OK, while pondering the bizarre motivations for evil alien monsters (must…destroy…schoolbus…which can dodge plasma blasts even as fighter jets cannot), GG wondered, why would any alien civilization want to conquer or destroy Earth?
Arguably the most likely reason would have something to do with our biosphere. Maybe there are cool new medicines to be found–the cure for some intergalactic plague. Or maybe they really are into zoos (hmm, didn’t Kurt Vonnegut go there?). Our biosphere is presumably highly unique and probably pretty rare (current enthusiasm for planets possibly harboring life not withstanding).
Not knowing anything about alien ecosystems or diseases or the like, can’t really go any further. Is there anything else special about Earth? In the past, movies and some science fiction have used the water on Earth as a main motivation (see Oblivion for a recent example). But water is simply hydrogen–which is widespread–and oxygen, which is also pretty common. If you have the muscle to move spaceships all over the place, making water is probably not that hard to do.
Oddly enough, one possibility is one that feels more like motivation for a spy movie and not for some extra-terrestrial invasion: gold.
Now gold on Earth isn’t the most common thing, but the funny part is that there is a lot more of it near the earth’s surface than you’d expect. If you make Earth by condensing all the material in the solar nebula at about this distance from the Sun, you kind of expect the gold to all end up in the core [woo-hoo! Another motivation for a movie about the core–travel there to get gold!]. Although this difference might be related to other elements present in early Earth and issues with experimental simulation of the partitioning of gold between core and mantle, if this is real, a decent proposal is that things like gold and iridium were emplaced on the earth’s surface in the Late Heavy Bombardment period just under 4 billion years ago (a review of much of this can be found here; a popular science story here and a 2011 Nature article providing observational support is here). What this might mean is that the earth might be uncommonly rich in metals like gold. And if our solar system were unusually rich in gold to start with (the production of gold in stars requires either supernovae or even more exotic events), we might be quite unusual. So maybe a good ET movie might combine sci-fi and a Ft. Knox heist….
Of course you’d have to have some big reason for wanting gold (hint: probably not to make coins with). But gold is exceptionally malleable and resistant to corrosion; it is also an exceptional conductor. Perhaps there is some kind of gold-based superconductor out there (so Earth could be Avatar’s Pandora for some other species).
GG will wait for that call from Hollywood….