Spent the past several hours trying to make a "simple" model for what kinds of gasses can exist in a planetary atmosphere. This only models Jeans escape; solar wind erosion is a lot more complicated and isn't modeled, so just because a planet could retain water doesn't mean that it does (Venus, Mars).
Some things I learned:
- Mass has a much stronger effect on atmospheric retention than insolation (temperature)
- It's pretty rare for a planet with less than 1e23kg of mass to have a significant atmosphere
- A planet doesn't need to be much more massive than Earth to enter gas giant territory
- I had to cheese this model a bit to allow Titan to retain methane, despite the fact that it does IRL
Once I figure out the deal with that Titan issue I might use this for a planet generator, we'll see