The aliens being held out of the innersphere by the Minnesota Tribe is an inside joke of the Battletech setting. There are no aliens. Well, there was ONE book, but it was a one off and never acknowledged again.
I try not to get into battletech conversations too much. Back in the day I was pretty obsessed with it to the point of collecting a lot of the harder to find stuff like the Stardate and Battletechnology Magazines. I even have the first editions of the source books. Including the Star League and Periphery books. I could easily relapse. It's still one of the most detailed socio-political settings ever developed that I have seen, surpassing Tolkien in a lot of ways.