by Lee Billings
• • •
In 1990, while Sagan was scrutinizing Earth from afar, Sara Seager was beginning her freshman year at the University of Toronto, tearing through her introductory coursework in math and science. She had convinced her father, a doctor who had left medicine to start a small hair-transplant business, that she would pursue a premed track. He encouraged her to specialize in something lucrative, dependable, and relatively stress-free, like dermatology. Instead, to her father’s chagrin, Seager soon switched her focus to physics and astronomy. She had been curious about the night sky ever since she was a young girl, when on nighttime family car rides she would wonder why the Moon always seemed to follow overhead no matter where they went. Soon after, Seager’s father took her to a “star party,” where an amateur astronomer explained the Moon’s orbit and let her gaze at it through a telescope. When she was ten, on a camping trip into the Canadian backwoods, Seager’s view of the world had drastically expanded as she stepped out of her tent at night beneath a clear sky suddenly free of city lights. Looking up at so many stars, she for the first time sensed the continuum that began with the Earth beneath her feet and extended out into the endless heavenly depths above. At sixteen, while attending a university open house, she learned that some privileged people actually studied stars, planets, and all else beyond Earth for a living.
“It was one of the most exciting days of my entire life,” Seager later recalled to me. “You can do this as a job? I rushed home and told my dad. He was so hard on me, and discouraged me with the harshest lecture he ever gave. He said, ‘You have these natural skills, but you need to be able to support yourself and not rely on any man!’ He wanted me to be independent, and just didn’t think it was a good career choice.” Seager’s father valued practicality, but time and time again, he told her she must think big, set goals, and visualize herself reaching them. Otherwise, she should not expect success.
Despite that advice, Seager often described her early path toward astronomy as an unfocused “random walk,” like that of a photon bouncing chaotically around the seething heart of a star. She appeased her father by first concentrating on physics, reasoning that would boost her chances of employment both within and outside of academia, but the more she learned, the less interest she could muster. “I believed you could perfectly describe everything with equations,” she said. “Then I learned that approximations were rampant. I was working so hard for three, four years, why should I suffer my whole life and work so hard for something that’s not enjoyable?”
Approaching graduation, she took a risk and tried her luck applying to astronomy programs at graduate schools. She decided to think big, and submitted an application to Harvard in the fall of 1994. She was twenty-two. To her astonishment, Harvard replied in February of 1995, offering her a grad-school spot as well as a modicum of funding. Seager received the news while cross-country skiing with friends in Ontario. She accepted, and set about planning her move from Toronto to matriculate at Harvard in the fall. That summer, she had little to do other than wait. She decided to travel north and go camping, but she didn’t want to journey alone. Seager reached out to an occasional canoeing partner, Mike Wevrick, a robust thirty-year-old who loved cars and the outdoors. Wevrick looked rather like a grizzled Marine, blue-eyed and broad-shouldered, with long, powerful legs and biceps as big as Seager’s thighs. He sported a crew cut and a constant few days’ growth of stubble on his lean face, and had a reputation for quiet intelligence and kindness. They had first met skiing in Ontario, on the very day Seager learned of her Harvard acceptance. For both reasons, she would later call it the luckiest day of her life.
• • •
Together, Wevrick and Seager devised an ambitious canoeing trip deep into Canada’s Northwest Territories, to the “Barren Lands,” the treacherous tundra that exists past the northern limit where trees can grow, a trackless wilderness so desolate and remote that it was essentially unmapped until after World War II. They would begin by driving four and a half days from Toronto into the boreal forest of northern Saskatchewan, to where the northerly road ended at a lake. From there, for twenty days they would canoe farther north on a series of rivers before finally reaching Kasba Lake Lodge, an outpost with a small airstrip where they could replenish their supplies. From Kasba Lake, they would continue canoeing north past the tree line, into the Barren Lands, on an out-and-back journey that would take another forty days. They planned to be back at Kasba by the end of those forty days to catch a plane south. Wevrick was an expert whitewater paddler, and would guide them through the rivers and lakes. Seager would help with portages, the overland hauling of supplies and Wevrick’s red Old Town Tripper pack canoe between navigable waterways. They left Toronto on June 24, at the end of the summer thaw, planning to return in late August as northern autumn fell.
The weeks leading up to the trip had been arid, with hardly any rain. Departing Toronto, they felt optimistic the weather would hold, minimizing muddy slogs and soggy supplies. But the lack of rain also raised the risk of lightning-sparked forest and prairie fires. Arriving at the end of the road, they found the lake and surrounding forest blanketed in dense smoky haze. They paddled into the gloom and through the mouth of a river, stopping to tie wet T-shirts around their faces when they passed smoldering shorelines. They developed a routine, taking meals in the canoe and paddling through most of the twenty hours of daylight provided by the sub-Arctic summer Sun. If the wind blew at their backs, they rested and used a plastic tarp as a makeshift sail. They portaged as often as fifteen times a day to circumvent sequences of boulder-filled rapids and plunging waterfalls. When they ventured onto land, swarms of biting black flies and mosquitoes rose from the underbrush to assail them. When daylight faded, they made camp and fell into weary, dreamless sleep within their tent.
Seager and Wevrick found comfort in their quiet conversations, and in the harsh, untrammeled beauty of the surrounding wilderness. They walked upon the Precambrian, Archean, Proterozoic rock of the Canadian Shield, the oldest exposed rock on planet Earth. They portaged over the roots of tall mountains transformed to gentle nubs by four billion years of weathering, in a country that had been compressed and scraped clean by the weight of advancing ice sheets during the last glacial advance. Sediment-clogged streams and rivers of meltwater had run like veins beneath the ice sheets, so that when the ice withdrew, it left behind eskers—sandy ridges of pink granitic gravel that followed the twisting paths of the dried-up subglacial flows. The eskers wound between and around kettle lakes, each lake a ghostly puddle from some great hunk of ice long ago calved off the retreating glaciers. The land was still slowly rising up and finding its bearings, rebounding at one centimeter per year from the heavy load of ice that had been lifted tens of millennia ago. Distant columns of smoke from incessant fires lined the horizon in all directions. During their twenty-day trek to Kasba, they saw abundant wildlife, but not another human being outside of each other.
In mid-July, Seager and Wevrick reached the Kasba Lake Lodge, perched on the west end of an island-dotted expanse of water stretching as far as they could see. They picked up supplies, hobnobbed with the lodge’s caretakers, then continued north, up through the lake and into the Barren Lands. Day by day, mile by mile, the trees became sparse, then stunted, then entirely absent, replaced by moss carpets, hardy grasses, and brightly colored lichens. Just north of the tree line, they spotted their first caribou, gazing at them as if they were visitors from another planet. Without trees, a constant wind rushed unrestrained over the sinuous hills and through the rivers and lakes in the hollows. The wind made canoeing slow going, often pressing them ashore at midday. Sometimes on a windy shore Seager would pull an astrophysics textbook from her pack. Far below the tree line, in another world, Harvard awaited. Other times, she and Wevrick would have long talks, seemingly the only people in existence, inhabiting a universe built for two.
The sky overhead was hazy when they reached the northernmost point of their journey. Even though they were
more than 200 kilometers north of the tree line, smoke from the fire-ravaged southern forests still reached them on strange, unseasonal winds. Atop a treeless knoll, they stumbled upon five stone cairns. Old Inuit graves—the first sign of humans Seager and Wevrick had seen since Kasba. Scavengers or looters had scattered some of the rocks aside, revealing artifacts of rusted metal and wood, as well as a small sun-bleached human skull. Seager snapped a photograph. She wondered what the person had looked like, how they had died, and why they had chosen to live so far from the world she knew. She looked up from the skull to the surrounding hills, spotted with pale grass and summer wildflowers, rolling on and on. The silence was broken only by the whisper of wind rippling from horizon to horizon. Silver and blue circles of sky pooled in the clear, cold water of countless lakes. In that moment she understood why someone might abide in such everlasting solitude.
After crossing back south beneath the tree line, they entered what Seager called “Esker Territory,” a seemingly interminable warren of sandy pink hills coiled in complex double and triple ridge systems, with small lakes and woods lying in the valleys between. It was beautiful, but exhausting to cross. The days blurred, and the geography rolled by like even-spaced rumble strips on a lonely highway. All was esker. Then lake. Forest. Esker. Lake. Boulder-filled streambed, esker, and lake. They smoothly, silently paddled and portaged for hours, now adapted to the rhythm of the land long ago laid down in the ebb and flow of ancient ice. No words were needed. They were in unison, almost reading each other’s thoughts. On one of their last evenings in the far north, Seager stood alone atop a spruce-sheltered ridge, contemplating the blue lakes and pink eskers as the Sun sunk lower in the sky. They were in a different world, one made all the more real by its distance from the bright, baleful cities and ever-scurrying crowds. Perhaps someday the cities and crowds would encroach here, too, driven poleward by drowned coasts, but for now the land lay empty. They had seen no one else for over a month, yet they were not lonely. They ate when hungry, slept when sleepy, and lived simply, yet never yearned for more. “We had grown so content with each other’s company that we had no psychological or emotional cravings for anything or anyone from ‘outside,’” she later wrote. “The trip became our perfect life.”
When their plane lifted off from the airstrip at Kasba on August 28, Seager looked down with wistful thoughts at the windswept lake and the little rivers that blindly meandered through the grasslands and conifer woods. “In sixty days, ‘real’ life had become so dim as to seem partly impossible and mostly unbearable,” she would recall. “The solitude, the vast wilderness, the free and compelling lifestyle, the constantly changing terrain, and my excellent companion were a truly unbeatable combination.” She realized she had not only fallen in love with remote desolation; she had fallen in love with Wevrick, too. Soon after they returned, Seager asked him to move with her to live together in Cambridge. Without hesitation he agreed.
• • •
At Harvard, Seager initially focused on cosmology, specifically the basic physics behind “recombination,” an event that occurred less than a million years after the Big Bang. Back then, our universe was still just a hot expanding mass of plasma, an opaque fog of electrons and protons with no atoms, no molecules, no stars and galaxies. For hundreds of thousands of years the plasma cooled and expanded, until it reached a critical transition, becoming cold enough for electrons to “recombine” with nuclei, glomming together to form atoms. In a literal flash, the atoms froze out of the primordial plasma, transforming the expanding fog of plasma into a transparent cloud of hydrogen and helium, unleashing a flood of light that still reverberates through the universe today. We detect it as an omnidirectional all-sky glow of microwave radiation with a temperature less than three degrees above absolute zero. As Seager worked on recombination, the first discoveries of hot Jupiters were trickling in. She approached her advisor, Dimitar Sasselov, looking for ways to segue into exoplanets, which she saw as a more interesting topic. Sasselov steered her toward modeling hot-Jupiter atmospheres, since, as with the epoch of recombination, the associated calculations partially concerned the mechanics of high-temperature hydrogen and helium. From that seed sprang Seager’s subsequent PhD and her career-defining early work that led to the first detection of an exoplanet’s atmosphere.
Meanwhile, Wevrick forged a successful career of his own writing and editing high school science and math textbooks. Throughout Seager’s Harvard tenure they escaped the city for the countryside whenever they could, and they eventually married in 1998, the same year that Seager completed her PhD thesis. The following year they relocated to Princeton, New Jersey, where Seager had secured a five-year fellowship at the Institute for Advanced Study, the same establishment where Einstein had spent the last years of his life. There, with the encouragement of another mentor, the late astrophysicist John Bahcall, Seager began meeting with several exoplanet-oriented researchers at nearby Princeton University, developing concepts and techniques that could be used to characterize exoplanet atmospheres and surfaces with one of NASA’s forthcoming TPF telescopes.
After one such meeting, Princeton’s David Spergel was inspired to conceive the coronagraphic masks that became a technological pillar for TPF-C. After another, Seager and two Princeton astronomers, Eric Ford and Edwin Turner, devised an exquisite method to gain information about an Earth-like exoplanet solely from the fluctuating brightness of its pale blue dot as seen across interstellar distances. They began by developing a model to calculate the amount of scattered starlight any given planet could project toward a distant observer, and as a test case ran it based on Earth-observing satellite data. As our virtual pale blue dot turned in various viewing geometries beneath their model’s scrutiny, over time the team found they could discern what region of the planet they were looking at solely from its brightness, despite its reduction to an unresolved starlike point.
Looking down on the equator, for instance, each day like clockwork the relatively bright continents of North and South America would rotate into view, sandwiched on either side by long dark stretches of open Atlantic and Pacific Ocean. In their repetition, such patterns revealed the length of Earth’s days. With the rotation rate established, Seager, Ford, and Turner could attempt more granular mapping, trying to discern the bulk fraction of ocean versus land, as well as finer features such as forests, prairies, deserts, and ice sheets. They feared bright reflective clouds would confuse their observations, but they found that clouds tended to arise and dissipate in predictable ways—more often at land-sea interfaces and less frequently over open ocean and dry continental interiors. They learned to distinguish the reliably cloud-free Sahara Desert by the intense near-infrared brightness of its hot sand and the lush, verdant Amazon Basin by its constant blanket of clouds. They saw hints of ice sheets, lakes, and seas by occasional spikes in brightness, when their smooth, flat reflective surfaces glinted sunlight back into space like mirrors. Given enough time, they suspected, they could even discern the varying reflectances of shifting vegetation, clouds, and ice cover that would come through changes in weather, seasons, and climate. All purely from a single wavering point of light, without the need to first obtain planetary spectra using an 8- to-16-meter mirror in space. Of course, they had the advantage of already knowing what they were looking at; teasing apart such features for the unknown environment of an actual faraway terrestrial exoplanet would be much more difficult. But the technique offered hope that even a relatively small 2- to-4-meter space telescope might be able to roughly map any Earth-like planets around the handful of closest stars. Seager pressed on, churning out a series of papers outlining how extremely precise measurements of transits could reveal properties such as an exoplanet’s rotation and atmospheric structure.
Now midway through her fellowship, Seager began searching for what would come next. Despite her leadership in the rapidly growing field of exoplanets, she received polite dismissals from many potential employers, who seemed to believe Seager’s optimi
stic visions of finding other Earth-like worlds would never come to pass. The exception proved to be the Carnegie Institution, which offered her a job in 2002. With Bahcall’s blessing, she left the Institute for Advanced Study and moved with Wevrick to Washington, DC. At Carnegie, she became even more involved with planning for NASA’s TPFs, and was for the first time exposed to the rigor of geophysics. Seager began exploring how to theoretically and observationally constrain not only an exoplanet’s surface and atmosphere but also its deep interior—things like its bulk composition, or its likelihood of volcanic activity and plate tectonics. Transits were key, since they allowed astronomers to measure a planet’s radius, its size. Paired with mass estimates from radial-velocity measurements, this yielded a planet’s density. Seager and others developed mass-radius relationships for worlds of various compositions, estimating how planet hunters could distinguish between, say, one Earth-size planet made of pure water and another composed of mostly carbon, or iron. The work would later prove crucial as more and more worlds of intermediate dimensions were detected. When they transited and their densities were calculated, many of the so-called super-Earths astronomers were finding proved in fact to be “mini-Neptunes,” gassy worlds with thick, opaque atmospheres of hydrogen and steam, rather than rocky planets with thin layers of translucent air.
As an emerging leader in the burgeoning field of exoplanetology, Seager began receiving frequent invitations to speak at high-profile conferences, meetings, and colloquia, and her retreats to the wilderness with Wevrick grew few and far between. In 2003, the trips for work and play were both sharply reduced—Seager became pregnant, and gave birth to their first child, a boy. They named him Max. Another boy, Alex, followed two years later.