Alien showed up in English in the middle of the fourteenth century, but since extraterrestrials and all that only came in vogue in the last century, back then it just meant foreign. English alien came from an Old Frenchword with the same spelling and meaning, which in turn came from the classical Latin alienus, and again, the meaning was pretty much the same.
Alienus (a fun word to write, in my opinion) is a different form of another Latin word, alius, which means “another” and it shouldn’t surprise anyone that it’s also the origin word for alias. So an alias is another, foreign identity, am I right? Alius can also be traced back to the Proto Indo European al-, beyond, and that just happens to be the origin word of the Old Englishelles. Or as we call it these days, else.
Well, that was a quick one. I don't even think I need a TL;DR. Probably should do more like this ; ).
Tony Jebson’s page on The Origins of Old English