The Drake Equation

Last Updated 12/10/11 16:29


Devised by Frank Drake in 1961, The Drake Equation provides a way to determine how many intelligent and communicating civilizations are present in our galaxy.

The Drake Equation is not “an end in itself” but provides a number of questions which need answering in order to reach an accurate conclusion – as we learn more from such subjects as astronomy, biology etc…, our ability to answer the questions will be enhanced.


The Drake Equation is:

N = N* fp ne fl fi fc fL



N* represents the number of stars in the Milky Way Galaxy

Question: How many stars are in the Milky Way Galaxy?

Answer: Current estimates are 100 billion.

fp is the fraction of stars that have planets around them

Question: What percentage of stars have planetary systems?

Answer: Current estimates range from 20% to 50%.

ne is the number of planets per star that are capable of sustaining life

Question: For each star that does have a planetary system, how many planets are capable of sustaining life?

Answer: Current estimates range from 1 to 5.

fl is the fraction of planets in ne where life evolves

Question: On what percentage of the planets that are capable of sustaining life does life actually evolve?

Answer: Current estimates range from 100% (where life can evolve it will) down to close to 0%.

fi is the fraction of fl where intelligent life evolves

Question: On the planets where life does evolve, what percentage evolves intelligent life?

Answer: Estimates range from 100% (intelligence is such a survival advantage that it will certainly evolve) down to near 0%.

fc is the fraction of fi that communicate

Question: What percentage of intelligent races have the means and the desire to communicate?

Answer: 10% to 20%

fL is fraction of the planet's life during which the communicating civilizations live

Question: For each civilization that does communicate, for what fraction of the planet's life does the civilization survive?

Answer: This is the toughest of the questions. If we take Earth as an example, the expected lifetime of our Sun and the Earth is roughly 10 billion years. So far we've been communicating with radio waves for less than 100 years. How long will our civilization survive? Will we destroy ourselves in a few years like some predict or will we overcome our problems and survive for millennia? If we were destroyed tomorrow the answer to this question would be 1/100,000,000th. If we survive for 10,000 years the answer will be 1/1,000,000th.

When all of these variables are multiplied together when come up with:

N, the number of communicating civilizations in the galaxy.




Frank Drakea brief biography.


Born in Chicago on May 28th, 1930 to Richard and Winifred Drake and raised in Chicago's South Shore with sister, Alma, and brother, Robert, Frank Drake became interested in science at an early age.

He and his friends passed their time experimenting with motors, radios, and chemistry sets. He was very interested in astronomy and he soon began to consider the actual size of the universe and the possibility of the existence of other planets and life on other planets. This seemed quite possible to him but because of the religious convictions of his parents and teachers he had little chance to discuss the subject of extraterrestrial life.

After high school Drake enrolled at Cornell on an ROTC scholarship to study electronics. It was here that he fell in love with astronomy and finally found someone else who was considering the possibilities of life on other planets.

In 1951, during his junior year he attended a lecture by Otto Struve, one of the world's preeminent astrophysicists. Struve showed that there was mounting evidence that planetary systems had most likely formed around half of the stars in the galaxy, stating that life could certainly exist on some of those planets. Drake realised that he had found someone who shared his ideas.

Drake then served for three years with the Navy to repay his scholarship and became the electronics officer on the USS Albany where he became highly skilled operating and fixing the latest high tech electronic equipment.

Following his stint in the navy, Drake attended Harvard graduate school to study optical astronomy, the only summer position available being in radio astronomy. His electronics experience in the Navy made him a “good man for the job” as the radio astronomy equipment was constantly in need of tweaking and repair. Drake became totally fascinated with radio astronomy.

After graduate school in 1958 he got a position at the newly founded National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia and in 1960 the first search took place. This search was called “Project Ozma” by Drake,comprising of a two week observation of the stars Tau Ceti and Epsilon Eridani. During the search a false alarm, which turned out to be a terrestrial signal, caused some excitement but other than that no signals were detected. Drake was not discouraged.

In 1961 Drake and J. Peter Pearman, an officer on the Space Science Board of the National Academy of Sciences, organized the first SETI conference. The meeting, held at the NRAO, comprised of a dozen or so scientists who were interested in SETI and lasted 3 days.

 Drake came up with the now famous Drake Equation whilst preparing for this conference. Frank Drake was primarily interested in the detection of planetary systems and life, particularly intelligent life elsewhere in the universe and he participated in Project Phoenix, the most powerful system for searching for radio signals from other civilizations, employing radio spectrometers that can monitor up to 15 million frequency bands simultaneously, on two polarizations, as well as special signal detection instruments and software to search for a variety of possible intelligent signal forms in the captured data.

Being portable, the system can be transported to the world's largest radio telescopes, all over the world and is being used to search for intelligent signals from some 1,000 of the nearest solar-type stars and from star-rich regions of the Milky Way..

Frank Drake designs large radio telescopes for use in radio astronomy and in the search for extraterrestrial intelligent life. The ultimate aim is to achieve very large energy-collecting areas at very low cost, based on the design principles of the Arecibo 305-meter radio telescope. He is also involved in the theory of optimum strategies for the detection of rare objects and transient events in the cosmos. He has explored the possible observational effects in natural and technologically produced radiation caused by gravitational lensing by stars and other discrete objects. These have suggested that such gravitational lenses might be used in some circumstances to great benefit in detecting other planetary systems or intelligent radio transmissions.



 Alien expert illuminates audience on extraterrestrial possibilities. Leif Griffin


The following article was published in “The Wisconsin Technology Network” 3rd March, 2004


MADISON - Some of extraterrestrial expert Frank Drake’s beliefs may leave people bewildered. Speaking to an audience of 300 at the University of Wisconsin-Madison Feb. 19, Drake, director of the Search for Extraterrestrial Intelligence Institute in Mountain View, Calif., explained the reasons why he believes intelligent life exists in the universe and highlighted potential methods of communicating with far-off civilizations.

“I think all of us who are well-read at all, or know anything about the universe, have at some time realized that it could well be that there are other intelligent beings in space,” Drake said. “Many of them could be more advanced, more sophisticated and with more of a history. It would be fascinating just to find out what they are like.”

He emphasized the importance of persistence and repetition in the search for extraterrestrial life.

“Detecting alien life is like winning the lottery, and to win any lottery we have to make many, many bets,” Drake said.

Luckily, Drake asserted, there are also many, many solar systems that are that are capable of incubating civilizations. Of the estimated 400 billion stars in the Milky Way, 10 percent are similar to our solar system and are able to support life, Drake said. Furthermore, it is only certain that 40 billion stars are unable to support life due to their relatively brief existence and rapid resource exhaustion. According to Drake, the remaining 80 percent of stars in the Milky Way are smaller than our sun but may prove just as useful in the quest for life.

“For a while scientists thought [the less massive stars] were not suitable for life, but now it appears they too could be abodes for life, and even perhaps better ones than the stars that are like the sun,” he said.

The potential provided by myriad life-supporting stars bodes well for the discovery of intelligent life and Drake discussed methods of communication required to detect extraterrestrial activity.

“We have the means, if we work very hard, to detect intelligent life, and get access to all of the wondrous things they can provide us with,” Drake said.

Utilizing rockets to make contact with extraterrestrial life would be costly and a doomed venture because of the distance, Drake said, so the best method of detecting alien civilizations is through electromagnetic messaging. According to Drake, it is possible to send a 60-second telegram at the speed of light to the far side of the galaxy for only a dollar’s worth of energy. The conventional method of powering a space shuttle to complete the same task would take around 500,000 year’s worth of energy.

Drake also briefly discussed his famous equation, Drake’s Equation, and its implications. Drake created the seven-factor equation in 1951 to estimate the number of alien civilizations in the Milky Way. Using those factors, which account for the rate of star formation and civilization life span, Drake estimated there are at least 10,000 such civilizations in the Milky Way.


Phillip Morrison, emeritus professor at the Massachusetts Institute of Technology, wrote of Drake’s Equation: “Next to Einstein’s equation, it is the most recognized equation in science.”  



 Chances of aliens finding Earth disappearing


The following article was published in the “New Scientist”, 9th August, 2004

A pioneer of the search for extraterrestrial intelligence (SETI) has warned that for any intelligent aliens trying to search for us, "the Earth is going to disappear" very soon.

Frank Drake's point, made at a SETI workshop at Harvard University on Friday, is that television services are increasingly being delivered by technologies that do not leak radio frequencies into space.

But he added that in some ways the observation is good news for SETI, as it means that the failure of Earth-based observers to detect aliens so far may be less worrisome than it would otherwise seem.

Most SETI efforts have focused on detecting radio signals that might be emitted by intelligent beings on planets around nearby stars. For humans, such signals "are the strongest signs of our existence", Drake said, thanks to television.

Traditional television broadcast antennas put out one megawatt each, and this radio-wave bubble now extends about 50 light years out from the solar system.

Straight down
But that is changing fast, Drake says. More and more television is now delivered by cable, with no radio-frequency leakage to space, and by direct-broadcast satellites that put out just 20 watts per channel, all efficiently directed straight down the intended areas on the Earth's surface.

So from the point of view of being detected through such inadvertent broadcasts, the longevity of humanity's detectability may be just 100 years.

And longevity may be the most important figure in Drake's famous equation for estimating the number of detectable intelligent civilisations on other worlds. The best estimates show that all the other crucial factors nearly cancel out, so that the number of such civilisations in our Milky Way galaxy is roughly equal to their average longevity of detectability in years.

Laser beacon
Drake's insight has important implications for search strategies. It means that eavesdropping on unintended alien transmissions is unlikely to succeed and "argues for an emphasis on detecting beacons", i.e. signals intentionally sent our way.

Some SETI strategies have already begun shifting toward that approach, including efforts to find optical beacons based on high-powered lasers deliberately aimed at nearby stars.

While optical communications across interstellar distances was initially thought impractical, military research has led to lasers sufficiently powerful to make such signalling much more efficient than any radio beacon.

Nuclear-powered lasers on the drawing boards could produce pulses that would outshine the sun by a factor of 10,000, said Harvard University physicist Paul Horowitz, who has already been searching for such pulses. He has designed a new telescope that will soon be dedicated full-time to that search.

And other innovative ideas keep coming along. Planet hunter Geoffrey Marcy of the University of California, Berkeley, said someday we may learn to use the sun itself as a gravitational-lens telescope, with a detector parked at its focal length of 500 astronomical units.


SETI and the Cosmic Quarantine Hypothesis