but a more detailed discussion of its component theories of planetary formation, the evolution of life and intelligence, will be covered in the next chapter.


The Drake equation

The Drake equation was first presented by Frank Drake at a meeting of scientists gathered to discuss ‘Intelligent Extraterrestrial Life’ at the Green Bank Observatory under the auspices of the Space Science Board of the US National Academy of Science in November 1961. The equation has consequently become a SETI creed, its logical justification. Reflecting on the equation some thirty years later, Drake said that its basic premise ‘is that what happens here will happen with a large fraction of the stars as they are created, one after another, in the Milky Way Galaxy and other galaxies’ (Drake, 1990: 151). This is the equation:

N = R* × f p × ne × f l × f i × f c × L


It is not a definitive equation, not is it inviolable like E = MC2. Its components can be broken down as follows:

N = the number of technically advanced civilizations in the galaxy that are currently capable of communicating with other solar systems.

R* = the number of new stars formed in the galaxy each year.

f p = the fraction of those stars that have planetary systems.

ne  =  the average number of planets in each such system that can support life.  

f l = the fraction of such planets on which life actually exists.

f i = the fraction of life-sustaining planets on which intelligent life evolves.

f c = the fraction of intelligent life-bearing planets on which beings develop the means and the will to communicate over interstellar distances.

L = the average lifetime of such a technological civilization.


R*: the rate of star formation

There are held to be over 40 billion stars in the Milky Way Galaxy with enough loose gas and dust for millions more. Estimates put the rate of star formation at ten each year. This, however, is a theory-laden guess in that any attempt to assess the rate of star formation must be derived from a theory as to how stars are formed. The dominant theory is the Big Bang theory, the moment when the universe began. According to this theory the first stage was when the elements flew off in rotating clouds. Then they cooled and formed galaxies. The early stars were massive combinations of hydrogen and helium, and when they exploded heavy elements were created and flung about the universe. Eventually other generations of stars condensed from this matter, leaving their rocky substances that formed planets in their orbiting paths. There are countless of billions of stars in over a billion galaxies. A star’s colour is a clue to its surface









temperature and the scale for classifying stars is indicated by the letters O B A F G K M, running from the hot blue stars to the cool red ones. A star’s life-span is measured in millions of years. Our Sun, for example, is intermediate and is classified as a G type. It has existed for about 5 billion years and supports life on Earth. It is likely to survive for another 8 billion years. Similar stars to our Sun can be found in the F to K groups. Whether they can support solar systems similar to our own is a matter for speculation. Application of the Goldilocks criterion eliminates the remaining groups as either too hot or too cold. It is estimated that about 25 per cent of the total number of stars are found in the intermediate F–K groups. From this it is concluded that a  quarter of all the stars in the heavens are capable of supporting planetary systems with life.

The problem with this kind of reasoning is obvious. The figure of 25 per cent is only a theoretical possibility, not an inductive probability. The data available from astronomers concerning the existence of planetary systems are far too limited for the assigning of inductive probabilities. The potential for confusion between theoretical possibility and inductive probability re-emerges throughout the whole debate on ET life.


f p: the fraction of stars with planets

Excluded here are classes of stars unlikely to have satellites, such as some, but not all, of the double stars and stars with short life-spans. This, of course, does not provide any degree of certainty concerning the numbers of planets to be found in the vicinity of other types of stars. It is a theory-laden estimate, based on theories of planetary formation and indirect observation of the alleged effects of planets on their parent stars. When Drake formulated his equation no planets had been directly observed outside of the solar system, but during the past few years there have been over thirty claims identified as planets.

The belief that planetary systems are the rule rather than the exception is supported by current astronomical theory. Estimates concerning the number of stars with planets will vary according to which theory of planetary formation is adopted. The Catastrophe theory of the nineteenth and early twentieth centuries held that planets were the product of the debris from either collisions or the explosions of stars. Given the vast distances between stars and their durability this would suggest that planetary formation was infrequent. Catastrophe theory thus dramatically reduces the theoretical scope for f p. However, during the past thirty to forty years the nebular theory of Descartes, Kant and Laplace has re-emerged. The current interpretation of this theory maintains that planets are the offshoots of the large rotating clouds of dust, out of which the stars were formed. Accordingly, planets are very likely companions of stars as the theory maintains that they are products of the same creative forces. This is described as accretion by impact. Dust grains stick together, welding on impact. Once the planetesimals are large enough, their gravity will attract other particles. The most prevalent model suggests two things: first, that planetary systems are




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