The planet in the background is not a backdrop, it's a textured sphere, and it actually slowly rotates. In fact, all the spheres are textured, you just can't tell since they're so small and the textures don't have any coarse features.
The world contains 97,350 spheres, but most aren't visible in the scene--they're distributed all around the sphere you're hovering over. But at the beginning of each frame, the renderer starts with the complete set and has to winnow out just the visible ones in real time.
This follows on some significant optimizations to the shadowcasting part of the algorithm; I think this is fairly close to as fast as it's going to be. So I can't actually afford to build scenes as complex as this one and actually hit 15-20 fps on this machine.