Cities on a planet... at night. ;D
Good catch. This has been done before, but there are so many amazing things that haven't been...and I'm sure you've found things no one has yet. Keep sharing.
I also used a reflective surface layer for the oceans instead of water. I tried making a really big lake and TG crashed.
Make a sphere with the same radius/coordinates as your planet and use a water shader on that for planetary oceans.
The terrain displacement will make some nice seas/mountains/lakes.
Cool idea. In the past I've used PS to add in city lights, but this is indeed neat.
For the city light layer, I just narrowed it to nearly flat slopes above sea level and below mountain peak level. It worked out great.