How can I use it to render realistic atmospheres on planets (looking from out in space)?

What I usually do is make a hollow sphere just a bit larger than the planet, set its material to totally transparent, and use a scattering interior medium with spherical density falloff. Tweak the color map so that the density has more of an exponential curve. The maximum density is at the surface. The point where this is on the color map depends on the size of the spheres.. If the atmosphere has a radius of 1 and the planet is .98 then the max density will be at .02.

Thanks Fingers. I had a hollow, transparent sphere with media scattering just as you say, but it was too bright, and the best I could do looked pretty good, but it was kinda hazy. I will try this out.