Click here
for some veeeeeeery old Blender tutorial I made...
Keep in mind that all these tutorials relate to ancient releases of Blender and so, altough
for the most part they should still work, several thing have changed in last Blender 2.79 release.
Also, all of them relate to the Blender Internal rendering engine, no Cycles yet.

"Realistic planet in Blender" "How to make a realistic planet in Blender" (2004)
This is quite outdated too, infact it has been made with Blender version 2.30.
By the way, the idea behind still works; several things can now be made in different ways, but I think that the "three domes" technique is still a very good one to add a layer of clouds and the fresnel effect on the planet surface.