In 1995, for the first time, a planet was discovered orbiting a bright star other than the Sun. Today several thousand ‘extrasolar’ planets are known, some of them rocky worlds not much bigger than the Earth, orbiting their parent stars at distances which mean that liquid water could possibly exist on their surfaces. The fact that they are in the ‘habitable zone’ does not necessarily mean that they are habitable. Venus, for example, is a roughly Earth-sized rocky planet orbiting in the habitable zone of our Sun, but this has not stopped a runaway greenhouse effect there from causing all its water to evaporate into space, turning its surface into a searing-hot desert. Nevertheless, the discovery of potential ‘other Earths’ has made the subject of astrobiology respectable, even though astrobiologists do not actually have any astrobiological matter to study yet. This provides fertile grounds for speculation about the kind of life that might exist on other worlds, some of it sensible, some of it not.

These two very different books about the possibility of life elsewhere in the universe, and how and where we might find it, cover a large part of this spectrum of ideas. Aliens, edited by the physicist Jim Al-Khalili, is a collection of nineteen short essays squeezed into a mere 233