That's amazing! I started to understand how PBR works in Unity just recently, it's quite complicated!
Every material has a "metallicity" value, right? It makes it darker and changes way reflections work together with "shininess".
It's very important to set materials correctly or it would look faker than it would look in a legacy material.
SE is becoming very modern and complicated for what concerns graphics, it challenges all modern engines. The only thing I can think of that is still missing is ambiental occlusion, but I'm not sure if it's possible to be implemented also because SE has a very very wide range of scale. That's an incredible work by the way.
would it make the non-lit surface of a planet brighten by its own rings?
Wasn't this already implemented?