It there are aliens, they'll never be able to discern a human "popular" story from "true art," by definition.
I disagree; I think there are definitely attributes that determine whether or not a book should qualify as 'literature.'
Probably the major one is "high seriousness." I don't think Arnold ever gave a solid definition of the term, but Burroughs once summarized it as "does it touch on basic issues of good and evil, life and death and the human condition"