For normal sky, I will take a look later. It should not be too mush different from this one. Several suns with bitmap or sphere. Moon with bitmaps. Clouds with a 3D perlin noise (but it need a varation on the final function I think).
By the way, if you have planet bitmap or extra other bitmap (galaxie, black hole,...) that can be add, I will gladly to (there are png file with transparency)