Light of the Stars

by Adam Frank

  Many scientists pounced on Lovelock and Margulis for promoting the equivalent of snake oil. As microbiologist John Postgate, a fellow of the Royal Society, put it: “Gaia—the Great Earth Mother! The planetary organism! Am I the only biologist to suffer a nasty twitch, a feeling of unreality, when the media invite me yet again to take it seriously?”56

  The real problem with Gaia theory for many scientists was the issue of teleology. It’s a hallmark of biology that evolution that has no purpose, direction, or goal (telos is the Greek word for “goal”). The idea that the biosphere was somehow manipulating the chemical and physical conditions on the planet for its own good seemed inherently teleological (that is, goal-oriented). It smacked of intention, and evolution doesn’t have intention.57

  Lovelock and Margulis were unbowed in their defense of Gaia. In response to critics who claimed their proposed feedbacks were nothing more than fantasy, Lovelock produced his now-famous Daisyworld model. Developed with mathematician James Watson, the Daisyworld model used a simple set of equations to describe a planet with two species of daisies (black and white) and a gradually brightening sun. The solutions to the equations showed clearly how feedbacks from the daisies (the black ones absorbed sunlight, while the white ones reflected it) could naturally keep the planet at a steady temperature even as the sun heated up. It was a tour de force of representing a complex idea with simple math in the service of proving an essential point. As Lovelock put it, “Daisyworld keeps its temperature close to the optimum for daisy growth. There is no teleology or foresight in it.”58

  And Lovelock and Margulis made it clear, they were not claiming the planet should be considered alive in any true sense of the word. New Age Gaian Mother Earth ceremonies notwithstanding, Lovelock and Margulis were ultimately arguing for the central role of the biosphere in planetary evolution. They were picking up where Vernadsky left off, and putting in more science.

  With the publication of the Daisyworld model in 1983, the tide, at least partially, began to turn. Biospheric feedbacks were recognized as an essential part of planetary laws of operation. These feedbacks represent the definition of how to think like a planet, and researchers embraced the biosphere’s central role in their studies of the Earth. But in the process, the name “Gaia theory” was dropped and replaced with the less contentious “Earth system science.” While the concept of self-regulation remained contentious, researchers now knew that the linkages between the biosphere, atmosphere, and other systems were so tight that they had to be considered together as a single entity. The adoption of the Earth system paradigm represented its own revolution in how we think of planets, and today it forms the cross-disciplinary foundation for all researchers trying to understand climate change.59

  As these studies of Earth system science were extended to include the planet’s past, a crucial new idea would be added to the researcher’s lexicon. Building on Vernadsky, Lovelock, and Margulis, a new generation of scientists began speaking of a “coevolution” between life and the planet. That word, coevolution, would help rewire astrobiology. Life could no longer be isolated from the planet that gave it birth. Instead, a planet could be deeply transformed by the life it births, including when that life goes on to create its own globe-spanning civilization. Thus, within that single term, coevolution, lay the seeds of a new story waiting to be told about humanity and our Anthropocene.

  CHAPTER 4

  WORLDS BEYOND MEASURE

  HOW TO RUIN YOUR LIFE WITH PLANETS

  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 t
his 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 mean
s 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.