The Stardust Revolution

by Jacob Berkowitz

  But Rosenblatt's grand planet-finding idea lived on in print, and with it, Bill Borucki had the project he'd been looking for all his life. If there's extraterrestrial life, it has to live somewhere. So the first step in trying to make contact is to find out if there are indeed other Earth-like planets and, if so, where they are. At the time, the other exoplanet-hunting techniques considered for NASA missions, such as stellar astrometry—observing a star's infinitesimal movements back and forth across the sky as a result of an orbiting planet—were sensitive enough to possibly detect Jupiter-sized planets. The beauty of Rosenblatt's transit technique is that it would be able to spot smaller planets, maybe even Earth-sized ones. Here was a way to actually search for alien Earths.

  From his first paper on the topic in 1984, Borucki began to build a vision and an informal team of researchers with the goal of detecting Earth-sized planets orbiting other stars. As this vision developed, several things became clear. First, it would be more difficult than Rosenblatt imagined. To see small planets, they'd need to develop more sensitive light meters, or photometers. More importantly, Borucki and others calculated that the Earth's atmosphere was simply too turbulent and messy a lens to see through if they were hoping to spot minute changes in a faraway star's brightness. To spot the eighty-four-parts-per million fading of a distant star by a transiting exo-Earth, a telescope would need to be beyond Earth's atmosphere, in the darkness of outer space.

  By 1992, Borucki's vision had morphed into a mission proposal, one that, like Apollo, would go big from the start. The goal wouldn't be just to find a single other Earth but to take a census of tens of thousands of Milky Way stars to determine the percentage of them orbited by Earth-sized planets. It was a broader scientific goal: not just are we alone, but also just how populated—at least with Earth-like planets—is the Milky Way?

  To achieve this, Borucki's mission had to go beyond just spotting Earth-sized planets to identifying those in their star's Goldilocks zone. Starting in the early 1960s, astrobiology pioneers began considering what a planet around another star would require to sustain life. As in all real-estate questions, at the top of the habitability list was location. A planet would need to be in its star's Goldilocks, or habitable, zone—a distance from the star within which the temperature would be not too cold and not too hot but would be just right for there to be liquid water and thus, potentially, life. The exact position of the habitable zone would be different for each star-planet system, dependent primarily on the star's size and the planet's atmosphere. A small drop in star size makes a sizable decrease in energy output. Thus, for smaller stars, the habitable zone is much closer to the star than is the case in our Solar System, where the zone is thought to stretch from this side of Venus's orbit to before that of Mars. In practice, however, each alien world's habitability would require individual examination, because a planet's atmosphere plays a crucial role—as the difference in habitability between Earth and the Moon quickly makes clear.

  At NASA-Ames, Borucki now occupies the former office of longtime Kepler colleague and friend Kent Cullers, the outline of whose name is still visible as a palimpsest from the four-inch-high letters in black electrical tape he'd used to mark his locale. Cullers, the first astronomer to be blind from birth, turned not to light but to math and sound waves as a way of exploring the cosmos. He developed the first advanced mathematical codes to detect signals coming from alien civilizations, codes that became the core of the search for extraterrestrial intelligence (SETI) program. They were a perfect pair: Borucki wanted to find extraterrestrial intelligence; Cullers wanted to communicate with it. Together, the dreamer, the blind man, and other early members of the Kepler mission calculated the number of stars they thought they'd need to study in order to get a reliable Earth-like exoplanet census. In 1996, they suggested thirty-four thousand stars, but as Borucki continued to pitch the project and as technology improved, they broadened the search to one hundred and fifty thousand stars. If NASA was paying the same price, why not offer more stars? As Borucki put it, “Would you rather pay for something that does fifty thousand or one hundred and fifty thousand stars?”

  While he developed the mission concept, he also led the proof-of-concept and R&D to demonstrate the underlying technologies. He spent years developing quantum-perfect charge-coupled device, or CCD, photometers, which were able to sense the energetic ping of a single incoming photon. At the Lick Observatory in California, Borucki and his team showed that automated photometry was possible by building Vulcan, a robotic system that monitored the light of ten thousand stars simultaneously.

  Yet, for almost a decade, Borucki's exoplanet mission proposal repeatedly failed to make the grade at a NASA focused on the Space Shuttle, the International Space Station, and the Hubble Space Telescope. After all, why look for alien Earths if no one's even found a single planet of any size outside our Solar System? The discovery of 51 Peg b in 1995 changed all that. Featured on the cover of Time magazine and in scientific journals was the dawn of a new era: worlds around other Suns. Then, in 2000, the first exoplanet was discovered based on its transit of its star. Rosenblatt's technique worked. Finally, in December 2001, after four formal proposals, Borucki's mission, renamed Kepler, had wings. On the evening of March 6, 2009, Kepler lifted off from Cape Canaveral Air Force Station atop a Delta II rocket to begin its planet census mission. Borucki's dream of searching for alien worlds was now a $650 million space-based telescope, one of NASA's Great Observatories, and a key part of its Origins Program.

  On the eve of the Extreme Solar Systems II conference, the baton had passed for the moment to Natalie Batalha, Kepler's acting science team leader. Batalha didn't sleep the night before the conference. As an astronomer, she's accustomed to long, sleepless nights. That night, however, was different. She wasn't operating a telescope, the usual reason for pulling an all-nighter. On that cool night in September, the first snow forecast at altitude in the Teton Mountains for the end of the week, Batalha wasn't imagining finding alien worlds; she was hurriedly crunching the numbers on the worlds Kepler had already discovered. If she stepped outside her motel-like room at the Jackson Lake Lodge and peered into the night sky, she'd see Kepler's field of view: just off the plane of the Milky Way, to the right of Cygnus, the Swan constellation, a swath of night sky—about the area covered by your hand held out at arm's length—including more than 150 thousand stars, ones that Kepler relentlessly monitored for evidence of transiting planets. It was Batalha's job to summarize the last three months of Kepler's planet-hunting data, the endpoint of an exhaustive process of weeding out potential false alarms.

  Given the history of exoplanet false calls, the Kepler project treats possible planet detections in the same way that bartenders want to see two pieces of identification before they start pouring drinks. Whenever Kepler's data-analysis system senses three near-identical transits around a star, the team's software automatically produces a twenty-page Threshold Crossing Event (TCE) report. Then the humans, the TCE review team led by Batalha, get to work on scrutinizing the data and seeing if it truly fits a transit pattern as expected from an exoplanet, or if it is an aberrant light blip. They analyze the collection of photons detected by Kepler's photometer, or light meter, that form the stretched-out “U” of a transit—as if suddenly a star's straight line, its constant brightness, detoured sharply down and then back up. If it's a keeper, the TCE changes acronyms and becomes a KOI, a Kepler Object of Interest. Using this rigorous vetting process, the team estimates that nine out of ten planet candidates that make it to the KOI are indeed other worlds. Final confirmation that a KOI is an exoplanet—that there's indeed something there—depends on the KOI passing a series of further tests, including detecting the putative exoplanet via the radial velocity technique. For this, the Kepler team uses the Keck Telescope—at $90,000 a night, no refunds if it's cloudy—one of the few in the world with an accurate enough spectrometer to measure the tiny wobble induced by small exoplanets at the distance of those detected by Keple
r.

  Prior to the Wyoming meeting, Batalha had already gone over each of the individual possible planetary detections with the Kepler team—putting aside some as too uncertain—and now it was time to compute the detections into a summary form. By six in the morning, Batalha had finished writing the computer code to process the latest Kepler data.

  “I was sitting in my hotel room, looking out at the forest—it was a beautiful scene here in Jackson Hole, just savoring the moment before hitting the ‘go’ button to run my code to get the new numbers,” she says. In the minutes it took the code to run, the cosmos got a lot more crowded, and alien worlds got a little closer. “It was a surreal moment to see that the number of Earth-sized planet candidates went up by 100 percent.”

  In fact, the next day—with no time for a press release, let alone a scientific paper—Batalha announced that Kepler had discovered another five hundred putative planets around distant stars, some of them appearing to be not much bigger than Earth. It was a historic, unprecedented announcement, almost doubling the number of known planets in the universe. On the same morning, Michel Mayor's HARPS team—Kepler's main competition in the race to find an alien Earth—announced a haul of fifty new exoplanets. This total included sixteen exoplanets that the HARPS team called “super-Earths,” exoplanets they estimated weighed in at between one and ten times Earth's bulk, the smallest of them just three and a half times Earth's mass.

  For Bill Borucki, the thousands of exoplanets that Kepler has discovered until now are stepping stones on the way to the ones he wants to find, the ones he wants to share with the world. Kepler is working its way down to smaller, Earth-sized planets—if they exist. “If the answer is no, there aren't [a large number of stars with such planets in their habitable zone], then there probably isn't much life out there, and it's going to be extremely difficult to find it and you might as well wait a hundred or a thousand years until the technology is much, much more capable,” he says. “On the other hand, if you find lots of Earths, and particularly if a good fraction of them are in the habitable zone, then you can gather the enthusiasm to do the next steps…Kepler has to get such good data that we can convince the world to go out to explore the galaxy.”

  Less than two decades after the hard-won discovery of the first exoplanet, most astronomers would now bet their telescopes that a multitude of Earth-like exoplanets are there for the finding. This assessment is based on a solid planetary trail. It's clear that the Milky Way abounds not just in stars but in solar systems. The discovery of exoplanets has been a case of metaphorically finding the biggest, easiest-to-find ones first. Exoplanet hunters have burrowed down from big, Jupiter-sized worlds to mini-Neptunes and super-Earths. Astronomers are confident that the exoplanet size gradient doesn't hit a planet-building wall here—there's a consistently occupied neighborhood of exoplanet sizes, a gradient that gives every indication of continuing down to Earth-sized planets and smaller. It's a view that's corroborated by evidence from infrared observations of newborn stars whose protoplanetary disks, areas of planet formation, reveal the glowing, dusty raw materials for rocky planets. The most enthusiastic exoplanet theorists think the evidence points to the conclusion that every Sun-like star harbors in its light an Earth-like planet.

  On a Monday morning in December 2011—for most Americans, a back-to-work day wedged between America's Black Friday shopping frenzy and the Christmas end zone—Bill Borucki and the Kepler team announced they'd landed their first Earth-like planet: a confirmed super-Earth orbiting in a Sun-like star's habitable zone. After watching three transits and verifying the planet's presence via ground-based telescopes, they announced Kepler-22b, a planet just under two and a half times Earth's radius orbiting a star six hundred light-years away. What made it the stuff of news headlines was that this distant world orbits its star, one slightly smaller than the Sun, in about 290 days, putting it, according to the Kepler astronomers, squarely in its star's habitable zone. Here was a planet that the Kepler team believed could be a large, rocky planet with a surface temperature of about 72°F, comparable to a comfortable spring day on Earth. Or, as the Kepler team knew well, it could be a mini-Neptune, a largely gas planet, perhaps one with a thick atmosphere that, so close to its sun, made it more similar to scorching and lifeless Venus than to a blooming spring day on Earth. It was the latest success in a three-year spree that's opening our eyes to a new cosmos, one resplendent with planets. In early 2012, based on these successes, NASA extended the Kepler mission to 2016. This additional time is essential for Kepler to spot smaller planets with long orbital periods, those that require numerous years of observation to record three separate transits, and for confirming Earth-sized planets around stars that have variable baseline brightness, which makes a transit detection more difficult.

  Coming from the age of exoplanet speculation, we stand on the edge of the age of exploration. Thousands of exoplanets beckon. Somewhere out there in the night sky, circling another life-giving star, is in all likelihood another rocky world, its atmosphere the planet's churning breath. When we find it, we won't have found an alien world but a distant relation—a living planet, born of the same stuff, the same cosmic processes, chances, intense bondings, and violent breakings as our Solar System and the planet we call home. The discovery of that alien Earth will open the next chapter in the Stardust Revolution—one that, for the first time, we will be able to use to compare the story of planet Earth with that of another living planet; to see ourselves reflected in another's story.

  We have had a century in which to assimilate the concept of organic evolution, but only recently have we begun to understand that this is only part, perhaps the culminating part, of cosmic evolution. We live in a historical universe, one in which stars and galaxies as well as living creatures are born, mature, grow old, and die.

  —George Wald, Harvard University biochemist

  and Nobel laureate, “The Origins of Life,” 1964

  THE BIOLOGICAL BIG BANG

  I’ve come to the Harvard-Smithsonian Center for Astrophysics—the CfA—in search of the current intersection of astronomy and biology, and I find myself literally standing at it. The center sits on a pleasant knoll in leafy downtown Cambridge, Massachusetts, at one end of Linnaean Street, named for the great eighteenth-century Swedish botanist and zoologist Carl Linnaeus. It was Linnaeus who developed the modern classification system for plants and animals based on shared physical characteristics—stems and leaves, fins and feathers. Linnaeus also developed the binomial species-naming system, giving us in 1758 the name Homo sapiens (meaning “wise man”)—a mammalian species known for asking, in moments of self-reflection, such questions as “Who am I?” and “Where have I come from?”

  Atop the knoll, a minute's walk from Linnaean Street, sits the Harvard Observatory. Built in the nineteenth century, it's tiny in comparison with the modern behemoths atop isolated mountaintops, but in historical perspective, this red-brick observatory is a giant. It was here, a century ago, that a scientific cadre of female astronomers, working for male professors, did the legwork in developing the modern classification and naming system not for species but for stars. The astronomers at this observatory photographed and analyzed the light fingerprints of tens of thousands of stars, the original images stored today in the observatory's basement vault. Just as Linnaeus grouped animals on the basis of shared characteristics, two hundred years later, Annie Jump Cannon and her colleagues grouped the stars on the basis of their common characteristics, the atomic lines that dominated each star's light fingerprint—here calcium, there iron. The result is the O, B, A, F, G, K, and M (Oh Be A Fine Girl/Guy, Kiss Me!) stellar classification system, adopted in 1922, that opened the door to a deeper understanding of the stars’ origins and evolution, just as the Linnaean system pointed the way to questions about the origin of Earthly species.

  Charles Darwin died in 1882, forty years before Cannon and her colleagues grouped the stars into their own families, but in the last sentence of On the Origin of Spe
cies, Darwin alludes to the Earth's cosmic context, to the way in which the evolution of our planet is tied to a larger story. He refers to the Earth as a planet that has long “gone cycling on.” With this reference to our planet's yearly journey around the Sun, the father of evolutionary thinking leaves readers to contemplate the central importance of the Earth's astrophysical context to evolution. In his parting words for those thinking deeply about evolution, Darwin pointed the way to the cosmos. Today in the Stardust Revolution, our story—the epic journey of our origins, the tracing of our family tree—doesn't start with thinking about the origin of species but rather with thinking about the origin of stars. To understand the cosmic context of our origins, of cosmic evolution, is to trace the genealogy of stars back to the dawn of time. In the Stardust Revolution, the long-asked cosmic question “Are we alone?” has been transformed into the extreme genealogy question “How are we connected?” Perhaps in the not-too-distant future, the question may be “How are we related?”

  In search of the very base of our cosmic family tree, I’ve come to the CfA—which, in the words of its website, is “the world's largest and most diverse center for the study of the Universe”—to meet the distinguished astrophysicist Alex Dalgarno. He's e-mailed me to say that he's in office B324. I walk up to the third floor, make my way to the “B” hall, and follow the numbered door labels into a short side hall, which includes the photocopy room. Here's room B323 and then B325. I look again, and my initial disorientation is confirmed. I get a sense that, in something akin to The Hitchhiker's Guide to the Galaxy, all the other rooms are there, but there is no room B324. It's a cosmic joke. I ask a graduate student about this alphanumeric mystery. He explains that B323, a locked door, is the entrance to an anteroom that leads to B324. B324 is there, but you have to know how to get there. It seems a fitting introduction to Dalgarno, since I’ve come to ask about the very beginnings of cosmic evolution, a time metaphorically hidden behind a series of doors for which astrophysicists have discovered some of the keys—none more than Dalgarno himself.