Frank Drake is an American astronomer who was one of the first to seriously examine the question, "Is there intelligent life elsewhere in the galaxy?"
Drake was born in 1930 and began wondering about the possibility of extra-terrestrial civilisations as a child after learning that our Sun is one of billions of stars in the Milky Way galaxy.
While organising a conference to push for the scientific search for ET in 1961, Drake realised that each of the important issues to be discussed could be expressed as a number. This became the famous Drake Equation, which can be used to estimate the number of detectable civilisations in our galaxy:
N = R* p ne l i c L (R* - The Average Rate of Star Formation)
For there to be a civilisation, there needs to be a star for the planets to orbit. Drake estimated that on average one new star formed in the Milky Way every year. Latest estimates are that between 0.7 and 1.45 solar masses of material form new stars each year and the average star is 0.5 of a solar mass. This gives us between 1.5 - 3 new stars being formed each year.
p - The Fraction of Those Stars that Have Planets
This was long before scientists such as Tamworth's own Dr Stephen Kane confirmed that exoplanets do exist. Drake's original estimate was that perhaps 20% to 50% of stars have a planetary system. We now know that the fraction of stars that have orbiting planets is very close to 100%.
NE - The Average Number of Planets that Can Support Life
This is difficult to estimate. Some stars are too volatile, giving off frequent bursts of radiation. The planets have to be in the "Goldilocks" zone, not too hot, not too cold, but just the right distance from the star to allow liquid water. The planets also need to contain the right heavy elements to allow life to form. Drake's estimate was between 3-5 planets per planetary system. Modern estimates are lower, assuming only 10% - 40% of systems contain a single planet capable of supporting life.
L - Fraction of Planets that Can Support Life that Do Develop Life
Life began on Earth quite early, leading us to perhaps think that if a planet can support life, then the development of life may be inevitable. However a sample size of one is not very large. A counterargument is that life appears to have started on Earth only once (all living things share at least some DNA. If we were to detect even microscopic life elsewhere in our Solar System (Mars, Europa, Titan and Enceladus being the best candidates) then this would lend weight to the theory that life is ubiquitous. Drake's estimate for this number was 100% and this remains the most common assumption.
I - The Proportion of Planets that Have Life Where Intelligent Life Develops
Using Earth as an example, one would be tempted to say that once life develops, intelligent life is inevitable. The counterargument is that of all the billions of species on Earth, only one branch (humanity) developed intelligence, and that only one intelligent species (homo sapiens) has survived. Drake's estimate was 100%. Modern estimates vary between virtually 0 and 100%.
C - The Fraction of Civilisations that Develop Technology that Can Be Detected
Earth has sent very few deliberate signals into space, and none of those we have sent have reached their destination. We have also attached plaques and records to the Pioneer and Voyager probes, but the chances of them ever being discovered are vanishingly small. We have sent plenty of accidental signals into space via TV and Radio broadcasts since the 1920s, however, these are very weak and would only be able to be discovered within a radius of one light year. The nearest star is four light years away. Also humanity is quickly moving away from technology that broadcasts electromagnetic waves into space. Drake estimated that between 10-20% of planets would develop technology that could be detected.
L - Lifetime of a Civilisation
How long does a civilisation last? Do they use their technology to save or destroy? Can any civilisation become so advanced that it becomes indestructible? Drake's estimate was between 1,000 and 100,000,000 years. Modern estimates range from 300 years to 10 billion years.
Drake and his colleagues estimated there would be between 20 and 50 million civilisations in the galaxy at any one time. Modern estimates tend to be lower, between 0 and 15 million. So where are they? Despite all the efforts so far and advances in detection of exoplanets, no evidence has been uncovered that points to life, intelligent or otherwise, anywhere else in the galaxy. I will leave the final word to Monty Python, "pray that there's intelligent life somewhere out in space, 'cause there's bugger all down here on Earth."