Also, since I *want* Engineer units on new planets anyway, how in the world can I prevent this from happening again? To clarify, I am talking about the Ancient Artifacts trade item. It doesn't make sense that they would just *poof* once the ruins on the planet are explored.
Also when the ruins are fully exploited and there are no more things to recover then the planet will not produce ruins. It helps to have left a couple of things that have not been restored so that the planet will produce the trade goods.
From the creator: The demand for these is handled like any normal trade good but they are only produced by colonies with ancient ruins. The amount produced is equal to the level of the ruins (1-10) multiplied by the pop in millions.