Given the current state of play in the search for life beyond the Earth, where as yet we have no conclusive evidence, it might seem inappropriate to discuss ‘common misunderstandings’. But it’s not. There can be misunderstandings about the way we search for extraterrestrial life, as well as in relation to the scientific basis for our search, and it is these that I focus on here, rather than misunderstandings about extraterrestrial life itself. I discuss them below in the order in which they’re first encountered in the book.

I introduced exoplanets – planets beyond our solar system – in Chapter 1, discussed them in the context of planetary systems in Chapter 2, and considered them in relation to the concept of habitability from a general perspective in Chapter 4. But so far I’ve given little detail about them. How many exoplanets, or exoplanetary systems, have I mentioned so far? Very few. That’s about to change, but not in the sense of replacing a dearth of detail with a wealth of it. Rather, I’ll be very selective about the particular exoplanets I discuss. This is essential, given that the number now known is huge, and most of them are irrelevant to the search for alien life.

This is where we switch from the geography of alien life to its biology – in other words from its distribution across the observable universe to its ‘nature’ in many senses of that word, including its chemical composition, its physical form, its means of acquiring energy, and, in some cases, its intelligence. For me, the nature of life beyond Earth is even more interesting than exactly which planetary bodies it inhabits, and many other scientists feel likewise. However, in moving from geography to biology things also become more controversial, because the so-called ‘sample size of one’ problem comes into sharp focus.

While we wait for our first conclusive evidence of life beyond Earth, we can contemplate its possible nature. In particular, we can ask the following question. To what extent should we expect evolution elsewhere to take a similar course to the one it has taken on Earth? That could be described as the key question about the biology, as opposed to the geography, of extraterrestrial life. But the way I’ve just put it isn’t ideal – it’s too centred on our home planet as a reference point. Let’s try to rephrase it in a Copernican manner, so that Earth doesn’t occupy a special place. Here’s one such rephrased version. To what extent does evolution follow similar courses on different inhabited planets? Earth is implicit here, but just as one of many inhabited planets, and almost certainly not the first one.

To look for life in the universe beyond Earth, we need to understand what is meant by ‘universe’, just as we need to understand what is meant by ‘life’. In the end, we can probably ignore most of the universe and focus our search in some very specific places. However, those places are best understood against a backcloth of what can be called – it’s an understatement really – the big picture.

The thought of there being millions of planets with life in the observable universe is inspirational. But it’s only that – a thought. Or perhaps a bit more than that – a thought with probability on its side. But the gap between probability and certainty is a huge one. We won’t really feel the presence of extraterrestrial life until we know for sure that it’s there. So we need evidence. I started the book with a look at a paper that focused on the need for a cool assessment of evidence and the importance of not jumping to conclusions. In this chapter we’ll return to that issue.

With the exception of planets orbiting the most massive and luminous stars, planetary lifespans are measured in billions of years. Evolution on Earth has taken about four billion years so far, and probably has about another two or three billion to run, depending on when our ever-brightening Sun eventually boils away all our surface water. In the absence of evidence to the contrary, it’s probably a good idea to assume that evolution elsewhere takes billions of years too. It’s hard to imagine an evolutionary process in which intelligence is an early result rather than a late one. So, to look for intelligent alien life, we need to concentrate on planets that aren’t too young. Earlier, I suggested that good yardsticks for planetary age when looking for photosynthetic or intelligent life were at least two and four billion years, respectively. In general, we can imagine at least four stages in the life of a planet – no life at all, chemosynthetic life only, a stage characterized by a mixture of forms of energy acquisition including photosynthetic life, and a final stage that also includes intelligent life. In the present chapter, we’re concerned with the final one.

For many millennia, humans have gazed up in wonder at the night-time sky. The full panoply of the Milky Way is an awesome sight. The scale of space is immense. Is there life out there somewhere? If so, where, and what form does it take? In the space of a couple of sentences, we’ve already gone from generalized wonder to specific questions. The next step is from questions to hypotheses, or, in other words, proposed answers. Here are two such hypotheses that I’ll flesh out as the book progresses: first, life exists on trillions of planets in the universe; second, it usually follows evolutionary pathways that are broadly similar to – though different in detail from – those taken on Earth.

I started the book by considering the possibility that our generation may be the one to discover the first persuasive evidence of extraterrestrial life. Suppose this turns out to be true, and humanity’s first such evidence arrives in a decade or so. What would the possible impacts be? We can deal with them under four headings: scientific, social, religious, and philosophical, with all of these being defined very broadly.

People involved in the modern era of the search for life – from the early days of SETI in the 1960s to the present – have tended to think of some planets as being potentially ‘habitable’ (or ‘inhabitable’, though that synonym is rarely used) and others not. For example, in our own system, Mars might once have been habitable, but Jupiter never so. Why this apparent certainty about Jupiter’s unsuitability for life? There are two main reasons.