Here, I examine whether the definition of life that we arrived at earlier needs to be modified when considering the possibility of alien life. I also note that the search for alien life of any kind – astrobiology – is a much broader venture than SETI – the search for extraterrestrial intelligence. Then I deal with the question of whether we should expect trees of life, rather than a non-tree-like pattern, to characterize other inhabited planets. In order to focus in on those parts of the galaxy that may host life, I start by excluding most parts of the galaxy from consideration – life is unlikely to be found in interstellar space or on stars, which means that more than 99% of the galaxy is lifeless in terms of both its volume and its mass. I then focus on planets. Next, I argue against the Rare Earth hypothesis – that animal life is vanishingly rare in the galaxy (and the universe). Instead, I propose the alternative Common Earth hypothesis. Finally, I ask the question: are alien trees of life likely to run in parallel to the tree of life on Earth, or might those alien trees and their constituent life-forms be very different?

Here, I consider other factors than distance from a star that may affect a planet’s habitability. These include its atmosphere, its magnetic field, and whether it has any moons. However, I emphasize that it is important not to draw up a list of all the Earth’s specific features, for example its unusually large Moon (which helps to stabilize its axial tilt) and make the assumption that all of these are necessary for another planet to support life. Making such an assumption leads to the Rare Earth hypothesis, which I regard as flawed. For life to originate on a planet, there must be places where conditions favour the biochemical evolution that leads to proto-cells and hence to life. For life to continue and diversify, there must be places where organisms can survive. Even if conditions are normally benign, all planets are subject to occasional major threats, such as impacts and glaciations. I examine the mass extinctions on our own planet, some of which were caused by asteroids impacting our surface. Finally, I examine a problem that Earth is not subject to – tidal locking. This may be a major problem for planets in the habitable zones of red dwarfs.