National Geographic News
A photo of Frank Drake.

Astronomer and astrophysicist Frank Drake is most notable as one of the pioneers in the search for extraterrestrial intelligence.

PHOTOGRAPH BY MARK THIESSEN, NATIONAL GEOGRAPHIC CREATIVE

Nadia Drake

for National Geographic

Published June 30, 2014

A few days before Halloween in 1961, a young astronomer was mulling over a fairly serious problem.

Soon the astronomer, Frank Drake, would be convening a meeting at the National Radio Astronomy Observatory in Green Bank, West Virginia, to discuss what was still a fringe, eyebrow-raising topic: the search for intelligent extraterrestrial life. Drake had invited everyone he could think of with an interest in the scientific search for E.T.—all 12 of them—to the meeting.

It promised to be a great gathering, hot on the heels of Project Ozma, the media sensation of 1960 that had looked for radio signals around two nearby stars.

Problem was, the meeting's scientific agenda was in disarray. Drake, who was 31, had been busy acting as a one-man organizing and hospitality committee, and had been distracted by meeting logistics. One attendee, UC Berkeley biochemist Melvin Calvin, was rumored to be on the short list for the Nobel Prize in chemistry, which would be announced during the conference. So Drake had spent a chunk of the previous few days solving the pressing problem of where to buy champagne (which would clearly be needed if Calvin won) in an otherwise dry county.

A day before the attendees were to arrive, Drake sketched out a way to focus the scientific discussion on the likelihood of detecting alien civilizations in the Milky Way galaxy. He used the term N to describe the number of those worlds.

The Drake equation.
The Drake equation, formulated in 1961, estimates the number of alien civilizations we could detect. Recent discoveries of ­numerous planets in the Milky Way have raised the odds.

Then he wrote down seven factors that were relevant to N: the rate of sunlike star formation in the Milky Way (which Drake called R*), the fraction of those stars that have planets (fp), the number of planets, per star, that could support life (ne), the fraction of those planets on which life evolves (fl), the fraction of life that evolves intelligence (fi), the fraction of those intelligent civilizations that develop detectable technologies (fc), and the average amount of time those civilizations are detectable (L).

If I plug in numbers and multiply the terms together, Drake reasoned, it should give me the value of N. (Never mind that at the time, the only factor with a reasonably well known value was R*.)

Great, he thought. That should do it.

On November 1, Drake kicked off the Green Bank conference by scribbling his equation on a chalkboard in the observatory's conference room:

N = R*fpneflfifcL

He couldn't know that what he'd just written would not only serve as a lauded framework for a meeting of brilliant minds (Calvin did end up winning the Nobel, by the way) but would also continue to be known, a half century later, as the Drake equation.

"There have been a few books written about important equations in the history of science, and it's usually included there," says Drake, now 84, who's also my dad. "Which always amazes me."

A photo of Frank Drake with colleagues at National Radio Astronomy Observatory’s 300 Foot Telescope in 1962.
Frank Drake (right) and colleagues visit the National Radio Astronomy Observatory's 300-foot telescope in 1962.
PHOTOGRAPH BY NRAO/AUI/NSF

A Multitude of Planets

More than 50 years after it was written, the Drake equation still guides ways of thinking about how to find E.T. As the years have passed and instruments sharpened, astronomers have started to refine and fill in numbers for the equation's variables. But the variables themselves have stayed the same. My dad is repeatedly asked whether any factors are missing, he tells me, but "as far I know, they're not." He says that even when suggested missing factors seem "reasonable," they can already be found in one of the seven factors he came up with in 1961.

In the years since, though, the value of R* has changed—from an early, pre-1961 estimate of maybe one or two sunlike stars per year to as many as five or ten stars per year. This is in part because astronomers no longer count only sunlike stars. Smaller, redder, and cooler stars known as M-dwarfs have emerged in the past decade as being potential hosts for life-bearing planets.

"We have to include the M-dwarfs," Drake says. "They do have planets, and they do have them in places where the temperature is suitable for life." They're also the most common type of star in the galaxy.

The value of fpthe fraction of stars with planets—was completely unknown in 1961. "There was no data on that back then. They'd seen no planets at all outside of our solar system," says Steve Dick, astrobiology chair at the U.S. Library of Congress and former chief historian at NASA. "That meeting at Green Bank was the first meeting of its kind. It was a very daring thing to do."

Now, after many thousands of hours spent searching the skies for planets outside the solar system, and only two decades after the first exoplanets were found, we know that basically every star has planets. In other words, the value of fp is close to one. But how many of those planets are suitable for life?

Exoplanet searches are getting closer to determining the frequency of Earthlike planets. One recent estimate, based on data produced by NASA's Kepler spacecraft, suggests that around 20 percent of sunlike stars have at least one Earth-size, habitable planet. But the habitable zone is slippery and hard to define, and it's too soon to say whether Earthlike planets are as common as we suspect.

The Drake equation originally defined the term ne as the number of planets in a system that could support life. But Drake has contemplated tweaking the definition of ne to use the words "objects" or "bodies" rather than "planets." Scientists think the "bodies" in our solar system best suited for (potential) life are three planets (Venus, Earth, and Mars) and three moons: Jupiter's satellite Europa, with its deep, ice-capped ocean, and two moons of Saturn, the oily Titan and its geyser-spewing sibling Enceladus. (Read about the possibility of discovering life on Europa and beyond in this month's cover story in National Geographic.)

If there's one thing we've learned about life on Earth, it's that organisms keep showing up in surprising places. In the driest of deserts, buried beneath Antarctic ice, or at the extreme depths of the ocean—it's hard to find a place where life hasn't gained a foothold. "Life is much more robust that we used to think," Steve Dick says.

But scientists still don't know how life got started on Earth and whether similar processes are common in the cosmos. Fl, the fraction of potentially life-supporting worlds where life has actually evolved, is still an open question.

In the coming decades, as scientists continue to peer more closely at the exo-Earths in the galaxy and try to sniff out the signatures of life in exo-atmospheres, they'll eventually inch toward filling in the value of Fl.

But the real point of all this calculating, of course, is to find planets or satellites where the conditions are ripe not just for the evolution of extraterrestrial microbes but for the evolution of life as intelligent as ourselves—or more so.

"As I look back over the last 50 years, I think there was initially a sense, especially among astronomers, that once you have life, it will almost inevitably go on to become intelligent," says Doug Vakoch of the SETI (Search for Extra Terrestrial Intelligence) Institute in Mountain View, Calif. "And as we really take into account the vicissitudes of evolution, that's not at all obvious."

The last terms in the equation, those framing the grandest question of whether humans are alone in their conscious curiosity, will be impossible to define until we detect extraterrestrial intelligence itself. Until we hear those alien murmurs, all we can do is estimate the value of N by plugging in the numbers we know and making educated guesses about the numbers we don't.

It's this kind of guesswork that tends to inflame the Drake equation's critics, those who complain that the equation isn't predictive, is too open-ended, and doesn't provide any answers. But "predictive" isn't really what Drake ever intended.

"It's a way to frame the problem," says MIT astrophysicist Sara Seager, about the equation. "In science, you always need an equation—but this isn't one you're going to solve. It just helps you dissect everything."

Seager has written her own version of the Drake equation and applied it to a different astrobiological question. Using the same framework, the Seager equation estimates how many alien, breathing biospheres might be detectable using telescopes set to fly in this decade. (Best guess? Not many—unless we're really lucky, Seager says.)

A photo of the Arecibo Observatory in Puerto Rico.
Arecibo Observatory in Puerto Rico is one of the world's largest single-dish radio telescopes.
PHOTOGRAPH BY STEPHEN ALVAREZ, NATIONAL GEOGRAPHIC CREATIVE

The Future: Speaking Loudly to the Stars

Folded into Seager's equation is one of the least-known caveats of the Drake equation: Ultimately, the answer depends on the technological capability of the civilization doing the searching. In Drake's version, that limit is hiding in an unexpected place: the last variable, L. This most beastly of variables, the one we cannot know until we find E.T., is the average length of time for which alien civilizations are detectable.

This span of time depends not only on the noisiness of alien technology (in other words, how easy a civilization is to eavesdrop on) but also on the sensitivity of the technology we're using to search for our cosmic cousins. Even if all the other variables are the same, "another civilization, with a different sensitivity, will end up with a different N," Frank Drake says.

As communication technologies become more efficient, Earth is going quiet. The planet is leaking fewer strong, detectable radio signals into space. For a civilization with the same detection capabilities as ours, Earth might only be detectable for somewhere on the order of a century. But civilizations with vastly more powerful detectors will be able to spot our yammerings far longer; Earth's contribution to L, from their perspective, is larger.

Thirteen years after the Green Bank conference, my father mounted an effort to provide the cosmos with deliberate signs of humanity's presence and make Earth easier to find for alien SETI programs. This led, in 1974, to the creation of a message that Drake designed and broadcast from the Arecibo Observatory in Puerto Rico. It included information about chemical elements, the structure of DNA, and Earth's address in the galaxy. Flying through space at the speed of light, the message should be detectable by a civilization with an Arecibo-like receiver.

What if we again started intentionally sending signals into the cosmos?

What if other civilizations are already doing the same, and altruistically beaming their presence into the galaxy for the purpose of helping those with a shared curiosity?

Those radio beacons in the cosmos, the noisy worlds intentionally talking to the stars, would be a boon to SETI searchers. And because of the way the Drake equation's math works, those chattering worlds can boost by the value of N by a lot.

"As we move forward with SETI, it's important to keep open the possibility of active SETI, of humankind deciding to take the initiative to transmit," Vakoch says. "In the early days of SETI we always assumed it would be the extraterrestrials who would take the initiative."

If we were to send another message and it was received, we may still never hear that alien "Hello," hailing us back from across a sea of stars. But I like knowing that my dad's brave, early treks off the beaten path have helped guide a new field of inquiry, that his ideas have challenged generations of scientists to stare at the stars with open minds. After all, the only way to discover another planet full of curious beings asking the same questions he asked is to stay curious ourselves, and try and find them.

Join us today at our Life Beyond Earth event, which will celebrate the achievements of the Cassini-Huygens spacecraft as it enters its tenth year in orbit, with a panel of four leading space experts moderated by Jamie Shreeve, National Geographic's executive editor for science. The discussion will be live streamed starting at 7:30pm ET and will highlight the themes of our July cover story, "Life Beyond Earth": NatGeoSpace.com

Be part of the conversation on Twitter by following #OurUniverse for live updates.

34 comments
Alison Bellinger
Alison Bellinger

I am grateful to this person and others who have sparked a search in other minds and souls.


We broadcast into space all the time, however, so I wouldn't focus too much on "what to say" or how to do it. We've been observed for a long time. We're hardly discreet.


David Chang points out in a comment below that it would be inconsistent to think all ETs out there are friendly. Yes.. what about those who may be resource explorers who just came across this gem of a planet in the Universe?


From "Facing a Greater Community of Life" via Marshall Vian Summers: "This will become a dominating focus and concern, both for average people in the world everywhere and for nations and governments as well. However, to meet these immense and unprecedented challenges, you will need Knowledge and wisdom from beyond the world. Here it is not technology you need as much as it is the will and the commitment to unite for the preservation of this world as a habitable environment for the human family and to cease your endless conflicts to prepare to engage with intelligent races in the universe, many of whom will not be here to support you."


kaya lou
kaya lou

If human are real, Alien are also real! visit andromeda-lady.blogspot.com

ioan jones
ioan jones

ET exist. Space is infinite and we're sitting on a planet surrounded by an endless number of other planets. It's a no brainer!

Shawn Riddick
Shawn Riddick

In 1997 our government asked me to sign a secrecy agreement with a writ of order attached for my immediate execution if I violated the conditions. Aliens are well known to Armed Forces Command and many scientists. What purpose does science serve when scientists are forced to sign secrecy agreements? Our atmosphere can be damaged to the point of no return and catastrophic failure will occur ending most life on earth. warningfromgod dot. Thank you.

Chris Johnson
Chris Johnson

Something that is rarely considered is the impact we might already be having on those hapless civilizations within our 50ly sphere....

What are the religious pundits of Epsilon Eridani VI going to make of The Munsters?


David Chang
David Chang

Active SETI seems like a bad idea all around - you're placing a bet on the existence of friendly ET's while trusting the fate of humanity to the non-existence of neutral-to-unfriendly ET's. It doesn't seem intellectually consistent to believe in the possibility of ETI without also believing in the possibility of malicious ETI.


Neutral ETI could be just as devastating to humanity as anything else. Humans developing a subdivision don't intend to kill any insects or small animals, but it happens anyways. Yog-Sothoth style "Cosmic Horrors" similarly don't intend to kill or drive insane a vast # of humans, it's just a byproduct of their presence.


Assuming that an ETI has the technology to actually interact with humanity (not just send radio signals that take hundreds of years to reply to), there's an overwhelming chance that the ETI is more technologically advanced than humans. They may also be more socially and intellectually advanced, possibly to the point where the ETI refuses to acknowledge humanity as an intelligent life-form.


If that were to happen, ET-human interactions wouldn't be like Europeans vs Africans, it would be more like humans vs passenger pigeons. The higher life form feels a little bit guilty, the lower life form is extinct.

Todd Carnes
Todd Carnes

And when another civilization hears our message and they turn out to be the Borg, then what? It's not necessarily a smart idea to send out a "Here I am" broadcast like that. There's no reason at all to assume the answer we receive will be "friendly".

Paul Kuhlken
Paul Kuhlken

I was 5 years old in 1961, living with my family in the "Nut Bin" a few hundred yards from the 85' telescope used during the early phases of Project Ozma, in Green Bank, WV.  My Dad, William Kuhlken, worked as an electronic technician on the telescope's receivers and amplifiers.  I have always been awed with that and throughly enjoyed growing up in that area and having the NRAO as my playground.  Truly, a wonderful time.  I have been a fan of Frank Drake.  


Nadia, you have a right to be proud of him and what has been accomplished since that day in that boardroom, which, by the way, was one of the coolest places to hang out in when no one was around. 

Dave Keeler
Dave Keeler

42. Isn't that N? 


Also, I'm thinking of the proverbial dog chasing a car: What's he going to do once he catches one?

Richard Alexander
Richard Alexander

I am of the opinion that the likelihood of each term in the Drake Equation approximately decreases exponentially as one proceeds from left to right across the equation. Until we find a means for life to arise spontaneously, the most sensible assumption would be that none of the conditions we have observed anywhere in the cosmos would result in it. I should note here, though, that life need not evolve on a planet for it to be detectable there; after all, our signals on Mars would be detectable, too, even though we aren't native to that planet. 


I believe the most level-headed assumption is that Earth contains the only biological life in the Universe, until some specific alternate cause can be demonstrated. 

loring chien
loring chien

I was at Cornell during 1970-1975. I remember taking Astronomy 102 and the buzz that was going on about the Astronomy dept and Drakes equation. There was a lot of high powered astronomy faculty then... Drake, Sagan, THomas Gold, off the top of my head.

Fred Wilcox
Fred Wilcox

A couple of personal comments.  First... I was Frank Drake's paperboy for a year or two.  I vividly remember the house on Pine Tree Road and the Porsche he drove.  Second, I was smart enough to take Astronomy 102 in Spring 1973 taught by Frank Drake and Carl Sagan.  One of the best college courses during my undergraduate years at Cornell.

Jason Pastrick
Jason Pastrick

Drake's equation is more than a landmark; it was the birth of what will someday be recognized as a vitally important and monumentally rewarding field of human research and endeavor.  Very few scientific discoveries will rank alongside the first discovery of ETI. But I like to tell skeptics that there's an much more straightforward equation; Astronomers estimate that there are between 100 billion and 300 billion stars in the Milky Way galaxy; we know that at least one (the Sun, of course) hosts what we consider to be a technological civilization. Therefore, the straight odds AGAINST us being alone are between 100 billion to 300 billion to one... 

John Houston
John Houston

Well a SF writer I´m sorry I can´t recall once said the silence of the cosmos might have an different explanation, it´s not emptyness,it´s the fear of the hunted.


Like 11 th century settlements, afraid of the viking raiders, avoiding to let seafarers know what lies inland, planetary civilzaitions might be not broadcasting their presence in order to avoid being raided.


Of course this is a "Star Wars" approach to the SETI issue, but let´s hope there is no truth at all in it.

Larry Butler
Larry Butler

Hmm, we started seriously radiating radio power ABOVE what the ionosphere will stop reflecting back about 1945, radars and FM radio.  Lower frequencies bounce off it so good I can say one word into my 1000 watt ham radio station and hear my echo bouncing up and down between Earth and these charged regions high above us sometimes more than once.  Gigawatt directional radars would make the trip, briefly, but not far.  Since 1945 any signals have traveled 69 light years, so far. Not many star systems are that close.  Epsilon Eridani is 10th on the list and the first one with planets at 10.5 LY distant.  They might have seen Hitler's VHF television transmitter but, probably not as it wasn't very powerful.  If the waves depended upon human fantasy, instead of this damned Einsteinian Physics, it would be much easier.


The other problem is signal to noise ratio.  The noise is the Big Bang and it will be quite strong, forever.  The sun is another horribly noisy source, too close for good listening.  Any signals, ours or theirs has to cross the receiver STRONGER than these noises and other space noises to be heard.  It takes an immensely sensitive receiver with a huge antenna.  We have enough trouble getting Direct or Dish TV from a powerful satellite transmitter only 30,000  miles away.  "Why is that?", I hope you ask.


Anything that radiates from the old incandescent light bulb in the barn to the best, most powerful transmitter on earth suffers from "Inverse square law".  Each time the distance between the source (bulb) and receiver (eye?) doubles, the signal level is 1/2squared or 1/4 what it was.  10X as far, 1/100th as much.  Imagine how small the signal will be if it starts out at 24,000,000 watts, 24 megawatts from a UHF TV transmitter by the time it gets to Epsilon Eridani, 10.5 light years away.  There's no signal left to detect!


Dreaming is great.  Reality is kind of a slap in the face.....damned physics...



A J
A J

I was somewhat inspired by the Drake equation to earn my own doctorate understanding the history of life. In my estimation the equation is good. It's the application of it that is often flawed.


There are many things on this planet, contingent things, that led to the ability for us to be able to live a modern, technological society. Some of these are somewhat unique to human beings (evolutionary), some are possibly somewhat unique to Earth (geologically and biologically).


Take, for example, the evolution of wood. Wood provided a major stepping stone in the evolution of energy use on the planet. Wood was turned into charcoal, and hotter burning charcoal allowed the development of smelting. Smelting led to tools that allowed us to acquire coal, which burned even hotter than charcoal and thus had more uses. And so on. History is incredibly contingent.


Of course, there are other paths toward technological modernization, of which we can't conceive. Applications of the Drake equation often assume a near inevitability of some path.


Even this article fails at all to discuss the real importance of Fi and Fc. These very well may be the most important parameters in the equation, given that our knowledge of space actually has given us some evidence of the ubiquity of life-supporting planets. Drake, himself, and many after him, assumed that the Fi and Fc could be very near to 1, and thus irrelevant in an equation of multiplied probabilities.


http://en.wikipedia.org/wiki/Drake_equation#Estimates


I would argue that we aren't really much closer to values for Fi and Fc. But based on what we do know they could be closer to 10^-5, 10^-10, or even 10^-100, than to 1 or 0.1. Under "current estimates" the Wikipedia article goes into some discussion of the why this may be.


Most importantly, for humanity, is that the relative popularity of astrophysics and space in the common imagination has led many people to the mistaken assumption that space colonization, alien visitors, and a future of humanity among the stars is inevitable.


It may very well be the case that all of those things are exceedingly unlikely, that we are extremely lucky to be the universe made aware of itself, and that we need to shape up and take care of the one planet we're ever likely to know.

Conwaythe Contaminationist
Conwaythe Contaminationist

My wager is that the closest intelligent extraterrestrial life is at least 30,000 light years distant. 

Eduardo Santilli
Eduardo Santilli

Your father´s equation was really a vital input to the human´s knowledge of Astronomy.

Congratulations to him!

And what pride for you have had such genious as a father!

Brian Edwards
Brian Edwards

I have thought about this a bit since I first heard about the equation in the original Cosmos series and recently I wondered about adding some variables to work out the likelihood of finding relict technology from an alien civilisation in our neighbourhood and reverse engineering it.

Tracy Crook
Tracy Crook

True I think. From what I can tell, and at the risk of sounding 'religious', the chance of biological life evolving from natural causes is virtually zero, given the 13.8 billion years available for it to happen.

Bart Blommaerts
Bart Blommaerts

@Richard Alexander Microbe fossils have already been found in Mars rocks, and Microbial life is reported several times to have traveled on meteors. So your level-headed assumption was debunked even before u made it. And adds to the assumption that life is resilient more abundant in the universe then you think. 

mr peabody
mr peabody

@Richard Alexander even if we assume each and every of the variables in the drake equation are very close to zero, the virtually unfathomable size and stellar population of the known universe is so staggering that there would still be a high probability of millions or billions of highly intelligent alien populations existing. there is no need, use, or wisdom in assuming there is nothing like the human race alive on other planets

Tom Sosnovec
Tom Sosnovec

I too was a student in the mid 90's in Santa Cruz.  He took me to SETI to meet his colleagues; such an amazing thing for him to do since I was really just a common undergrad at the time.  He mailed me a term paper he had graded late the summer the movie independence day came out with a short apology: he was late getting his grading done because he was approached by the government with questions about how we might defend ourselves from an invading alien fleet(!)  He told his classes about it, we all had a laugh.  I remember him walking into lecture just a little bit late one day I think in '96; he wrote four names on the board and told us it was a big day- those were the names of the first 4 extra-solar planets just discovered.  When talking with him in his office after hours he always made a point of introducing me using my last name to every one of his colleges as if I was as important as they were. Frank is not only an inspirational scientist and thinker, but a fine and considerate man.

Bart Blommaerts
Bart Blommaerts

@John Houston Still worth the risk i say. Malevolent or not, contact with an alien species will force humanity to throw out the ancient views.

mr peabody
mr peabody

@John Houston i once read a very technical and informed article that explained why we earthlings will for better or worse probably never detect alien signals even if there are billions of inhabited planets. to sum it up and omit the stats and calculations that were used to convincingly back that contention up, the universe is too big, and the lifetime of such civilizations too finite for our signals to ever reach one another and be detected intact

mr peabody
mr peabody

@A J and im not sure if drake's equation addresses such things as a moon that stabilizes its parent planet's spin and magnetic field, and tectonic plates giving rise to continental drift which i have read is potentially crucial for life as we know if for stated reasons i dont recall but made lots of sense, but i too feel there is a multitude of very chance factors that ultimately make up a more fully ornate reckoning of whether life, and intelligent life at that will arise. in addition to coal, i also have to wonder what if there were no oil reserves on earth. even as evolved as we are, would we have ever gone to the moon if we hadnt been able to tap into the for all intents limitless and practically free power of concentrated 200 million year old sunlight?

Adam Hickenbotham
Adam Hickenbotham

@Bart Blommaerts No microbe fossils have been found on Mars and no life is reported to have traveled on meteors.  Your confusing TV with real life.  Go back and read credible scientific journals.


Tracy Crook
Tracy Crook

What ancient views remain to be thrown out? The current default position of most scientists is that life is virtually inevitable on those suitable planets. The real question is, what do we do if we eventually find out that we ARE alone in the universe? Do we revive those 'ancient views'?

Tracy Crook
Tracy Crook

Not that I am a biochemist but being somewhat obsessed with determining the value of Fi for many years, the factors you cite and many others point to a value of zero. Which in turn makes N = zero as well.

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