I'll update these as the campaign goes along.
Currently inhabited worlds:
Sol: Mercury (Terraformed), Earth, Mars (Terraformed), and Titan.
LHS 292: Vond (Terraformed), source of Gallicite for Sol.
Epsilon Indi: Shepherd (Terraformed), a small colony in a resource-poor dead end system, mostly a population dump and a PR project.
61 cygni: 4 terraformed worlds, major mineral deposits, first real extrasolar fleet base.
Aliens discovered so far:
Star Swarm (QY Aurigae, Gliese 526): A formidable threat in both systems, but it has been determined they are incapable of interstellar travel, and the systems they occupy have simply been declared off-limit until a sufficiently large fleet of effective combat ships can be constructed.
Cassiopeii (Provisional name): A disturbingly human-like species of aliens encountered in Mu Cassiopeiae, communications has proven impossible and an invasion force is being assembled to make sure they will not pose a threat to humanity.
Unknown (Probable, no proof): There may be an alien civilisation in Wolf 636, but there is no proof of this as the grav survey group that initially found the system decided not to take the risk of exploration due to their low speed and lack of weapons.