Five Billion Years of Solitude (5 page)

After the participants had discussed and debated
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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
L
might 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.

Our experience at Monterey Bay was a product of our planet’s physical characteristics—and the unlikely events that led to them. Earth’s abnormally large Moon, which stabilizes our planet’s axial tilt and bestows it with tides, was born when a Mars-size body collided with the proto-Earth early in our solar system’s history. Another impactor, a six-mile-wide asteroid, struck the Earth 66 million years ago and sparked a global mass extinction, ending the age of dinosaurs. Humanity’s small mammalian ancestors began their slow progress toward biospheric dominance, and the saurians that didn’t die out gradually gave rise to birds. Billions of years before the dinosaurs, the life-giving liquid we recognize as Earth’s ocean was mostly delivered by impactors, too, in a shower of water-rich asteroids and comets from the outer solar system. Earth’s aquatic abundance, it is thought, lubricates the planet’s fractured crustal plates and allows them to drift and slide in the geological process we call plate tectonics, a climate-regulating mechanism unique to our world out of all those in the solar system.

Turning away from the bay, Drake walked over to the center of his driveway, where the weathered stump of a giant redwood rose like a long-extinct volcano. He stooped and placed his hands upon the ancient wood. Years ago, he said, he had spread a thin layer of chalk on a section of the stump’s surface, allowing the growth rings to be easily seen, and set his young children to the task of counting them as an informal science project. They counted more than 2,000, one for each year of the tree’s life, which apparently began around the time of the birth of Jesus Christ.

“This tree saw the first light from the supernova that made the Crab Nebula, right about here,” Drake said, touching a point midway between the stump’s center and perimeter. Light from the supernova washed over the Earth in 1054, just as Western Europe was emerging from its Dark Ages. Sweeping his hand halfway farther out toward the
perimeter, he brushed over the Age of Discovery, past rings recording the years when Europeans first explored and colonized the Americas. His hand kept moving until it slid from the stump’s edge.

Over the course of the tree’s 2,000-year existence, the Milky Way had fallen nearly five trillion miles closer to its nearest neighboring spiral galaxy, Andromeda, yet the distance between the two galaxies remained so great that a collision would not occur until perhaps 3 billion years in the future. In 2,000 years, the Sun had scarcely budged in its 250-million-year orbit about the galactic center, and, considering its life span of billions of years, hadn’t aged a day. Since their formation 4.6 billion years ago, our Sun and its planets have made perhaps eighteen galactic orbits—our solar system is eighteen “galactic years” old. When it was seventeen, redwood trees did not yet exist on Earth. When it was sixteen, simple organisms were taking their first tentative excursions from the sea to colonize the land. In fact, fossil evidence testified that for about fifteen of its eighteen galactic years, our planet had played host to little more than unicellular microbes and multicellular bacterial colonies, and was utterly devoid of anything so complicated as grass, trees, or animals, let alone beings capable of solving differential equations, building rockets, painting landscapes, writing symphonies, or feeling love.

By its twenty-second galactic birthday, some thousand million years hence, our planet may well return to its former barren state. Astrophysical and climatological models suggest that by then the Sun, steadily brightening as it ages, should increase in luminosity by about 10 percent—a seemingly minor change, but enough to render Earth’s climate too hot and its atmosphere too anemic to support complex multicellular life. Around that time, the oceans will begin evaporating, and most of Earth’s water will rapidly cook off into space. The loss of oceans a billion years from now marks the most likely expiration date for all life on Earth’s surface, though the omnipresent microbial biosphere might endure for billions of years further, shielded deep within the planet’s parched crust. Somewhere in the neighborhood of five
billion years from now, the Sun will exhaust its supply of hydrogen and begin fusing its more energy-rich helium, gradually ballooning 250 times its current size to become a red giant star. Astronomers debate whether the Earth will be submerged within the hot outer layers of the swollen red Sun or whether it will escape relatively unscathed and only suffer its crust being melted back to magma. Either way, at that late date the life of our planet will be brought to a decisive conclusion.

Considering the long concatenation of astrophysical events that led to our habitable planet, and the unknown synergies of technology and geology that could shape its fate, the distinction between chance and necessity blurs. Given a few hundred million years, would life arise on any rocky, wet, warm world? Would intelligence and technology emerge only on worlds with histories that mirrored our own, replete with the equivalents of Earth’s Moon, mobile crust, and blue sky? Or was a focus on these features merely a failure of our Earth-bound imaginations? Was our planet and its history a useful template or a stumbling block in the search for alien life and intelligence? Would we even recognize our own planet as “Earth-like” if we glimpsed it a half billion years in its past or in its future? Answers to questions like these would be elusive as long as scientists only had one living world to study—our own. Drake didn’t believe they would remain intractable forever.

•   •   •

B
ack in 1960, I thought that the possibility of detecting extrasolar planets in my lifetime was very, very low, though Otto Struve had already given us ideas about how it might someday be done,” Drake had told me back in his living room. “I thought our only hope of detecting evidence of other planets was to receive radio signals from any intelligent creatures on them. We’re seeing a similar pessimism play out now with characterization of planets around other stars. The techniques are there before us.”

Already, planet hunters had found a handful of worlds that in their
most basic details didn’t appear too dissimilar from Earth. Those planets, their numbers growing every year, could potentially be much like our own. But the methods used to find them relied on closely observing a planet’s bright, beacon-like star, not the dim planet itself; the gravitational pull of a planet on its star, or the shadow a planet cast toward Earth as it transited across its star’s face, generally only revealed such things as a world’s mass, size, and orbital properties. Without actually seeing these worlds—that is, collecting and analyzing photons reflected off their atmospheres and surfaces—scientists would be unable to determine whether any potentially habitable, potentially Earth-like planet was actually either of those things. They would be stuck where Drake had been fifty years before, hoping against all odds for a message from the stars to come streaming from the sky, filled with information on the flora, fauna, and environment of a place far, far away.