The Smallest Lights In The Universe
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First we adopted a gray-striped tabby that I named Minnie May, after the sick girl saved by Anne in Anne of Green Gables. (I had subconsciously inherited my mother’s penchant for naming animals after literary characters.) Mike had been resistant to pets—he was so afraid of having anything like dependents, the thought had given him chest pains—but then we agreed that Minnie May needed a friend, and we took in another kitten who turned into a big fat cat named Molly. She became Mike’s cat, curled up near him whenever he did his work. Later came a feral black cat named Cecilia that I never managed to tame.
Cecilia’s spectral nature surprised me. I have always felt connected to animals. I think it’s because, unlike people, they are easy for me to read. Their needs are finite, physical as often as emotional, and I know how to meet them. Animals don’t get puzzled or angry when I say the wrong thing. They have short memories. They don’t cast judgments or see weakness in difference. They don’t take my energy and concentration; they give those precious things to me. Animals are blind to everything but love. Animals forgive.
A dog seemed like the next logical addition to our kind-of family. Soon Mike and I were back at the shelter, adopting what we thought was a Labrador cross. We named her Kira—well, Mike named her Kira, after one of Ayn Rand’s spirited protagonists—and she turned out to be some mix of terrier and pit bull. She was strong and willful, with a giant ridge on her head that worked like a flying buttress for her massive jaws. Mike had second thoughts about keeping her, but I thought she was beautiful, and he began to see what I saw in her. He started taking Kira in his canoe, and she became the best kind of ballast.
I thought of Mike and me as celestial bodies, distinct from each other but tied together by the universe’s invisible forces. We were like the two moons of Mars: Phobos and Deimos follow different trajectories, but they act in strange, satisfying concert, like the twin sons of Ares and Aphrodite for which they are named. The two moons are relatively small, so they remained hidden from us until Asaph Hall, an American astronomer, found them in 1877. Hall must not have been filled with the warmest thoughts when he named his discoveries after the personifications of horror and terror, but there was an overarching logic to his nomenclature. Mike and I somehow named pets after characters from Ayn Rand and Anne of Green Gables.
At least we both loved to read. Mike was a libertarian who believed in the supremacy of the individual; I believed in the supremacy of the universe. Mike thought deeply about philosophy and history—he could lose an entire day reading a biography of a long-dead president, whereas I might last two pages before I fell asleep. Philosophy was too abstract, too aimless for me. Mike would say the same was true of my work. For him, particle physics was witchcraft; advanced mathematics was sorcery. He would edit my writing for grammar and structure without being able to understand the meaning of a single sentence.
And yet: We were both capable of intense focus. We were both parsers. Neither of us accepted easy answers; we just asked different questions. It was as though our brains were the same machine, only wired for different purposes.
I would be leaving Harvard soon, and we had to decide what would come next. Our choice, it seemed to me, was binary: Either we were going to get married or break up. I gave Mike six months to decide between our possible paths. When Mike’s time was up and he still didn’t seem certain, I reminded him how long it had taken us to find each other. The odds of his finding another us were infinite. It was a life with me or a life alone, I told him. He stood in the light of our carriage house and chose me. I jumped out of my shoes.
We were married in the fall of 1998 at the University of Toronto. I didn’t want a traditional wedding—I never believed that love needs an audience or public confession to be real—but my family wanted one. Mike and I stood in front of a small audience in Hart House, a beautiful Gothic building on campus. I’ll never forget how he looked at me that day: with love, with hope, with pride, the way only a groom sees only his bride. After the ceremony, we turned to walk down the aisle, his hand in mine, and my hand in his, and I was surprised to see everyone rise to their feet and burst into applause. Our families and friends could see that our love story had reached its right and happy end. Our Unitarian minister chose to compare us to rivers, not moons: two streams that had run parallel until they finally, inevitably, fell into each other. I still thought of us as twin satellites. Formally joined in the eyes of the law and our loved ones, we remained, to my mind and I think to his, two separate pieces.
But somehow the pieces fit.
*
Late in my time at Harvard, not long before my PhD defense, I was invited to speak at the Institute for Advanced Studies in Princeton, New Jersey, most famous for serving as Albert Einstein’s academic sanctuary after the war. I was awestruck when I arrived. The Institute seemed like a place where monks should gather, convening in circles on the grass, under the branches of enormous trees. I was put up in an elegant bedroom in the corner of a mansion. The walls were all painted white, and the silence was almost eerie. It was a place for thinking about the universe.
I was met by John Bahcall, my host and the “boss” of the astrophysics group at the Institute. He was a legendary figure but didn’t carry himself that way. His glasses softened his already kind face. His gray hair clung to his head in tight curls. He reminded me of a rabbi, and in a way he was one. In the 1960s, when most Americans were looking at the moon, John was enraptured with solar physics, determined to answer fundamental questions about stars. He wanted to know why the sun shines. Later, he was one of two people crucial to the existence of the Hubble Space Telescope, and the standard model of the Milky Way is named the Bahcall-Soneira Galaxy Model, half after him. He asked me how my walk over from the mansion had been, fatherly from the start. “If you were my daughter,” he said, “I’d want to be sure the walk wasn’t too far.”
My work in exoplanets, like the field itself, was still incipient. I had decided to speak instead about my contributions to the timing of the atomic events that followed the Big Bang. My talk was in the library, and I took in the smell of the floor-to-ceiling bookshelves as I sat and waited for the audience to arrive. Postdocs and the occasional luminary began filing in. I pulled up my long hair and clipped it back, one less distraction. John soon arrived, and then in walked Jim Peebles, a giant in the field of cosmology. Today Jim is the Albert Einstein Professor Emeritus of Science at Princeton and winner of the Nobel Prize in Physics. He also happened to have made some of the original calculations that I had corrected.
I began my talk, again using an overhead projector to illuminate my work. I hadn’t been speaking long when someone asked, a little abruptly: “How many electron energy levels did you use in your model hydrogen atom?” The answer was on my next plastic sheet.
“Three hundred,” I said.
Another hand went up. “Did you include helium in your calculation?”
Once again the answer was on my next sheet, and I flipped to it. “Yes. Both helium and ionized helium.”
A few sheets later, another question: “What is the specific reason the recombination of electrons and protons proceeded more quickly than predicted?” I would learn that this was the Institute style: Nobody is allowed to finish a thought unchallenged. Yet again the answer was on my next sheet. The audience laughed. I laughed, too. I looked across at Jim Peebles and saw him nod. That nod felt like acceptance.
The next day, John gave me a ride to the train station. I had no sooner slipped into the car and shut the door when he turned to me. “Sara,” he said, “I’d like to offer you a job here.”
I looked out the window for less than a trillionth of a second. I turned back to him with the widest smile. “I’m so pleased to accept,” I said.
For years afterward, John joked about how quickly everything had unfolded, poking fun at me especially for not talking to Mike first. I should have. But the Institute just felt like the right home for me. In a field as vast and daunting as ours, mentors are so important. The best ones show you not only where to look but also how to see. I felt certain that exploring the universe with John would be as close as I could come to standing at the shoulder of Galileo.
The line between lunacy and scientific fact, John would tell me, is forever shifting. Former impossibilities become accepted truths, which means that astrophysicists can be judged only from a distance. To this day I don’t know how much faith John had in the future of exoplanet discovery. Our research interests didn’t overlap much, and he never told me what he thought was within the realm of possibility. He had been surprised often enough by physics, and by people, to have been humbled.
Maybe, as a young scientist, you have an idea that can’t be proven despite its having a solid foundation in physics. It might make intuitive sense, but some theories, especially the most revolutionary, resist experimental evidence. John told us not to fear. When better instruments almost inevitably come along, and some future scientist, following your earlier hunch, uses them to make an important breakthrough, then your work was still worth pursuing. What you did this week or this month or even this year wasn’t important to him. What mattered was the sum value of your lifetime.
What could make us more ambitious in our thinking? What could make us more daring? More and more, my life felt like the product of the decisions that I made. I could feel an unfamiliar certainty rising in me, that I was where I was supposed to be, doing what I was supposed to do, with the people I was supposed to be doing it with. It had taken me a long time to find my way, but for the first time in my life, I wasn’t lost and lonely. I wasn’t an electron anymore. I was atomic.
CHAPTER 4
In Transit
Einstein’s oasis at the Institute for Advanced Studies felt more like a launchpad to me, the seeds of ignition in every blade of grass. I sat under those enormous trees throughout the fall of 1999 and pondered the next step in my journey to the farthest reaches of the galaxy.
Not long after I arrived, NASA sponsored an effort to find the first Earth-like exoplanet. By then, about forty exoplanets had been discovered using radial velocity. Astronomers were still stuck sensing the presence of other worlds rather than seeing them, and those they found were all too big and hot to give life a chance. The agency set a higher bar: It wanted to find a rocky planet of reasonable size that orbits its sun-like star in what’s known as the “Goldilocks zone,” neither too hot nor too cold to sustain life.
NASA also wanted evidence of that life’s existence.
It was a virtually impossible ask, but NASA had sponsored countless unfinished efforts to find extraterrestrial life over the years. Now they wanted to wipe the slate clean and start again. They called the program the Terrestrial Planet Finder. Four teams were chosen to participate; one included people from Princeton, and I was invited to join. I was surprised to learn that the belief that we could find another Earth was flourishing, at least in some circles. The truth was, we’d be thrilled to find a planet that was Earth-like. But engineers yearn toward specificity; they want to know what exactly they are designing their machines to do. We would tell ours that we wanted to find another Earth: a perfect copy, an identical twin. We planned to find another us.
From those first nights I spent looking through my telescope with my father at my side, I had wondered what else—who else—might be out there. I had always had the gut sense that we weren’t the only light in the sky that life called home. Why us? We couldn’t be that special. Now, for the first time in my embryonic career, I was being asked to help turn that feeling into fact. We needed to know that there are other Earths, to be able to point at a celestial map and say: That one, right there. That was our goal.
The word “no” was banned from our gatherings. David Spergel was our team’s local committee lead, and we met every week at Princeton’s Peyton Hall. A practically visible current leapt like voltage from one dreamer to the next, each new idea lighting up the room a little more brightly. For a brief spell, we had the budgets and youth to imagine a seriously fantastical future.
Apart from the ever-challenging problem of distance, we had to reconcile the fragilities of light. At its essence, astrophysics is the study of light. We know that there are stars other than the sun because we can see them shining. But light doesn’t just illuminate. Light pollutes. Light blinds. Little lights—exoplanets—have forever been washed out by the bigger lights of their stars, the way those stars are washed out by our sun. To find another Earth, we’d have to find the smallest lights in the universe.
We began with a simple question: If aliens were observing distant exoplanets with their own version of the Terrestrial Planet Finder program, what would Earth look like to them? We all realized that Earth’s brightness wouldn’t be constant. The continents reflect light. The oceans absorb it. So if you were an alien who happened to look at Earth when North America was facing you, our light would be brighter than it might be a little later, when the Pacific Ocean slipped into dimmer view. Maybe we could use that curve in the light to infer the presence of oceans on distant planets, and thus water, and so perhaps a planet that might prove suitable for life. Oceans might be the biggest windows in the universe.
Given the chasm in relative brightness between stars and planets—a sun is about ten billion times brighter than an Earth—our main challenge was figuring out a new way to make the stars go dim. Our committee’s most tangible achievement, credited to David, was a new design for an instrument to do just that.
Coronagraph: I love that word. It’s an umbrella term for any kind of light-snuffing device built within a telescope. The first coronagraph was invented in the 1920s by a pioneering French astronomer named Bernard Lyot. He was studying the sun, and he had used two small, circular light blockers inside his telescope to create an artificial eclipse. His system worked well enough for looking at the sun. But, like the concentric ripples made by a stone when it’s dropped in water, light waves radiated around Lyot’s pair of internal shields. Those thin halos would obscure the far more distant exoplanets that we wanted to see. David suspected, and others on our team helped prove, that a shape more like a cat’s eye would push those same light ripples farther out of sight. The darkness our coronagraph left behind wasn’t a perfect darkness, but it was very, very dark.
Building a new coronagraph to go inside a new telescope might take decades, however. The Terrestrial Planet Finder program remained a product of our imaginations, not our hands. I wanted more immediate returns. Inspired in part by my committee work, my mind began wandering to those forty or so exoplanets that we had already found. Even if they couldn’t harbor life, maybe they could reveal something about how to find it. It’s easier to come up with a new way of seeing when you already know what you’re looking for.
Theoretically, there was a way to find and study exoplanets other than radial velocity. If, for the moment at least, astronomers couldn’t fight the brightness of stars, maybe we could use their power to our advantage. Bodies in transit sometimes align—not always, but every now and then. If we were lucky, it was possible that a planet might pass between us and its star. It stood to reason that the effect would be something like a miniature eclipse. The moon looks giant when it blocks out the sun. The Transit Technique, as it would come to be called, applied the same principle to exoplanets: We would find them not by the light they emitted, but by the light they spoiled. Nothing stands out like a black spot.
The Transit Technique made perfect sense to me. Working in conjunction with radial velocity, it could tell us more about any particular exoplanet than radial velocity could on its own. You can learn a lot about an object from its shadow. A few pioneering, risk-seeking astronomers began monitoring the most favorable of the few dozen or so stars that we already believed hosted at least one exoplanet, waiting for a transit. I called my former adviser at the David Dunlap Observatory to see whether we could try, too. The telescope’s camera at my old haunt wasn’t sensitive enough for the job, but Hot Jupiters were capable of eclipsing about 1 percent of their star’s light—more than enough for us to measure with better tools. We calculated that each suspected short-period planet had a one-in-ten chance of passing in front of its star. Those aren’t the best odds, but they are far from infinite. I woke up every morning wondering whether someone had detected a transiting planet in the night. I was filled with the feeling that the world might change with every email, with every ring of the phone.
It happened so fast, I almost missed it. That November, I decided to take a weekend off. Work had been threatening to overwhelm me; I was too caught up in the frenzy of potential discovery. I took Kira for a long walk across the Institute campus. Most of the leaves had fallen, and the weather was just starting to turn. I tried to breathe a little more deeply than I had been. I felt the muscles in my shoulders start to release.
On Sunday night, I checked my email. There was one from Dave Charbonneau, a student I knew. He had been at the University of Toronto behind me and was now doing his graduate work at Harvard. From the opening sentence, he sounded crestfallen.
Back in September, he wrote, he had collected data while testing his thesis adviser’s tiny telescope in Colorado. The eventual goal was to be able to survey a wide field of stars in one swoop, increasing the chances that his adviser’s team might find a transiting planet. For some reason, he hadn’t looked at the data culled from the test star until November. When he did, it was a revelation: He had detected the first transit. He had seen a transit of a known planet—HD 209458b, a Hot Jupiter. It was absolutely fantastic news. He had erased the last shred of doubt that exoplanets exist.
Right around the same time, however, Geoff Marcy and Greg Henry, two far more established astronomers, had seen the same black spot on the same star. (Marcy was something of a celebrity in our field. He and his research partner would eventually claim seventy of the first hundred or so exoplanet discoveries.) Marcy, Henry, and their team had observed the star later in the season, as it was setting, so they saw only a partial eclipse. Dave had seen a beautiful full transit. I don’t know exactly what happened next, but Dave’s adviser might have confessed something about his discovery to Marcy. Rather than waiting to do his own follow-up, Marcy seemed to use that conversation to confirm his team’s find. Within a week, they had issued a press release: The first transiting planet had been found, and they had found it. Dave Charbonneau had come in second.
Pioneers can be ruthless. I wrote Dave: You are a fantastic scientist and time will show that. Then I told him what he already knew. He should still publish his findings, regardless of what his competitors did. That paper would matter more in the end than a press release. Time will tell the truth, I wrote.
*
The more immediate truth was that one race was over, and a new one had begun. That walk across campus with my dog was the last break I would have for weeks. I had been turning over an idea—a genuinely original one—and the successful use of the Transit Technique gave it a greater urgency. I was practically swollen with it. A lot of science, especially pioneering science, relies on intuition. You’d be surprised to learn how many of our most significant discoveries began as a hunch, as a feeling. I didn’t have any evidence that my idea would work. But I was doubtless.
I had realized that the Transit Technique might help reveal something more than the black silhouette of a planet. Immediately around that tiny, partial eclipse, the same starlight that was being blocked by an exoplanet would pass through its atmosphere. The starlight would reach us, but not the way regular starlight reaches us. It would be filtered, like water running through a screen, or a flashlight’s beam struggling through a fog.
If you look at a rainbow from a distance, its many colors form a perfect union. But if you look at a rainbow more closely, using an instrument called a spectrograph, you can see gaps in the light, minuscule breaks in each wavelength like missing teeth. Gases in the solar atmosphere and Earth’s own thin envelope interrupt the transmission of sunlight, the way power lines cause static in a radio signal. Certain gases interfere in telltale ways. One gas might take a bite out of indigo, while another gas might have an appetite for yellow or blue.
Why couldn’t we use a spectrograph to look at the starlight passing through a transiting exoplanet’s atmosphere? That way we could determine what sorts of gases surround that exoplanet. We already knew that large amounts of certain gases are likely to exist only in the presence of life. We call them biosignature gases. Oxygen is one; methane is another. Maybe the way to find Bigfoot really was by seeing his breath. We could start with Hot Jupiters, the planets we already knew, and their more easily detectable atmospheres. Like a skunk’s spray, their traces of sodium and potassium would stand out amid the company of less potent atoms.
I kept my idea to myself, because I knew it was great—I was the first to see the potential of the Transit Technique for studying atmospheres—and now I knew, too, that great ideas get stolen. Dimitar Sasselov, my former PhD supervisor, was the only person I told about my theory, and he offered to help me bring it closer to practice. When we had worked out the details, I published a paper extolling what Dimitar and I called “transit transmission spectra.” Reading the gaps in rainbows.
My paper received considerable attention. NASA was accepting proposals to use the Hubble Space Telescope; within a few months of publication, one team cited my work and won the rights to study the light that passed through the atmosphere of a transiting Hot Jupiter. I was furious not to be included on that team, which chose an older male scientist over me.
Within two years, their work revealed the first exoplanet atmosphere. It didn’t surround another Earth, but my premise had worked. We had seen our first alien sky. I had profoundly mixed emotions on the day of the announcement. My theoretical efforts had triumphed, but I still felt as though I hadn’t been invited to the party I had helped to throw.
News of the discovery was embargoed from the public until one o’clock on a Tuesday afternoon. I told John Bahcall that I’d be ready to talk about it at 1:01 P.M. Every Tuesday there was a formal lunch called (imaginatively enough) Tuesday Lunch, at which John took his seat at the head of a horseshoe-shaped table filled with some of Princeton’s finest minds. I was proud to join them. John had a habit of cutting off boring speeches by clanking his glass, terrifying some of my colleagues, but I wasn’t scared. He was won over by careful, quantitative work, and I had done the work. I stood up and explained what others had done with what I had done, the chain of succession of my idea, from my lamp-lit desk to deepest space. The thought made me breathless. We had traveled light-years. It hadn’t taken any time at all.
John smiled with pride, and I warmed in his admiration. But he was far from satisfied. “What’s next?” he said.