This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Given the current state of our home planet, and indeed our species, it can seem a bit surplus to needs to speculate on whether extraterrestrials are going to one day come and eat us (or, well, you know, do anything equally unpleasant).
Nonetheless, the question of whether there's anyone or anything 'out there' that might ever show up to cause trouble does seem to come around again, and again, and again. For example, it's become a bit of a perennial topic for Stephen Hawking to mention, and he's not alone among serious scientists in speculating on the outcome of any such encounter.
As with any effort to decode the likely thought processes or intents of entirely hypothetical lifeforms, the options are - shall we say - fairly loosely constrained.
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I do agree with Hawking's general thinking that if a species goes to all the trouble of undertaking interstellar travel itself (rather than sending robotic scientific probes) the chances are that it wants more than a few nice anecdotes for the next edition of the Really Lonely Planet Guide to the Galaxy.
But contrary to many Hollywood-style sci-fi tales, I doubt that what an alien species needs are most of the usual tropes: water (what, one of the most abundant compounds in the cosmos?), human slaves (seriously, you can traverse interstellar space and you need slaves?), or some mystical 'life-force' to be drained from us all (no, just no).
A semi-plausible motivation would actually be the need for a functional biosphere, because an alien home-world has gone belly-up. Although here too, if you can sustain life on a mothership (for the sake of argument) for what would be at least decades, if not centuries of travel, would the need for a planet be quite so desperate? Maybe it would be. As anyone with half a cup of grey-cells in their head is painfully aware, a planetary ecosystem, replete with chemical recycling, photosynthesis, and moderately stable climate regimes, is actually pretty useful, hard to replicate, and possibly the very best life-support system money can buy.
Of course it's easier to set up a colony if the indigenous lifeforms don't include anything particularly intelligent, so the cost-benefit analysis has to include the effort of handling the locals who might get in your way. Option A is to try to be nice and hope you get asked to co-exist, Option B is to follow the template of what humans have successfully, and desplorably done to each other over the millennia: "I know you were here first, but now it's ours and we're going to make it GREAT (again)".
Which brings me to the real point of this post. If the single greatest motivation for a species to go interstellar, and to show up in a hostile fashion, is because their home system can no longer support them, we should be trying to find those nearby exoplanets where it looks like life is on its way out.
Being able to actually do this is a way off, so bear with me. Suppose though, in the not-so-distant-future, we do have the capability to confirm the presence of a biosphere on an exoplanet, and we have refined our techniques so that we can measure stuff like industrial pollution, climate change, and perhaps even the infra-red excess of active technology. In this case we might be able to gauge the odds that life has messed things up good and proper.
With this information in hand we could go as far as assigning a probability that a technological species on one of these dismal worlds will pack up and come looking for a new home. And, given that this would tell us the best directions to be looking in, we might conceivably be able to pick up signs of an interstellar launch, or deceleration, or other activity.
There you have it. The best way to avoid hostile alien invasion, or at least be forewarned, is to continue funding all that wonderful exoplanetary science; research that will answer a bunch of other critically interesting questions along the way.