Space: the final frontier. These are the voyages of the Starship Enter-prise. Her five-year mission: to explore strange new worlds, to seek out new life and new civilizations, to boldly go where no man has gone before.

(Star Trek, beginning 8 September 1966)



Theories regarding extraterrestrial intelligent life-forms must be supported by evidence of potentially habitable sites, such as stable planets in a moderate temperature zone. This chapter surveys contemporary claims concerning potentially habitable planets, theories of planetary formation and methods of observing ex-solar planets. The investigation then focuses on the development of life, stressing an important distinction between conditions which may support the origins of life and conditions which life can tolerate, reorganize or adapt. Given the fact that there are probably many more sites where life can survive than  those where life can be produced from scratch, the thesis that life on Earth is the result of seeding is defended as a more plausible account of the development of life on Earth. Arguments concerning the contingency of life and the tenacity of life are assessed and the emergence of intelligent life and exotic life-forms is also considered.


The cosmological principle

Appeals to a high probability of intelligent life elsewhere in the galaxy provide SETI research with its primary rationale. The appeal to large numbers, the vastness of the universe, frequently referred to as the cosmological principle, features strongly as a factor in SETI’s plausibility. Ben Bova, a science fiction writer, SETI enthusiast and former advisor to President Ronald Reagan  on the ‘Star Wars’ project, expresses this belief: ‘It is difficult to believe that around all those billions of billions of stars we are the only living creatures, the only intelligence’ (Bova, 1990: 8). Jill Tarter taps the richness of the SETI programme













with an appeal to large numbers: ‘we have explored so little of the Universe and the space around us that we cannot guarantee that “they” are not here’ (1991: 185).

We have already argued that the appeal to the vastness of the  universe is a good reason for optimistic expectations in favour of pluralistic hypotheses, and provides a good reason for conducting a search. But while such appeals may support a campaign for exploration, by themselves they carry no probative weight. The error in drawing definite conclusions from appeals to large numbers can be exposed with reference to our friends, the monkeys in the British Museum who have been set the task of randomly typing out the complete works of Shakespeare, given enough time and suitable word-processors. According to the  mathematician, Frank Cousins (1972: 263), if we allow reasonable assumptions about typing speed the chance of an occasional Hamlet appearing is once in 10460,000 seconds, which is effectively never. Michael Crowe (1988: 553) illustrates the problem as follows:

Consider a universe containing a billion galaxies, each made up of a billion stars, around each of which a hundred planets revolve. Place a billion monkeys on each of these planets and set them typing for fifteen billion years [the approximate age of the universe]. This would produce only 1046 seconds of typing time towards the 10460,000 seconds needed.

The intuitive strength of the appeal to an infinite amount of time and space lies in our familiarity with practical tasks. Richard Dawkins (1988: 139) appeals to the large number argument when formulating theories on the origin and evolution of life: ‘Given infinite time or infinite opportunity,’ he says, ‘anything is possible.’ But this claim is soon qualified with reference to what is, or is not, physically possible. Given enough time and a willing supply of driving instructors many an incompetent driver will pass the driving test. One might also predict that given time, the manufacturers of sun cream will provide a totally effective screen against UV radiation. But not even in an infinite amount of time will they provide a barrier that will enable people to take a holiday on the Sun. Appeals to infinity are obviously restricted, but they can indicate what is plausible within a particular theoretical domain. This point is made forcibly by Jean Heidmann who asks:

How can we reject the idea that in billions of years, in billions of galaxies, each containing billions of stars, physical evolutionary processes may have produced more advanced results than those found in our small globe toward the end of what we insignificantly call our 20th century?

(1997: 115)




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