It must be emphasized that this is an argument for the plausibility of a hypothesis; not a proof but an appeal for experiment. As Heidmann notes: ‘SETI is, in fact, the only method at our disposal of trying to verify, by observation, that extraterrestrials exist’ (ibid.: 116). The appeal to the vastness of the universe can be self-defeating. There are about 1010 galaxies in the universe. Suppose there are 10,000 million habitable planets, this would be roughly one planet for each galaxy. If this were the case, we would be unlikely to see them or even their parent stars. And if they were very far away, we could only see what they were several million years ago. Even to say that there are several thousand sites where ETI could flourish does not lend encouragement to expectations of an encounter. We need specific directions. If contact is made, it is likely to be due to luck.
Closely related to the appeal to large numbers is the appeal to analogies with the Earth. Given the vastness of the universe and a high probability of numerous Earth-like planets, then inferences drawn from Earth’s natural history can be applied to theories concerning life on other planets. It is not clear as to how much can be inferred from Earth’s natural history. Quite clearly the fact that Earth exists and supports intelligent life is proof that similar environments can exist, but in the absence of further evidence, claims concerning actual probabilities cannot be given much weight. This is not to say that such claims are valueless, or even unscientific, but that they belong to the generative stage of scientific inquiry rather than in the finished scientific report.
A frequent methodological problem in SETI research arises from the misuse of analogy, usually between the Earth and allegedly similar planets. Evidence for the existence of similar planets to Earth might be derived from appeals to the existence of similar G-type suns to our own. Without strong additional evidence such analogies have little probative value. This is not to say that analogy should have no role in scientific reasoning. In fact it has an important generative and heuristic role if properly applied (Lamb, 1991), but an appeal to analogy should not be offered as a conclusive proof. A total rejection of the use of analogical reasoning in science may, however, be a case of cutting off one’s scientific nose to spite one’s philosophical face. The use of what might be called ‘timid’ analogies is to be applauded. These only permit speculation around what is already scientifically possible. Thus, for example, speculation about possible extraterres-trial life might be contained within analogies drawn with sites where there is carbon-based life and liquid water.
A cautious use of analogical reasoning is clearly advised when appeals to the Earth’s natural history are made in SETI research. Earth is not an adequate sample, as one cannot draw a statistical conclusion from a sample of one – although a sample of one is sufficient to refute a claim that something cannot happen. If extraterrestrial life exists, our knowledge of it cannot be based on inferences drawn from the Earth. While studies of the Earth can serve as a useful laboratory model for the development of life, they cannot provide reasons to suppose that other life-forms in the universe exist and will be intelligent. In short,
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there is no logical inference to extraterrestrial intelligence. Its discovery will have to be empirical, not by means of an appeal to logic and large numbers.
So much for the empirical requirements for SETI. But speculation and analogy need not be excluded altogether. One of NASA’s ground rules for SETI is that life is likely to originate on planets orbiting a sun like ours. It is a fact that intelligent life evolved on one planet – the Earth – so the evolution of intelligent life cannot be ruled out a priori. After all, in 1491 most Europeans had no evidence that America existed.
If there is intelligent life elsewhere, the most sensible place to look for it is on ex-solar planets orbiting similar suns to our own. Life on other planets will require two necessary features. First, a stable source of energy from the parent star, a nearby gas giant or an internal heat source; second, plenty of raw materials in usable form. Although calculations abound concerning the possible number of planets like Earth which are capable of supporting life, until recently no planets have been directly observed beyond the solar system. It is widely believed that many stars have planets, but until recently ex-solar planets have proved extremely difficult to find, as they are likely to be distant and have no light of their own.
Ex-solar planets
The existence or non-existence of ex-solar planets is one of the most important issues facing SETI. Recent successes in the detection of ex-solar planets have led to optimistic forecasts. In 1999 Roger Angel at the University of Arizona forecast a ‘revolution in planetary science’ (Adler, 1999: 4). There is no doubt that many theories about the formation and composition of planets will be revised in the wake of new planetary observations. Nevertheless, evidence of ex-solar planets is not itself revolutionary, as most astronomers have believed in their existence for a considerable time. But without ex-solar planets there is little hope of making contact with other life-forms.
According to J.D. Scargle (1988: 79): ‘The search for planets is the first step in the search for extraterrestrial intelligence, as the surface of a planet is probably the only viable location for the origin and evolution of life.’ Since 1995 planetary masses have been detected by various means and we are now on the brink of discovering rocky Earth-type planets.
How common is our solar system? Is it typical of such systems throughout the universe? The answer to these questions depends very much on theories concerning the formation of stars and planets. Most astronomers believe that planets are commonly distributed throughout the galaxy, but until the end of the twentieth century there was no clear observational evidence beyond the solar system. Estimates of the number of possible planets have been based on a combination of intelligent guesswork, theories about the nature and formation of planets, and indirect observations of their alleged gravitational influence on other bodies.
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