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.

View Images 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 (f p ), the number of planets, per star, that could support life (n e ), the fraction of those planets on which life evolves (f l ), the fraction of life that evolves intelligence (f i ), the fraction of those intelligent civilizations that develop detectable technologies (f c ), 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*f p n e f l f i f c L

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."

View Images 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 f p —the 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 f p is close to one. But how many of those planets are suitable for life?

View Images

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 n e as the number of planets in a system that could support life. But Drake has contemplated tweaking the definition of n e 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. F l , 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 F l .

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.)

View Images 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

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