[2300AD] The GDW 2300AD Worldbuilding system: Part 1 – Investigation

Gas Giant orbiting a triple G V system (created in Space Engine)

Gas Giant orbiting a star in a triple G V system (image created in Space Engine)

Now that I’ve got the Realistic Near Star Map Project out of the way I thought I’d take a look at the world generation system from the original GDW 2300AD Directors Guide, which I haven’t really examined in detail before. To do this, I automated the generation system so that I could generate thousands of systems and go over the statistics of the results.

However, I had to make a few tweaks to the system while writing the program so it’s not entirely faithful to the original system in the Directors Guide. First, the 2300AD system was designed to be used with existing stars, so I had to add a stellar generation system to generate those on the fly. I used a slightly modified version of my Revised Stellar Generation Tables (the original is available on my Worldbuilding page), that only generated solo stars (largely because it was easier to program that way). I used the stellar data tables from 2300AD to determine the stars’ luminosity and radius, but I just used the mass from the “V” column since in reality stars don’t change their masses significantly when they evolve into sizes II, III and IV. I also assumed fixed masses for the II and III Giants (1.5 Ms) and the White Dwarfs (0.5 Ms).

Another issue was that 2300AD is missing an Orbital Zone – there is a gap between the outer edge of the Habitable Zone and the inner edge of the Outer Zone. This “Middle Zone” is where Mars is located in our own Solar System, and cold rocky bodies (as opposed to icy bodies) would dominate there.

Additionally, in the original system, Gas Giants could snowball to immense sizes – the radius multiplier could go up to 16x the original rolled radius, which meant that you could potentially get Gas Giants with a radius of up to 384,000 km – over half the size of Sol! Obviously, this is not realistic – in reality, the maximum radii of jovians (that aren’t in star-hugging “Torch orbits”, at least) is between 70,000 and 80,000 km, because at that point adding more mass just causes them to self-compress further. In other words, Jupiter is about as big as a Gas Giant can get in terms of radius, though more mass can be piled into it and it wouldn’t get much bigger. So I set the upper limit at 70,000 – 80,000 km here, which results in an upper mass limit of about 8 Jupiter Masses (still too small for a Brown Dwarf though).

Finally, I simply calculated the MMW (and blackbody temperature and mass) of each planet instead of attempting to encode the tables directly. The MMW table in the book isn’t accurate in a general case anyway – this is discussed further in the Observations section.

Aside from these changes, I was able to translate the worldgen rules pretty faithfully into the program. Once that was done, I could set up runs that would generate 100,000 systems to investigate how many of which types of stars and worlds were created.


Observations about the 2300AD system: In the process of going over the system in the book and translating it to code, I noticed several odd and/or inaccurate features about the 2300AD worldgen system:

- O/B/A stars are the only stars that can have “Chunks” (‘planets’ less than 500km radius). The logic is that they are too young to form planets, but A V and late B V stars can exist for long enough to at least form planets around them, though they won’t be habitable.

- Oddly, only the atmosphere type that can result in Garden worlds is considered to be “Dense”, while “Standard” atmospheres are actually unable to hold gases as light as Earth can retain!

- Gas giants only snowball outside the inner zone. This means that there will be more small or very small gas giants in the Inner Zone (equal to their original radius) – though if they’re not snowballing then it’s technically more justifiable to call those “Failed Cores” than it would be for the ones described in the rules!

- Although the book implies that Brown Dwarfs could form if a Gas Giant ‘snowballs’ enough, it doesn’t actually go into any more detail about them at all – which is ironic given that one of the iconic worlds of 2300AD (Aurore) orbits a Brown Dwarf.

- Asteroid belts just appear in 1 in 6 empty orbits, regardless of what worlds are in the orbits on either side of them. In reality, nearby gas giants are required to disrupt planet formation and form asteroid belts.

- At the low end, Rocky and Icy cores have unrealistically low densities. A rocky world with a density of 0.4 earths would have a density of 2200 kg/m³, which is far lower than any planet-forming rocky material, and an icy world with a density of 0.1 earths would be half the density of water ice!

- Since 2300AD doesn’t calculate temperatures and my automated system does, Glacier worlds can be located in orbits in the habitable zone that have calculated temperatures that are actually too hot for them (it’s hard to justify a glacier world when the base blackbody temperature in that orbit is 50°C!). To fix this, the Garden World table would have to be changed to be based on calculated temperature, rather than dictating the temperature based on the results.

- As mentioned earlier, the MMW tables in 2300AD are actually signficantly flawed. The MMW retained by a world depends on its mass *and* its blackbody temperature, but the MMW table in the Directors Guide is only based on the planet’s density and diameter, from which mass can be derived. The values appear to be roughly appropriate to the MMW retained by a world with a given density and diameter in the Optimum Orbit in the Habitable Zone, but that makes it completely useless for any worlds outside the habitable zone since for a given mass colder worlds will retain lighter gases and hotter worlds will be unable to retain those lighter gases.

- The description for Failed Cores in 2300AD doesn’t really make sense – I think what they meant is that these are simply cold worlds that have atmospheres (I have no idea why they picked “Triton” as an example of a ‘Failed Core’ – it makes a lot more sense if “Titan” is used instead). I won’t redefine them at this point though – I’ll change that when I present my own fixes to the system in a later post.

- K and M V stars can’t have Pre-Garden worlds using the rules as written (they have at least a +1 modifier to the Garden Worlds roll, and Pre-Garden only appears on a result of 1). M V stars are also very likely to have Post-Garden worlds (assuming that results higher than 11 are also Post-Garden). I’m not sure what the justification for that is, but it sounds like they’re assuming that K and M stars are more likely to be old systems. This isn’t necessarily true – there are still numerically lots of comparatively recently-formed K and M V systems around even though many others may be older.

- One would expect Subgiants (IV) stars to have more Post-Garden worlds since they’re Main Sequence stars that are evolving and heating up their planets as they do so, but in this system the Garden World modifiers are only for Star Type, not Size.

Results: These are the results (shown as percentages) from a typical run of 100,000 generated systems (other 100k runs were similar too). In this instance, 576,644 worlds were generated in those 100,000 systems – usually there are about six times as many worlds as there are systems. The star size and type are percentages of the total number of systems. The World types are percentages of the total number of worlds generated. The “Gardens” row shows how many Garden Worlds around each Main Sequence star type as a percentage of the number of V stars of that star type.

100k results (default 2300AD system)
star type: A: 2.744% F: 5.973% G: 8.336% K: 13.654% M: 63.313%
star size: V: 91.020% IV: 1.536% III: 0.965% II: 0.499% D: 5.980%
size/type: A V: 2.533% F V: 5.215% G V: 7.541% K V: 12.547% M V: 63.184%
size/type: A IV: 0.127% F IV: 0.596% G IV: 0.553% K IV: 0.260%
size/type: A III: 0.064% F III: 0.074% G III: 0.135% K III: 0.664% M III: 0.028%
size/type: A II: 0.020% F II: 0.088% G II: 0.107% K II: 0.183% M II: 0.101%
size/type: D: 5.980%
World type: Rockball: 5.662% Iceball: 5.043% Gas Giant: 43.416% Hothouse: 2.995% Glacier: 0.464%
Pre-Garden: 0.119% Garden: 0.779% Post-Garden: 0.580% Desert: 3.949% Failed Core: 35.590% Chunk: 1.403%
Gardens: F V: 10.757% G V: 18.048% K V: 9.596% M V: 1.817%

As expected, the vast majority of stars are size V (main sequence) and the majority of those are M V stars. Subgiants, Giants and Bright Giants are much rarer, and about 6% of all stars rolled up were white dwarfs. Less than 1% of all worlds were Garden worlds, which is partly explained by the stellar distribution – many stars are red and orange dwarfs (M V and K V stars) whose habitable zones are usually within 0.2 AU, meaning that the initial orbit generated will often be beyond the habitable zone and of course all subsequent orbits will be too.

While about 0.8% of all the worlds generated were Garden worlds, about 4500 garden worlds were generated – so on average about 4.5% of the 100,000 systems contained a Garden world. The systems of M V stars are full of Gas Giants and Failed Cores, since the habitable zone for most of them is within 0.1 AU (this is the closest orbit to the star that can be generated). Only about 2% of M V stars have garden worlds around them (all of which which will be tidelocked, further limiting their habitability) compared to about 18% for G V stars and 10% for F V stars and K V stars – though the actual number generated was similar for G/K/M V stars (between 1100-1300 gardens for each of those star types, compared to 561 for F V stars). The vast numbers of Gas Giants and Failed Cores generated here (almost 80% of all worlds are those two types alone) are in part because the outer system is almost always much larger than the inner system, and also because the very common M V systems are pretty much entirely contained with their Outer Zone.

Sample system The G2 V system below illustrates some of the issues raised in the Observations section. The Garden world in Orbit 3 is a large world (slightly bigger than Earth) but its density is low – similar to the Moon or Mars. Some of the “Failed Cores” are very small (the second one is only 1500km in radius – half the size of Mars!), which supports my idea that maybe these are actually just supposed to be small Titan-like worlds with atmospheres and not “failed gas giants”. The second gas giant is on the small side too – only 10,000 km in radius, less than half the size of Neptune and therefore more rightly a “sub-Neptune”. It is actually possible to generate Mars-sized Gas Giants, which seems rather silly. The outermost gas giant is separated from the rest of the system by an Empty Orbit (not listed here), and is possibly a little too distant to have formed there (at least assuming that it formed in the same way as the rest of the planets). The jovian itself is slightly larger than Jupiter but about 5.5 times more massive (but not a Brown Dwarf).

Sample G2 V system
G2 V lum: 0.994 mass: 1.000 rad: 0.982 (0.005 AU) orbits:14
Zones: I: < 0.718 AU H: 0.718-1.446 AU M: 1.446-2.528 AU O: > 2.528 AU
a (AU) Radius (km) Density (earths) Mass (ME) BBTemp (K) MMW World Type
0.4 1500 0.8 0.01 439.939 196.467 Rock
0.52 3000 0.9 0.094 385.852 38.292 Hothouse
1.092 7000 0.6 0.796 266.263 7.28 Garden
1.966 1000 0.8 0.003 198.461 199.414 Rock
3.538 40000 0.9 222.81 147.924 1.321 Gas Giant
5.661 5000 0.6 0.29 116.944 6.267 Failed Core
11.888 1500 1.1 0.014 80.699 26.21 Failed Core
17.832 10000 1.1 4.255 65.891 1.926 Gas Giant
26.748 2500 1.3 0.079 53.799 5.323 Failed Core
45.471 4500 0.1 0.035 41.262 16.379 Failed Core
59.113 19000 1.3 34.492 36.189 0.248 Gas Giant
248.215 72000 1.2 1732.569 17.661 0.185 Gas Giant

Conclusions: As a generation system, the default 2300AD worldgen system isn’t terrible, but it does have a lot of issues that limit its realism (that said, I think it’s definitely better than Traveller’s default world generation system, since it at least tries to be based on science). Many of the issues described above are solvable without too much tweaking though – I’ll be discussing that in my next post though. Stay tuned!

2 Responses to “[2300AD] The GDW 2300AD Worldbuilding system: Part 1 – Investigation”


  • Back in the day my brother and I realised the MMWR table should have a modifier for temperature. He gave me a formula which I think worked out the temperature. (I can’t find it now. He’s the more math minded of us, although I’m not afraid of equations.)

    Speaking of which, in Asimov’s Planets for Man, there’s a density equation for Rocky cores: 2^(r-1) . I think it’s a curve-fitted kludge though.

    We used 2d6 on the Rocky Core Density table, (2=.4 and 12=1.4), and a result of 1 on the Icy Core was also .2; as a kludge we used the Icy Core generator for the final density (after snowball) for Gas Giants; they could achieve a density of .1 (On the basis we knew Saturn is less dense than water, which is .18 on the Equivalent Densities chart.

    • Technically the densities of the gas giants should be recalculated here, since it’s their *cores* that have the densities rolled up on the Density table, and all that hydrogen and helium above them will be less dense. That said, the rolled densities actually work well enough as bulk densities for the ‘snowballed’ gas giants so it probably doesn’t matter too much.

      For the MMW formula, I posted it on the SFRPG boards: http://www.sfrpg-discussion.net/phpBB3/viewtopic.php?f=30&t=2718

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