I haven't read the whole thing because I've been a little preoccupied lately and I have to take off in a few minutes but the planet seems flat. If it was darkened around the edges it would look more spherical which I think would be a good thing for this. Or you might just make a sphere and use it as a merged texture behind/above it to get the rounding effect. It is also possible that you already have this well in hand. Like I said, I'm zipping out in a minute and haven't been able to read the whole thing.