Star System Generation

I get that STA tends to shy away from crunchy details, but every once in a while, I want to throw crunchy details at players.

One thing that falls into that category is Star Systems. I kind of like being able to give my players a run-down of star system that they’re checking out. The thing is, I don’t think I’ve seen a really good set of RPG Star System generation rules in quite some time. Is anyone aware of a good set of Star System Generation rules? (Bonus points it those rules have taken into consideration the things we’ve learned since the discovery of exo planets).

What I’ve been doing up until now is use Donjon’s Star System Generator page; that mostly works, but I can’t say “generate a system for me, but I need an earth-like planet in the fourth orbit.” So I do a lot of “spinning the wheel”. Anyone have anything better?

You are aware of the Star System creation rules on p. 302 ff. of the Shackleton Campaign Setting?
If you need crunch regarding gravity etc. have a look at p. 145 ff. of the Core Rulebook. Take the gravity spans for each class, take min/max and 9 equidistant numbers in between and take 2d6 for a nice gaussian distribution.

Maybe this writeup of using the random tables of STA is of interest to you…? :slight_smile:

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Say what?!? Huh. I don’t know how I overlooked that. I’ll need to brush up on those rules.

Thanks for opening my eyes.

“If only the character generator supported this, I would have found it more easily…”

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That is of interest. Thanks.

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If you do want lots of detail: it’s a bit long in the tooth, but one of the best RPG rulesets for this in my experience is the GURPS 3rd edition Space rulebook. It produces a quite detailed set of “real world” data (as opposed to game stats) but focusses on what’s useful for RPGs, so you can place the worlds you want then fill in the gaps. It’s about US$9 on Warehouse 23 (GURPS Classic Space).

It was written when astronomers were just beginning to realise the frequency of exoplanets was higher than they thought, so is a little out of date!

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If you want to focus more on the societal aspects, have a look at Mongoose’s Traveller.

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I actually do have a generator here on my machine. It’s a python3 commandline tool, but it shouldn’t be too hard to build a simple website. Not sure how about Modiphius’ position on this (@Modiphius-Jim).

Thanks for that suggestion: I actually have a copy of that book (although I don’t think it’s come off my shelf in more than a decade). I’ve never actually played a game with the Gurps rules. But I’m re-reading Gurps: Space, now.


It looks like it’s a 1999 copyright so, yeah, I think your take is totally correct. I just did “Internet research” (which, as we know, never gives us inaccurate results) and apparently the first exoplanet was discovered in 1992, and the first exoplanet around a main sequence star was 1995.

I’ve also sometimes used the rules for star system generation from Hero Games’ Star Hero, Fifth Edition, which has lots of math, but starts with a big assumption (that I speculate is no longer believed, because exoplanets) about where the orbits should be.

Astrosynthesis from NBOS software is quite good.

The problem with most RPG planetary/solar system generation systems is we keep finding things that will break them. I play 2300AD which is one of the harder SFRPG’s out there, and the system generation in that was, for many years, the gold standard. Then we discovered Hot Jupiters, Panthalassa worlds, Super-Earths, Chthonian Worlds and so on. The original system could not comprehend the infinite diversity possible. As soon as you think you have a good system, some gas giant decides it’s going to barge it’s way in-system and occupy orbit 2 apparently just because it feels like messing up my diagram…


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How interested would you be in having it merged with the character generator? The character generator currently everything in-browser, which probably means some retooling of what you have. But I kind of like the idea of one-stop-shopping for the STA rules. (Obviously, if Modiphius hates the idea, that’s a thing we have to acknowledge).


If it involves showing you the embarrassment I dare to call “code” the answer is: probably not too much. :wink:

No, honestly: We can for sure talk about that.

And while we’re at it: I have a little tool that spits out plot components as in the Division supplements. Maybe we (+ maybe Modiphius/Jim) should have a mail conversation… :wink:

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The old RPG Spacemaster had a good star/system generator of the old paper and pencil type. I still use it today if I need to randomize a star system. As a system the RPG was too chart and detail heavy for me to enjoy playing it. But as a reference in support of other games it is fantastic.

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Sorry, I haven’t completely dropped the ball on this. I’ve just been busy with a lot of non-STA stuff lately. I’d still totally like to chat. If you’re up for email, you can reach me a bc at bcholmes dot org.

I also just logged a GitHub issue about planetary systems to remind myself that I’d like to get around to doing this.

BCing you


Don’t worry. I think I even researched an email address of yours, yet had to drop it due to non-STA stuff, as well. I’ll drop you a message these days, promise! :slight_smile: