Here, I inspect the layout of our own solar system, and consider the extent to which other planetary systems are similar or different. Discoveries so far show that there are many possible layouts, and suggest that quasi-replicates of our system, with four inner rocky planets and four outer giant planets (gaseous or icy) are rare. Every system is different from every other one. This is a consequence of the chaotic process of collisions that leads from a protoplanetary disc to planets. Planets can be found orbiting large, medium, and small stars – with consequences for their maximum lifespans. Some planets are found in binary systems and even in systems with more than two stars. The nearest system to ours – Alpha Centauri – has three stars. This system has at least one planet, which orbits the smallest of the three stars. Planetary systems are also thought to typically contain smaller bodies than planets, as seen in the solar system – moons, minor planets, asteroids, and comets. Life is most likely to occur on planets, but life on moons is also possible. Finally, there are some lone planets that do not orbit a star at all. These are the least probable homes for life.

Here, I examine the link between intelligence and life. Unlike a skeleton, which is a requisite for any large organism, intelligence is less crucial for survival. It is much more thinly spread in the animal kingdom than are skeletons, and is absent entirely from the plant kingdom. After considering how intelligence might be defined, I consider the question of where it is found in the animal tree of life. I then focus on four examples – tool use by octopuses and crows, mirror self-recognition in certain mammals, and space travel by apes (both humans and chimps). I finish by considering the link between intelligence and Darwinian fitness. Over the course of animal evolution, some groups have prospered without having brains, others have evolved small brains, and others still – notably humans – large ones. Strangest of all, perhaps, is the case of the starfish group (echinoderms), where all current species are brainless, in contrast to their ancestors, which possessed brains, albeit small ones. This combination of evolutionary trajectories in brain size shows that the link between intelligence and fitness is complex.

Here, I examine the shape of the tree of life on Earth that is the result of four billion years of birth, death, reproduction, and relatedness. This extended family tree has been produced by four billion years of using energy from the environment to power biological systems. I then consider the question of how to define life, from both evolutionary and metabolic perspectives. Defining life is not easy, but it is possible if we accept that viruses constitute a grey area. Next, I deal with the main driver of evolution on Earth (and probably elsewhere), namely Darwinian natural selection. This driver only works when there is variation among organisms, so our next port of call is how variation arises – gene mutation and related processes. Finally, I examine the origin of life. The emphasis here is on the hypothesis that life arose here rather than arriving pre-formed from another planet – the Terraspermia hypothesis. The alternative Panspermia hypothesis is considered to be fatally flawed.

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 investigate the concept of a habitable zone. Given that all life on Earth – and possibly all life in general – needs water, the defining of a habitable zone as the belt around a star in which liquid water could exist on a planet’s surface seems sensible. However, habitable zones may shift in position over time. One reason for this is the gradual increase in energy output from a star as it progresses through its main-sequence phase. To take account of this, we can define a continuously habitable zone (CHZ) in which liquid water could exist on a planet’s surface throughout its history. Inevitably, the CHZ will be narrower than its ‘instantaneous’ counterpart. I consider the question of whether life might exist, or might have existed, on Mars – a planet that is near the outer edge of the Sun’s habitable zone. I also look at whether life might exist on the moons Europa and Enceladus. Although these are far outside the habitable zone, they are thought to have sub-surface oceans. Finally, I ask whether in addition to there being stellar habitable zones there may also be larger-scale habitable zones – galactic ones. The answer is a qualified ‘no’.