Because i not have the enough skills if not i will made something like that.
Seems is a Seamless texture added in a sphere, i guess in C2 using the Sphere effect or the Crystal ball one, and add a light/shadow to get the final effect.
Also with a X/Y coordinate change of the texture you can get it rotating like a real one.
At least i think actually is possible create a "planet template" editor using what i said. After that the people "only" nedds to get the seamless terrain texture they want to use for the planet. An that with programas like FilterForge or similar not have to be too complicated.
I made this using NormalMaps and the FilterForge texture:
https://dl.dropboxusercontent.com/u/659 ... index.html
For the other type of rotation using sphere/crystal ball i guess comething like a real planet can be done, hope to do some test soon.