The tower looks sweet, the only thing I would change is how fast the light bit transitions into the dark bit, I think it's slightly too fast, looks like there is a color junction where it turns bluey instead of a continuum of shading.
And the skull located at the "junction" is all dark, yet the one on the back side (on the blue side) has a bit of lighting from our direction, I would switch it so the one at the junction has a little bit of lighting from the left side and the one in the blue area is fully dark.
Detail is AWESOME, love it.
Edit: Could anyone give advice on manual AA on my favorite piece I've done so far? (first post in my post history) it has a lot of views (~300) but no-one has commented...
Edit2: I agree with bacca that the text isn't easy enough to read, perhaps less glow?
The opposite is needed for the planet, as bacca said it needs to be more interesting - my suggestion? Add glow.
Reference:
http://www.colorado.edu/engineering/BioServe/lgplanet.jpg you can see a "halo" around the Earth, makes it look more interesting. Maybe adding a very slight bluish glow to the clouds (scattered reflection from the clouds of the sky, which itself is scattered reflection from the atmosphere).
Edit3: Also, realistically a photo from earth show's land as a brownish color, though green is fine if you're not bent on everything being realistic.