The Drake Equation

Estimating the Number of Advanced Civilisations in the Milky Way

Jan 10, 2009 Robert Harrand

The Drake Equation is used to estimate the number of intelligent civilisations in the Milky Way galaxy, based upon several estimated terms.

In 1960, Dr. Frank Drake developed an equation for a meeting, where the work of SETI (the Search for Extraterrestrial Intelligence) was to be discussed. The equation, which Drake never intended to be anything more than a useful way of organising the content of the meeting, has gone on to become one of the most well known in popular science.

The Content of the Drake Equation

The Drake equation is made up of 7 terms, all of which are multiplied together to give an estimate for the number of intelligent alien civilisations in the Milky Way galaxy.

The equation is as follows,

N = R x p x e x l x i x c x L

The terms refer to the following,

N = The number of intelligent civilisations,

R = The average rate of star formation,

p = The fraction of developed stars which possess planets,

e = The number of potentially life-supporting planets for a given star,

l = The fraction of these planets that develop life,

i = The fraction of these planets where the life develops intelligence,

c = The fraction of these planets where the intelligent life develops technology,

L = The time during which the intelligent, technological life send signals into space,

The Result of the Drake Equation

In 1960, when the Drake Equation was first used, the numbers were estimated to be the following,

R = 10, p = 0.5, e = 2, l = 1, i = 0.01 (1%), c = 0.01 (1%), and L = 10,000

These numbers give the number of civilisations as 10.

Modern Estimates of the Drake Equation

Over the past 5 decades, the ability to estimate the numbers contained within the Drake equation has vastly increased. This is due mainly to advances in technology, such as the Hubble space telescope, radio astronomy, and computer models of star development.

Today, the number N is estimated to be lower, and is based upon the following numbers,

R = 7, p = 0.5, e = 2, l = 0.33, i = 0.01 (1%), c = 0.01 (1%), and L = 10,000,

Giving N = 2.3.

The Consequences of the Drake Equation

The scarcity of intelligent life in the Universe could be due to any one of the terms contained within the equation. For example, it may be that simple life is common, but the development of intelligence is rare. Or it could be that the development of planets which are suitable for the development of life is rare. If, however, all terms relating to the development of intelligent life are reasonably large, then the sad conclusion is that the term 'L' is small, i.e. such civilisations tend to rapidly destroy themselves.

Many factors in the Drake equation are unknown, but the low results answer the question of why no signs of intelligent life have been detected outside of the Earth. The answer is simply that, if they exist, intelligent life is too far away to be contacted. If the modern estimate is correct, then there may be just 2 civilisations in the Milky Way galaxy, Humanity, and 1 other.

This underscores the rarity and fragility of life, and emphasises the need to make every effort to protect the Earth and it's civilisation.

References

The copyright of the article The Drake Equation in Scientific Inquiry is owned by Robert Harrand. Permission to republish The Drake Equation in print or online must be granted by the author in writing.
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