Are we are alone in the Universe? Almost every adult human alive today has asked this question. About 400 years ago, when Galileo watched the four large moons of Jupiter orbit the giant planet, he probably asked himself that very same question. Perhaps even when Homo sapiens first arose in Africa some 200,000 years ago, our ancestors looked up at the night sky and wondered if there might be life ‘out there’ somewhere – albeit their notion of the Earth and what lay beyond it must have been very different from ours.

However, the question ‘are we alone?’ needs some dissection. The answer may depend on what is meant by ‘we’. Does the ‘we’ of the question refer to life, animal life, or intelligent life? The probability of us being alone must increase as we move through this series of gradually more restrictive categories. And yet, it may be that for all versions of ‘we’ the answer is no – we are not alone.