Light of the Stars

All of Thomas See’s fellow astronomers hated him. That was a particularly ironic position for See to find himself in, given that he was one of the most popular of “popular” astronomers in the late nineteenth century.1

See began his career full of promise. He was considered an expert with a telescope, and his skill as a writer for non-scientific audiences made him the astronomer reporters turned to when they needed a quote or an explanation. But his meteoric ascent would later be followed by a fall into the depths of scientific scorn. In the end, See was so despised by his colleagues that his experience became an object lesson in ruining a scientific reputation.

It’s a story that begins with a planet.

See was born in rural Missouri in 1866. Though he was clearly a gifted child, his family would not allow him to attend school full-time until his teens. Once there, his natural aptitude for science and mathematics caught the attention of teachers who helped him get into the state university. Later, his natural talent led him to work with some of the best astronomers of the era, studying pairs of orbiting suns called binary stars.

See’s work involved precision mapping of the sibling stars as they changed position on the sky. He was tireless in these “astrometric” studies. He worked eighteen-hour days, translating the information in photographs produced over many nights of telescopic observation into positions on sky maps. This astrometric data was then fed into calculations that spit out the exact shape of the binary stars’ orbits. Finally, estimates of the stars’ masses could be extracted from the orbits by using laws of physics. No one knew very much about how much mass stars contained back in the 1890s, and See’s work was hailed as cutting-edge science.

See was hired at the University of Chicago, and then at the observatory that Percival Lowell, the rich, Mars-crazy amateur astronomer, was building in Flagstaff, Arizona. It was at Lowell’s observatory that the trouble began.

Astronomer Thomas Jefferson Jackson See.

In 1899, See published a letter in the prestigious Astronomical Journal claiming the binary system called 70 Ophiuchi was “perturbed by a dark body.” He meant the orbits of the two stars seemed to be distorted by the gravity of a third, unseen object. Later, See would claim to see other binary stars with invisible companions, reporting, “They seem to be dark . . . and apparently shining by reflecting light. It is unlikely that [the unseen objects] will prove to be self-luminous.” See was being coy with his language, but the implications of his statement were unambiguous. He was telling the world he’d discovered other planets orbiting other stars.

The question of whether other stars in the sky might have planets goes back to the ancient Greeks. For millennia, astronomers and philosophers argued about the existence of other solar systems in the cosmos. Giordano Bruno risked his life arguing that other worlds exist. That is why direct evidence for the existence of even one planet orbiting one other star would have been an epoch-making discovery. See was making an extraordinary claim with his orbit-perturbing “dark body” of a planet. But in science, extraordinary claims require extraordinary proof. For the practicing scientist making such claims, a healthy dose of skepticism is essential, because someone else is sure to check your results very, very carefully.

See lacked that internal skepticism, and he would pay a steep price for its absence. In May 1899, a former student of See’s named Forest Ray Moulton published a paper in the same Astronomical Journal, demonstrating that See’s planet around 70 Ophiuchi couldn’t exist because the laws of physics would not allow it.

Science is a “call and response” kind of business. Much as blues or jazz musicians will pick up on a riff that’s played by one of their bandmates, See could have taken Moulton’s results and built on them. He could have conceded that, with cutting-edge observations such as his, there were bound to be misinterpretations. He could have learned from the episode and built better science.

Instead, he doubled down.

In a blistering letter to the Astronomical Journal, See attacked Moulton and tried to weasel his way out of mistaken claims about planets. He wrote that he already knew about Moulton’s objections and then waffled about the nature of the orbit and the planet. The editors of the journal were so taken aback by the acid tone of See’s letter that they took the extraordinary step of printing only a few pieces of its text. Then they handed See the Victorian version of a smackdown: “The present is as fitting an opportunity as any to observe that heretofore Dr. See has been permitted, in the presentation of his views in this journal, the widest latitude that even a forced interpretation of the rules of catholicity would allow; but that hereafter he must not be surprised if these rules, whether as to soundness, pertinency, discreetness or propriety, are construed within what may appear to him unduly restricted limits.”

The Astronomical Journal was essentially threatening See with censure.

Things went downhill from there. See’s resentments and temperament led him from the world’s greatest centers of astronomy down to the “Naval Observatory” at Mare Island, California. This was little more than a timekeeping station attached to a huge naval shipyard. Mare Island had no telescope worth mentioning.

Lacking access to a good instrument for observations, See turned his attention to theory. Unfortunately, while he had clear talents with a telescope, his instincts for fundamental physics were terrible. See managed to miss the boat on every major revolution happening in physics at the turn of the century. He consistently rejected the profound discoveries about atomic phenomena in the new science of quantum physics, and he opposed Einstein’s triumphant theory of relativity, claiming that his own ideas about cosmic structure had been proven by observation. (They had not.)

The final nail in the coffin of See’s scientific reputation was a 1913 book called The Unparalleled Discoveries of T.J.J. See. The author called See “the greatest astronomer in the world.” Upon further investigation, however, some suggested that it was See himself who’d written the book. He would never regain the respect of his peers, and he died in 1962, rejected by his chosen profession.

THE PROBLEM OF PRECISION

See would not be the last astronomer whose claims of a planet discovery would prove tenuous or career-threatening. A number of times in the years that followed, astronomers claimed to have detected a planet, only to see their claims evaporate. The difficulty in finding exoplanets can be summarized in a single word: precision. Planets are small, and stars are big. Planets are dim, and stars are bright. Planets are cold, and stars are hot. Planets have small masses, but stars weigh in as behemoths. The Sun, for example, would appear a trillion times brighter than the Earth when seen from the stars. That means trying to see an earthlike planet across interstellar distances would be like looking from New York City to AT&T Park in San Francisco, where the Giants play, and making out a firefly next to one of the stadium spotlights.

So for scientists to “see” a distant planet, they must pull the tiny signal it produces out of the enormous impact of its star. There are a number of strategies astronomers can pursue to detect an exoplanet, but all demand high-precision measurements.

The basis for the oldest methods of detecting planets is the astrometry T.J.J. See was using, which focuses on the orbital motion of the star and planet. We usually think that planets orbit around their stars. The truth, however, is more interesting: objects always orbit each other. Binary stars of equal mass both circle around a point halfway between them. But if the mass of one of the objects is less than that of the other—as the case would be when a tiny planet orbits a big star—the orbit’s center will be nearer to the center of the heavier object. So even though it looks like a planet orbits its star, the planet’s gravity is still forcing the star to shuffle around in a tiny orbit. The center of that little dance is just slightly displaced from the star’s own center.

See’s astrometric studies were designed to see that tiny stellar motion. The idea was to track the position of a star over many years. In this way, astronomers would see the star zigzag as it was “perturbed” by the gravity of its unseen planet. But changes in the star’s position as it wobbled back and forth would be minuscule. For example, aliens looking at the Sun from fifteen light-years away would have to strain to see the orbital wobble caused by even the most massive planet in our solar system. The precision needed to measure these tiny shifts in position was beyond the technology See had at his disposal.

There is another way of tracking the gravitational dance of a star and its planet, one that relies on tracking changes in the star’s velocity rather than its position. As the star executes its little orbit, the gravity of the planet will cause it to swing first toward observers on Earth and then away. If astronomers could detect these changes in velocity—called reflex motion—it would constitute a detection of the orbiting planet. But like the orbits themselves, the changes in stellar velocity caused by orbital reflex motions are so small that taking measurements at the needed level of precision presented a huge technical challenge.

A third way of seeing an exoplanet focuses only on a star’s brightness, meaning its total light output. During any given year, between two and five solar eclipses are visible from the Earth’s surface. Each occurs when the Moon lines up just right for earthbound observers, passing in front of the Sun and either partially or totally blocking its light. The same principle can be applied to planet hunting.

Imagine a distant star that hosts an exoplanet. Now imagine that the planet’s orbit around its parent lines up perfectly with the “line of sight” between Earth and the star. That kind of alignment means the exoplanet will briefly swing between Earth and the star once during each of its orbits, just as the Moon swings between Earth and the Sun during an eclipse. Each time the planet gets between us and its star, it will block a fraction of the star’s light, and from Earth we will see the star dim ever so slightly.

Astronomers use the term transit to describe a planet crossing the face of a star. Seeing an exoplanet transit its own star would require hyper-precise light detectors. Aliens looking at the Sun from interstellar distances would see its light dim by just one percent when Jupiter crossed its face. An Earth transit would dim the Sun by just 0.01 percent. Along with this demand for precision, there is another complication. Stars can naturally produce light variations of the same order as an exoplanet transit. Dark regions on stars, called “spots,” caused by powerful stellar magnetic fields, are just one of many sources of natural variation. Any successful transit-based exoplanet-hunting method would have to be exact in both its measurements and its understanding of the star being measured.

By the early 1970s, planets had been hiding beneath their veils of imprecision for so long that many scientists had given up on trying to find them. In addition, throughout the 1950s and 1960s, there had been enormous progress in other arenas of astronomy, like the study of distant galaxies. Hunting for other worlds came to seem like a dead end.

“I remember how, in the early 1990s, people would look down at the few researchers who were pushing for planet hunting,” recalls one scientist. “There were NASA administrators who’d walk the other way just to avoid being bugged by them. It was a hard time for those guys.”2

But the fortunes of the planet quest were about to change. The first steps toward taking exoplanets seriously began in the mid-1970s, and the motivation came directly from Frank Drake’s original questions about the search for alien intelligence.

THE PATHS TO AN ANSWER

Frank Drake and Carl Sagan’s very public discussions about exo-civilizations established the scientific basis for the search for extraterrestrial intelligence, or SETI. But the search itself would require a new generation of scientists. Chief among their number was Jill Tarter.

Like Drake, Tarter began her scientific training at Cornell, in the engineering physics program. But by the time she completed graduate school at the University of California, Berkeley, she’d decided to focus her work on SETI.3 Over a long and distinguished career, Tarter carried out observational programs at radio observatories across the world, served as project scientist for NASA’s SETI program, and was given the Bernard M. Oliver Chair at the SETI Institute.⁴ She has seen firsthand how the question of exo-civilizations and the question of exoplanets converged.

In the 1970s, Tarter’s dedication to SETI took her to a series of meetings where questions of precision and planet detection were first taken on in earnest. “Technology for finding planets just didn’t exist back in the early 1970s,” she says. “That means astronomers needed to get together and figure out exactly what the barriers were and how we could beat them.”⁵ With this goal in mind, in 1975 a workshop was organized at NASA’s Ames Research Center in San Jose, at which the general problem of SETI technologies was first laid out. This workshop focused on search strategies for signals from exo-civilizations, but the attendees agreed that the factors in the Drake equation needed to be explored on their own as well. The most important of these sub-questions was the fraction of stars with planets and the fraction of planets in the habitable zone.⁶

“The original workshop led to two others that focused explicitly on planet-hunting methods,” Tarter told me in an interview. “There was a meeting at [Ames] in 1978. This was the first time the different methods of planet hunting were drilled down into to see which one had the highest chance of success.”

Records from that meeting show that most of the discussion focused on astrometric sky mapping, the approach that See used. Searches based on detecting reflex motions were discussed in detail, too. Direct detection—actually seeing the light from a planet—was also on the table.7 But the transit method, based on the dimming of starlight due to a passing planet, didn’t even make it into the report. The future would show the irony of this exclusion.

Though the problems with all the methods were acknowledged to be vast, the report ended on a positive note. “The prospects of increasing our confidence concerning the frequency and distribution of other planetary systems are good,” the authors concluded.⁸ Later, another SETI-inspired NASA workshop was held at the University of Maryland to explore technical details in more detail.

“People came away from that [second] meeting with a sense of what was possible,” Tarter told me. “The reflex motion approach was seen as particularly promising if the technology could be hammered out. I think a lot of folks were really excited.”9

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Astronomer and SETI research leader Jill Tarter.

Not everyone was so happy, however. While the transit method was raised at the Maryland meeting, its prospects were deemed to be dim. The final report concluded, “The Workshop considered the role of photometric [transit-based] studies in an effort to detect other planetary systems and upheld the conclusions of earlier studies, namely, that photometric studies are not practical.”¹⁰

That conclusion didn’t go down well with one tenacious scientist. “There was a young NASA researcher named Bill Borucki,” Tarter said. “He felt the transit method had a lot of promise, even if everyone else thought it was hopeless. I think he was determined to prove them wrong.”

THE FALL OF A THREE-THOUSAND-YEAR-OLD QUESTION

In 1995, at an astronomy conference in Florence, Swiss scientist Michel Mayor walked up through the audience and took his place at the podium. The other astronomers present looked around the room and wondered why a film crew had just appeared. That’s when Mayor dropped his epoch-making bombshell. He and his partner Didier Queloz had firm evidence for the existence of another planet orbiting another star.¹¹ When it came to solar systems, at least, we were not alone.

In the decade and a half following the Ames and Maryland meetings, the hurdles blocking the way to reflex motion–based planet searches had been overcome. In the US, astronomers Geoff Marcy and Paul Butler had built a series of ever more sensitive instruments to monitor a long list of nearby stars. Theirs was the world’s most complete planet-hunting program.

But Marcy and Butler were expecting other solar systems to look like ours. They thought they’d need years of tracking before the signal of a Jupiter-sized planet in a Jupiter-sized orbit would appear in their data (Jupiter takes twelve years to make one swing around the Sun). The European researchers Mayor and Queloz had an observational program oriented toward finding close binary stars. They’d gotten lucky with their planet detection, but also had the insight to recognize what they’d found.12

Mayor and Queloz discovered their planet orbiting the star 51 Pegasi, which is fifty light-years from Earth (one light-year equals six trillion miles). The planet, called 51 Pegasi b, was Jupiter-sized, but swung around its star once every four days. That meant it was almost ten times closer to its star than our innermost planet, Mercury, is to the Sun.¹³ A giant planet on a tiny orbit was not what astronomers were expecting when it came to solar systems.

Back in the US, Marcy and Butler quickly began looking for planets on such short orbits. It didn’t take long for results to appear. At a press conference just a few months after Mayor’s talk in Florence, Marcy and Butler announced their own discovery of two more Jupiter-sized worlds.¹⁴

New planets began to pile up after the discovery of 51 Pegasi b. As the shock of discovering exoplanets wore off, astronomers got down to the work of building a census of the new worlds.

But the real prize still lay in the search for Earth-sized worlds living in the star’s habitable zone, where water and perhaps life could exist on the surface. The Earth’s mass is one three-hundredth that of the Sun, a fact that meant even more precision was needed to detect Earth-sized worlds. That need for greater precision was also coupled with the problem that reflex motions only worked on one star at a time. What astronomers desperate for data needed was a precise way to discover planets wholesale. That threshold would be broken by the stubborn resilience of a man who simply refused to accept rejection.

Bill Borucki was a longtime NASA scientist who had cut his teeth on the physics of spacecraft heat shields. In the late 1970s, he decided to switch fields. The problem of planet detection offered the kind of technical challenges he loved, and after the Maryland meeting where transit-based planet-hunting methods had been dismissed, Borucki became determined to show these methods could work. In a now-famous 1984 paper, he and a coauthor laid out the basic framework for how to build a precise device to detect tiny changes in a star’s light output. Then, in 1992, he proposed a space-based telescope using the same technology for planet hunting.15

While NASA thought the idea was interesting, it didn’t believe Borucki’s detectors would work. Unperturbed by the proposal’s failure, Borucki began systematically addressing NASA’s concerns. He built prototypes on the cheap to demonstrate that his system could hit the needed goals. After months of exhaustive work, Borucki’s designs worked exactly as he said they would. In 1994, he spent months putting together the documentation needed to propose his transit-based telescope again. The proposal was rejected a second time.

A different set of concerns was raised in the second rejection. The new questions focused on Borucki’s claim that he could do transit-detection on many stars at once. Once again, Borucki cobbled funds together and carried forward the extraordinary efforts needed to address each and every objection. Four years later, he and his team sent in their new version of the proposal. The proposal was shot down a third time.16

A reasonable person might have given up at that point. But in this regard, Borucki was not reasonable. He knew he was right. He knew the transit method would be a game changer. The only direction he could allow himself to move was forward.

Eventually, Borucki prevailed. After more than two decades of working on the same idea and having that idea rejected as scientifically unsound, Borucki’s proposal was finally accepted. What would come to be known as the Kepler Mission was given the green light.¹⁷

Kepler was designed to stare at a single portion of the sky. In that small patch of cosmic real estate, about 156,000 individual stars had been identified as worthy of attention.¹⁸ The satellite would patiently watch the same stars week after week, year after year. The patience was needed to accumulate enough transits—enough dips in light output—to provide an unambiguous signal of an orbiting exoplanet.

On March 6, 2009, the Kepler telescope rode into space on a Delta II rocket.¹⁹ The launch was flawless. After so many years of rejection, Borucki and his team were staring across the frontier, ready to see how well his decade-spanning vision would work. They didn’t have to wait long.

“As soon as the data started coming in from the spacecraft, we could see transits,” recalls Natalie Batalha, a NASA astronomer who joined Borucki ten years earlier. “You could see the dips as clear as day. We were literally just sitting there in our office, watching as new planets were discovered with each transit.”20

The first confirmed detections of exoplanets by Kepler came in January 2010, but they weren’t the real news. Along with these detections, thousands of Kepler “candidates” were identified. These were stars showing dips in light that hadn’t yet been confirmed as real planet detections. With so many exoplanet candidates, the Kepler team was sitting on the equivalent of a cosmic piñata. By 2014, that piñata had been busted wide open. That year, the Kepler team announced the discovery of 715 exoplanets in a single news release.²¹ Wholesale planet hunting was the new reality. By 2015, the combination of Kepler and other methods had given astronomers 1,800 new worlds that were ready for detailed investigation.²²

As the list of exoplanets grew, one of the first and most important conclusions was how different the architectures of other solar systems could be from our own.

Here on Earth, we all grew up learning about our solar system’s tidy arrangement of small, rocky worlds tucked close to the Sun and larger gas giants splayed out at ever-greater orbital distances. The very first exoplanet discovered, 51 Pegasi b, showed that this arrangement was anything but universal. It’s an example of what is called a “hot Jupiter”—a gas giant that somehow ended up on an outrageously tight orbit. Big planets on small orbits are easy to find in reflex-motion searchers, so many more of these hot Jupiters were quickly added to the exoplanet tally. Lots of stars were also found to have Jupiter-sized worlds on orbits the size of Earth’s, rather than out at the farther reaches of their solar systems.

Eventually, other kinds of planets living close to their parent stars would be found—“hot Neptunes” and even “hot Earths.” Inner rocky worlds and outer gas giants were clearly not the only way nature laid out her planetary families. Systems with hot Jupiters were the most dramatic examples of “weird” solar systems, but there were many other surprises. Systems consisting of only smaller rocky worlds were found, and even they looked weird by our standards.

“One of the big surprises was our discovery of what we call ‘compact multis,’ ” says Batalha. “These are planetary systems with a bunch of small planets clustered very close to each other.”23 In our solar system, Earth and Venus are the nearest neighbors, coming as close to each other as twenty-five million miles. That’s why it takes many months for us to reach these worlds. But in the compact multiplanet system Kepler 42, for example, there are three planets stuffed into remarkably tight orbits. These worlds get one hundred times as close to each other as Venus ever gets to Earth.²⁴ If you lived on one of Kepler 42’s worlds, you could travel to your neighbor planet in just a week or so, using the kind of spacecraft that got us to the Moon back in 1969.

The architecture of planetary systems wasn’t the only surprise. “We found a whole class of planet out there that don’t even occur in our solar system,” says Batalha. There are no planets orbiting our Sun with a mass between those of Earth and Neptune. That represents a considerable gap since Neptune is a big mix of gas and ice and weighs in at fourteen times the mass of Earth. Earth and Neptune are, in other words, very different kinds of planets. But as the exoplanet revolution matured, astronomers soon found worlds—a lot of them—with masses right in that gap between one and fourteen Earth masses. They called these “super-Earths,” and it soon became clear that this new kind of planet, which doesn’t even occur in our solar system, might be the most common in the universe.25

“We don’t even understand what these worlds will look like,” says Batalha. “Some of them could be rocky. But some could be water worlds with deep oceans surrounded by thick water-vapor atmospheres. Others could be a mix of rock and ice and gas. The possibilities are pretty broad.”

Beyond the general findings, there were the incredibly weird specific cases. For example, there’s J1407B, the “super-Saturn” located 434 light-years from Earth. The rings orbiting this gas giant stretch two hundred times farther than the gossamer disk surrounding Saturn.²⁶ Then there’s 55 Cancri e, which is forty light-years away. Its diameter is only twice as great as Earth’s, but it has a mass almost eight times higher, resulting in a density so great that it may be a planet made of diamond.²⁷ And not to be missed is the ominously named WASP-12b. It’s a hot Jupiter with a temperature of nearly 4,100 degrees Fahrenheit, making it one of the hottest exoplanets ever discovered. Astronomers can see a trail of debris surrounding the planet as WASP-12b boils away in a torrent of evaporating gas.²⁸

In the end, though, what matters most are not hot Jupiters, super-Saturns, or super-Earths. The numbers as a whole are what make the exoplanet revolution so important for us. At the beginning of the second decade of the second millennium of the Common Era, humanity finally learned that, in one very real sense, we were not alone. There were other worlds out there. Just as important, with a full census of planets being built, the first three terms in the Drake equation were now fully known. With that advance, questions not only about planets, but even about civilizations other than our own, could be seen in an entirely new light.

DRAKE AND THE EXOPLANET REVOLUTION

The first term in Drake’s equation describes the rate of making stars (called N_*). It has been known with some accuracy since the late 1950s, and subsequent studies have only refined that value (about one star per year).29 But when Drake first wrote his equation in 1961, the second term, describing the fraction of stars with planets (called f_p), and the third term, describing the number of planets in a star’s habitable zone (called n_p), were anyone’s guess. By 2014, in the wake of Kepler and other exoplanet studies, there was enough data in hand to give scientists meaningful—that is, statistically significant—values for those numbers.

The implications of this advance are stunning enough to change our experience of the night sky. Let’s consider the fraction of stars with planets first. Remember that, during the early part of the twentieth century, astronomers believed planet formation was a rare event, meaning the fraction of stars with planets would be very low. But by 2014, the agreed-upon value for f_p was about 1.³⁰ In other words, pretty much every star you see in the night sky hosts at least one planet.

The next time you find yourself outside at night, take a moment to stop and consider the implications of this result as you gaze at all those pinpricks of light. Every one of them hosts at least one world, and most stars will have more than one planet. Solar systems are the rule and not the exception. They’re everywhere.

The advent of Kepler also allowed astronomers to reach a firm conclusion about the average number of habitable-zone planets orbiting each star. Remember that the habitable or Goldilocks zone is a band of orbits around a star where liquid water can exist on a planet’s surface. That means any planet in a star’s habitable zone might be a world of rain and rivers and oceans—a world potentially capable of supporting life. There are currently two planets in the Sun’s habitable zone—Earth and Mars—and both have had water running in torrents across their surfaces.

From the exoplanet data, astronomers can now say with confidence that one out of every five stars hosts a world where life as we know it could form.31 So, when you’re standing out there under the night sky, choose five random stars. Chances are, one of them has a world in its Goldilocks zone where liquid water could be flowing across its surface and life might already exist.

The importance of the achievement represented by nailing these two numbers cannot be overstated. Through the hard-won efforts of a generation of astronomers, we increased the number of known terms in Drake’s equation by 200 percent. Where there was darkness, there now is light. Where there was ignorance, there now is knowledge.

YES, THERE PROBABLY HAVE BEEN ALIENS

But what, if anything, could the trove of data leading us to these numbers reveal about the possibility of other worlds inhabited by technology-deploying, civilization-building species? We still have zero evidence that such civilizations exist. Is there any way to leverage the achievement of the exoplanet revolution to say something—anything—about exo-civilizations? Addressing exactly that question was the task Woody Sullivan and I took on at the beginning of 2015.

I first met Woody Sullivan in the late 1980s, when I was a physics graduate student at the University of Washington. He’s tall and slender with a wry sense of humor and a passion for sundials and baseball (the Seattle Mariners, in particular). Most importantly, Woody is a radio astronomer with an unwavering interest in SETI. When I was a graduate student, he was the only person on the faculty at the University of Washington who worked on the question of exo-civilizations. This was well before NASA began serious funding for astrobiology. The exoplanet revolution was still a decade from its inception. In the 1980s, SETI and its astrobiological surroundings were still considered a bit “out there” for many folks. But Woody didn’t care. He was interested, and he thought there was science to be done. So he pressed on and wrote a number of important papers on the subject.

I once helped Woody teach a course called “Life in the Universe.” He set the class up to deal with everything from the nature of physical law to the prospects for life on other worlds. His perspective was broad and imaginative. I loved being involved with that course, and its perspectives shaped my thinking for decades. It was also the first time Woody and I started talking about exo-civilizations. Those conversations have been going on ever since, even before I did any direct work in astrobiology.

In 2014, Woody and I found ourselves asking if all the new exoplanet data could be used to infer a definite conclusion about technological civilizations on other worlds. The astonishing progress made since the first exoplanet discovery had to be good for something. Wasn’t there some way to use it with an eye toward answering Drake’s original question about our uniqueness in the universe? We soon saw there was a path forward, but to take it, we’d have to turn Drake on his head.

Drake built his famous equation on a simple question: How many exo-civilizations exist now? He chose that focus because his real interest was in finding signals from alien civilizations. For his equation to make sense, those aliens had to be out there, emitting radio signals right now (relatively speaking). But to make the kind of progress Woody and I were interested in, we realized we had to change the focus. We had to ask a different question—one that could be answered by the exoplanet data. Our new question was only slightly different, but the small change we made would mean everything in terms of results. Our question was this: How many exo-civilizations have there ever been across the entire history of the universe?

Taking this approach gave us a strategy for getting an empirically based number concerning the existence of exo-civilizations. First, we combined all the astronomical terms in Drake’s equation into one. This was easy, since they were all known. Then we began thinking differently about three unknown probabilities involving life in Drake’s equation (f_l, f_i, and f_t). Rather than dealing with them separately, our approach lumped them all together, too. We were interested in the process as a whole, going from the origin of life all the way up to an advanced civilization. We called our new term the “bio-technical probability,” f_bt, and it is the product of multiplying all the usual life-centric terms in the Drake equation together. In the language of math:

f_bt = f_lf_if_c

Finally, by asking about the total number of exo-civilizations that had ever existed, rather than limiting our interest to those existing now, we took the issue of the average lifespan of a civilization out of the problem. We didn’t care if the exo-civilization overlapped with our own. It didn’t matter. We just cared that they had existed at some point in cosmic history. Effectively, that allowed us to ignore the final factor—the pesky lifetime term, L—in Drake’s equation.

Our approach gave us a new form of Drake’s equation that looked a lot simpler:

A = f_af_bt

In this version of the equation, A was just the total number of civilizations that had ever existed. We thought of A as standing for “archaeology” because, in a weird way, that’s what we were interested in. Because we took the whole of cosmic history as our playing field, most of the civilizations we’d be describing in our approach would probably be long gone. But all that mattered to us was that they had existed at some point in cosmic space and time. That was the archeological bent our approach took. We saw that the Kepler data could tell us more about what had happened than what was happening right now.

Meanwhile, f_a represented all the astronomical terms in the original equation. The important point was that, since all of those terms were now known, f_a was also known. That left just the biotechnical probability (f_bt). It represented all the unknown, life-oriented probabilities in Drake’s equation. This was what we were after.

By rewriting the equation without L and using the new exoplanet data, we then saw that we could recast the question of the probability of alien life in a way that turned our new form of the equation into a very specific and scientifically meaningful formulation. Our new question, therefore, was: What would the biotechnical probability per habitable zone planet have to be for humans to be the only civilization nature had ever produced over the entire history of the universe?

In other words, what were the chances that ours is the only civilization ever? Putting in the exoplanet data, we found the answer to be 10^–22, or one in ten billion trillion.32 We called this number the “pessimism line,” for reasons we’ll unpack below. To me, the implications of this number are staggering.

To understand how to think about the pessimism line, imagine you were handed a very big bag of Goldilocks-zone planets. Our results say the only way human beings are unique as a civilization-building species would be if you pulled out ten billion trillion planets and not one of them had a civilization. That’s because Kepler has shown us that there must be ten billion trillion Goldilocks-zone planets in the universe. So the pessimism line is really telling us how bad the probability of a civilization forming would have to be in order for ours to be only one that has ever existed.

Ten billion trillion planets is a lot of worlds to go through without finding anything. The sheer size of that number is enough to make it seem like we are not the first time nature has ever created a civilization-building species. By comparison, think about getting killed by lightning, an event most of us think of as unlikely. The probability that you’ll be killed by a lightning strike in any given year is about one in ten million. But, based on the pessimism line, your lightning-induced death is a thousand trillion times more likely than humanity being the only civilization in cosmic history. Surely nature is not that biased against evolving civilizations? It can’t be that perverse. Or can it?

Drake’s question—How many civilizations exist now?—still can’t be answered. But our question—What limit can be placed on the odds that it’s ever happened?—could be. We could put a stake in the ground and say that if nature’s processes of evolution led to odds less than the pessimism line, then yes, ours is the only energy-intensive, technological civilization that’s ever existed. But if nature’s value for the biotechnical probability is higher than one in ten billion trillion, then we are not the first.

After our paper was published in the journal Astrobiology, I wrote an op-ed for the New York Times about our result. The Times ran the piece with the headline “Yes, There Have Been Aliens.” Within days, I was inundated with requests for interviews from outlets ranging from the large and established, like CBS, to small websites run by avid UFOlogists. Some of those folks might have been discouraged from contacting me if the headline had been closer to what we really meant, which was “Yes, Aliens Probably Existed.” But either way, our result was bound to generate controversy. The critiques are worth looking at closely, since interpreting the pessimism line correctly is critical.

Our goal, after all, is to see how astrobiology and the study of life on other planets can help us understand climate change and the project of civilization on our own world. In that pursuit, the pessimism line marks a critical boundary where we might see our project set against the stars. But to truly understand what the pessimism line can do for us in that endeavor, we must first understand what it cannot.

THE CRITIQUE

One of the principal objections raised to our paper (and the New York Times op-ed) was straightforward. Just because the probability that we’re the only civilization in cosmic history is low (10^–22), that doesn’t constitute a proof that exo-civilizations have existed before us. This was the argument made by Ross Andersen, the science editor for the Atlantic, and Ethan Siegel, an astrophysicist who writes for Forbes.33 Andersen and Siegel are excellent thinkers, and their criticisms contained a lot of insight. Their essays cut to the heart of key issues in what Woody and I were trying to explore. Most of all, their skepticism made me think even harder about the ideas in our paper, and I was grateful for that.

There was one point in particular that Andersen took issue with, and it was embodied in this sentence from my Times op-ed: “The degree of pessimism required to doubt the existence, at some point in time, of an advanced extraterrestrial civilization borders on the irrational.”³⁴ He was right to criticize that line. In spite of the bar set by the pessimism line, it’s not “irrational” to think we are unique in cosmic history. In fact, the only empirically valid claim Woody and I can make is this: we can say with certainty where the pessimism line lies. In the absence of more data, it is rationally possible to construct an argument that nature’s value for the biotechnical probability lies below 10^–22.

Questions were also raised about the values for the individual pieces that make up our biotechnical probability. Some argued that the probability of making just simple forms of life would be too low to allow civilizations to ever form. Or perhaps it was the probability of life evolving its way up to intelligence that was really low. But these considerations don’t change our result. Our biotechnical probability, f_bt, does not hide the fact that each of the life-centric terms in the Drake equation might be small on its own. We didn’t establish our pessimism line by ignoring possibly small values for the individual life-centric terms. Instead, our reworking of the Drake equation bundled them all together. Our approach let us go for the whole enchilada at once: the entire evolutionary process, from abiogenesis up to the creation of a technological civilization. No matter how improbable you think each individual step is, it’s the total probability of other civilizations existing that matters. That’s what you have to pay attention to, and that’s what the pessimism line represents.

We called our result the pessimism line for good reason. The whole history of the debate about life beyond Earth is an argument between optimists and pessimists. It’s a debate that began with the opposition between Aristotle and Epicurus, extended through the 1800s to Flammarion versus Whewell, and took its modern turn with the Drake equation, through which the battle between pessimism and optimism became quantitative.

Since the 1961 Green Bank meeting, many scientists have argued that exo-civilizations are rare. What is rarely specified, however, is exactly what “rare” really means. Scratch below the surface, and you’ll see that many self-described pessimists’ version of rare is way above our pessimism line. That’s why the history of the debate can’t be ignored.

Looking across that debate since the Drake equation appeared, we see that optimism always has a clear upper limit. You can’t get more optimistic about the possibility of life evolving on an exoplanet than if you say it always occurs (that would mean setting the value of f_l at 1). The same holds true for the other life-centric terms in the Drake equation. You can’t assign a value greater than 1 to the probability of intelligence—or high technology—evolving. Making all these choices implies that every exoplanet in the habitable zone will create life that goes on to form an intelligent technological civilization.

But pessimism is another story. How low is low? How pessimistic do you have to be—expressed in terms of the Drake equation—to be truly pessimistic about exo-civilizations? That was what Woody and I were asking. Our answer provided a line marking the limit of true pessimism. If nature had a biotechnical probability that was lower than our limit—one in ten billion trillion—then human beings had to be the only example of a high-tech civilization in the history of the observable universe. In that case, we’d be truly and deeply alone in the most absolutely cosmic sense of the word. But if the forces of evolution led to a number higher than the pessimism line, then what’s happened with us on Earth has happened before.

Of course, we still don’t know what nature has chosen. But to see what our next steps might be in thinking about exo-civilizations and our own fate, we can look at how our pessimism line compares with what actual pessimists have proposed for the biotechnical probability.

Pessimist #1: Ernst Mayr. Chief among the exo-civilization pessimists was the renowned German evolutionary biologist Ernst Mayr. Mayr was a brilliant scholar who was instrumental in linking classical ideas from Darwin to the revolution in genetics that occurred after the discovery of DNA. But Mayr never bought Carl Sagan’s optimism about SETI or the existence of other intelligent forms of life. In 1995, the Planetary Society gave both men the chance to voice their opinions on the subject and respond to each other’s criticism. While Mayr never provided an explicit value for the biotechnical probability, from his essay 35 we can extract an estimate of his pessimism.

Mayr had no doubts about life forming on other planets. Of the probability that life exists elsewhere in the universe, he wrote, “Even most skeptics of the SETI project will answer this question optimistically.” Because molecules necessary for the formation of life had been found in cosmic dust, he conceded that it was very possible there was life elsewhere.

The development of intelligence, however, is where Mayr’s pessimism kicks in. Looking at the history of Earth, Mayr wrote, “Only one of these [approximately fifty billion species that have existed on Earth] achieved the kind of intelligence needed to establish a civilization.” And on the subject of intelligence leading to a civilization, Mayr wrote, “Only one of [the twenty or more civilizations that have risen in the past ten thousand years] . . . reached a level of technology that has enabled them to send signals into space and to receive them.”

From Mayr’s statements, we can estimate what he thinks the biotechnical probability might be. Given that he argues that the formation of life is not a hard step, let’s assume he would be happy with a value of one in a hundred for that factor (10^–2). After all, something that happens once every hundred times is not really very rare.

Given his statement about the total number of species evolved on Earth versus the single one that became intelligent, we can infer that Mayr might say that the odds of evolving intelligence on any given exoplanet with simple life would be one in fifty billion (or about 10^–11). That certainly seems pretty pessimistic. Finally, from his statements about civilizations becoming high tech, we might infer he’d consider the probability for that term to be one in twenty. Let’s err on the side of pessimism and call this one in a hundred (10^–2).

If we put all of these together, we would find that Mayr seems to be arguing that the value of the biotechnical probability is around one in a thousand trillion (10^–15). That is certainly pretty small. Recall that if Mayr is right, you would have to sort through a bag of one thousand trillion planets to find a single technological civilization. Given that there are “only” one hundred billion stars in our galaxy, Mayr’s brand of pessimism would mean we were alone in our galaxy.

But being alone in the galaxy and being the only civilization the universe has ever produced are two different things. Comparing Mayr’s pessimism with the limit expressed by the pessimism line Woody and I derived shows something remarkable.

Even if civilizations were as rare as Mayr proposes, there is still a vast gulf between Mayr’s “one in a thousand trillion” and the pessimism line’s “one in ten billion trillion.” To be exact, even if Mayr is correct, there will still have been ten million high-tech civilizations appearing across space and time. That means ten million individual stories of a species waking up to itself. Ten million different versions of science being harnessed to harvest a planet’s resources and build a civilization. Ten million different histories of civilizations either going on to become long-lasting or collapsing under the weight of their own choices.

If you tried to imagine the history of each of these civilizations, giving each one an hour of your time, it would take 1,140 years to get through them all. That’s how many exo-civilizations would have existed in what Mayr thought to be a pessimistic universe.

Pessimist #2: Brandon Carter. In 1983, the physicist Brandon Carter developed an absolutely ingenious argument against exo-civilizations. Carter was famous for using simple observations to infer immensely vast conclusions about the universe and our place in it.

His thinking about exo-civilizations began with the simple observation that the time required for intelligence to arise on Earth was close to the total age of the Sun. In particular, while the Earth has been habitable for four billion years, it will only remain so for another billion or so years because the Sun is continually heating up. It will eventually grow so hot that the Earth’s orbit will no longer be in the habitable zone. Thus, a technological civilization (ours) has only appeared on Earth close to the end of its period of habitability. Using this one fact, Carter made the case that intelligence must have required evolution to pass through a series of “hard steps.” Fulfilling each of these hard steps would itself be highly improbable.36

Looking at Earth’s evolutionary history, Carter argued that there were ten evolutionary hard steps. These included the evolution of oxygenic photosynthesis or of multicelled animals. Based on these ten hard steps, he devised a calculation to predict the probability of exo-civilizations, which was just our biotechnical probability by a different name. Carter’s value came out to be 10^–20. He claimed this was “more than sufficient to ensure that our stage of development is unique in the visible universe.”

What is wonderful about Carter’s calculation is that it leads to an explicit number for the biotechnical probability. The number he calculated was so small, it implied to him that no technological civilization other than our own could ever have existed across all cosmic space and time.

But that’s not what Carter’s number implies! A comparison of the pessimism line Woody and I found with Carter’s result shows that his 1983 calculation still allows for one hundred exo-civilizations. Carter intended his calculation to be hyper-pessimistic, but it turns out to be optimistic instead. Carter’s original argument still leaves us with the remarkable conclusion that we are not the first. If Carter is correct, a hundred other civilizations had passed through the processes of civilization building that we find ourselves in now.

It should also be noted that researchers who have followed Carter’s line of reasoning now believe only five hard steps exist, if any exist at all.37 This consideration, combined with the other values in Carter’s original paper, implies a biotechnical probability of 10^–10. Compare that with our pessimism line, and you end up with a trillion exo-civilizations across cosmic history. Allowing for the existence of a trillion other civilizations is anything but pessimistic.

Pessimist #3: Hubert Yockey. Of course, one can find ways to argue for a hyper-hyper-pessimistic viewpoint. This is exactly what Hubert Yockey did in a 1977 paper. Yockey was a physicist and information theorist. His argument focused on the first life-oriented term in Drake’s equation—the probability of life forming on an exoplanet. What are the odds, he asked, that random chemical combinations would produce the right kind of self-reproducing molecule for getting life started? His answer was less than an astonishing one in a trillion trillion trillion trillion trillion (his actual value was 10^–65).38 This number is certainly below our pessimism line, and if Yockey is right, then we represent the only time in cosmic history that life of any kind has emerged.

But this kind of argument is balanced by the fact that there are strong counterarguments that life’s emergence may not be so hard to achieve. Many of these responses come from advances in biology. For example, biologist Wentao Ma and collaborators used computer simulations to show that the first replicating molecules could have been short strands of RNA (a molecule closely related to DNA and an integral part of cellular machinery). These are much easier to form than what Yockey was thinking about.³⁹ Many researchers also take the fact that life appeared so quickly after the Earth’s formation as an indication that abiogenesis may not be extremely hard to achieve. Either way, Yockey’s hyper-hyper-pessimism seems to be an outlier in the debate about alien life.

THE BIG STEP

The pessimism line doesn’t prove that other civilizations on other worlds ever existed. It doesn’t help us in our search for signals from other civilizations that may overlap with our own. So what, exactly, does it allow us to say or to do?

More than anything, what Woody Sullivan and I did was use exoplanet science to raise a key philosophical point that drew its potency from real observations. It was an opening into a way of thinking about our place in the universe and the challenges of our Anthropocene moment in a radically different way from what we’re doing now.

While the Drake equation was all about making contact with other civilizations, our perspective was straightforward: the exoplanet data now lets us make a reasonable argument that there have been many other civilizations before ours. If you agree that the pessimism line is low enough to make those other civilizations far more probable than we could have known before, then you can also take the step of considering them worthy of serious consideration. With that step, something remarkable can follow in facing the challenge of the Anthropocene.

Before we go any further, let me be clear that you don’t have to make that step. Remaining deeply agnostic about the existence of other civilizations in cosmic history is certainly a stance that science cannot argue against. So, if you don’t think it’s worth considering those other civilizations seriously, that’s fine. Everything we have already explored about astrobiology and the Anthropocene will still hold true. Our understanding of the climate change we are driving today must still be seen as grounded in what we learned by studying other planets in the solar system. And our questions about what to do next must still be informed by the understanding we gained by studying our own planet’s long history of coevolution between the biosphere and Earth’s other coupled systems. We know what we know because we have already learned a lot about what it means to think like a planet. That means there are rules to the game when it comes to the evolution of the Earth, including the Earth with us on it. That perspective alone undercuts the arguments of climate denialists and represents a fundamental shift in how we understand ourselves and the challenges before us.

But if you are willing to see the pessimism line as the universe’s invitation to consider other civilizations seriously, then we can begin to ask what other civilizations mean for us. The purpose here is not to consider them as the source of science fiction stories, but to recognize that we are probably not the first experiment in civilization building the universe has run.

Throughout all of history, our mythologies have told us who we are, what we are, and where we stand in relation to the cosmos. But those stories ignored the possibility that we are one of many. Our stories did not—because they could not—include the possibility that our civilization was a planetary phenomenon that was not unique. That is why the exoplanet revolution and all the astrobiology we have explored so far can be a kind of wake-up call for us. It can be part of our coming of age as a civilization.

The discovery that the universe is teeming with habitable-zone planets connects the challenges we face in the Anthropocene directly with the questions Fermi, Drake, and Sagan asked fifty years ago. The pessimism line tells us that the universe has had lots of opportunities to do what it did on Earth. With that information, we can begin to seriously consider that there have been many other stories, meaning many other histories, beyond our own. It’s an invitation to begin putting ourselves and our choices into a more accurate and fully cosmic context. If we take that step, then everything we’ve learned about planets and climate and biospheres becomes relevant to those other civilizations, too. We can treat those other civilizations as objects of study. That is why a science of those civilizations—a theoretical archaeology of exo-civilizations—is the territory we must explore next.