He suggests that aliens might simply raid Earth for its resources and then move on: “We only have to look at ourselves to see how intelligent life might develop into something we wouldn’t want to meet. I imagine they might exist in massive ships, having used up all the resources from their home planet. Such advanced aliens would perhaps become nomads, looking to conquer and colonise whatever planets they can reach.”
Let’s look at this from a resource management view. Any alien race which has the need to raid to sustain its resource consumption would probably be better off gathering supplies from celestial bodies other than our own little ball of dirt.
The first reason for this is that our ball of dirt actually isn’t so little. Any resource that you want to gather from the Earth’s surface and schlep up into some sort of mothership in orbit requires you to break the gravity well of the Earth. This is not cheap, at least, not using any sort of engine that requires reaction mass.
You need hydrocarbons? Get them from Titan, which has much less of a gravity well than Earth’s and oceans of hydrocarbons. The asteroid field has lots of nice loose chunks of industrial metals and would be far easier to mine than the Earth’s crust for anybody that has the capability of interstellar travel (it’s very nearly easier for us, and we don’t even have interstellar travel). Earth trumps all the local system bodies for liquid water supplies, that’s certainly true, but Triton and Charon both have water ice on them, they’re farther away from the gravity well of the Sun. This makes them easier to get to from outside the solar system (and away from, assuming your nomadic reavers are actually nomadic).
You have to get the supplies up onto the ship, and that takes some sort of energy, presumably. Even the heavy elements like thorium and uranium are present elsewhere in the solar system.
On the other hand, if your technology is advanced enough for interstellar travel, you might not care about reaction mass… but if you don’t care about reaction mass, you probably don’t need to raid planetary bodies for supplies. Whatever your technology looks like, you’re capable of violating a few of our understood laws of physics.
It’s likely that anybody that might want to drop by for a visit has to be some distance away… where “some distance” == “really, really far”. There’s only so many candidates for likely life-supporting environments within 50 light years of our little ball of iron (and of course, it goes without saying that if your lifeform isn’t sufficiently like ours, you probably don’t regard liquid water as a necessary resource).
I dunno, Professor, I’m thinking that the likelihood of roving bands of aliens is pretty far-fetched, logistically speaking.