Tutorial – How to Build a Solar System

This is a little deviation from my usual project posts. Instead of showing a project I’ve finished I’ll run through a bit of a Tutorial/How to on designing your own Fantasy/Sci-Fi World Solar system. This is the process I used to design the Sarilo system in the previous post. In looking for this kind of information on the WWW, I pretty much came up blank -everything was either very high level or assumed your world was an earth clone except for the continental layout :). A word of caution: I’m no expert mathematician or physicists so I may have made some liberal assumptions with some of the formulas and maths below, if your reading this blog post for some sort of school project (unless it’s “Build a Fantasy/Sci-fi Solar system”), turn back now.

Jaw Dropping Picture for your Players

First up you’ll need to design  a Solar System layout, how many planets what types etc. If you’re going to all this trouble at some point your going to want to casually insert some kind of picture into your gaming session so the players can go “coooool”. So behold I have assembled the ultimate in solar system development tools, I’m possible over selling it a little. Below is a link to a very basic drag and drop solar system assembly page best viewed at 1280×1024 (+). It uses only CSS and Javascript to run no fancy flash around here :). I checked and it seems to function best in IE7 and 8 (just say yes to allowing active content if your asked). It functions tolerably well in Firefox although changing label names is irritating,  right click them first and then it’ll let you adjust the label name. Go forth and design a Solar System using the below or if your so inclined use Photoshop,  GIMP or something else:

Link to Builder

Link to Builder

Hopefully your back with a semi-pretty Solar system picture to throw at your players. Now we need to flesh out some of the details, how far is each planet from the sun how long does it take to orbit the sun etc. Now if you’ve stuck your habitable planet way out near where Pluto normally sits, now would be a good time to adjust it to somewhere closer to the sun rings 2-4 (possibly 5)are the best places for habitable planets. However it’s your Solar system so feel free to keep it that way but you’ll see the interesting impacts below.

Specify Planet Distance in AU

Don’t get to hung up on how far an AU is.  Just be aware a good range for Terrestrial (Earth like) planets is approximately 0.8 AU’s to 2 AU’s. Earth in our Solar System sits at 1 AU from the sun, Pluto hangs way out at 39 AU and Mercury at a toasty .4 AU’s. Now Assign each planet an AU Distance from the Solar System Star. The planets don’t have to be as spread out as much as our Solar System but do be aware big planets to close to each other could in reality have interesting effects on each others orbits, Feel free to play around with this Solar System builder. I punched in an approximation of the Sarilo System and ran it for a 100000 years (about 4-5 days) with no problem.  It’s interesting to try to create a stable solar system with one planet on an elliptical orbit that doesn’t wipe out your other planets, I have no idea why a comet hasn’t run into Earth yet :).

A little bit of Math

This bit is fairly glossed over, if you want to get into the full workings and understanding feel free to check out  Keplers Laws, for me I’m happy with the below. Basically grab your AU distance for your first planet and place it to the power 3/2, basically 1.5 and you’ll get the orbit of your planet in equivalent earth years. An example My primary planet Etena is 1.24 AU from the Star Ravi, 1.24 to the power of 1.5 equals 1.381 earth years or 504 earth days:

To make things super simple here is an Orbit calculator online (right at the very bottom of the page)or use this spreadsheet I assembled . Using the above does assume certain similarities between our Solar System and the fantasy one but unless you really want to get into some heavy math just live with the above :).

A little bit more Maths

So now you’ve got your planets and you know how many earth days it is to year for each planet. You can just see all your NPC’s running complex mathematical conversions every time they try to calculate what season it is. Time to give your planets unique day lengths etc. I’ll admit I stuck with an hour being 60mins for all the calculations I’m just not that dedicated to make up whole new time system. For each of your planets allocate the number of hours in a day, before giving your earth like planet a 500 hour day, think what impacts this would have on your planets society. I’d recommend to keep your earth like planets hours in a day between 20-30 for easier game play.

Once you have the number of hours in day for a planet  simply take your number of earth days in a year x by twenty four then divide by the number of hours in your planets days, this is the number of the planets days in a year, as an example Etena has 504 earth days in an orbit and 26 hours in a day. 504 x 24 = 12096 hours then 12096/26= 465.2. So there are 465 Etena days in a Year. If you like you can do the above calculation for all your planets. My handy spreadsheet will do these calculations for you :).

Adding the detail from the above

So you now have a Custom Year Calander for your planet which will if you need it to stand up to at least cursory mathematical checking. But what are some of the impacts gamewise? Below is a bit of dot point list of things to consider:

  • If your year is very long (in earth days), the seasons will be longer, more food might need to be stored for winter or a double cropping season might be carried out, plants might mature slower.
  • If your year is shorter (in earth days), plants might mature quicker, as winter is shorter less food needs to be stored.
  • Longer/Shorter Day length could impact working day length etc (shop opening hours)
  • People’s ages will be different younger or older depending on shorter or longer years, maybe you age by each season instead.
  • Certain events could be more or less important, ie if spring comes around every 100 days it’s going to seem less important to celebrate, however if winter is 250 days long your definitely going to have a big party when that’s over.  (or if your planets out by Pluto your whole life might be lived in “winter”)
  • Is your habitable planet closer or further to the sun, this could make it hotter or colder.

Basically think about how things would impact you if the days and years were of different lengths. Now that you know how many days are in your planets year you can design up a calendar for your players to use. Remember to include seasons, approximately one per each quarter of the year.

Hope this information is useful to other people building up a campaign world.

Orbit Calculator – Spreadsheet

Solar System Builder

4 Responses to “Tutorial – How to Build a Solar System”


  1. 1 行銷 December 6, 2010 at 7:40 pm

    About time! Someone with some information on this. You’d think considering how popular Comic Book Adaptations are nowadays, some information would actually be pretty easy to find. Apparently not. Anyway, thanks for this! I appreciate it!

  2. 3 Sirrob01 October 26, 2014 at 12:59 am

    Hmm my builder link appears down, I’ll try and find a new hosting location.


  1. 1 Gimp Planet Making – Blue Green Globes (Earth Like) | Sirrob01 Trackback on September 7, 2013 at 5:46 am

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