Looks like your planet and rocks could do with a load more contrast, and a bit more variety in the hues.
I'd also take a look at the outlines on your ship. You seem to go very dark at the base of each vertical line, which creates these 2x2 pixel chunks that cause it all to look a bit rough. It looks like the style might benefit from not having an outline. I wonder how it would look if you replaced those near black outline pixels with the normal outline color, then remove the rest of the outline?