The cave system is cool. This could become a fun (cave/)dungeon crawler, with a space theme .

As for your first question, what is precisely the error message? To many functions? Or to many ents? Anyway the solution for this would be to not generate the planets all at the same time, but only when the player is near and remove them when the player is (far) away. Or have a map for each planet if you don't mind loading time.

something funny: I clicked your link to your Star Explorers video on youtube, and now the right bar which shows recommended videos in youtube is filled with Dora the Explorer stuff