A couple of months ago I created this image with the help of this Blenderguru tutorial. The Blender Render engine is used. Although only one sun lamp is used you may note that the shadow side of the earth is not black.
Now I have a little understanding of cycles I tried to build the planet earth in cycles with this result. Note that the shadow part is very dark / black although I used a very tiny Ambient Occlusion
I really prefer the blender render result because
1) the blender render planet is a "true blue planet" with lots of water and blue tints. In the cycles render there seems to be much less water 2) the africa continent seems very large in the cycles image 3) the clouds are very present in the cycles image and make it a more greyish image.
It looks like that the proportions are different. Did I make some mistakes with the texturing. Or is this simply the kind of differences that are normal ? I thought that Cycles was the more photorealistic render engine, but is this true and are there better engines ? Were did I go wrong with my cycles render ?
After the answer of Gandalf I applied his volume shaders to my Atmosphere and I also added subsurf modifier to the atmosphere. This is the result. I used the same shader as Gandalf. I guess I should change the distance between the atmosphere and the earth ??