Five Billion Years of Solitude

by Lee Billings

  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 hydrogen, methane, ammonia, and water 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 that fp , 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 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 mu
lticellular 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.