On a nerdy tangential note: the internal consistency of fantasy/sci-fi plots is of passing interest, and the same goes for the alien speculation game in real life. The idea that a race of aliens capable of interstellar travel would arrive to loot Earth presupposes the idea that Earth has something so rare and valuable that looting it would be worth the effort. If you stop to think about it, this pushes back in the direction of "rare Earth" theorizing, which militates against the likelihood of aliens in the first place.
In all the vastness of the universe, what makes Earth so special? Water? We think water is special because we depend on it. An alien race might not share that dependency -- but if it did, it is difficult to imagine that it would not have solved its own water problems long before it attained interstellar travel. Or that it would not have learned to melt ice. Or to combine hydrogen, oxygen, and an appropriate energy source. A water dependent star-faring race would probably understand the chemistry.
So what else does earth have that is found nowhere else, or that is exceedingly rare? It's hard to think of anything other than life itself, which as far as we know (so far) is still unique to Earth.
That leaves us with something on the order of dilithium crystals, which Star Trek technobabble tells us are exceedingly rare and -- surprise, surprise -- cannot be replicated. The latter is important because, in a sci-fi universe in which almost anything can be easily synthesized or replicated, the problem of scarcity has to be artificially reinvented so that it can be mined for plotlines. (Without scarcity problems, scriptwriters are reduced to sex, power, and insanity, which are all well and good but which tend to get stale after awhile). So: where are our dilithium crystals?