I dunno if you are familiar with a software package called "Vue 9" from e-on? in that software you can create environments from rocks and plants to terrains and ecosystems, clouds, weather, etc, and it also has an option that will take your scene and "bend" it into a spherical planet, you then export the scene into a folder take the textures, import them into a singular file in Photoshop in different layers, and then export it or save it as a .dds file, with a tool download from nvidia... import on sandbox and you have your planet with atmosphere, clouds, terrain and anything else you want... seamless textures, bumps, normal maps, everything...