First of all, those spheres you used as planets could use a significant higher polycount. They're planets, for crying out loud, not ping-pong balls. They need more detail if you take close-ups of them like that.
Secondly, the lighting is not realistical unless there's a system with 2 suns...And the luminosity of the blue planet is excruciating.
And don't use bump maps on planets. Planets don't show any bumps on the surface like that, it looks all flat.
Don't use any more reflection maps like you did on that (exagerated low-poly) space station, don't you think they look ugly and messy? And use higher resolution maps and better realised. For close-ups like u did for the planets you need a minimum 1024x1024 map(s).
Oh, and try to be more careful with the background, just because it's not in the spotlight it dosen't meen you don't see it. Here's what I mean:
Use a soft round brush to erase stuff like that next time.
