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Seems that spawn_system (it would be nice to see the exact code of the function ~_~) actually creates a new system... from what I can see (and it's a bit hard for me to understand for certain), depending on the scope it's run on, it creates a nearby system using whatever system initializer and with a min/max distance. However... I don't see any spawn_planet or other spawn function that might be used to create planets.
Is there a way to save everything in a system and perhaps respawn it with desired additions? Somehow I doubt it.. and it would probably break things