The Drake Equation

Is it possible to estimate the number of technological civilizations that might exist in our galaxy? According to radio astronomer Frank Drake, there is a way to do it. While working at the National Radio Astronomy Observatory in Green Bank, West Virginia, Dr. Drake conceived of an approach to define and put limits on the terms involved in estimating the number of technological civilizations among the stars. He presented this method to the scientific community in 1961, and it has since become known as the “Drake Equation”. There is no exact solution to this equation, since we don't have accurate numbers for the terms involved; but it's fun to play with the equation and see what one comes up with.

The Drake Equation consists of seven terms which are all multiplied together to give the answer. It appears as follows; each of the terms is explained below.

Here are the meanings of the terms. We'll put in our own estimates for them along the way.

N is the number of civilizations in the Milky Way Galaxy whose radio emissions are detectable. Since Drake is a radio astronomer, he was particularly interested in searching for extraterrestrial life with his radio telescope. We can assume that it means the number of civilizations which have a technological capability “somewhere” near our own. N is the answer for which the equation is trying to solve.

R* is the rate of formation of stars (per year) with a large enough “habitable zone” and a long enough lifetime to be suitable for the development of intelligent life. Astronomers currently estimate this number to be about 10,000 in our Milky Way galaxy.

f_P is the fraction of stars which have planets. Recently, astronomers have for the first time discovered proof that a number of stars in our visible neighborhood actually do have planets. These findings suggest that the number of planet-bearing stars is higher than was originally believed. Perhaps estimates of 10% to 20% would not be far out of line. Let's say 15%, so f_P would be expressed as 0.15.

n_E is the number of “earths”, or earth-like planets per planetary system. All stars have a “habitable zone” at some distance out from the star, where a planet would be able to maintain a temperature that would allow liquid water to exist. A planet within this habitable zone could have the basic conditions for life (as we know it). In our own system, the number is 1. Let's assume that this could occur in 5% of planetary systems, so we will express n_E as 0.05.

f_L is the fraction of earth-like planets where life actually develops. This term is a big question mark. Given the correct conditions, is the emergence of life inevitable, or is it a very rare phenomenon? Some people believe that life could be “seeded” on suitable planets by outside agencies. However, let's take a middle ground and say that the chances of life arising are 1 in 10, giving f_L = 0.1.

f_I is the fraction of life sites where intelligence develops. Intelligence on Earth took a long time to develop. On other life-bearing planets it could happen faster, or slower, or not at all. However, given already-existing life, intelligence seems to be a pretty powerful survival weapon, which argues in its favor. Let's say half of life-bearing planets will develop some form of intelligence, so f_I = 0.5.

f_C is the fraction of worlds inhabited by intelligent life, on which a technological civilization develops. It's one thing to have intelligent life on a planet, and another matter entirely to have intelligent life that is capable of producing a technological civilization. On our own planet, which has two intelligent species (humans and cetaceans), only one developed technology. Given the large number of paths that intelligent development could take, we might pessimistically assign a value of 0.05, or 5% to f_C.

L is the lifetime (in years) of a technological civilization. Dr. Drake sees this as the length of time that such civilizations could release detectable signals into space, where they could be picked up by his radio telescope. This is another big question mark. What is the tendency of a civilization to eventually destroy itself, or die out from some other cause? Or would it develop to the point where it no longer needed to use the kind of technology that we could relate to? We'll take a plunge here and say that a civilization would stay somewhere near our level of technology for 10,000 years.

Now that we have all our terms for the Drake Equation, let's pull out the old calculator and put them together:

N = R* × f_P × n_E × f_L × f_I × f_C × L
= 10,000 × 0.15 × 0.05 × 0.1 × 0.5 × 0.05 × 10,000
= 1875

This estimates that there are, on the average, 1875 technological civilizations existing in our Milky Way Galaxy at any given time - one of which is our own. Of course, these numbers are from our own estimates. Try supplying your estimates for the various factors, and see what you get! Carl Sagan, in his book “Cosmos”, played with the Drake Equation and came up with the much more pessimistic value of 10.

Perhaps not surprisingly, Dr. Frank Drake is now President of the SETI Institute, which has several ongoing projects, despite current lack of congressional funding, to try to detect evidence of life among the stars.

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