Here's a planet package I made to answer this particular request: http://forums.sc2mapster.com/development/artist-tavern/4423-planet-models/#p9
It contains 76 planets, actually it only contains one planet model :P but it has 76 textures for you to choose between. To change the "planet" all you have to do is rename the whatever texture you wish to use so you can import them to the editor. Better instructions inside the package.
The package is rather large so you can contact me to upload a single planet or give you the textures at least.
I take no credit for the diffuse textures, all of them you can find at www.filterforge.com, the rest (normal maps, model, renders, stuff) was made by me.
Enjoy it!