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Here on Earth we live on a planet that is in orbit around the Sun. The Sun itself is a star that is on fire and will someday burn up, leaving our solar system uninhabitable. Therefore we must build a bridge to the stars, because as far as we know, we are the only sentient creatures in the entire universe. . . . We must not fail in this obligation we have to keep alive the only meaningful life we know of.

–WERNHER VON BRAUN,
ARCHITECT OF NASA’S APOLLO PROGRAM,
AS RECALLED BY TOM WOLFE

This story properly begins 4.6 billion years ago, with the birth of our solar system from a cloud of cold hydrogen and dust several light-years wide. The cloud was but a wisp from a much larger mass of primordial gas, a stellar nursery manufacturing massive stars destined to explode as supernovae. One by one, the giant stars popped off like firecrackers, ejecting heavy elements that sizzled with radioactivity as they rode shock waves through the murk like so much scattered confetti. One of those enriching shock waves may have compressed the cloud, our cloud, in its passage. The cloud became dense enough for gravity to seize control, and it collapsed in on itself. Most of its material fell to its center to form a hot, simmering protostar. Eventually, the protostar gained enough mass to kindle a thermonuclear fire at its core, and the Sun began to shine. What was left of the cloud settled around the newborn star in a turbulent, spinning disk of incandescent vapor.

Microscopic grains of metal, rock, ice, and tar rained out from the whirling disk as it slowly cooled. The grains swirled through the disk for millennia, occasionally colliding, sometimes sticking together, gradually glomming into ever-larger objects. First came millimeter-scale beads, then centimeter-scale pebbles, then meter-scale boulders, and finally kilometer-scale orbiting mountains called “planetesimals.” The planetesimals continued to collide, forming larger masses of ice, rock, and metal that grew with each impact. Within a million years, the planetesimals had grown into hundreds of Moon-size embryos, protoplanets that through violent collisions grew larger still, until they became full-fledged worlds.

After perhaps one hundred million years of further collisions, the embryos in the inner solar system had combined to make Earth and the other rocky planets. The inner worlds were likely bone-dry, their water and other volatiles blowtorched away by the intense light of the newborn Sun. In the outer solar system, freezing temperatures locked the volatiles in ice. The ices provided more-solid construction material, allowing the cores of Jupiter and the other outer planets to rapidly form and sweep up lingering gas within the disk in only a few million years. As they grew, the giant planets created zones of instability where embryos could not assemble, leaving behind pockets of primordial planetesimals and bands of shattered rock and metal. These remnants are the asteroids. The giant planets also catapulted many icy planetesimals far out into the solar hinterlands, to drift in the dark out beyond the orbit of present-day Pluto. When jostled by perturbing planets, galactic tides, or close-passing stars, those icy outcasts fall back toward the Sun as comets.

Finally, sometime between 3.8 and 4 billion years ago, a complex, chaotic, hazily understood series of gravitational interactions between the giant planets stirred up most of the outer solar system, sending barrages of asteroids and comets hurtling sunward to pound the dry, rocky inner worlds. This event is called the “Late Heavy Bombardment,” and was the last gasp of planet formation. We observe its effects in the cratered surface of the Moon, and also in the rain that has eroded its geographic scars from our own planet—much of Earth’s water seems to have arrived during the Bombardment, express-delivered from the outer solar system. Afterward, Earth’s crust had partially melted, and its original atmosphere had been mostly swept away. But as those first torrential rains fell from the steam-filled sky, our planet gained the gift of oceans. Slowly, the Earth cooled, and gas-belching volcanoes gradually replenished the atmosphere. Soon, perhaps uniquely of all the new-formed worlds of the solar system, ours would somehow come alive.

Slightly less than four billion years later, I was four years old, standing with my mother, father, and sister in our backyard in Jasper, Alabama. It was January 1986, shortly after sunset. My father had built a small bonfire, and we clustered around it against the evening chill, roasting marshmallows as the stars came out overhead. Lower in the sky, just above the treetops, a soft white smear was barely visible. It was Halley’s comet, passing near Earth on its trip around the Sun. I remember asking whether I could visit it. I had recently seen the 1974 film adaptation of Saint-Exupéry’s The Little Prince, and, like the small boy living on an asteroid in the story, I, too, wanted to fly through space to see all the solar system’s strange places. “Maybe someday,” the answer came. Weeks later, I and the rest of a generation of children would learn that space travel is no fairy tale, watching as NASA’s space shuttle Challenger broke apart on its way to orbit.

I didn’t know then that Halley’s comet would not be coming back until far-off 2061, and I was much too young to feel the weight of that date. The comet didn’t feel it, either—when it returned, it would be practically unchanged. I, on the other hand, would be nearing my eightieth year on Earth, if I was so lucky. With a great deal more luck, my parents would see it through centenarian eyes.

When I was ten, after we had moved to Greenville, South Carolina, my mother spent much of one summer teaching illiterate adults to read at a local library. She always brought me along, letting me wander the shelves unsupervised. I began reading enormous amounts of science fiction about alien civilizations and interstellar travel, as well as books about astronomy, which tended to gloss over the possibility of planets and lives beyond our solar system in favor of bigger, flashier things—exploding suns, colliding galaxies, voracious black holes, and the Big Bang. Such was the spirit of the times: for most of the twentieth century, astronomers had been all-consumed by a quest to gaze ever deeper out into space and time, pursuing the fundamental origins and future of existence itself. That quest had revealed one revolutionary insight after another, showing that we lived in but one of innumerable galaxies, each populated by hundreds of billions of stars, all in an expanding universe that began nearly fourteen billion years ago and that might endure eternally. I thrilled at the cosmological creation story but couldn’t help but think that it was missing something. Namely, us. Lost somewhere in between the universe’s dawn and destiny, a ball of metal, rock, and water called Earth had given birth not only to life, but to sentient beings, creatures with the intellectual capacity to discover their genesis and the technological capability to design their fate. Creatures that, before their sun went dim, might somehow touch the stars. Maybe what had happened once would happen many times, in many places. My father saw the galaxies and stars on the covers of my checked-out library books and bought me a department-store telescope.

Looking through my telescope, I was soon disappointed to learn I couldn’t see many of the cosmic fireworks described in the astronomy books, or any evidence for the galactic empires of science fiction. Everything out there looked awfully, deathly quiet. It seemed in all that cosmic space, and thus in the great minds of many learned astronomers, there was paradoxically no room for living beings and their tiny home worlds. Such things were too small to be searched for, too insignificant to be of notice. I kept looking every now and then anyway, half-hoping I might catch a UFO in my viewfinder as it streaked across the sky, or see the bright flashes of some interstellar battle in the twinkling of a star. One day I asked my father whether any planets at all existed around other stars. He thought a moment, and replied that other stars probably had planets, but that no one really knew; none had ever been found, because they were all so far away. After that, most times when I gazed up at the night sky, I would wonder what those planets might look like. Would they be like Earth? Would they have oceans and mountains, coral reefs and grasslands? Would they have cities and farms, computers and radios, telescopes and starships? Would creatures there live and die as we did, or look up and wonder about life’s purpose? Would they be lonely? Staring at the trembling stars, I dreamed of worlds I thought I would never see.

By the mid-2000s, I had followed my curiosity into a career in science journalism, where instead of pestering personal friends and acquaintances with my questions, I could simply pester the experts themselves. Answers to some of my earlier questions had emerged over the intervening years: planets proved quite common around other stars, and since the mid-1990s astronomers had found hundreds of them. These worlds were called “exoplanets,” and most were far too large and far too near their suns to be hospitable to life as we know it. Using large telescopes on the ground and in space, astronomers had even managed to take pictures of a few that were very hot, very big, and relatively nearby. But other questions remained unaddressed: Were there other Earth-size, Earth-like exoplanets in our galaxy and in the wider universe? Was our situation here on Earth average, or was it instead quite special, even unique? Were we cosmically alone? I decided to write this book when I learned just how soon we might gain answers to some of these seemingly timeless questions.

It was 2007, and I was interviewing the University of California, Santa Cruz, astrophysicist Greg Laughlin for a story. During our chat, Laughlin explained that since exoplanet searches were becoming progressively more sophisticated and capable, there would soon be thousands rather than hundreds of known exoplanets to compare with our own. Astronomy’s next big thing, he suggested, would focus not on the edges of space and the beginning of time, but on the nearest stars and the uncharted, potentially habitable worlds they likely harbored. Near the end of our conversation, he guessed that the first Earth-size exoplanets would probably be found within the next five years. He had graphed the year-to-year records for lowest-mass exoplanets, drawing a trend line through the data that suggested an Earth-mass planet would be discovered in mid-2011. It suddenly seemed I had stumbled upon some magnificent secret, hidden in plain view. The more exoplanet-related press releases and papers I read, the more convinced I became that somewhere on Earth there were scientists who would be remembered in history for discovering the first habitable worlds beyond the solar system, and perhaps even the first evidence of extraterrestrial life. Yet they were largely anonymous, utterly unknown to the average person. I wanted to learn more about them, and tell their stories. One by one, I sought them out.

Most welcomed me with open arms, and the ones who didn’t still politely tolerated me. Many planned for a bright near-future, one in which they would use great, government-built techno-cathedrals of glass and steel on remote mountaintops and in deep space to wring secrets from the heavens and investigate any promising exoplanets for signs of life. Looking further out in time, some even envisioned our culture eventually escaping Earth entirely to expand into the wider solar system and beyond, driven by a curiosity so insatiable and restless that it would forever propel us outward into the endless immensities of new, far-flung physical frontiers. And yet, as I researched the book, I saw many of their boldest hopes dashed as crucial telescopes and missions were delayed or canceled, deferring all those dreams for generations, if not forever. On the verge of epochal revelations, their work had faltered, but not because of any newfound limitations of celestial physics. Instead, rapid progress in the search for life beyond Earth had succumbed to purely human, mundane failings—negligent organizational stewardship, unsteady and insufficient funding, and petty territorial bickering. Time and time again I felt I was witnessing the planet hunters reach for the stars just as the sky began to fall. And so I became committed to telling not only their personal stories, but also the story of their field, where it came from and where, with a reversal of fortune, it might still go.

The result is the book you now hold in your hands. By necessity, it glosses over or fails to mention numerous discoveries and discoverers that deserve entire shelves of dedicated literature. I hope the knowledgeable reader will forgive my omissions in light of all that this work does encompass. It is a portrait of our planet, revealing how the Earth came to life and how, someday, it will die. It is also a chronicle of an unfolding scientific revolution, zooming in on the ardent search for other Earths around other stars. Most of all, however, it is a meditation on humanity’s uncertain legacy.

This book’s title, Five Billion Years of Solitude, refers to the longevity of life on Earth. Life on this planet has an expiration date, if for no other reason than that someday the Sun will cease to shine. Life emerged here shortly after the planet itself formed some four and a half billion years ago, and current estimates suggest our world has a good half billion years left until its present biosphere of diverse, complex multicellular life begins an irreversible slide back to microbial simplicity. In all this time, Earth has produced no other beings quite like us, nothing else that so firmly holds the fate of the planet in its hands and possesses the power to shape nature to its whim. We have learned to break free of Earth’s gravitational chains, just as our ancient ancestors learned to leave the sea. We’ve built machines to journey to the Moon, travel the breadth of the solar system, or gaze to the edge of creation. We’ve built others that can gradually cook the planet with greenhouse gases, or rapidly scorch it with thermonuclear fire, bringing a premature end to the world as we know it. There is no guarantee we will use our powers to save ourselves or our slowly dying world and little hope that, if we fail, the Earth could rekindle some new technological civilization in our wake of devastation.

In the long view, then, we are faced with a choice, a choice of life or death, a choice that transcends science to touch realms of the spiritual. As precious as the Earth is, we can either embrace its solitude and the oblivion that waits at world’s end, or pursue salvation beyond this planetary cradle, somewhere far away above the sky. In our lives we all in some way contribute to this greater choice, either drawing our collective future down to Earth or thrusting it out closer to the stars. Some of the people in this book have devoted themselves to seeking signs of other, wildly advanced galactic cultures, hoping to glimpse our own possible futures via interstellar messages carried on wisps of radio waves or laser light. Others closely study the evolution of Earth’s climate over geological time, trying to pin down the limitations of habitability on our own and other worlds. A few have become makers of maps and crafters of instruments, and strive to find the most promising worlds that untold years from now could welcome our distant descendants. All seem to believe that in the fullness of planetary time any human future can only be found far beyond the Earth. You will find their tales, and others, recorded in these pages.

I won’t pretend to know what our collective choice will be, how exactly we would embark on such an audacious adventure, or what we would ultimately find out there. I am content to merely have faith that we do, in fact, have a choice. Similarly, I can’t suggest that we simply ignore all of our planet’s pressing problems by dreaming of escape to the stars. We must protect and cherish the Earth, and each other, for we may never find any other worlds or beings as welcoming. Even if we did, we as yet have no viable way of traveling to them. Here, now, on this lonely planet, is where all our possible futures must begin, and where I pray they will not end.

LEE BILLINGS
NEW YORK CITY, 2013

Looking for Longevity

On a hillside near Santa Cruz, California, a split-level ranch house sat in a stand of coast redwoods, the same color as the trees. Three small climate-controlled greenhouses nestled alongside the house next to a diminutive citrus grove, and a satellite dish was turned to the heavens from the manicured back lawn. Sunlight filtered into the living room through a cobalt stained-glass window, splashing oceanic shades across an old man perched on a plush couch. Frank Drake looked blue. He leaned back, adjusted his large bifocal glasses, folded his hands over his belly, and assessed the fallen fortunes of his chosen scientific field: SETI, the search for extraterrestrial intelligence.

“Things have slowed down, and we’re in bad shape in several ways,” Drake rumbled. “The money simply isn’t there these days. And we’re all getting old. A lot of young people come up and say they want to be a part of this, but then they discover there are no jobs. No company is hiring anyone to search for messages from aliens. Most people don’t seem to think there’s much benefit to it. The lack of interest is, I think, because most people don’t realize what even a simple detection would really mean. How much would it be worth to find out we’re not alone?” He shook his head, incredulous, and sunk deeper in the couch.

Besides a few extra wrinkles and pounds, at eighty-one years old Drake was scarcely distinguishable from the young man who more than half a century earlier conducted the first modern SETI search. In 1959, Drake was an astronomer at the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia. He was only twenty-nine then, lean and hungry, yet he already possessed the calm self-assurance and silver hair of an elder statesman. At work one day, Drake began to wonder just what the site’s newly built 85-foot-wide radio dish was capable of. He performed some back-of-the-envelope calculations based on the dish’s sensitivity and transmitting power, then probably double-checked them with a growing sense of glee. Drake’s figuring showed that if a twin of the 85-footer existed on a planet orbiting a star only a dozen light-years away, it could transmit a signal that the dish in Green Bank could readily receive. All that was needed to shatter Earth’s cosmic loneliness was for the receiving radio telescope to be pointed at the right part of the sky, at the right time, listening at the right radio frequency.

“That was true then, and it’s true today,” Drake told me. “Right now there could well be messages from the stars flying right through this room. Through you and me. And if we had the right receiver set up properly, we could detect them. I still get chills thinking about it.”

It didn’t take long for Drake to discuss the wild prospect with his superiors at NRAO. They granted him a small budget to conduct a simple search. During the spring of 1960, Drake periodically pointed the 85-footer at two nearby Sun-like stars, Tau Ceti and Epsilon Eridani, to listen for alien civilizations that might be transmitting radio signals toward Earth. Drake called the effort “Project Ozma,” after the princess who ruled over the imaginary Land of Oz in Frank Baum’s popular series of children’s books. “Like Baum, I, too, was dreaming of a land far away, peopled by strange and exotic beings,” he would later write.

Project Ozma recorded little more than interstellar static, but still inspired a generation of scientists and engineers to begin seriously considering how to discover and communicate with technological civilizations that might exist around other stars. Over the years, astronomers used radio telescopes around the world to conduct hundreds of searches, looking at thousands of stars on millions of narrowband radio frequencies. But not one delivered unassailable evidence of life, intelligence, or technology beyond our planet. The silence of the universe was unbroken. And so for more than fifty years Drake and his disciples played the roles of not only scientists but also salespeople. For the entirety of the discipline’s existence, SETI groups had been searching nearly as ardently for sources of funding as they had for signals from extraterrestrials.

Early on, governments were quite interested—SETI was briefly one of the scientific arenas in which the United States and the Soviet Union grappled during the Cold War. What better propaganda victory could there be than to act as humanity’s ambassador to another cosmic civilization? What invaluable knowledge might be gained—and exploited—from communication between the stars? In 1971, a prestigious NASA commission concluded that a full-bore search for alien radio transmissions from stars within a thousand light-years of Earth would require an array of giant radio telescopes with a total collecting area of between 3 and 10 square kilometers, built at a cost of about $10 billion. Politicians and taxpayers balked at the price tag, and SETI began its long descent from political favor. The trend of null results stretched out over decades, and already scarce and fickle federal funding for American SETI efforts progressively dwindled. A glimmer of hope emerged in 1992, when NASA launched an ambitious new SETI program, but congressional backlash shuttered the project the following year. Since 1993, not a single federal dollar had directly sponsored the search for radio messages from the stars. Drake and a group of his disciples had suspected what was coming, so in 1984 they formed a nonprofit research organization, the SETI Institute, to more easily solicit financial support from both the public and private sectors. Headquartered in Mountain View, California, the SETI Institute began to thrive in the mid-1990s through a combination of research grants and private donations from starry-eyed and newly wealthy Silicon Valley technologists. Drake served as the Institute’s president from its founding until 2000, before transitioning into an active retirement a couple of years after the turn of the millennium.

By 2003, the Institute had secured $25 million in funding from Paul Allen, the billionaire cofounder of Microsoft, to build an innovative new instrument, the Allen Telescope Array (ATA), in a bowl-shaped desert valley some 185 miles north of San Francisco. Rather than construct a smaller number of gigantic (and gigantically expensive) dishes, the Institute would save money by building larger numbers of smaller dishes. Drake had spearheaded much of the ATA’s design. Three hundred fifty 6-meter dishes would act together as one extremely sensitive radio telescope, monitoring an area of sky nearly five times larger than the full Moon on a wide range of frequencies. Allen’s millions, along with $25 million more from other sources, were sufficient to build the ATA’s first forty-two dishes, which were completed in 2007. Significant funds to operate the fledgling ATA came from California state funding and federal research grants to the Radio Astronomy Laboratory at the University of California, Berkeley, which jointly ran the ATA with the Institute. Though only partially completed, the ATA still functioned well enough to support a SETI effort as well as a significant amount of unrelated radio astronomy research. It operated on an annual budget of approximately $2.5 million—at least until 2011, when funding shortfalls forced the entire facility into hibernation.

As I spoke with Drake in his home in June 2011, weeds were already growing up around the idle dishes at the shuttered ATA. Only a skeleton crew of four Institute employees remained attached to the facility, merely to ensure it wouldn’t fall into irreparable disarray. The ATA would not restart operations until December, buoyed by a brief flurry of small donations. The money raised was sufficient to fund only another few months of operations. The Institute was seeking a partnership with the U.S. Air Force, which later purchased time on the ATA to monitor “space junk”—cast-off rocket stages, metal bolts, and other debris that can strike and damage spacecraft. But that funding, too, proved only temporary, and time spent surveying space junk was time sucked away from the ATA’s SETI-centric goals. Unless more wealthy patrons swooped in with heavyweight donations, the ATA had very little hope of reaching its original target of 350 dishes—and during the long recession after the 2008 turmoil in the global financial system, potential donors were proving at least as elusive as any broadcasting aliens. Drake’s greatest dream seemed to be collapsing.

Aside from political and economic difficulties, there was another factor in SETI’s decline that was at once more scientific and particularly ironic: the rise of exoplanetology, a field devoted to the discovery and study of exoplanets, planets orbiting stars other than our Sun. Beginning in the early 1990s, as radio telescopes intermittently swept the skies for messages from extraterrestrials, a revolution occurred in astronomy. Observers using state-of-the-art equipment began finding exoplanets with clockwork regularity. The first worlds discovered were “hot Jupiters,” bloated and massive gas-giant worlds orbiting inhospitably close to their stars. But as planet-hunting techniques grew more sophisticated, the pace of discovery quickened, and ever-smaller, more life-friendly worlds began to turn up. Twelve exoplanets were discovered in 2001, all of which were hot Jupiters. Twenty-eight were found in 2004, including several as small as Neptune. The year 2010 saw the discovery of more than a hundred worlds, a handful of which were scarcely larger than Earth. By early 2013, a single NASA mission, the Kepler Space Telescope, had discovered more than 2,700 likely exoplanets. A small fraction of Kepler’s finds were the same size as or smaller than Earth and orbited in regions around stars where life as we know it could conceivably exist. Emboldened, astronomers earnestly discussed building huge space telescopes to seek signs of life on any habitable worlds around nearby stars.

When the ATA briefly came back online in December of 2011, it began to survey those promising Kepler candidates for the radio chatter of any talkative aliens who might live there. No signals were detected before the ATA was sent back into hibernation, starved once again for money. SETI’s half century of null results could not be further from the ongoing exoplanet boom, where sensational discoveries could lead to media fame, academic stardom, and plentiful funding for researchers and institutions. For those interested in extraterrestrial life, exoplanetology, not SETI, was the place to be. As the search for Earth-like planets came to a boil, SETI was being frozen out of the scientific world.

When I asked Drake if we were witnessing the end of SETI, his blue eyes twinkled behind a knowing Cheshire Cat grin. “Oh no, not at all. This, I think, has been just the beginning. People presume we’ve been somehow monitoring the entire sky at all frequencies, all the time, but we haven’t yet been able to do any of those things. The fact is, all the SETI efforts to date have only closely examined a couple thousand nearby stars, and we’re only just now learning which of those might have promising planets. . . . Even if we have been pointed in the right direction and listening at the right frequency, the probability of a message being beamed at us while we’re looking is certainly not very large. We’ve been playing the lottery using only a few tickets.”

• • •

Drake’s confidence that there are other life forms out there at all had its roots in a private meeting that took place shortly after Project Ozma. In 1961, J. P. T. Pearman of the U.S. National Academy of Sciences approached Drake to help convene a small, informal SETI conference at NRAO’s Green Bank observatory. The core purpose of the meeting, Pearman explained, was to quantify whether SETI had any reasonable chance of successfully detecting civilizations around other stars. The “Green Bank conference” was held November 1–3, 1961.

The invite list was star-studded and short. Besides Drake and Pearman, three Nobel laureates attended. The chemist Harold Urey and the biologist Joshua Lederberg had both won Nobel Prizes in their fields, Urey for his discovery of deuterium, a heavier isotope of hydrogen, and Lederberg for his discovery that bacteria could mate and swap genetic material. Both were early practitioners in the still-nascent field of astrobiology, the study of life’s origins and manifestations in space. Urey was particularly interested in the prebiotic chemistry of the ancient Earth, and Lederberg worked to define how alien life on a distant planet might be remotely detected. As the conference was underway, one of the guests, the chemist Melvin Calvin, was awarded a Nobel for his elucidation of the chemical pathways underlying photosynthesis.

The other attendees were only slightly less celebrated. The physicist Philip Morrison had coauthored a 1959 paper advocating a SETI program just like the one Drake undertook in 1960. Dana Atchley was an expert in radio communications systems and president of Microwave Associates, Inc., a company that had donated equipment for Drake’s search. Bernard Oliver was vice president of research at Hewlett-Packard, and already an avid SETI supporter, having earlier traveled to Green Bank to witness Drake’s first search. The Russian-born American astronomer Otto Struve, the director of Green Bank observatory, invited one of his star pupils, a soft-spoken NASA researcher named Su-Shu Huang. Struve was a legendary optical astronomer, and one of the first who seriously considered how to find planets orbiting other stars. He and Huang had worked together studying how a star’s mass and luminosity could affect the habitability of any orbiting planets. The neuroscientist John Lilly came to Green Bank to present his ideas on interspecies communication, based on his controversial experiments with captive bottlenose dolphins. A dark-haired and brilliant twenty-seven-year-old astronomy postdoc named Carl Sagan was, at the time, the youngest and arguably least distinguished name on the guest list. Lederberg, one of Sagan’s mentors, had invited him.

It fell to Drake to arrange the agenda. A few days before the conference began, he sat down at his desk with pencil and paper and tried to categorize all the key pieces of information needed to estimate the number, N, of detectable advanced civilizations that might currently exist in our galaxy. He began with the fundamentals: surely a civilization could only emerge on a habitable planet orbiting a stable, long-lived star. Drake reasoned that the average rate of star formation in the Milky Way, R, thus placed a rough upper limit on the creation of new cradles for cosmic civilizations. Some fraction of those stars, fp, would actually form planets, and some number of those planets, ne, would be suitable for life. From astrophysics and planetary science, Drake’s musing entered into the field of evolutionary biology: some fraction of those habitable planets, fl, would actually blossom into living worlds, and some fraction of those living worlds, fi, would give birth to intelligent, conscious beings. As his considerations shifted to the rarefied realms of social science, Drake became restless. He sensed he was nearing the end of his categories and the outer limits of reasonable speculation. He doggedly forged ahead. The fraction of intelligent extraterrestrials who developed technologies that could communicate their existence across interstellar distances was fc, and the average longevity of a technological society was L.

Longevity was important, Drake believed, because of the Milky Way’s sheer size and immense age, and the inconvenient fact that nothing seemed able to travel through space faster than the speed of light. Approximately 100,000 light-years wide, and thought to be almost as old as the universe itself, our galaxy presented a huge volume of space and time in which other cosmic civilizations could pop up. If, for example, the average lifetime of an advanced technological society was a few hundred years, and two such societies emerged simultaneously around stars a thousand light-years apart, they would have essentially no chance of making contact before various forces brought the communicative phases of their empires to an end. Even if one somehow discovered the other, and beamed a message toward that distant star, by the time the message arrived a millennium later, the society that sent the message would no longer exist.

If one were to multiply all of Drake’s factors together using plausible figures, conceivably a ballpark estimate of N would emerge. The terms were interdependent; if any one of them had a vanishingly low value, the resulting N, the estimated number of detectable technological civilizations at large in the Milky Way, would drop precipitously. Strung together, they formed an equation of sorts that, if it did not yield an accurate estimate of contemporaneous cosmic civilizations, at least helped quantify humanity’s cosmic ignorance.

• • •

On the morning of November 1, after the guests were seated and sipping coffee in a small lounge in the NRAO residence hall, Drake rose and strode forward to present what he’d come up with. But rather than address the room from the central lectern, he kept his back turned and scratched out his lengthy figure on a nearby blackboard. When he put down the chalk and stepped aside, the board read:

N = R fp ne fl fi fc L

That string of letters has come to be known as the “Drake equation.” Though Drake had intended it only to guide the next three days of the Green Bank meeting, the equation and its plausible values would come to dominate all subsequent SETI discussions and searches.

At the time, only one term, R, the rate of star formation, was reasonably constrained. Astronomers had already closely studied several star-forming regions in the Milky Way. Based on that data, the astronomers in the group quickly pegged R at a conservative value of at least one per year within our galaxy. They also chose to focus on Sun-like stars. Stars much larger than our own were also far more luminous, and burned out in only tens or hundreds of millions of years, leaving little time for complex life to arise on any orbiting planets. Stars much smaller than the Sun were far more parsimonious with their nuclear fuel, and could weakly shine for hundreds of billions of years. But to be sufficiently warmed by that dim light a planet would need to be perilously close to the star, where stellar flares and gravitational tides could wreak havoc on a biosphere. Sun-like stars struck a balance between the two extremes, steadily shining for several billions of years with sufficient luminosity for habitable planets to exist far removed from stellar fireworks.

In 1961, no planets beyond our solar system were yet known, so the estimate of fp relied only on indirect evidence. It emerged from a discussion between Struve and Morrison. Struve had performed pioneering work decades earlier, measuring the rotation rates of different types of stars. He found that the very hot, very massive stars larger than our Sun spun very fast, while stars like our own, as well as those that were smaller and cooler, spun more slowly. The difference, Struve thought, was that spinning planets accompanied the stars more like our Sun, sapping the stars’ angular momentum and reducing their spin rates. However, roughly half of the known Sun-like stars were in binary systems, co-orbiting with a companion star that could also affect their spin. In such a system the pull of each star upon the other, it was thought, might disrupt the process of planet formation. Struve speculated that only the other half, the singleton suns, would be likely to form planets. He was so convinced that planets were common around Sun-like stars that almost a decade earlier, in 1952, he had published a paper laying out two observational strategies to find them, presaging the exoplanet boom by a half century. Struve’s estimate that half of all Sun-like stars had planets was too high for Morrison, who guessed that even around many solitary stars only scattered asteroids and comets would form. He thought fp might be as low as one-fifth.

Next, the group turned to ne, the number of habitable planets per system. Huang and Struve marshaled their years of work together to posit that our own solar system’s architecture was typical, with a large number of planets in a wide distribution of orbits. In any system, they suggested, at least one world would fall within Huang’s “habitable zone,” the broadly defined circumstellar region where liquid water could exist on a planet’s surface. Sagan concurred, and pointed out that abundant greenhouse gases in a planet’s atmosphere could act to warm an otherwise frigid planet, greatly extending the habitable zone’s expanse. Looking to our own solar system, the group focused on scorched Venus and frigid Mars, two borderline worlds that, if they possessed moderately different atmospheric compositions, would likely be quite Earth-like indeed. Accounting for Sagan’s proposed greenhouse extension of Huang’s habitable zone, the attendees decided that a planetary system would likely harbor anywhere from one to five planets suitable for life. They pegged ne at somewhere between one and five. Of course, billions of habitable planets could exist in the galaxy and none other than Earth might be inhabited, if life’s origin was a cosmic fluke.

As the discussion turned to the value of f l , the number of habitable planets that gave birth to life, it entered Urey and Calvin’s realm of expertise. In 1952, Urey had teamed with one of his graduate students, Stanley Miller, to investigate the origins of life on the primordial Earth, where geothermal heating, lightning strikes, and ultraviolet light from the tempestuous young Sun would have suffused the environment with useful energy. The duo decided to run a modest electric current through a sealed vessel of hydrogen, methane, ammonia, and water vapor—a mixture of gases thought at the time to mimic Earth’s ancient atmosphere. After only a week the Urey-Miller experiment had synthesized a “primordial soup” of organic compounds—sugars, lipids, and even amino acids, which are the building blocks of proteins. Acting over millions of years on a planetary scale, such reactions could easily synthesize the organic ingredients for life from inorganic chemical precursors. On our own planet, the fossil record suggested that life must have already been thriving only a few hundred million years after our planet cooled from its formation; it seemed to have appeared as soon as it possibly could.

Calvin argued forcefully that on geological timescales the emergence of simple, single-celled life was a certainty on any habitable world. Sagan noted that astronomers had already detected some of life’s key chemical ingredients in clouds of interstellar gas and dust, and that even some varieties of meteorites were proving to be rich in organic compounds. All this suggested that planets with atmospheres similar to the early Earth’s would be common outcomes of planet formation, Sagan said. And, since the laws of physics and chemistry were everywhere the same, when warmed by their stars’ light these worlds would become enriched with life’s organic building blocks. Through innumerable iterations and permutations of organic compounds in the primordial soup, crude catalytic enzymes and self-replicating molecules would gradually emerge, and life’s genesis would be at hand. The rest of the group agreed: given hundreds of millions or billions of years, single-celled life would likely spring up on each and every habitable world, yielding an fl value of one.

When the time came to discuss fi, the fraction of habitable, life-bearing planets that develop intelligent inhabitants, Lilly discussed his experiments with captive dolphins on the island of Saint Thomas in the Caribbean. Lilly began by noting that the brain of a dolphin was larger than that of a man, with similar neuron density and a richer variety of cortical structure. He recounted his various attempts to communicate with the dolphins in their own language of clicks and whistles, and told stories of dolphins rescuing sailors lost at sea. He focused on one case in which two of his captive dolphins had acted together to rescue a third from drowning when it became fatigued in the cold water of a swimming pool. The chilled dolphin had let out two sharp whistles in an apparent call for help, spurring the two rescuers to chatter together, form a rescue plan, and save their distressed companion. The display convinced Lilly that dolphins were a second terrestrial intelligence contemporaneous with humans, capable of complex communication, future planning, empathy, and self-reflection.

Morrison broadened the discussion by introducing the concept of convergent evolution, the tendency for natural selection to sculpt creatures from very different evolutionary lineages into common forms to fit shared environments and ecological niches. Hence, fish such as tuna or sharks shared a streamlined body form with mammalian dolphins, and features such as eyes and wings had independently evolved across the animal kingdom several times. Perhaps, Morrison said, intelligence was another example of convergent evolution, and had emerged not only in humans and dolphins but also in other primates and cetaceans, such as whales and now-extinct Neanderthals. Like eyes or wings, intelligence might be an extremely successful adaptation that would emerge time and time again in a planetary environment—provided life first made the fundamental evolutionary leap from simple solitary cells to complex multicellular organisms. Moved by Morrison’s arguments, the Green Bank scientists optimistically placed the value of fi at one.

Morrison also proved decisive in framing the Green Bank debate over the two final and most nebulous terms of Drake’s equation: fc, the fraction of intelligent creatures who would develop societies and technologies capable of interstellar communication, and L, the average longevity of an advanced technological civilization. He first noted that while creatures like dolphins and whales might well be intelligent, in their current aquatic forms they seemed destined for cosmic invisibility: supposing they had language and culture, they still lacked a way of assembling or using even relatively simple tools and machines. None of the attendees could easily imagine any cetacean civilization ever building anything like a radio telescope or a television broadcast antenna. But on land, Morrison said, history suggested that the emergence of technological societies might be another convergent phenomenon. The early civilizations of China, the Middle East, and the Americas all arose independently and generally followed similar lines of development.

And yet, the drivers of social change and technological progress were not at all clear. Despite China’s development of technologies such as gunpowder, compasses, paper, and the printing press hundreds of years before Europeans did, China experienced nothing equivalent to the European Renaissance and the successive scientific and industrial revolutions. When Spanish and Portuguese explorers, rather than the Chinese, used great ocean-faring ships to discover the Americas, they found indigenous civilizations using Stone Age technology that was no match for European steel and gunpowder. Sending ships across oceans or messages between the stars appeared to be a matter not only of technological prowess, but also of choice. Whether any given technological culture would attempt interstellar communication seemed unpredictable. Facing a somewhat arbitrary decision, the Green Bank attendees eventually guessed that between one-fifth and one-tenth of intelligent species would develop the capabilities and intentions to search for and signal other cosmic civilizations. That left only L, the typical lifetime of technological civilizations, for the group to consider.

During a break in the proceedings, Drake noticed something that made him suspect his equation could be substantially streamlined: Three of the equation’s seven terms (R, fl, fi ) appeared to be equal to one, and hence would have little effect on the product N, the number of detectable civilizations in our galaxy. Similarly, the plausible values of the other three terms (fp, ne, fc ) could easily cancel each other out. For instance, the group had guessed that the average number of habitable planets per system, ne , was between one and five, and thatfp , the fraction of stars with planets, was between one-half and one-fifth. If the value of ne was actually two, and fp’s value was one-half, multiplied together the result was one, and N was scarcely affected. After considering the best evidence that was available, some of the brightest scientific minds on planet Earth had concluded that the universe, on balance, was a rather hospitable place, one that surely must be overflowing with living worlds. It stood to reason that, on other planets circling other suns, other curious minds gazed at their night skies wondering if they, too, were alone. And yet, Drake announced, more than the number of stars, or the number of habitable planets, or how often life, intelligence, and high technology emerged, what he suspected really controlled the number of technological civilizations currently extant in the cosmos was almost solely their longevity. N=L.

The thought made Morrison shudder. Of all the Green Bank attendees, he alone could viscerally appreciate just how fleeting our modern era might be. He had worked on the Manhattan Project during World War II, and had witnessed the detonation of the first atomic bomb, at Alamogordo, New Mexico, on July 16, 1945. A month later, on the South Pacific island of Tinian, Morrison had personally assembled and armed an atomic bomb that was later dropped on the Japanese city of Nagasaki. Tens of thousands of civilians were incinerated in the bomb’s fireball, and tens of thousands more died slowly from secondary burns and exposure to radioactive fallout, all from the nuclear fission of about two pounds of plutonium. When Japan’s surrender drew the war to a close, Morrison was among a contingent of American scientists who toured the cities of Hiroshima and Nagasaki to evaluate up close the devastation wrought by atomic warfare. Shortly after, he became a vocal proponent of nuclear disarmament, but it was too late. The Soviet Union had already begun a crash program to develop atomic bombs, and would successfully test its first nuclear weapon in 1949. In the ensuing arms race both the United States and the Soviet Union succeeded in harnessing the far more powerful process of thermonuclear fusion, squeezing the destructive force of hundreds of Nagasakis into individual bombs. The resulting arsenals of thermonuclear weapons were more than adequate to extinguish hundreds of millions of lives in a single nuclear exchange. Those who survived such a nuclear holocaust would face a severely damaged planetary biosphere and a world plunged into a new Dark Age. Less than a year after the Green Bank proceedings, the Cuban missile crisis would bring the world to the brink of thermonuclear war, and as time marched on, more and more nations successfully weaponized the power of the atom. Humans had developed a global society, radio telescopes, and interplanetary rockets at roughly the same time as weapons of mass destruction.

If it could happen here, Morrison gloomily suggested, it could happen anywhere. Perhaps all societies would proceed on similar trajectories, becoming visible to the wider cosmos at roughly the same moment they gained an ability to destroy themselves. In fact, he went on, running the numbers in his steel-trap mind, if the average civilization endured only a decade before passing into oblivion, at any time there would most likely be only one communicative planetary system throughout the galaxy. We would have already met the Milky Way’s only culture, for it would be us. One of the most compelling reasons to search for evidence of extraterrestrial civilizations, Morrison thought, would be to learn whether our own had a prayer of surviving its current technological adolescence. Maybe a message from the stars could provide some inoculation against humanity’s self-destructive tendencies.

Sagan attempted to counter the doomsaying, noting that we could not rule out some technological civilizations achieving global stability and prosperity either before or even after developing weapons of mass destruction. They might master their planetary environment, and move on to exploit resources in the rest of their planetary system. He thought that such a society, flush with power and wisdom, would have a fighting chance to prevent or withstand nearly any natural calamity. It could, in theory, persist for geological timescales of hundreds of millions or even billions of years, potentially lasting as long as its host star continued to shine. And if, somehow, that civilization managed to escape its dying sun and colonize other planetary systems . . . well, perhaps then it would endure practically forever. Of all the attendees, Sagan was by far the most optimistic that technological civilizations could solve not only their many planetary problems, but also the manifold difficulties associated with interstellar travel. Somewhere out there, if not in our galaxy then in at least one of countless others, immortals passed their unending days amid the stars. Sagan thought we might yet be included in their number.

After the participants had discussed and debated L to the point of exhaustion, Drake stood up and announced that they had reached a consensus. The lifetimes of technological civilizations, he said, were likely to be either relatively short, lasting at most perhaps a thousand years, or very long, extending to one hundred million years and beyond. If indeed longevity was the most crucial consideration of the Drake equation, that implied there were somewhere between one thousand and one hundred million technological civilizations in the Milky Way. A thousand planetary civilizations translated to one per every hundred million stars in our galaxy. If the number was that low, we’d be hard-pressed to ever find anyone to talk to, as our nearest neighboring civilization would most likely be many thousands of light-years away. Conversely, if a hundred million civilizations existed, they would occupy one out of every thousand stars, in which case we might expect to have heard from them already. Drake’s best guess in 1961 walked the line between these extremes: He speculated that Lmight be about ten thousand years, and that consequently perhaps ten thousand technological civilizations were scattered throughout the Milky Way along with our own. It was probably not coincidental that Drake’s personal estimate rendered the successful detection of alien civilizations still quite difficult but not entirely beyond our capabilities: by his reckoning, only ten million stars would need to be monitored to obtain an eventual detection, though the search could take decades, even centuries.

At the conference’s end, as the guests drank champagne left over from celebrating the news of Calvin’s winning of a Nobel Prize, Struve offered up a toast: “To the value of L. May it prove to be a very large number.”

Drake’s Orchids

A half century later, as we chatted in his living room, Drake expressed his conviction that most of the Green Bank conference’s conclusions were, if anything, too pessimistic. In the last few decades the astrophysical case for a life-friendly universe had grown immensely, he said. Estimates of the rate of star formation had scarcely changed since 1961, but many new studies hinted that “red dwarfs,” stars smaller, cooler, and far more plentiful than ones like our Sun, were more amenable to life than previously believed. Statistical analyses of data from the exoplanet boom suggested that hundreds of billions of planets existed in our galaxy alone, around all varieties of stars; the Green Bank group’s original estimates of planet-bearing stars had been far too low. Inevitably, a good fraction of all those planets would orbit within habitable regions of their systems. Spacecraft visiting Venus and Mars had pieced together tantalizing evidence for oceans of water on both worlds billions of years ago, though the planets’ periods of habitability were brief, and after hundreds of millions of years each had lost its ocean. Meanwhile, researchers had discovered oceans of liquid water in the outer solar system, vast sunless seas beneath the icy crusts of gas giants’ moons like Jupiter’s Europa and Saturn’s Titan. Extrapolating from these results, astronomers speculated that perhaps habitable Earth-like moons orbited some of the warm Jupiter-size worlds already known around other stars. A few even spoke of habitable planets free-floating through the depths of interstellar space after being slingshotted away from their stars. A thick atmospheric blanket of greenhouse gas or an icy crust over a deep ocean could insulate such nomadic worlds and preserve their habitability for billions of years. It could well be that most planets suitable for life in our galaxy don’t orbit stars like our Sun, Drake said. Perhaps they didn’t even orbit stars at all.

He thought the biochemical case had grown, too. A half century of progress in studying the origins of life had found a plethora of possible chemical pathways that could lead to membranes, self-replicating molecules, and other fundamental cellular structures. Multiple lines of evidence indicated that the jump from single-celled to multicellular life had occurred several times on the early Earth in a wide array of organisms, suggesting that the transition was yet another instance of convergent evolution, not a rare fluke. Researchers had discovered microbes flourishing in rock miles beneath the Earth’s surface, in boiling-hot pools of hypersaline acidic water, in the icebox interiors of glaciers, in the deepest, darkest ocean abysses, and even in the radiation-riddled containment chambers of nuclear reactors. Once it arose, life as a planetary phenomenon appeared to be supremely adaptable, prospering in every possible ecological niche and enduring almost any conceivable environmental disruption.

I asked what all that meant for the later terms of his equation.

“We’ve found a truly great number of potentially habitable places, but the number of places where you could expect to find intelligent, technological life really hasn’t increased that much,” Drake replied. “That suggests to me there are probably significant barriers to the development of widespread, powerful technology. To surpass them, you might need a planet quite a lot like Earth. That may sound discouraging, until you realize just how many stars there are. Their sheer number suggests the equivalent of Earth and its life has probably happened many times before and will occur many, many times again. They’re out there.”

He chuckled, coughed, and creakily unfolded himself from the couch, clearly weary of sitting. We went outside to breathe fresh air.

Afternoon sunlight warmed our faces, and a cool breeze sighed through the towering redwoods to tousle Drake’s silver hair. The air smelled of green, growing things. Drake pointed out the Moon’s thin waxing crescent, faintly visible high in the cloudless sky. It was adjacent to the passing silver needle of a high-flying passenger jet. As we walked down into the yard, I gingerly stepped over the pale blue remnants of a robin’s egg cracked open on the front steps, fallen from a nest in an overhanging tree. The tide was rolling in far below us, down past the forested hills and beachfront suburbs, and surfers rode big waves toward the shore of Monterey Bay.

The scene from Drake’s front door encapsulated many of the essential facts of life on Earth. Fueled by raw sunlight, plants broke the chemical bonds of water and carbon dioxide, spinning together sugars and other hydrocarbons from the hydrogen and carbon and venting oxygen into the air. Sunlight scattering off all those airborne oxygen molecules made the sky appear blue. Animals breathed the oxygen and nourished their bodies with the hydrocarbons, utterly dependent upon these photosynthetic gifts from the plants. In death, plants and animals alike gave their Sun-spun carbon back to the Earth, where tremendous heat, pressure, and time transformed it into coal, oil, and natural gas. Mechanically extracted from the planet’s crust and burned in engines, generators, and furnaces, that fossilized energy powered most of humanity’s technological dominion over the globe. Built up and locked away for hundreds of millions of years, the carbon stockpile was gushing back into the planet’s atmosphere in a geological instant.