I’ve been playing around with different ways to fake the atmosphere of a planet in a 2D canvas context.
That’s interactive, by the way, so mess around with it (Chrome/Chromium is recommended!). Clicking on/around the planet changes the “angle” of light. You can get some pretty nifty looking results. I made this to determine what were reasonable parameter-ranges to use to generate plausible-looking planets.
The way I get the sort of “volumetric” look for the atmosphere is really simple: I just stack many semi-opaque layers of the same gradient-filled-circle over the planet, each one having a radius slightly bigger than the previous.
I actually threw this together using with a JSFiddle which you can view/hack here.
Of course, I still need to figure out how to combine this with a surface/cloud texture (procedurally generated or otherwise…).