In fulfilling an ancient dream, Mayor and Queloz opened the door to a modern one. The search for other solar systems and ultimately another living planet is the beating heart of the Stardust Revolution. This quest is about much more than finding extraterrestrial cosmic cousins with whom to talk or visit, or the joy of turning science-fiction visions of alien worlds into reality. Without planets revolving around other stars, the Copernican Revolution was a philosophical dead end. There might be billions of galaxies and trillions of stars, but as far as concrete evidence went, the only known planetary system was ours. Earth wasn't the geographic center of the cosmos, but science fiction aside, it appeared to be the planetary and biological center, and a unique one at that.
For the scientists of the Stardust Revolution, the absence of other planetary systems presented the central conundrum in developing a natural history of cosmic evolution. Astronomers could study the natural history of everything from galaxies on down to stars, molecules, and even atoms. Yet when it came to planets, they hit a wall or, rather, a gap. Without other planets, the cosmic story—our cosmic story—didn't really make sense. Here was a universe in which common forces and processes—from gravity to the nuclear processes in stars to astrochemistry—were clearly visible and responsible for the cosmos that astronomers observed, but this physical continuity fell into a black hole when it came to other solar systems. Did the cosmos really behave one way around our Sun and differently everywhere else? Were we, as some leading planetary geologists posited in 2000,a rare Earth, a planet with diverse life that might be “utterly unique…in the visible universe”? There was an intellectual chasm at the core of the Stardust Revolution.
It was a gap that was as much about biology as about astrophysics. Without other planets, there was no way of truly understanding our cosmic origins. Evolutionary theory is grounded in comparative biology. Darwin's evolutionary insights came from famously comparing the beaks of different finches, comparing animals on one continent with those on another, and comparing the fossil skeletons of ancient animals with the skeletons of animals walking and swimming during his day. Stardust scientists could compare interstellar molecules and stellar energy output across the Milky Way, but astronomy and evolution could never truly meet without the existence of other worlds with which they could compare biological notes. Astrobiologists needed comparative exoplanetology if they were ever going to deepen their knowledge of our astrophysical heritage.
Mayor and Queloz's historic exoplanet discovery now seems like it took place long ago, like the sepia-toned images of the old West that adorn the Jackson Lake Lodge's walls. In less than two decades from the discovery of the first exoplanet, a new cosmos has emerged. We're no longer alone—at least not when it comes to solar systems; and astronomers already know of hundreds of others. On the conference's first day, Mayor announced his research group's discovery of more than fifty new exoplanets.
But for all the palpable excitement and intense buzz at the Extreme Solar Systems II conference about the number and nature of alien worlds, it was the absence of two pioneering exoplanet hunters that bore testament to just how desperately, dirt-scrabble-hard it had been to cross into this cosmic frontier of alien worlds—and what might lie ahead.
A NEW VISION
Bruce Campbell and Gordon Walker met for the first time in September 1969 at the University of British Columbia in Vancouver, Canada, as classes resumed for the fall session. As Campbell, a twenty-one-year-old, third-year engineering student, walked into his first astronomy class, at the front of the UBC lecture hall was Walker, a Cambridge-trained thirty-three-year-old recent immigrant from Scotland, in his first year as a professor. The two men quickly found they shared a common passion: they were astronomy tech geeks, drawn together by a love of and fascination for building better and faster telescopic equipment. Campbell scored the top mark in Walker's class, Astronomical Measurements, graduated from UBC in 1971, and returned to work with Walker as a postdoctoral researcher in 1976, after earning his PhD at the University of Toronto.
Campbell returned because Walker was, in the Star Wars movie jargon of the day, a Jedi knight of stellar spectroscopy in his ability to sift starlight for its secrets. Campbell was Walker's Luke Skywalker—brilliant, talented, and ambitious. Their technical curiosity and prowess led them to realize they could create a radically better way to study starlight, one so accurate they'd be able to see something no one else ever had: the slight gravitational tug of a distant planet on its star.
Although we're used to thinking of stars as fixed points of light in the night sky, stars move. A lot. They expand and contract due to their thermonuclear nature, boiling up in some spots and shooting out arcing fountains of plasma. Stars, like planets, rotate on their axis, a rotation that on Earth creates day and night. And, just like planets, stars have an orbit. Although we think of planets orbiting a stationary star, the star and its surrounding planets actually orbit a common center of mass. Much like when a large adult and a small child balance on a teeter-totter—to make it work, the adult sits very close to the center of mass, and the child sits farther away—the “parent” star is very close to the center of mass, giving it an orbit not much larger than its own diameter. It's this to-and-fro movement that astronomers measure with astrometry—measuring a star's back-and-forth movement across the sky over the course of many nights.
However, there's another less obvious way of seeing a star's movement. Rather than trying to measure a star's side-to-side movements, it's now easier to measure its movements toward and away from the Earth. The secret is in its light. Spectroscopy can be used to see not just what stars are made of but also how they move. Measuring a star's speed around its orbit depends on Doppler spectroscopy, a 150-year-old standard of astronomy. Doppler spectroscopy had already made its claim to cosmological fame in the red-shifted galaxies that Edwin Hubble observed, the red shift indicating that these spirals of stars were speeding away from the Earth and from one another—that the cosmos is expanding.
The Doppler method is based on the same wave behavior that occurs when the pitch of a siren changes as an ambulance speeds toward us, passes us, and then goes away. In the case of stars, astronomers look for changes in the pitch, or frequency, of starlight to measure a star's speed toward or away from us, using a technique called radial velocity. It's colloquially known as “the wobble method” because, plotted on the axis of a graph, the star's speed varies, creating a wave, or wobble, in the curve as the star appears to move slightly faster when approaching the observer and slower when moving away. The bigger the wobble, the more massive or closer the exoplanet is to its star.
In 1952, Russian American astronomer Otto Struve—whose great-grandfather Friedrich had first calculated the interstellar extinction of starlight a century earlier—predicted that the radial-velocity method could be used to detect exoplanets. In the 1970s, the technique was being widely used to detect stellar binaries—pairs of stars in orbit around one another, each gravitationally altering the other's speed, though one star might be too faint for anyone to observe its light. But these spectroscopists were working with errors of four hundred meters per second, margins far too large to detect the slight stellar speed changes induced by a planet.
Campbell and Walker realized they could improve the radial velocity method by creating a better light ruler. The key to measuring the red shift is to have a reference against which to measure changes in a star's light fingerprint. In the early 1970s, Walker's research group developed a technique to use the light fingerprint of water in the Earth's atmosphere as the ruler, significantly boosting the accuracy of their measurements. But it turned out that this technique was like using a rubber ruler, because atmospheric changes smudged water's spectral fingerprint. Campbell's stroke of genius was to turn back to the technique that Gustav Kirchhoff had used to determine the nature of the Fraunhofer lines. Campbell envisioned that by passing starlight through a known gas placed in front of the spectrometer, the gas's distinctive spectral fin
gerprint would act as a rigid, finely grained ruler against which to measure changes in starlight.
The key was to choose a gas with regularly spaced spectral lines. Molecular spectroscopy expert Gerhard Herzberg—who'd arrived in Canada as a refugee from the Nazis with five dollars and his beloved spectroscopy equipment—said he knew just the right gas for the job: hydrogen fluoride. Campbell and Walker agreed. Hydrogen fluoride produces a spectrum of evenly spaced absorption valleys, like the tines on a comb across the spectrum. The gas's only drawback is that it's highly toxic and corrosive; mixed with the slightest amount of moisture, such as is found in the lining of the respiratory tract, it becomes a powerful, deadly acid. Campbell and Walker developed a clear, sealed, inert metallic cell with sapphire windows (hydrogen fluoride dissolves glass) to hold the gas that could then be placed in front of the spectrometer. They were concerned that they might not survive to tell the tale of their first test of the system, but the hydrogen fluoride cell worked perfectly, providing an unprecedented level of accuracy. For the first time in history, the velocities of stars could be measured to plus or minus ten meters (approximately thirty-three feet) a second, about the speed of an Olympic sprinter. Thus a telescope and spectrometer could be used (like a police speed-radar gun) to clock a distant star's velocity.
A radial-velocity accuracy of ten meters a second was the magic number for exoplanet hunting. Astronomers knew that Jupiter's massive gravitational tug causes our Sun to speed up and slow down by about twelve meters per second. Thus, Campbell and Walker's technique would spot the wobbles induced by Jupiter-sized exoplanets on stars elsewhere in the Milky Way—if they were out there. “I heard myself say to Bruce, ‘We could start looking for planets,’” Walker recalls. “I don't know where the idea came from. That's how these things happen in science—suddenly a light turns on. It's the art of the possible.”
Walker and Campbell were moving into an astronomical minefield. If other worlds were out there—which many astronomers believed was the case—they were devilishly hard, perhaps impossible, to detect, judging from a half century of experience. During the five decades from World War II to 1995, numerous exoplanets were thought to have been discovered. In the equivalent of a Harvard Business School case study of the vagaries of scientific discovery—of turning years of research into a hard-won accepted truth—this half century was marked by flattened hopes, faulty technologies and calculations, and off-base news reporting. Headlines with messages like “First Planet Found outside Our Solar System!” appeared in newspapers dozens of times over this period, at least twice in the New York Times and once on its front page. It wasn't the reporters who were getting it wrong; it was the astronomers.
The most dramatic and high-profile exoplanet discovery claim—the great cautionary tale among exoplanet hunters—is that of a putative Jupiter-like planet around Barnard's Star, which was heralded by American astronomer Peter van de Kamp in April 1963. He'd hardly rushed to the conclusion. His assertion was based on a painstaking quarter century of careful observations, after which van de Kamp's planet made it into some astronomy textbooks as the first exoplanet. A decade later, van de Kamp's planet disappeared. A fellow astronomer noted that the measurements recording the movement of Barnard's Star on the sky—the basis for concluding there was an unseen planet tugging at its star—were almost identical to the movements of another star. While it was improbable that both stars had absolutely identical movements, the astronomer noted that the apparent shifts in position coincided with equipment upgrades in 1949 and 1957 to the telescope van de Kamp had used. The shocking conclusion was that van de Kamp had recorded the movement not of the star but that of the telescope itself.
In another case of mistaken exoplanet identity, a venerable American astronomer arrived at what was planned to be the announcement of an alien world he'd found; unfortunately, he was forced to announce that what he thought was a planet was in truth a mathematical error. He was nonetheless hailed by his colleagues for having been brave enough to break the bad news himself. And it wasn't only human error that produced supposed exoplanets. The most remarkable coincidental erroneous exoplanet spotting occurred on December 11, 1984, when two American research groups simultaneously raced to announce the detection of an object orbiting a distant star. Ostensibly, the detection of something by two competing groups would encourage any poker player to bet in favor of the object's existence. But on closer inspection, there wasn't anything gravitationally tugging at the star. The postmortem analysis concluded that, somehow, subtle systematic effects of the Earth's atmosphere had similarly skewed both observations. When van de Kamp published his last book, Dark Companions of Stars, in 1986, the frontispiece included the biblical inscription “Blessed are they that have not seen, and yet have believed.” It was a fitting testament to the deep challenge faced by those who hoped to fulfill the ancient quest of beholding alien worlds.
Walker and Campbell knew they faced a formidable challenge, but they had a plan. After a trial run with the Doppler spectroscopy system on the telescope at the Dominion Astrophysical Observatory, north of Victoria, British Columbia, in 1980, Campbell installed their star speed system on the new, much larger Canada-France-Hawaii Telescope atop the dormant volcano Mauna Kea in Hawaii. They were fortunate to get telescope time, leveraged as it was by Walker's stature in the astronomy community rather than by great enthusiasm among the community for their program. “It is quite hard nowadays to realize the atmosphere of skepticism and indifference in the 1980s to proposed searches for exoplanets,” Walker wrote in chronicling his exoplanet search.
Walker and Campbell's exoplanet hunt plan was straightforward: they assumed that other solar systems existed and were like ours; Jupiter-like exoplanets would take about twelve years to orbit their star, just as Jupiter does. Walker, Campbell, and University of Victoria astronomer Stephenson Yang began a decade-long search of twenty-six stars, looking for Jupiter-sized exoplanets—ones large enough, they reasoned, to tug at their stars to a degree visible with the scientists’ telescope. Looking patiently at more than two dozen stars and using the best spectroscopic tools in the world, they thought that if there were Jupiter-sized planets around other stars, they'd find at least one. Three or four times a year, one or all members of the team spent several nights atop Mauna Kea, fourteen thousand feet above the Pacific Ocean, searching for other worlds. They'd start just after dusk and work through the night, wearing winter parkas and enduring altitude-induced headaches and dehydration until dawn's light drowned out the stars.
“It was deadly,” Walker says of the psychological challenge of searching for exoplanets no one knew existed or that could even be found. “We never knew from one night to the next if we were observing anything. We could only tell when the data were reduced, about once a year, whether we were seeing trends.” The work truly was potentially deadly. Working with altitude-induced clouded thinking, the astronomers had to heat the hydrogen fluoride gas to just over 200°F and repeatedly mount and dismount the container. In case it ruptured, they carried gas masks and hydrogen fluoride antidote gel for treating external burns.
By 1987, the astronomers believed they were seeing something no Earthling had ever seen before—stellar wobbles caused by orbiting planets. That summer, at a press conference at the annual meeting of the American Astronomical Society in Vancouver, Campbell announced their preliminary results: a half-dozen stellar wobbles indicative of possible exoplanets. One star's motion was particularly intriguing: that of Gamma Cephei.
It was a fitting star for the Canadian planet hunters. A bright star forty-five light-years away, Gamma Cephei is always visible from Canada in the night sky, shining near Polaris, the pole star. Speaking in a charged, paparazzi-esque atmosphere to a group including the astronomers who packed the press conference room, Campbell described how, from their detailed measurements, he and his associates had seen that Gamma Cephei had a periodic 2.5-year-long wobble. As viewed from Earth, the star moves toward us and then away over a 2.5-yea
r cycle—evidence, they thought, of an exoplanet gravitationally tugging at the star.
The New York Times headline read “Planets outside Solar System Hinted.” Campbell recalls, “They were calling us planet hunters. We were on the track.” His professional colleagues weren't as impressed. In an arena of supercharged skepticism, Campbell's tentative announcement generated more doubt than accolades. An astronomer quoted in the New York Times article said, “I probably won't call it a planet until I can get out and walk on the surface of it.” No researchers attempted to confirm the results; at the time, none could. Walker says the press conference, ironically, was more like the beginning of the end rather than a high point, increasing the pressure to deliver results from a project that was, by its nature, deeply long-term. Two years later, Campbell, Walker, and Yang still had nothing conclusive, and, worse, Campbell didn't have a secure job. A Vancouver native, he was determined to stay in the area, but a decade of effort hadn't earned Campbell a permanent position at UBC, the University of Victoria, or the National Research Council's Victoria-based Herzberg Institute of Astrophysics. In Walker's view, Campbell's not getting a job “had a lot to do with the fact that planet searching was still not above the radar…. It all started to unravel.”
Campbell alienated those who controlled Canada's professional astronomy appointments by publicly bemoaning the state of astronomy funding in Canada. Increasingly frustrated, disheartened, and distanced from the academic community, Campbell took a final, renegade approach with the help of an influential supporter—the guru of astronomy popularizing, Carl Sagan. “I remember telling Carl Sagan, we think we're going to be able to do this,” recounts Campbell, who met Sagan in the late 1980s while the two were on a University of Toronto panel on the search for extraterrestrial life. “He was somewhat incredulous at first, but then I convinced him, and he became a great friend and supporter.”
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