The Stardust Revolution

by Jacob Berkowitz

  Buoyed by Sagan's endorsement, Campbell raised $125,000 in private funding to support an endowed planet-hunting chair at UVic. However, the Canadian government's university research-funding guidelines at the time specified that only tenure-track professors could receive funding. This disqualified Campbell, an adjunct professor, from receiving the necessary matching federal funding for the position. By 1991, at forty-two years of age, a salary and family stability were more important to Campbell than unlocking the secrets of the stars. “It had been so frustrating to try to secure some sort of position, and then to try to set up an endowed chair, that when it all came to naught, I decided to walk away,” says Campbell. When he did, he went supernova. In a final burst of anger, and in a major breach of research etiquette, Campbell erased his computer hard drives, deleting a decade's worth of compiled and analyzed data. After the deep uncertainty of looking for other worlds, he turned his back on the stars for good and took up a career dealing with one of life's great certainties: taxes. “It was the advent of electronic filing of tax returns in Canada that got me involved,” says Campbell, who started work as a personal tax consultant in 1993. “Many accountants were hesitant to get into computers, and I pretty much knew how computers worked, so I had the edge there.”

  Stunned and bruised by the loss of his younger colleague, Walker was left to lead the conclusion of the exoplanet project. It took almost a year of painstaking work for Stephenson Yang—fortunately by then holding the additional position of UVic's computer systems manager—and UVic colleague Alan Irwin to recover the original data and reanalyze it. In 1992, having reassessed the data, Walker came to a conclusion about the most promising of the possible exoplanets, the one around Gamma Cephei. In a scientific paper, he and several coauthors concluded that the star's 2.5-year wobble was probably due to its own cyclical expansion and contraction, giving the appearance that the star was in fact moving toward and away from us. As the star expanded, it would appear to move toward the Earth, and as it contracted, it appeared to move away. There was no invisible exoplanet, just a turbulent star. Nobody disputed the conclusion, except Walker himself. In fact, he'd quietly agonized over whether the wavy line in the data was in fact a star revealing the presence of another mysterious world.

  “I had written the paper as it being a planet,” he says. He was sitting in his office when a recently arrived postdoctoral student, Jaymie Matthews (now a UBC professor), entered and looked at the data. Matthews pointed out that the supposed planet's two-and-a-half-year orbital period coincided with what appeared to be periods of the star's heightened surface activity. “I think Jaymie had a very valid point,” says Walker, who deferred to the dominant view and rewrote his paper, adding a question mark to the title and saying that what might be an exoplanet might also be stellar rumblings. He'd been right the first time. Canada's planet hunters had found their prize. Yet it wasn't until 2003, after assessing almost twenty years’ worth of data, that exoplanet hunters finally concluded definitively that every 906 days—about two and half years, as Walker had calculated—a Jupiter-sized planet completes its orbit around Gamma Cephei. The final vindication of the detection of the planet Gamma Cephei b for Walker “was like a Eureka moment in tortuously slow motion.”

  David Charbonneau, a leading exoplanet hunter at the Harvard-Smithsonian Center for Astrophysics, says that Walker's experience is a testament to the excruciating nature of the search to discover new worlds. “We like to imagine that the scientists looking through the microscope or telescope see something and then they know that this is the thing they've been looking for and it's just a matter of getting the news out,” says Charbonneau. “But the point is that when you're actually involved in a true discovery, it's a very uncomfortable process, because you really don't believe that this is the thing you saw, and you want to make absolutely sure you're not being confused by some spurious signal and you start to question the data yourself.”

  Within months of Walker's about-face, Polish American astronomer Alexander Wolszczan announced the discovery of two Earth-sized objects around a pulsar, the radio wave emitting the dense remnant of a supernova. This was completely unexpected—astronomers are still uncertain as to how planets survive or result from a star's detonation—and these objects were the first planet-like objects found outside our Solar System. However, in the scheme of understanding planet formation around stars and the search for possible Earth-like exoplanets, these heavily radiated cinders were space oddities rather than guiding lights.

  In June 1995, at an astronomy conference in Calgary, Alberta, Gordon Walker told the assembled star watchers that if other Jupiter-sized planets orbited distant stars, they were far less abundant—and much harder to find—than had been assumed. In the audience sat a man who would have loved to speak his piece, but the timing wasn't right. A year earlier, in April 1994, Swiss astronomer Michel Mayor had started his own exoplanet hunt. For Mayor, it was a natural progression. With his graduate student Antoine Duquennoy, he'd recently completed a landmark census of relatively nearby Sun-like stars and had discovered, using radial velocity measurements, that our naked eye deceives us when we look into the night sky. About 60 percent of all Sun-like stars are binaries—two stars in an orbital dance. Mayor was aware of Campbell and Walker's spectroscopic work—he'd met Campbell and talked spectroscopic shop at the first bioastronomy conference in France in 1990. Having conducted the definitive survey of stars orbiting one another, Mayor set his sights on using radial velocity to search for smaller orbiting objects, such as the hypothesized yet elusive brown dwarf—an object a smidgeon too small to ignite as a star—or exoplanets. For the search, he helped build a new spectrometer at France's Haute-Provence Observatory, which used new state-of-the-art digital technologies to achieve a stellar-wobble precision of just fifteen meters (about forty-nine feet) per second. Mayor was in the exoplanet game.

  It didn't take long. Relying on his stellar binary work, Mayor and his graduate student Didier Queloz chose 142 Sun-like stars that they knew were single stars. Starting in April 2004, they spent a week of nights each month monitoring the stars for spectroscopic wobbles. It took only several weeks of watching to get the first inklings that something was up around the star 51 Pegasus. But by Christmas, the star disappeared from the observatory's view of the night sky, and the astronomers had to wait until July 1995 to double-check their results. After that, it took only a single week of observing to confirm their find, an exoplanet game winner that stunned astronomers: 51 Pegasi b, commonly called 51 Peg b, a Jupiter-sized planet so close to its star that it orbited in only four days.

  “Mayor was one of the last people to enter the field,” says Alan Boss, an exoplanet theorist who has closely followed the historical trajectory of the search. “But he came from a very different mindset. He came from the field of binary stars. So he was used to searching his data for even very short period companions, because binary stars can be so close that they're almost touching to so far apart that we're not sure if they're actually going around one another. So he was used to searching his data for all kinds of variable periods.”

  Gordon Walker—one of the scientific referees sought out by the editors of the scientific journal Nature to review Mayor and Queloz's historic paper before it was announced to the world—was among the first to receive the news. “Nobody, but nobody, suggested there were going to be Jupiters in few-day orbits,” says Walker. “In looking for the familiar you miss the obvious.” In fact, while Mayor and Queloz were waiting to confirm their finding, one of the world's leading exoplanet theorists published a paper in the journal Science arguing that Jupiter-sized planets would be found only at distances from their star that are similar to the distance Jupiter is from our Sun. Michel Mayor and Didier Queloz had cracked a millennia-old frontier and turned talk of probabilities and possibilities into the real realm of exoplanets.

  DR. SEUSS'S UNIVERSE

  In Jackson Hole in 2011, there was little time for looking back. The pioneer era in the search
for alien worlds had turned into the hottest field in twenty-first-century astronomy. Whereas the pioneers found it difficult to get telescope time if they mentioned looking for distant worlds, exoplanet hunters today go to the front of the telescope line. In the second decade of the twenty-first century, exoplanet exploration has become big business: around the world there are now about a hundred ground- and space-based missions in search of exoplanets.

  “Bruce Campbell is the father of all of the exoplanet work happening in the world right now,” says University of California–Berkeley astronomer Geoff Marcy, one of the world's leading exoplanet hunters and the chair of the Extreme Solar Systems II conference. “There are several thousand people working on exoplanets, and it all goes back to Bruce Campbell.” Marcy has codiscovered hundreds of the exoplanets found so far. He's done it using a technique pioneered by Campbell and Gordon Walker, who, he says, “invented the technique that we stole.” In the cast of characters involved in the exoplanet search, Marcy plays the role of a central witness, having worked in the field since 1982. “If it wasn't for Bruce Campbell, you wouldn't be talking to me.”

  The Doppler technique is now one of a half-dozen proven ways to detect these once-unattainable alien worlds. Today, the search for exoplanets has the feel of the urban morning commute—focused, forward-looking, a little frantic. Many searchers worry they'll be just a little late to the big find. One bellwether of where the scientific world is at in its surveying of alien worlds is this: only fifteen years after the discovery of the first alien world, the discovery of a single, run-of-the-mill exoplanet will probably be lumped into an overview scientific paper rather than getting its own marquee billing. Just another alien world.

  The irony in this age of exoplanet discovery is that the more we learn, the less certain we are. Twenty years ago, astronomers were more confident about the types of planets and how they form than they are today. After all, they had our Solar System as a shining exemplar. Stargazers had spent several hundred years figuring out Earth's Solar System siblings—where they were, how they behaved, and what they're made of. As a result, many astrophysicists thought they understood what a solar system is and thus had a template for all they'd find elsewhere in the cosmos. This is what they went looking for around other stars. Just as at home, any other solar system would start closest to its central star with smallish, rocky planets. Next, after a sizable safety zone of a gap inhabited by the asteroidal leavings of solar system formation, would come the giant gaseous planets, akin to Jupiter and Saturn. Still farther out are the medium-sized icy planets, the Neptune and Uranus lookalikes.

  The planets orbit the Sun, all in near-circular orbits; not perfect, divine circles but only slightly squeezed ellipses. Gravitationally bound to one another, the whole system lies on a single, simple plane, flattened out like a spinning ballerina's skirt around the star's midline. We had our Solar System family photo, the one that adorned schoolrooms around the world, and that picture guided us in our search: every planet with a place, and every planet firmly in its place. It was a twentieth-century scientific version of fifteenth-century Ptolemaic order and beauty. Exoplanets changed all this.

  If there's one thing astronomers have learned on their journey out, it's that our Solar System isn't the cosmic norm. “Almost every discovery in the domain of extrasolar planets was not expected,” says Michel Mayor, who now leads the team that uses HARPS, the High Accuracy Radial Velocity Planet Searcher, a spectrograph on the 3.6-meter telescope at the European Southern Observatory's La Silla Observatory in Chile. HARPS holds the world record for parsing a star's speed down to just half a meter per second, the pace of a fast-crawling toddler. Rather than living in a cosmic-template solar system, we live in one amid an incredibly diverse cosmic zoo of exoplanets, more akin to a Dr. Seuss menagerie of colorful worlds than to a planetary artist's neat view of our home Solar System. This discovery has required the creation of a new language of exoplanets, from hot Jupiters, dwarf planets, and orphan planets to water worlds, carbon planets, and super-Earths. In a classic case of extrapolating from singular anecdotal evidence, we've had a severe case of solar system myopia. The psychological impact of our discovery of exoplanets is akin to a small-town girl arriving in a big cosmopolitan city and spending the first week rubbernecking at people sporting multiple piercings, speaking foreign languages, and displaying unimaginable behaviors. Our Solar System, for so long immense, all-encompassing, and defining, is now a mere village amid a multicultural galaxy.

  At the Extreme Solar Systems II conference in Jackson Hole, much of the buzz concerned the fact that exoplanet hunters had entered a new era. No longer focused on singular planets, the larger debates are about understanding overall galactic rates of occurrence of different types of planets. With the number of putative exoplanets pushing several thousand, astronomers are drawing broad conclusions about what's “out there.” They are getting the first broad-brush view of other worlds, and it is already clear that there aren't just the occasional exoplanets around other stars, but there are also exo–solar systems. When we look up into the night sky, we're seeing stars; but what our eyes can't see is that around many of those stars are swarms of orbiting planets. In the Stardust Revolution, the age of speculation is over: solar systems aren't an exception but rather a constant; in at least a quarter of the known instances, and probably more, stars and planets are born together.

  These alien solar systems come in a smorgasbord of types. The first exo–solar system discovered was a trio of planets around the Sun-like star Upsilon Andromedae. The closest planet in the system, about two-thirds Jupiter's mass, zips around the star in just 4.6 days. Farther out, orbiting in about 241 days, is a planet with about twice Jupiter's mass, and orbiting its star once every three and a half years is the system's big brother, a planet of four Jupiter masses. The discovery of Upsilon Andromedae as not just a star but as a solar system awakened astronomers to a string of exo–solar system discoveries. The analysis of twenty years of Doppler measurements of the star 55 Cancri revealed an extended solar system with at least five planets and an architecture that looks more like a scaled-up version of our own Solar System, with four of the exoplanets closer to their star than the Earth is to the Sun. To date, our Solar System has the largest number of planets, but in time it will almost certainly lose this cosmic distinction. The Kepler-11 solar system, discovered in 2010, is a remarkable young solar system that includes at least six smallish, gassy planets, five of which form a tightly packed bunch that orbit between ten and forty-seven days. The alien solar system that looks most like ours so far is that around the star HD 10180, a Sun-like star in the southern constellation of Hydrus, or the Male Water Snake. Around this star, 127 light-years away, the HARPS team untangled the gravitational tugging of at least five Neptune-mass planets—between thirteen and twenty-five times the Earth's bulk—orbiting in as few as six days and as many as six hundred days.

  Using space-based telescopes, it's now even possible to see alien solar systems in the throes of being born. Many people know the Hubble Space Telescope for its spectacular Hubble Deep Field image of early, and now very distant, galaxies. In 1994, after a major refurbishing mission, this eye-in-space caught glimpses just as stunning and revelatory of the process of cosmic creation—much closer to home. Hubble peered into the Orion Nebula, the closest area of intense star formation to Earth. There, amid the cold molecular clouds, were dozens of newborn stars, the stellar winds from them clearly carving out their natal clouds. Around dozens of these young stars was something no telescope had ever before resolved in such detail: protoplanetary disks. The Hubble images of individual stars were only thirty pixels square, the kind of blip that would make you think your computer monitor was failing. But the fuzzy blurs around the new stars let astronomers shorten the term “protoplanetary disk” to a cozier and more biological-sounding name, proplyds. Here weren't just new stars but new solar systems.

  Later, when the Spitzer Space Telescope looked at stars in the infrared
(seeing objects that Hubble couldn't), there again were stars with disks around them. The Spitzer's observations were informed by earlier infrared space-based telescopes that had measured infrared excesses coming from many stars—too much infrared light to be explained by the star itself. Something else, very near the star, was glowing in the infrared. On closer inspection of the data, the Spitzer astronomers realized they were looking at dust located in single, narrow rings around dozens of stars—warm dust glowing in the infrared. Protoplanetary disks are now thought of as an inevitable consequence of star formation, and they are orbiting almost all Sun-like stars. It's estimated that about ten solar systems are born in the Milky Way every year.

  Differences notwithstanding, the language of our Solar System has been extended to describe the planets of other solar systems. Much like weight categories in boxing, exoplanets are grouped on the basis of their mass relative to that of the planets in our Solar System. In the heavyweight category are the Jupiter-like exoplanets—massive gas planets. Jupiter's mass is approximately 317 times that of Earth's, and exo-Jupiters range from about a quarter to about thirteen times Jupiter's bulk—at which point the giant ball of gas is a brown dwarf, a middling creature between planet and star. The midweight contenders are the Neptune-like exoplanets—worlds of about seventeen times Earth's mass. Farther down the planetary chain come the sought-after planets that weigh in at low multiples of Earth's weight.