[geoschmo]

I don't recall the exact formulas, but it goes something like this: (This only applies for the stock game)

The planets type/atmospheres are not "balanced" for each map, but they are random. Because of a mistake in teh sector type file Methane ice has one less selection then teh others and so averaged out over the entire quadrant, and over several maps they will tend to be slightly less frequent in total numbers.

The other rock and ice planets of all atmoshpere types have an equal chance of being used.

Gas giants are significantly less frequent, but this fact is compensated for at least partially by the fact that gas giants are larger on average and thus have more facility spots.

None atmosphere are more frequent because of the fact that all moons are but this fact is partially compensated for by the fact that all moons are tiny and so have fewer facility spots.

In the end though it really does not matter which planet type/atmosphere is more common on average for all maps. It only matters which type is more frequent on your map, and in the general vicinity of you homeworld. That is random with only small probability variances between types so trying to base your empire selection on those variances is useless. Play what you like or want to and roll the dice, keeping a couple things in mind.

If your play style prefers many small planets spread out all over, don't choose gas giant. If you prefer fewer planets that are larger and thus more defended consider takeing gas giant.