Really good work on the lighting ambiance. That's what impressed me about this. Its difficult to get a nice smooth background without dithering. Planets are very well textured, but they look very similar, which is easily remedied. Main light source is nice. Did you use Color Dodge blend mode for it?
I worked alot on the background in this. making sure i got it as smooth as it could be. For the lightsource i used several layers with a compination of overlay, Dodge blend and lighting. I also added one more normal layer with a big fluffy white dot, just to "erase" some of the flaws that appears in the senter og the lightsource.