The problem is that your planets have an unnatural glow. there is no definite light source, so the glow is unaccounted for, and thus lacks realism/sense of believablity (if thats even a word >_>).furthermore, the render of the ship is tiny and insignificant, leaving alot of unused space in the sig. about the stars, you should probably add another layer of noise, but much more brighter and extremely scattered,so it looks like the bg has a mix of dull and bright stars, making the scene look more "real"