Alien hunters have spent 60 years finding new solutions for the Drake Equation
Astronomer Frank Drake came up with the famous formula as he prepared for a last-minute meeting in 1961. It still guides the search for intelligent life beyond Earth.
Published November 30, 2021
15 min read
SANTA CRUZ, CALIFORNIA“Dad, did you ever imagine that your formula would become so famous?” I ask the kind-eyed, 91-year-old person next to me on the patio.
My dad, Frank Drake, is quiet for a moment. Perhaps he’s thinking back to the November day 60 years ago when he unceremoniously drafted a formula that went on to shape humankind’s hunt for extraterrestrial civilizations. Or maybe he’s listening to the shrieking scrub jays and chattering woodpeckers in the yard, confirmation that noisy lifeforms are thriving in at least one part of the cosmos.
“No,” Dad says after the pause. “I never imagined that it would become of such widespread interest. I also expected that there might be alterations made to it, and that has not happened.”
Now known as the Drake Equation, Dad’s formula provides a framework for scientists looking for intelligent life beyond Earth. By considering a series of variables, the equation allows scientists to estimate the number of detectable alien civilizations that might be scattered across the Milky Way.
The formula has since become one of the most recognizable equations in science. It’s a common tattoo. It’s a beer. It’s written on the side of U-Haul trucks. Its logic has been borrowed and parodied in cartoons about finding a date or calculating the number of credible-seeming alien sightings.
Scores of scientists are still guided by the equation today, and the latest discoveries about other planets both within and beyond our solar system are helping researchers to fill in the variables. It’s a remarkable legacy considering he only wrote the thing down in 1961 when he was strapped for time and needed to organize a meeting.
“It was the best conversation starter, ever,” says astrobiologist David Grinspoon of the Planetary Science Institute. “He was trying to kick off a conversation, and in a way, he did such a good job the conversation is still going on.”
Dreams of alien life
Dad first started wondering whether humans are alone in the cosmos when he was growing up in Chicago in the 1930s and his dad mentioned one day that “there are other worlds out there.”
My grandfather was talking about the other planets in the solar system—at the time, they were the only planets humans knew of—but eight-year-old Frank didn’t know that. To him, “other worlds” meant “other worlds like Earth,” places populated by smart, exotic beings who might be broadcasting their presence to the stars. The idea made sense to Dad, and he began thinking about how to detect such worlds.
“To find the existence of intelligent creatures that are conscious—that would be very exciting,” Dad says now. “I wonder how widespread that situation is, in the universe.”
As a young adult, Dad was one of the few scientists who took the search for intelligent life beyond Earth seriously. In 1960, he got his first chance to search for alien transmissions among the maelstrom of radio waves that crisscross the galaxy. He designed an experiment to detect signals from any civilizations orbiting two nearby sunlike stars, Tau Ceti and Epsilon Eridani. He named the experiment Project Ozma, and for three months, he aimed the Green Bank Observatory’s Tatel Telescope at those two stars. Neither were known to host planets at the time, though worlds orbiting both stars would be discovered nearly a half-century later.
The two stars were quiet, but Project Ozma attracted so much attention that in 1961, the National Academy of Sciences asked Dad to convene a meeting in Green Bank, West Virginia, to discuss the scientific search for extraterrestrial intelligence, or SETI. He could invite anyone he wanted, they said, and organize the meeting however he desired.
Dad, the first person to use a modern radio telescope to search for alien life, needed a way to describe the search to some of the world’s leading scientists—and to justify the endeavor.
The beginning of modern SETI
Dad invited about a dozen people to Green Bank, including astronomer Carl Sagan, who at the time was fresh out of his PhD program; Manhattan Project leader Philip Morrison, who had independently designed an experiment to search for alien life; and the biochemist Melvin Calvin, who was rumored to be on the short list for that year’s Nobel Prize in chemistry.
As West Virginia’s leaves withered and fell in the fall of 1961, Dad realized he had no idea how to organize several days of discussions about a topic that was still on the fringe of science. For a few months, he’d been thinking about the various factors that influence our ability to detect life in the galaxy, starting with the birth rate of the stars around which worlds revolve. He reasoned that each of his factors would provide a rich topic for discussion, so he jotted it all down—and saw that he had created a viable formula, depending, of course, on which numbers you feed into it.
“It’s a great way to organize our ignorance,” says the SETI Institute’s Jill Tarter, a pioneer in the field.
On November 1, as the meeting opened, Dad wrote his formula on a blackboard in the observatory’s lounge, and for the next two days, he and his colleagues discussed each of the variables—although they did take a break during the second day to “get smashed on champagne,” as Sagan recalled, when Calvin won the Nobel Prize for his work on photosynthesis.
The nature of the equation, Dad says, is that everything is equally important. It’s all written in the first power—no exponents, no logarithms, nothing fancy.
He defined the terms like so:
Plug in your values for each term, multiply them together, and you’ll get a number for N: the number of detectable civilizations in the Milky Way. Ask Dad what his value for N is, and he’ll say it changes each time he thinks about it. But in general, he says with a wink, the number is likely between one and a billion, maybe around 10,000.
Even though the equation can be “solved,” it was never meant to provide concrete values like Einstein’s E=mc^2 in the theory of special relativity or Newton’s second law of motion, F=ma.
“People will critique the whole field of SETI by critiquing the equation,” says Jason Wright of Pennsylvania State University. “This is silly. Frank has said he never intended it to be precise. It just gets misunderstood and misused.”
The equation is a thought experiment, a probabilistic argument, and a framework for thinking about life in the cosmos. Wright says that Dad carefully defined his variables so the formula would answer a specific question about detecting radio signals from alien civilizations. And he notes that the equation makes some key assumptions—namely that civilizations stay put rather than leapfrogging across the galaxy.
Over the years, various scientists have proposed missing factors or sought to modify the equation—but Grinspoon says those are really just attempts to slice the variables a bit differently, or to ask a slightly different question than Dad posed 60 years ago.
“I’ve seen a lot of attempts to improve on the Drake Equation, or critique it in various ways, all of which are welcome and interesting, but I haven’t seen any argument or paper that renders it obsolete,” Grinspoon says. “It has stood the test of time. Any attempts to improve upon it are just validating its worth.”
When Dad first devised the formula, he only knew of a rough value for one variable: the rate at which stars are born, which for sunlike stars is roughly one per year. Everything else was a total mystery.
In 1961 there were no known planets outside the solar system, but in the 1990s, astronomers finally observed the first planets orbiting faraway stars. Since then, planet-hunters have spotted thousands of exoplanets in the Milky Way, and using observations amassed over the last decade, NASA’s Kepler mission has revealed that on average, each star hosts at least one planet.
“We went from complete ignorance to really substantial knowledge, as far as planets are concerned, since the equation was posed,” Grinspoon says. “That’s a big shift.”
Those alien star systems may be nothing like our own, with massive worlds snuggled close to their stars, planets orbiting a star’s poles rather than its equator, or an abundance of worlds that come in sizes we simply don’t see orbiting the sun.
But Kepler also revealed, as recently as last year, that potentially habitable worlds are common. There could be as many as 300 million Earth-like worlds in the Milky Way, defined as rocky worlds in temperate orbits around sunlike stars. That number increases if worlds orbiting non-sunlike stars are included, and it gets even larger if “worlds” is broadened to include moons in addition to planets.
Now we also know that about half of the planetary systems orbiting sunlike stars have at least one habitable planet—and that’s a very conservative estimate.
Scientists are quickly chasing down the rest of the variables. NASA’s Perseverance rover is currently looking for signs that life may have once existed on Mars. Soon probes will launch to a handful of icy moons in the outer solar system, where all the ingredients necessary for life as we know it can be found. And scientists are getting ready to peer through the atmospheres of alien worlds to look for molecules that could suggest the presence of alien metabolisms.
Scientists are also looking for signs of alien technology, as Dad first did with Project Ozma. Originally defined in the equation as the fraction of civilizations that develop “communicative technology,” many SETI scientists today consider a broader definition that includes any manifestation of extraterrestrial handiwork, such as radio waves, optical laser beams, or energy-harvesting megastructures.
To reflect this range of possible signals, Tarter coined the term “technosignatures,” and she says such a detection would likely be much less ambiguous than plucking clues from alien atmospheres or searching for fossilized microbes. As Earth’s next generation of sharp-eyed telescopes come online, Tarter and other SETI scientists are hoping to use computer programs to sift through the mountains of incoming data, looking for anything unusual or anomalous—a way to scan for technosignatures that we may not have even imagined.
“We might get totally blindsided by something that turns out to be a serendipitous signal from a different observing program altogether,” Tarter says.
Solving the last few variables—the fraction of worlds with life, intelligence, and technology—will take more than a single detection. As with exoplanets, multiple observations will be needed to reveal how common life is among the galaxy.
“With life we tend to think: Oh man, if we discover it in one place that’s going to be so revolutionary. Of course it will be, but again, if we find it in one place, we haven’t gone from ignorance to substantial knowledge,” Grinspoon says.
“So even if we get a signal next Friday, it’s not like we don’t need the Drake Equation anymore.”
The trickiest variable
Dad has often said that the last variable in the Drake Equation, L, is the most vexing. L is the average length of time that civilizations are detectable—a definition that’s often conflated with survival or extinction, but which isn’t necessarily linked to either.
“It’s unfortunate that people refer to the longevity term as the longevity of a technological civilization. That’s not what it is. It’s the longevity of the emission mechanism,” Tarter says. “We could in fact build something, some sort of technosignature, that has a longevity well beyond that of our civilization.”
Because L is an average, even one incredibly long-lived alien transmission could dramatically change its value—for example, if a civilization found a way to beam its presence into the galaxy for billions of years, perhaps for no other purpose than to help others in the search for cosmic companions.
“Since everything in the equation has equal weight, I knew the answer was going to be only as good as the thing we knew the least about,” Dad once told me. “L is definitely the thing we know the least about.”
Unlike the other variables, the value of L also depends on the detection capabilities of the civilization doing the searching. Humans can search for technosignatures by studying a variety of electromagnetic signals; if a civilization like ours were observing Earth, it would first see the chirps of military radar emitted in the early 1900s. But a civilization with superior detection capabilities could search for finer clues.
Aliens that have figured out how to read the signatures of technology in planetary atmospheres, for example, may have been able to sniff out emissions released during the Industrial Revolution in the mid-1800s—a detectable technosignature if you know what to look for and have the patience to watch as a planet slowly changes. Civilizations with even more advanced technology could quietly watch as life emerges and toddles around on numerous planets, sometimes failing, other times thriving.
Humanity doesn’t yet have that power, and there’s no guarantee that our species will survive for long enough to perfect the art of finding alien life. But one day, if we keep at it, we may finally make contact.
The prospect of superintelligent aliens looking for us is a heady thought to ponder as I gaze into the thick, thousand-year-old redwood trees surrounding my Dad and me in our little patch of California forest. Over a millennium, these trees have quietly witnessed countless lives unfolding beneath them, countless struggles for existence, countless new survival strategies. For long-lived civilizations—the redwoods of the galaxy— the value for L, at least in theory, is enormous.
Advanced human civilization may be just a blip in the age of the universe, but Dad thinks it’s only a matter of time and sufficient will before we find evidence that the galaxy is populated with more brainy beings, in whatever form they might take.
“We don’t know what we’re looking for,” Wright says. “We are our only example of the thing we’re sure we’re looking for.”
If anything, the Drake Equation’s most enduring legacy is not a numerical solution, but a mirror: It asks us to think about Earth, and about humankind, from a cosmic perspective—to consider the fragility of our existence in this galactic sea.
“Its durability, longevity, persistence, the reason it sticks around, the reason we keep using it, the reason it keeps popping up is because it’s such a great guide to the whole problem of life in the universe,” Wright says. “It’s a very versatile equation in that it lets you explore all sorts of aspects of life and humanity, depending on which term you want to grab onto.”