Something like:
- when designing, for each of your planet levels, separate the lighting and game objects into different sub-levels
- when running the game, stream in only the object sub-level for the new planet, without the lighting
- put some type of “lighting info” in your object levels, which saves the lighting settings for the planet, so the existing skylights and stuff can be changed appropriately
This might enable you to interpolate the lighting from one level to another? Kinda like how World of Warcraft or other games do when you seamlessly walk between map areas, and you get a smooth lighting transition.
But yeah, the default situation for the engine is entirely based on all players being in the same level with no streaming. If you do level streaming extra work will be involved. Is your game multiplayer? Same thing applies. There is no support for players being in different physical levels in the same multiplayer game/server.