Illustration depicting alien constructions on an extrasolar planet. Astronomers are finding more and more planets around nearby stars. Planets, it seems, are the norm rather than the exception. As our techniques for finding these worlds improve, smaller and smaller planets - beyond the scope of previous technology - are being identified. These small words are important because, by analogy with the planets in our own Solar System, they are likely to be rocky, with a solid surface. It is these planets, rather than gas giants, which are more likely to be conducive to life. This image depicts an Earth-like world - lush with vegetation - orbiting within the habitable zone of a yellow star. The strange tower-like constructions betray the presence of an intelligent civilisation.