The Seti Institute » The Drake Equation

The Green Bank meeting was the first gathering to use the formula that came to be known as the "Drake Equation.". When Frank Drake (whose picture is shown on the right above) came up with this formula, he had no notion that it would be used by SETI theorists for decades to come. In fact, he thought of it as an organizational tool - a way to order the different issues to be discussed at the Green Bank conference, and bring them to bear on the central question of intelligent life in the universe. The Drake equation was closely inspired by the famous remark of Enrico Fermi, “where is everybody?”, known as the Fermi Paradox. Drake thought nit possible that a large number of extraterrestrial civilizations would form, but that the lack of evidence of such civilizations suggests that technological civilizations tend to disappear rather quickly. This theory often stimulates an interest in identifying and publicizing ways in which humanity could destroy itself, and then counters with hopes of avoiding such destruction and eventually becoming a space-faring species. Perhaps it is very rare for intelligent life to arise, or the lifetime of such civilizations may be relatively short.

The question of the number of communicating civilizations in our galaxy could, in Drake's view, be reduced to seven smaller issues. His equation suggests that


N = the number of civilisations in our Galaxy with which communication might be possible, and
R* = the average rate of star formation per year
fp = the fraction of those stars that have are at the centre of planetary systems
ne = the average number of planets that can potentially support life in a star that has planets
f = the fraction of the above that actually go on to develop life at some point
fi = the fraction of the above that actually go on to develop intelligent life
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space, and
L = the length of time such civilizations release detectable signals into space.

Because it is the of great interest to the SETI community, many additional factors and modifications of the Drake equation have been proposed. These include the number of times a civilisation might re-appear on the same planet, the number of nearby star systems that might be colonised and form sites of their own, and other factors. It has been proposed that the Drake Equation should include additional effects of alien civilisations colonising other star systems.

» Addition of a reappearance number to the Drake equation «
The Drake equation may furthermore be multiplied by how many times an intelligent civilization may occur on planets where it has happened once. Even if an intelligent civilization reaches the end of its lifetime after, for example, 10,000 years, life may still prevail on the planet for billions of years, allowing for the next civilization to evolve. Thus, several civilizations may come and go during the lifespan of one and the same planet. This raises the interesting conjecture that if our species died out another type of animal – bear,dog,orang-outang for example may develop in intelligence and fill the ecological niche left vacant by us and set up a technological civilisation. The most intelligent of all next to human with large frontal lobes is a species of dolphin but it is difficult to see how they could develop a technology in the sea instead of dry land.

Ray Goodwin

Somewhere there are mountains
Glistening in the snow
Somewhere there are mountains
That we shall never know

Somewhere there are rivers
Flowing fast and free
Somewhere there are rivers
That we can never see

Somewhere there are oceans
And sun drenched island sands
Forests full of creatures
In vastly distant lands

Somewhere there’s a planet
Beneath an alien star
The people watch our tiny sun
And wonder where we are

One day perhaps we’ll find them
Across the void of space
Perhaps through ways as yet unknown
We’ll meet them face to face

The author of this web site Ray Goodwin holds B.Sc. Degrees from London University in Chemistry, Geology and Physiology and an M.Sc. in Biochemistry. He has spent most of his professional life teaching in Colleges of Technology. On his retirement he has entered the fields of astronomy, astrochemistry, astrobiology and space sciences. He has spent a great deal of his retirement in visiting amateur astronomy societies and in attending European Space Agency Symposia in ESTEC in the Netherlands and other scientific conferences in England and Sweden. He regularly attends the yearly European Astrofest in South Kensington London and other meetings in the UK. He has written scientific articles and given a number of lectures on diverse scientific subjects.

Readers of this web site are invited to e-mail the author ( and discuss their opinions of the topics dealt with and suggest any changes which they think may be helpful.

Life in the Cosmos Website
Version 01.00 - April 20, 2015.