Is there anyone out there? Drake’s Equation gives a clue
Life beyond our berth in space is a matter of conjecture . . . perhaps even a numbers game
Plaque with the Drake Equation, at the National Radio Astronomy Observatory, in West Virginia, US.
The Drake Equation is a formula for the number of developed civilisations in our galaxy, the Milky Way. This number is determined by seven factors. Some are known with good accuracy but the values of most are quite uncertain. It is a simple equation comprising seven terms multiplied together (see illustration).
There are no exponentials, no power laws and no logarithms; the equation is linear in each of its factors. All terms are equally important, and the overall uncertainty is dominated by the most uncertain terms.
The equation was formulated by astronomer Frank Drake to serve as a framework for a conference at the National Radio Astronomy Observatory, Green Bank, West Virginia in 1961. This meeting was to investigate the possibility of detecting intelligent life in the Milky Way.
The equation had terms representing the key quantities that would need to be determined in order to estimate the likelihood of life on other planets. It was intended as an instrument to facilitate discussion, but has proven to be very durable. It is a valuable teaching aid and is found in many university textbooks on astronomy.
Each of the seven terms in the equation was estimated at the conference. The first term is the rate at which new stars are formed in the galaxy. It has a value of about 20 per year. Next is the fraction of stars with planets. At the time of the conference, this was completely unknown.
We have now found evidence of thousands of stars with their own solar systems and we estimate about half of all stars have planets. But could they sustain life? Only if they are in a favourable region known as the Goldilocks Zone – not too hot and not too cold. This is determined by the third term.
The next two terms specify the proportion of these planets that contain life and the fraction of these in which there is intelligence. The sixth term represents the fraction of intelligent civilisations that are technologically advanced and that could communicate with us.
The last term measures the average longevity of an advanced civilisation. This is a crucial factor, and it could vary from a century to billions of years. Multiplying all seven terms together we get N, the number of detectable civilisations in our galaxy.
Since the formulation of the Drake Equation about 60 years ago, our knowledge has advanced greatly, and many of the factors determining N are now much better known. The recent discovery of thousands of planetary systems was a major scientific advance. Our understanding of biology suggests that, wherever conditions are right, life will emerge, so it must be abundant in the cosmos.
The Drake Equation does not quantify precisely the extent of extra-terrestrial life in the Milky Way; it is essentially probabilistic in nature. It has been a great aid in guiding discussions and has served as a stimulus to research. The search for exoplanets – those around stars other than our Sun – continues and there is now a vibrant field of investigation called astrobiology.
We have no idea when life outside Earth will be found but the ultimate success of the quest is virtually certain. When we do detect intelligent life elsewhere, it will almost certainly be far in advance of our own civilisation. Let’s hope that, when we get what we wish for, we will not regret it.