I have found something about the sentient creatures.
In the space age, you can kidnap sentient beings on other planets- Tribes, cities and space peeps- and when you place one sentient being on a planet which is terraformed, or T1 or above, the sentient creature places a campfire and stands by it. The same happens with multiple creatures, they make a campfire. I think there might be a secret there that noone has noticed yet, and I can proudly say I found out myself.