A typical galaxy contains some hundred billion stars, most of which are not unlike our own Sun. The largest telescopes can photograph some billions of galaxies. The observable Universe therefore contains some 1020 stars! How likely is it that the Sun is the only one among these which is orbited by a life-bearing planet? This fundamental question is not one that can easily be answered. The only life-forms we know of are here on Earth, and we are only
just now beginning to understand terrestrial life; the genetic code was cracked only twenty-five years ago. Given only a shallow understanding of life on Earth, one might well wonder whether discussion of hypothetical extraterrestrial life-forms is any more than a speculative exercise. Even more so, one might well ask whether life on Earth has any relevance to possible extraterrestrial life-forms. It is clear that if we are to make any progress towards answering the question ‘are we alone?’ scientifically, we must, at the outset, base our discussion on facts deduced by observing the terrestrial environment. We are, of course, immediately guilty of terrestrial chauvinism, but anything else would belong firmly in the domain of science fiction. The fundamental question should therefore be asked in a different, more scientific way: what does modern science have to say about the possibility of life similar to our own existing elsewhere than on Earth ?

There are several facets to the problem. There is the astronomical problem of deciding what kind of stars are likely to be accompanied by a retinue of planets. There is the biological problem of finding a suitable definition of life and evaluating the circumstances under which life may evolve. Then there are the problems of interstellar communication: if there is intelligent life somewhere else, should we try and let it know of our existence ? Such life-forms may be trying to communicate their existence to others: should we search for their signals and if so, how ? To answer each of these questions scientifically we must inevitably fall foul of terrestrial prejudices simply because life on Earth is the only kind we know. The way we are going to approach the problem is as follows. We shall examine the evolution of a star and discuss whether or not we should expect a planetary system to form. We shall then look at the formation of the planets and ask ourselves how likely it is that planets similar to the Earth are to be found orbiting stars like the Sun. Strong arguments can be presented suggesting that terrestrial planets are not uncommon. So encouraged we shall follow the various stages of the development of life on Earth, as we understand it, and try to gauge how likely it is that a similar sequence of events has occurred on another Earth-like planet. Then we may make the final extrapolation, and speculate about the emergence of intelligent life. To do that requires some understanding of what has happened on —Earth: what distinguishes Man from other terrestrial life-forms? This is a highly Earth-orientated picture, but at least it minimizes the speculative aspect of the subject. Speculation is, however, one of the most exciting, and occasionally fruitful aspects of scientific research. We should not ignore the possibility that totally dis¬similar life-forms exist elsewhere in the Universe.

Since we are going to adopt such an Earth-biased basis for discussion, it is important to lay bare the prejudices that underly the whole approach. There are two basic sources of bias. First we live on a planet m an almost circular orbit around a rather typical G star. The almost constant distance to the Sun has ensured a temperature range in which the water that is essential to terrestrial life is a liquid. Second, all organic compounds are based on the element carbon. We could certainly imagine life developing on a planet orbiting a smaller star than our Sun, an M-dwarf, for example. The temperature range would be right for liquid water if the planet were nearer the star than the Earth is to the Sun. On the other hand, it is often argued that because stars more massive than the Sun have shorter lifetimes, life is less likely to develop on any planets they may have. This argument rests on the assumption that life takes billions of years to evolve. But what about the liquid water requirement? There is clearly a very strong terrestrial bias on this point. Crystalline or gaseous life-forms are by no means in¬conceivable, and indeed the pages of science fiction abound with such ideas. What is perhaps more relevant, and less terrestrially biased, is the need for a particular range of temperatures. Life must depend on complicated chemical reactions, otherwise such a sophisticated level of organization could not be achieved. At too high temperatures, complex molecules cannot survive, whilst at very low temperatures, chemical reactions proceed relatively more slowly. Of course, the carbon-based organic chemistry underlies all such arguments, yet it is not inconceivable that somewhere life may be based on a nitrogen or silicon organic chemistry. Our present knowledge of chemistry is inadequate to provide a firm basis for ruling out such a scheme, so we must simply admit that it is yet another possibility.

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