[2300AD] The GDW 2300AD Worldbuilding system: Part 2 – Realistic Version

In the first part of this series, I looked at the distribution of systems that the default 2300AD system generated, and examined some of the issues and assumptions that it made. In this article, I’ll present the results of a modified system that addresses these issues and corrects the flawed assumptions, and explain why I made the changes.

Modifications

Orbital Zones: The Outer Zone now begins at 2.5356*SQRT(L) AU from the star. This is roughly where the “frost line” is (where the blackbody temperature is 175K), beyond which volatiles can condense and gas giants are more likely to form. The new zone between the outer edge of the habitable zone and the inner edge of the outer zone is called the Middle Zone, and cold rocky planets are likely to form there (Mars is located in the Middle Zone in our Solar System, and Jupiter and the more distant gas giants are in the Outer Zone).

The boundaries of the Habitable Zone also need to be adjusted. The K multipliers used in the Life Zone equation in 2300AD don’t match the values shown in the table there – the values in the equation are K=0.72 and 1.45, but the table shows values for K=0.82 and 1.2. Unfortunately, those are both pretty unrealistic! Recent papers have put the inner boundary much closer to 1 AU than 2300AD assumed (between 0.9 and 0.99 AU) and while the outer boundary is less well defined but seems to be around 1.4 to 1.6 AU (see the Circumstellar Habitable Zone wikipedia article).

I will assume here that the habitable zone goes from K = 0.9 to K = 1.4 – this lets us have a full range of environments in the habitable zone without things getting too extreme. So K = 0.9 for the inner edge of the Habitable zone, K=1.4 for the outer edge of the Habitable zone, and K=2.5356 for the border between the Middle and Outer Zones.

New/updated World Types: A few updates and additions are necessary to the list of World Types. Failed Cores originally showed up on Rocky and Icy bodies in the Outer Zone that had Dense, Standard, or Thin atmospheres. Although the text description implied that they were ‘worlds that never accumulated enough mass to become a gas giant’, in practical terms they actually just represent smaller icy worlds that had atmospheres.

Titanian: All examples of what were formerly known as “Failed Cores” are now considered to be “Titanian” worlds. Titanian worlds are defined here as small (roughly earth-sized or less) outer system bodies that have appreciable atmospheres, named after Titan in our own Solar System (essentially, this is what Failed Cores were supposed to be, but with a more appropriate name). Titanians are not habitable, will most likely have icy surfaces even if they have rocky cores, and may have liquid methane, nitrogen, or other cryogenic compounds on their surfaces depending on temperature and atmospheric composition and pressure.

Failed Cores: Failed Cores are now a sub-type of “Gas Giants”, and are redefined simply as “Gas Giants that are smaller than 12,000 km in radius” (about half the size of Neptune). As such, they represent what we now refer to as ‘Sub-Neptunes’ (a.k.a “Mini-Neptunes”, “Sub-giants”, or “Gas Dwarfs”). They may be found in any orbital Zone and may have Rocky or Icy cores, and most will have a hydrogen and helium atmosphere that extends a few thousand km thick above their stated radius (technically the top of this atmosphere would be their ‘surface’), with pressures at the base of a few hundred or thousand atmospheres. Unlike a true gas giant, the ‘solid’ surface of a Failed Core (at the radius that was actually rolled) is likely to be entirely covered with supercritical liquid water overlying layers of high pressure ices with a rocky surface hundreds of kilometres below. It is possible that some can be “Superearths” that actually do have rocky surfaces, but those are more likely to be the larger worlds with Dense atmospheres rather than the Massive ones that Failed Cores have.

Frozen: Frozen worlds are a new class of rocky worlds found in the outer parts of the Habitable Zone and in the Middle Zone, that are generally too cold to support complex life. The difference between Frozen and Glacier worlds found in the habitable zone is that Glacier worlds are only temporarily and partially frozen (usually due to a global-scale ice age that lasts thousands of years), whereas Frozen worlds are always frozen because they are too far from their primary star. Frozen worlds might have oxygen in their atmospheres as a result of life in the liquid oceans under their ice sheets, and they could actually retain some liquid water at their equators – but ice will always cover the majority of the planet all year around. Frozen worlds are more likely to have “pre-garden” exotic atmospheres and are unlikely to have complex surface life. They don’t have to be entirely covered by water/ice (though the land would probably be covered in permafrost), but if they are then they could be considered as “Super-Europas” – larger version of Jupiter’s moon Europa – with a liquid ocean beneath a global ice shell.

Core Types: Icy Bodies are the only bodies found in the Outer Zone (replace Rocky entries with Icy). The Middle Zone is Rocky on 1-5, and Icy on a result of 6. Inner and Habitable Zones are entirely Rocky.

World Density: Planetary bulk densities are adjusted for more realistic results. Roll 1d6 for Icy cores and Rocky cores, with a DM+3 for Rocky cores with radius >= 7000 km. (larger worlds will tend to be denser due to self-compression).

Orbits and Planets: All main sequence stars have 2d6 planets (including A V stars). All subgiants (IV) and K and M II and III Giants will also have 2d6 planets, limited by temperature and stellar radius. A/F/G Giants are the only stars that have 1d6 chunks, and D stars (white dwarfs) have 1d6 planets – in both cases, they start at 2 AU+(1d10*0.1, 0 is zero) AU (for giants this is due to the heat, and for white dwarfs because closer worlds would have been destroyed during the red giant phase). If any O and B V stars are present, they would have 1d6 Chunks. The maximum possible orbital distance from any star is 100 AU. Additionally, the minimum orbital distance has now been reduced to 0.05 AU (given that we know of many planets that are closer than 0.1 AU to their star).

Garden worlds: Garden worlds are determined in a different way now since blackbody temperatures are also calculated here. Blackbody temperatures in the Habitable Zone now range between 294K (21°C) and 235K (-38°C), and it’s a lot more likely that there will be colder worlds than hotter ones. If the blackbody temperature is below 253K (-20°C) then there is a chance that a Garden world would be replaced by either a Glacier world or a Frozen world. The logic is that if a world is a Garden in the outer habitable zone then it is assumed to have a dense atmosphere and/or a high greenhouse effect in order to still be warm in such conditions. However, it could be such a world that happens to be going through a temporary ice age, making it a Glacier world – but if its atmosphere is too thin or it does not have a high enough greenhouse effect then it would instead be a permanently Frozen world at that distance.

Stellar Data: The Stellar Data tables in 2300AD are not accurate. Fixing all of them is beyond the scope of this exercise, but I can at least use the correct masses for the stars (derived from stellar evolution tables) even though the radii/luminosities are different. So the stars in 2300AD now have realistic stellar masses, but unrealistic radii and luminosity (they’re mostly in the right ballpark, but they’re still inaccurate). This is important for determining tidelocking.

Tidelocking: Whether a world is tidelocked or not depends on many variables and is hard to simplify realistically – the key factors are the age of the system, the mass of the star, the planet’s distance from the star, and the planet’s radius. In practical terms, the closer a planet is to its star, the more likely it is to be tidelocked. If a planet is very close to its star – as is the case for habitable worlds around M V stars and most K V stars – then it will be tidelocked within a few hundred million years of formation (i.e. not long after it forms). The habitable zone of brighter stars is further away, making it much less likely (if not effectively impossible) for a world to be tidelocked there. Additionally, larger worlds will tidelock faster than smaller worlds at a given distance from the star.

However, it is impossible to calculate the extent of tidelocking if the age of the system is unknown. If we determine the age of the system then a lot of other things in this world generation process will become much more difficult to deal with – we would have to account for the star’s increasing luminosity over time, move the habitable zone out over time, link the age to the type of Garden world, etc. Accounting for all of this is far too complicated and far too much effort for the system described here. Instead, for tidelocking purposes only, we will assume that all the systems created here are a specific age (2 billion years old) – this age will have no effect on anything else in the system.

This means that we can calculate the distance at which a world would become tidelocked to its star in a span of 2 billion years assuming that the planet has the same size, mass, moment of inertia, and tidal dissipation factor as Earth. This is obviously a ballpark figure, since the actual age of the system is unknown and it assumes that all worlds are the same size and mass. It should be noted that Chunks are too small to ever be tidelocked, and Gas Giants will not be tidelocked either because they do not dissipate tidal friction in the same way as rocky planets.

Results

So how do all these changes affect the statistics of world generation? The table below shows the averaged results from five different runs that generated 100,000 systems.

The percentage of stars is about the same as before, since I used my own rules for that when generating them for the 2300AD rules. Now though, about 60% of all the worlds generated are Gas Giants and Titanians (icy worlds with atmospheres), and another 15% are Failed Cores (sub-neptunes).

Gardens now make up only 0.24% of all worlds (about a third as many as in the default 2300AD rules), and are on average found in about 1.3% of the systems generated. About 61% of all Garden worlds will be tidelocked. Pre-Gardens are more common now, since it’s more likely to encounter a system in its longer pre- garden stages, but Post-Gardens are less common than before (realistically so) since K and M dwarfs are not assumed to be old anymore. Numerically, M V systems have the most Gardens of all the main sequence star types, but since there are vastly more M V stars than other types, only 0.85% of M V systems will actually have a Garden.

Sample Systems
Some sample systems generated using this Realistic 2300AD system are presented below:

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This M3 V system contains no habitable worlds, and the innermost four of its six worlds are tidelocked to the star. A large Failed Core is the closest thing this system has to a gas giant.

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This G8 V system contains only four planets – a Garden world in the first orbit, a Failed Core in the Middle Zone, and a large Titanian and Failed Core in the Outer Zone. The radius of the outermost Failed Core including its cloudtops is probably about the same as the Titanian. The Garden world is slightly larger than Earth and may be colder, depending on atmosphere thickness and greenhouse effect.

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This K4 V system contains six worlds. The first is a Garden (not tidelocked), and the next two are smaller Frozen worlds in the Middle Zone. Two large gas giants and a small Titanian world occupy the Outer Zone, separated from the inner worlds by an Asteroid Belt.

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Finally, this G2 V system is an extensive one that contains eight planets and an asteroid belt. The second planet is a Garden world, and the third is a Frozen world larger than Mars. The system contains four gas giants, including a superjovian nearly three times more massive than Jupiter, and a small jovian which has a very low mass and orbits almost 90 AU from the star.