I wondered if that might be intentional! But it doesn't look quite right to me; shadows are usually soft on planets which have an atmosphere:
http://antwrp.gsfc.nasa.gov/apod/image/ ... esa_c1.jpg
...and the dividing line isn't totally clean even on airless bodies (because of variations in terrain height):
http://www.astromax.org/planets/images/ ... -Moon5.jpg
You could replicate the former effect by putting a sprite over each planet (with a gradient sphere, filled black and white, and an effect like Lighten). Then you could adjust opacity of the lighting effect with distance from sun.
Or you could keep the clean and simple art style. I'll butt out.