Astrobiology is an unusual science in that there’s no clear, undisputed evidence that its subject of study – extraterrestrial life – even exists. It’s still an active field of research, though, with observational astronomers scouring the skies for telltale signs of life, and theoreticians tying themselves in knots trying to explain why we haven’t seen any of those signs yet. This book deals almost exclusively with the second of those topics. Before going any further, it’s worth defining exactly what we mean by ‘life’ in this context. If you’d asked Aristotle or Shakespeare or Charles Darwin to define life, they’d give pretty much the same answer, relating to biological processes. If we can’t detect such processes at a distance of many light years, that’s not really a great surprise: the latest generation of telescopes has only just got to the point where it might be possible even in principle. But many popular discussions of extraterrestrial life, this book included, aren’t talking abo...