For Star Trek (remastered version) I’ve created about 39 planets I painted texture maps (see sample below) for the surface color, clouds, and for the displacement maps (that give the surfaces of the planets the 3-dimensional surface). Then I mapped the textures to a 3D sphere, added lights, and rendered them. After that I touched up the rendered images as needed  (and to eliminate or soften the 3D look). For their natural chaotic randomness, and - at the same time - their dynamic order following wind patterns, the clouds were the hardest elements to paint and apply to the 3D spheres.