The Meaning of the Drake Equation

A comic pertaining to our search for extraterrestrial life, courtesy of xkcd. It brings up a fair point about our methods of listening for interstellar communication.

The Drake equation was originally created by (and is named after) astronomer Frank Drake. Given the values of a number of variables, his equation may be used to estimate the number of species in the galaxy capable of interstellar communication. Drake’s proposed equation is as follows:

N = R * fp * ne * fl * fi * fc * L

N will be the number of species we could possibly communicate with in the galaxy
R is the number of new stars forming in our galaxy per year, on average
fp is the fraction of those stars which have planets
ne is the average number of planets per star which could potentially develop life
fl is the fraction of these planets that actually will develop life
fi is the fraction of these life-hosting planets that will eventually have intelligent life
fc is the fraction of these intelligent species that will eventually have the technology for interstellar communication
L is the amount of time for which this civilization will be detectible from the outside

By multiplying these variables together, it is possible to come up with an estimate for the number of communicable species within our galaxy. In 1961, Drake estimated using his own equation and a range of reasonable values that there are likely somewhere between 1,000 and 100,000,000 communicable species in our own galaxy–assuming of course that only one inhabits a planet; if two or more intelligent species share a planet, the number would be higher. The lowest defensible estimates according to the Rare Earth hypothesis suggest we are alone in the galaxy, offering a value of 8×10^-20 communicable species. Contrast that with the more liberal proposed estimate of 36.4 million species, and it is easy to see why some may consider this equation scientifically untrustworthy.

The current most plausible estimate based on what we know so far is as follows:

R = about 7, according to NASA

fp = almost 1, as the vast majority of stars seem to have at least one orbital planet

ne = 0.34 +/- 0.14, according to survey data of ~150,000 stars by the Kepler mission. For this calculation, we’ll go with the maximum and minimum values.

fl = 0.13, according to extensive study (geological, etc.) done by the Australian Centre for Astrobiology.

fi = There are two schools of thought on this one, some favoring a value almost 1, while others believe it to be almost zero. We’ll use 0.99 and 0.01 for our range.

fc = almost 1, as if species develop radio-based technology it is bound to escape the planet at some point and become detectible. Whether or not species will intentionally communicate is another debate entirely. That is likely to be determined by social beliefs, etc.

L = 420, according to a survey of about 60 human civilizations throughout our history. This would likely change based on the species, and is largely connected to the idea that intelligent species may be doomed to destroy themselves–this assumes that all intelligent species are, like humans, war-like in nature.

The final result for N is a range from as few as 0.7644 (rounded up to one, since humans are confirmed to exist) to as many as 182 intelligent, communicable species alive right now.

Very few of these variables are known–or can even be guessed with any measure of accuracy–so it is inadvisable to take the numbers produced at face value; instead, the purpose of this equation is to provoke discussion about the possibility of the existence of life beyond Earth. If a world has life, how likely is it to ever become intelligent? How do we define “intelligence”–is mere sentience enough, must they exhibit problem-solving abilities, or do they even need social capacities?

This also leads to the idea of the Fermi paradox which observes that even if there is evidence suggesting the existence of intelligent extraterrestrial life, we still have neither seen nor heard anything from another communicating species. Possible explanations for this paradox could be that there simply are no other civilizations (Rare Earth hypothesis); that they once existed but destroyed themselves through war, overpopulation, pollution, etc.; that we lack the technology or ingenuity to detect their communication methods; or that for a variety of reasons they have chosen not to communicate with us at all. Other less-likely explanations have also been offered, such as the possibility that communication actually has been received but the information is being withheld by our governments, or even that extraterrestrials are currently on Earth undetected.