When will city lights be savable in US^2? I've save a solar system that had a planet with city lights and when I loaded it up, they were gone. Maybe there was a war and they all died. Sad face.
If you want to load up planets with city lights, you need to follow the following steps.
1. Pause the simulation immediate (I believe there's an option to pause a simulation on load up)
2. Go to the planet(s) with lights
3. Change their temperatures to 10 degrees
4. Play the simulation.
If this doesn't work then make sure your planet(s) are in the habitual zone of their star.