff you want to read." I'm going to try to take this advice, as amended: "Write the tables you want to roll on."
One-Roll Star System Generator
First, to determine the type of star your planets orbit.
Because the point of this process is rapid development, I’ve limited the following table to the least-exciting Main Sequence Stars (dwarfs), presumed to be in the middle of their lifespan, because this makes them the most-likely candidates for evolutionary success. They're more stable and long-lived than other stellar options.
If you want a more “realish” distribution of star results, use d100.
If, on the other hand, you don’t want to roll Red Dwarfs over and over, use 1d10.
Main Sequence Star
M-Type Stars (Red Dwarf)
K-Type Stars (Orange Dwarf)
G-Type Stars (Yellow Stars)
F-Type Stars (Yellow/White)
A-Type Stars (blue/white)
Determine Planetary Orbits
The second roll is the big one. Here we determine what planets go where.
Each die represents a planet (don't use the 10s die):
d4: this die represents your Small Terrestrial Planets (ST). e.g., Mercury, Mars, the Moon, etc.
d6: represents Medium Terrestrial Planets (MT). e.g., Venus, Earth
d8: represents Large Terrestrial Planets (LT). e.g., super-Earths or mini-Neptunes
d10: represents asteroid belts (AB)
d12: represents Ice Giants (IG) e.g., Neptune, Uranus
d20: represents Gas Giants (GG) e.g., Saturn, Jupiter
Steps:
Drop your dice
Interpret Results: each die result represents the planet’s distance to its parent star
Adjudicate Ties:
If a Small and Medium terrestrial planet are on the same orbit, replace them with an asteroid belt
If an asteroid belt coexists with any planet, the planet “wins” and gains a ring system
In all other circumstance, the larger planet “wins” and is upgraded to a larger class of planet: e.g. Large Terrestrial Plant → Ice Giant → Gas Giant → Super-Jupiter
Determine Number of Moons: subtract 2 from each die roll
Example 1
G-Type Star
d4: 4
d6: 4
d8: 4
d10: 6
d12: 8
d20: 20
d8 would normally be a Large Terrestrial planet, but since over the course of its development, it ate up d4 and d6, it’s an Ice Giant instead. But it’s also too close to its parent star (coming before the asteroid belt), so its atmosphere has been stripped away and resulted in a massive Ocean Planet (OP).
d10 is an interrupted Asteroid Belt (AB).
d12 is an Ice Giant (IG)
d20 is a distant Gas Giant (GG)
Moons:
d8: 4 - 2 = 2
d10: asteroid belts don’t have moons, but they may have moon-sized planetesimals. 6 - 2 = 4.
d12: 8 - 2 = 6.
d20: 20 - 2 = 18
We’ll determine moon types and orbits later.
Example 2:
M-Type (Red Dwarf)
D4: 1
D6: 4
D8: 5
D10: 1
D12: 5
D20: 13
d4 & d10: I’m doubtful that a Small Terrestrial planet could hold onto a ring this close to a star, but Red Dwarfs are pretty small, so let’s say it managed to maintain a ring of dust and small debris.
d6: A Medium Terrestrial planet which would have been in the CHZ of a star like the Sun, but here is frozen, out in the cold.
d8 & d12: The Ice Giant eats the Large Terrestrial planet.
d20: 13 is normal Gas Giant (GG)Moons:
d6: 4 - 2 = 2
d12: 5 - 2 = 3
d20: 13 - 2 = 11
Example 3:
K-Type Star
D4: 2
D6: 6
D8: 7
D10: 5
D12: 1
D20: 3
This is a chaotic one! It’s sort of a weird, dense, inside-out star system.
D4: Small Terrestrial Planet, right in the middle of the CHZ.
D6: Medium Terrestrial Planet, quite frozen.
D8: Large Terrestrial Planet, also frozen.
D10: Asteroid Belt
D12: Is an Ice Giant--but orbiting its star at less than 1 AU! That makes it an Ultra-Hot Neptune. These weird planets which are not well-understood by science. But see Glossary.
D20: to make things crazier, this star also has a close-orbiting Gas Giant. Most likely, the GG and UHN migrated inward from their original, unstable orbits. They probably threw other orbits out of whack, which is maybe why the terrestrial planets are on the outside of the system rather than on the inside. The exterior planets might even be migrating out of the system depending on how jarring the transitions of the UHN and GG were.
What a weird doomsday scenario for anyone that might have evolved on the MT planet.
Moons:
UHN: 0
ST: 0
GG: 1
AB: 4 planetesimals
MT: 6 - 2 = 4, but if it originated from further in the system it probably really only has 1 or 2.
LT: 7 - 2 = 5
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