The image of visiting aliens as ruthless invaders hell-bent on destroying mankind in order to colonise our little blue planet for themselves, may be good for the Hollywood box-office, but how likely is it to reflect reality? The deeper we delve into the cosmos, the more likely it appears that we not alone. Even if bio-genesis – the appearance of life from non-living chemical components – is a mind-numbingly rare event, the Universe is so vast, and the numbers of potential star systems so large, that even the slimmest odds could result in millions of occurrences. Of course, even assuming they’re out there, there’s no guarantee they’ll come knocking at our door. For this to happen, they would not only need to have evolved enough intelligence to have mastered interstellar travel, but would need to have done so in a part of the Universe not too distant from our own (unless faster-than-light travel is one day found to be possible). We know that it in our case it took 3.5 Billion years for life to evolve to the point of starting to explore the cosmos and although we’re not quite there yet, we’re probably not more than a few hundred years or so from achieving interstellar travel. So back to the original question: Is ET likely to be friendly or hostile? Back in 2010, Stephen Hawking, in a series he did for the Discovery Channel, said, “We only have to look at ourselves to see how intelligent life might develop into something we wouldn’t want to...