The plannets look a bit like blurred smudges, you should do a circle, and think, what planet will this be?
For example, say i wanted to do something like the moon, id make it white. Then mix with with shades of yellow and grey, then investigate whats on the moon, what revolves around the moon, for example, craters, so i would add craters.
Then create a glow, either in the program im using or with a gscript.
http://www.noao.edu/image_gallery/images/d4/moon.jpg
a lovely example.
As for the building tiles, they look too repetative, the left one looks to flat with flat shading, and the outside looks too buttonized.
How my critz helps
