beetleking that looks awesome. The sketchiness of it gives it kind of a clay look.  I think the perspective would be more like this:



that, or make everything in a strict side view.

what if you had the sky showing the sun breaking out through storm clouds, like the storm just ended.  Quick edit cause I gotta go somewhere right now

some quick edit on how you can shade the planet



If you google planet, youll see alot of images where the dark side of the planet is nearly black, or even just blends in with the space behind it.  You can go pretty high with the contrast on them.

AlcopopStar, you should realize there are two ways to make a picture grayscale.  Converting the image to grayscale, and setting the saturation to 0, which is not an indicator of actual contrast of value between colors.  I see you did the later.  Heres a simple image I made to demonstrate this to someone else a while ago



the HSL color system is a little different than you might think.  The LUM value (lightness or darkness) does not represent an absolute lightness or darkness.  Basically how it works is a LUM of 120 at highest saturation is the brightest/purest version of every hue.  The purest blues happen to be dark and the purest yellows happen to be light, even though they have the same LUM.  So when you just set the saturation to zero that blue and yellow are both gonna turn into the same gray.  Thats why you have to convert to grayscale if you wanna compare value contrast between colors.

theres another example in megaman x4, though it moves so fast its kind of hard to see
http://youtu.be/EI3OLfPmcww
starts at 15 seconds.  Cant think of any examples that are more 2d like your mockup though.