From: Brighton, UK
I see the devs as having sorted planet types by both water levels and surface temperature, with volcanic planets being the high extreme of temperature (get close enough to the star and it won't matter how much water is in the atmosphere!), then ice being the extreme of lots of water and low temperatures.
Naturally surface temperature is dependent on distance from star, star type, and star attributes (I've often wondered in game why it tells us the different energy outputs of stars). The different combinations of water level and surface temperature allow for the variations in planet types you see in game. Though sitting here now, not in game, I can't remember if volcanic planets occur through star systems. I know ice planets definitely occur further out, and deserts are found further in, but all of the others I'm not sure about.
The marshy swamp planets confuse me particularly. On the face of it you'd think they have more water than a continental planet, but if you look at the images in game you'll see the oceans are much smaller. The forest planet images on the other hand appear to have a similar amount of ocean coverage as the continentals.
My theory when it comes to the marshy planets, one I would apply if we could mod planets and star systems, is that they do have more water than continentals - a similar amount to ocean planets - but they are found (or should be found at least) closer to a system's star, resulting in more of the water existing as vapour in the air. Forest planets, and I stress that this part is not based at all on science as far as I know, would have a similar amount of water to continentals but be colder, further away from the system star but not as far as ice planets, resulting in a planet with no tropics, deserts or savannahs to speak of; just temperate forests, alpine forests, and tundra.
Boiling it down to numbers:
Volcanic, 3.0 temperature, any water
Desert, 2.0 temperature, 0.5 water
Marshy, 1.5 temperature, 1.5 water
Continental, 1.0 temperature, 1.0 water
Ocean, 1.0 temperature, 2.0 water
Forest, 0.5 temperature, 1.0 water
Ice, 0 temperature, 1.5 water
Barren, any temperature, 0 water
It's not quite that simple, the numbers would be more like ranges than single figures, but that's the gist anyhow.
That post was a lot longer than I originally intended, but I hope it makes for interesting reading. I have made a request on the master wish-list thread that the ability to mod planets and stars be added to the game, but it wasn't very detailed. I'll edit in a link to this.
Going all in on this, the image I've emedded below shows what I'm talking about:
By raising or lowering the surface temperature and water levels of a planet, you alter what 'biome' it is merely by decreasing the variety of biomes found on its surface. Looking at Earth, if you lowered our planet's surface temperature far enough, and increased the amount of water, you could theoretically (I stress, in the most unscientific way possible) end up with a planet dominated by subtropical forests.
< Message edited by gerishnakov -- 6/14/2014 6:15:51 PM >