Nest three spheres, each one a little bigger than the previous one. Map the innermost one with an earth map (check NASA's immage database). The middle sphere, you would make a cloud map (image, procudural...whatever gets the job done). The largest, outermost sphere would be for your atmosphere. Do a search on x-ray shaders and you'll get the general idea of how to set up a shader for it (sampler info node into the v coordinates of a ramp). Then just tweak everything til your heart's content.