Do you mean something like this?
http://www.renderosity.com/mod/gallery/index.php?image_id=1741954

BTW, maybe this image is familiar to you... This image was chosen for the box cover of Vue 7 Pioneer.

The atmosphere is a sphere slightly larger than the planet, with a volumetric material, which is set like this: lighting model "shaded", difuse 100%, ambient 0%, density production made with a "incident light angle" node (to make it "thicker" at the borders and more transparent in the center. You have to adjust density and fuzziness to suit the size of your sphere (in my case, density is 50 and fuzziness 10).