We have means for interrogating systems for specific criteria. It's not something you'd want to run on the entire galaxy though
Michael
I'm not sure that I want to be on the receiving end of something that can interrogate a whole star system!
(bold for emphasis, mine)
On topic:
Using seeds is normally the way that procedural generation works, and then you either save the seeds more permanently or make a new one when needed.
For discovered systems, the seed would be saved, so that it isn't something new for every player (Although, that would be ... Interesting ...
But, considering how fast the loading when entering a system that haven't been visited before is (either by you, or no-one at all), I'd assume that at least -some- info regarding the system is saved and it isn't generated from scratch upon first visit..