Here, I outline the various sources of energy that different life-forms use. The most fundamental division of life from an energy perspective is that between self-feeders that can utilize non-biological forms of energy such as light (autotrophs) and those that feed on other organisms (heterotrophs). Further subdivision of the autotroph category shows that not all of these organisms conduct what can be called ordinary photosynthesis – the type that yields oxygen. Some autotrophs conduct non-oxygenic photosynthesis, while others do not use light at all, but rather utilize chemical energy of various kinds in processes that are collectively called chemosynthesis. Next, I consider the flow of energy in ecosystems and note the limitations in the efficiency of energy transfer between trophic levels, but also the limitations of the trophic-level concept itself. Finally, I note that the biosphere is a realm of decreasing entropy: processes that contribute to this decrease include evolution, embryological development, and ecological succession. The decreasing entropy of the biosphere is perfectly compatible with the second law of thermodynamics as this law only applies to closed systems.

Here, I describe the Earth’s biosphere, which is not a sphere at all but rather a spherical shell. I discuss how far it extends in both directions. The question of how deep the biosphere goes takes us underground and to the bottoms of deep ocean trenches. The question of how far up it extends takes us to the stratosphere and the ozone layer that it contains. Next, I deal with plate tectonics. The recycling of the mobile plates that form the base of the biosphere has many consequences, including the obliteration of impact craters, in contrast to their near-permanence on the Moon. I then consider the extent to which the biosphere can be divided up into areas in which the predominant life-forms are different from each other. The marine component covers about 70% of Earth’s surface and is sometimes referred to as the global ocean, to emphasize its lack of real boundaries. Although the land component is smaller (about 30% of Earth’s surface), this component can be divided into biogeographic realms in which evolution has operated quasi-independently. Finally, I look back at the biosphere’s history, including such phenomena as glaciations, supercontinents, and the Great Oxygenation Event that occurred some 2.5 billion years ago.

Here, I ask the question: how many planets are there with life in the Milky Way? This question has many variants. We can ask about simple microbial life. Alternatively, we may ask about more complex life – for example multicellular animals and plants. Then again, we may ask specifically about intelligent life. An approach that can be used for all of these variants is the one pioneered by the American astronomer Frank Drake. In this chapter, I use the Drake equation to estimate the number of microbial worlds and the number of worlds with animals. (In a later chapter I use the same approach to estimate the number of worlds with intelligent life.) When Drake first devised his equation, we were hard put to come up with meaningful numerical values for any of its parameters. Now we have reasonably good values for at least some of them. Hence our estimates are better than before. However, there are still wide errors, so I investigate the effects of these errors on our estimates. Bearing them in mind, I only attempt estimates to the nearest order of magnitude. These estimates are: 1 billion planets with microbial life; and 10 million planets with animal life.

Here, I outline the nature of the electromagnetic spectrum. I note that various living organisms can detect ultraviolet, visible, and infrared wavelengths, but that none can detect the longer wavelengths of radio. This may be partly due to the fact that there is little to be gained from evolving such an ability and partly because unrealistically large eyes would be required to ‘see’ these long wavelengths. I then turn to human use of radio, which began in the early twentieth century. I consider the question of when the ‘radio age’ started, from the perspective of our transmissions into space – both accidental and deliberate. Leakage of broadcasts could have occurred since the 1920s; messages specifically aimed into space began in the 1960s. When the radio age will end is hard to predict; guessing how long other broadcasting civilizations will last is even harder. However, using both optimistic and pessimistic figures, I use the Drake equation to guestimate how many broadcasting civilizations there are in the Milky Way right now. The result: anything from just one (us) to about a quarter of a million. I end by stressing the difference between radio waves and radio signals.

The biological universe was born more than 10 billion years ago. The vast majority of the life that comprises it is carbon-based; most but not all life-forms are constructed on a cellular basis; on many planets large (multicellular) organisms have evolved; in a tiny proportion of these organisms, intelligence has evolved.

Here, I describe the size and structure of our home galaxy, the Milky Way, and the place within it of our own solar system. The number of stars within the galaxy is somewhere in the range from 100 billion to 500 billion. It now seems likely that the vast majority of these stars have planets. Habitable planets may be commoner in the galaxy’s disc than in its central bulge or outer halo. I use the constellation of Orion as a way to think about how to connect two-dimensional and three-dimensional pictures of the galaxy. Constellations are products of visual astronomy and are patterns in just two dimensions that disappear if we take a 3D perspective. In contrast, the Orion arm of the Milky Way is a real 3D structure whose existence is independent of our vantage point. The stars of the Orion constellation are all within the Orion arm. However, this correspondence is unusual. For example, the stars we see in the Sagittarius constellation are not in the Sagittarius arm. The final section of this chapter deals with the fact that galaxies are not static – they evolve over time. This evolution has consequences for the probability of life originating.

Here, I use the phrase ‘intelligent universe’ to refer to all intelligent entities everywhere. Whether the set of all such entities overlaps with, or is a subset of, the biological universe depends on whether we include artificial intelligence in it. I focus here on biological intelligence. On Earth, evolution to high intelligence has proceeded via a series of milestones. These include: multicellularity, bilaterality, brain, and dexterity. To what extent does evolution towards high intelligence elsewhere proceed via the same milestones? I suggest that similar steps would often be found to characterize evolution on other inhabited planets, providing it can continue for long enough. I put forward the hypothesis that there are at least a trillion radio-level intelligences in the observable universe right now. Then I consider the possible implications of ‘first contact’ between humans and one of them. Such contact could pose a threat for human survival. Finally, I look at home-grown threats, including the fixed mind-sets that underlie religious fundamentalism and science denial. I end by urging a robust defence of both science and humanity against such unthinking views.

Here, I look at the concept of a biosignature. The presence of certain gases in a planet’s atmosphere may represent signatures of life-forms. Oxygen is of particular interest. However, while finding a high concentration of oxygen would be suggestive of life, it would not be conclusive if abiotic means of maintenance could be envisaged. Since studies of exoplanet atmospheres are in their infancy, I start in the better-known realm of our solar system, and look at the atmospheres of planets without life, to see the extent to which these vary. Both Mars and Venus have more than 95% carbon dioxide. In contrast, the only solar-system moon to have an atmosphere – Saturn’s Titan – has more than 95% nitrogen. Mercury has virtually no atmosphere. No body in our system has a significant fraction of oxygen except Earth, with about 20%. I examine the techniques of spectroscopy and show how they allow us to see the signatures of particular gases. Then I mention some recent exoplanetary results – such as detection of atmospheric sodium. Finally, I look at proposed direct-imaging space telescopes, notably NASA’s HabEx and LUVOIR, which, if approved, will be missions of extraordinary importance.

Here, I use a particular type of mental picture to contemplate the possible extent of the biological universe. The idea is that at various levels of spatial scale we can use green to depict entities with life. Such entities could be planets with life, stars that have planetary systems with life, or galaxies that include one or more such planetary systems. Whatever the extent of the biological universe, another question is: what form does it take? A pessimistic answer is that we have no idea because so far we only have a sample size of a single inhabited planet. However, I advocate a more optimistic view by recommending what I call Copernican biology. This is the view that life on Earth is more likely to be ordinary than special. I consider which laws of science should apply to the biological universe and which should not. I then ask to what extent alien life-forms would be expected to resemble terrestrial ones in their construction, at both cellular and morphological levels. I end by outlining a view of the extent and nature of the biological universe that I call the Huge hypothesis.

Here, I deal with the issue of how organisms support themselves against the downward pull of gravity. First, I look at the microscopic skeletons of cells, which help both to maintain the shape of a static cell and to alter that of a mobile one. Then I consider the higher-level skeletons that are needed by large multicellular creatures, whether animals or plants. In the animal kingdom, I focus on the solutions to this issue that we find in the three largest phyla. Vertebrates and arthropods have solved the problem of support in what might be called opposite ways – skeletons on the inside and outside respectively (endo- and exo-skeletons). Molluscs are harder to generalize about. An external shell is the commonest hard structure, but in some groups the shell is reduced and internal. Shells are different from endo- and exo-skeletons in that they have to be carried, so are a mixed blessing from a gravitational perspective. Outside the three main animal groups, other skeletal solutions are found. These range from the hydrostatic skeleton of worms to the woody skeleton of trees. Wood has allowed the evolution of the tallest organisms on the planet – coastal redwoods.

Here, I start by discussing Stephen Jay Gould’s famous thought-experiment of ‘replaying the tape of life’. If we could wind back to the early days of evolution and reboot, would the tape play out in a similar way? Gould thought not, but his hypothesis was untestable since a real version of his thought-experiment is impossible – at least on Earth. However, other inhabited planets represent independent playings of the tape of evolution, and when we can observe enough of these we will know to what extent evolution is repeatable in a broader context than the one that Gould considered. We can hypothesize in this broader context, confident in the knowledge that our hypotheses will ultimately be testable. Plausible hypotheses are: (1) most life is based on carbon (not carbon chauvinism – the assertion that all life must be based on carbon); (2) most life is based on cells; (3) many features of large life-forms will recur often across different inhabited planets, including skeletons and muscles; (4) intelligence will be absent from some inhabited planets, just as it initially was on Earth – where it occurs, it will be the exception rather than the rule, just as it is here.

This is a generalized version of the Terraspermia hypothesis for all inhabited planets. Wherever life has evolved, its origin was on the same planet as its evolution. If successful space travel and planetary invasion by dormant spores occurs anywhere, it is only within very small-scale planetary systems, and even this seems rather unlikely.

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.

Here, I examine whether some of the exoplanets that we have already discovered might be inhabited. However, I start by cautioning against an overly optimistic stance. Although we now know of thousands of exoplanets, and although the Drake equation estimates of Chapter 12 suggest that millions of planets are inhabited, a quick calculation suggests that of the exoplanets discovered so far, only a few are likely to have microbial life and none to have animal life. Against that background, we look at four planetary systems that are reasonably promising. One of these is the Kepler-186 system, where planet f may be habitable. Another is the Alpha Centauri system, where Proxima b may be habitable. A third is TRAPPIST-1, where there are three potentially habitable planets. The final one is Kepler-452, where planet b may be habitable. Whether any of these planets are actually inhabited will only be answered by particular kinds of observation – most likely spectroscopic studies of their atmospheres. How realistic such studies are depends on the distance to the system concerned. The four systems used as examples here range from 4 to nearly 2000 light years – from doable to quasi-impossible.

The ubiquity of gravity and topography, coupled with the presence of surface water, means that the broad range of habitat types on most inhabited worlds will parallel that of the Earth.