So, i gave it a quick try and yeah, something is wrong with translucent materials. I got half, blocky shadowing when i used translucent material for the ring. But this is the result i get with opaque material;
You can see my settings for the light. The planet is a single mesh with a radius of 8k units inside the editor. Those may help you as starters, if you want to go with opaque ring for now.
You may want to start another thread for translucent shadows tagged as bug though.
Hope it helps!