So Latham was working on HD 114762, a star in the Henry Draper catalog. His calculations suggested a wobble with a period of about eighty-four days, about the same as Mercury’s. The magnitude of the wobble suggested an object with a mass about eleven times that of Jupiter—or rather, that was the minimum possible mass. This was a crucial point. Since the object pulling on the star was itself invisible, Latham couldn’t be certain its orbit was truly edge-on. If it was, the mass was eleven times that of Jupiter. If the orbit had been at right angles to Latham’s telescope, you wouldn’t see any motion at all toward or away from the telescope; it would all be side to side. But if the orbital plane was tilted somewhere in between those extremes, you’d see something between the full effect and zero. Any motion toward or away from the telescope would reflect part of the tugging. A much bigger object, pulling at an angle, could mimic an eleven-Jupiter-mass body seen directly edge-on.
Latham sent an e-mail to Mazeh, with a copy to Michel Mayor, a Swiss astronomer who was also looking for wobbling stars. Mayor wasn’t looking for planets either; he was looking for brown dwarfs, objects bigger than planets but smaller than stars. A brown dwarf could be as much as eighty times as massive as Jupiter before it would start fusing hydrogen into helium in its core—the same reaction that powers an H-bomb, and the one that makes stars shine. At this point, brown dwarfs were purely theoretical (they’ve since been shown to exist), but looking for them had made Mayor an expert on radial-velocity measurements too. The e-mail said, in part, “This is interesting—the minimum mass is well under the stellar limit. It could even be a giant planet.” By the time Latham hit “send,” the clock had ticked past midnight. It was now April 1, and he was a little bit worried that Mazeh and Mayor might think it was an April Fool’s joke.
But Mayor went out and did his own measurements, and got the same results. Mayor also determined that whatever this object was, its orbit was eccentric—it was somewhat oval rather than nearly circular. That ruled out a planet, because, as Latham said, “everyone knew that giant planets had to have circular orbits.” Everyone also knew you couldn’t have planets bigger than about twice the mass of Jupiter. And with a “year” just eighty-four days long, well, said Latham, “that was just a killer. Three strikes, you’re out! Tsevi and I had a bet—we still do, in fact—I said it was a small star, he insisted it was a big planet.” When Latham, Mayor, Mazeh, and two others reported the discovery in Nature on May 4, 1989, they wrote: “The companion is probably a brown dwarf, and may even be a giant planet.” Latham told me Mayor “wasn’t too happy with that wording.” He was on Mazeh’s side.
In any case, Latham was too busy with other things to keep looking for objects like this one, but Mayor, he says, “picked up [the project] and ran with it.” Like Marcy and Butler, he and a French instrument builder named Andre Baran began beating down the errors in their own spectrograph. Marcy and Butler had chosen to do it with iodine cells and horrifically complex software. Mayor and Baran chose instead to make their spectrograph as utterly stable as they could. They used a reference spectrum from a lamp outside the telescope, but they piped it into the spectrograph with fiber-optic cables in such a way that it was as undistorted as it could possibly be.
When they’d done everything they could think of, Mayor told me at a conference on the Isle of Capri in 1996, they had beaten down their errors to thirteen meters per second, or about 30 mph. They couldn’t find a Jupiter like the one in our solar system, but then, they weren’t looking for one. Mayor still cared mostly about brown dwarfs, and the spectrograph, far more sensitive than Dave Latham’s, could detect them easily. In early 1994, Mayor and a graduate student named Didier Queloz began taking measurements of wobbly stars. By now, Marcy and Butler had been at it for half a dozen years.
Mayor and Queloz had put more than a hundred stars on their observing list to maximize their chances of finding something. Brown dwarfs might be relatively rare, after all, and their orbits would have to be nearly edge-on for the astronomers to make a strong detection. Not all of them would be, of course. So the European astronomers began methodically ticking through their list. Within a few months, they noticed something very odd. A star named 51 Pegasi (the fifty-first brightest star in the constellation Pegasus) seemed to be wobbling, but in an impossible way. It was moving back and forth, not with a Jupiter-like rhythm of 11 years, not with an Earth-like rhythm of 365 days, not even with a cadence of 84 days, like HD 114762. This star was moving toward Mayor’s telescope and dancing and advancing again once every four days. If this motion were truly caused by an orbiting body, it was hugging its star an absurd ten times closer than Mercury hugs the Sun.
Not only that, but based on how hard it was yanking on the star, this was no brown dwarf: It was only half as massive as Jupiter. Mayor’s spectrograph wasn’t sensitive enough to find a Jupiter in a Jupiter-like orbit, but something this close in had a huge amount of leverage on the star. It looked just the way a giant planet should look—if a planet could exist in this location. Theorists said it couldn’t. But as Tsevi Mazeh had said ten years earlier, “maybe the theorists are wrong.”
In fact, not all theorists had said such a close-in planet couldn’t exist. Douglas Lin, of the University of California, Santa Cruz, had proposed back in 1992 that giant planets might migrate inward from where they originally formed. He figured they’d just spiral all the way into the star and be destroyed—but for a while, they could take up an orbit like the one Mayor was describing. And forty years before that, the legendary theorist Otto Struve had written a paper for the October 1952 issue of a journal called The Observatory titled “Proposal for a Project of High-precision Stellar Radial Velocity Work,” in which he foreshadowed not only Mayor’s and Marcy’s and Dave Latham’s work, but Bill Borucki’s as well. In part, Struve wrote:
We know that stellar companions can exist at very small distances. It is not unreasonable that a planet might exist at a distance of 1/50 astronomical unit, or about 3,000,000 km. Its period around a star of solar mass would then be about 1 day. If the mass of this planet were equal to that of Jupiter … [it] might be just detectable … There would, of course, also be eclipses … This, too, should be ascertainable by modern photoelectric methods.
Struve’s idea had been largely forgotten, however, and Lin’s work was considered highly speculative. If an observer is wrong as much as 10 percent of the time, goes the astronomical rule of thumb, he or she is a pretty careless observer. But if a theorist is wrong as little as 10 percent of the time, he or she isn’t taking enough creative risks. Lin was, and remains, a very good, creative theorist, so his colleagues took his predictions with a grain of salt.
Mayor and Queloz returned to the telescope, trying to make absolutely certain that they weren’t somehow kidding themselves. Maybe the star was pulsing, its outer atmosphere bulging toward and then away from Earth in a four-day rhythm. Maybe the star wasn’t perfectly spherical, and they were seeing the bulgy part moving toward them and then away. “The first principle [of science],” the physicist Richard Feynman said in a commencement talk at Caltech in 1974, “is that you must not fool yourself—and you are the easiest person to fool.” Peter van de Kamp had fooled himself with his “discovery” of a planet orbiting Barnard’s Star (he fooled others as well; Otto Struve’s 1952 paper refers to “results announced … by P. Van de Kamp”). Mayor and Queloz, like Marcy and Butler half a world away, were determined to avoid destroying their reputations. But hard as they tried to make the impossible conclusion go away, it refused.
In the end, Mayor told me on Capri, “It’s a difficult thing to decide you’ve done all you can, that you’re ready to leave your office and go public.” They submitted a paper to the journal Nature claiming the discovery of a planet-like object they called 51 Pegasi b (the b meaning that it is a secondary object orbiting the star 51 Pegasi). Before the paper could be published, he spoke about the discovery at a conference in Florence, Italy, in October 1995. According to Nature’s strict
rules, he was allowed to do this, but he wasn’t allowed to discuss the findings with reporters until the paper was actually published. If he did so prematurely, Nature wouldn’t publish it after all—and Nature was prestigious enough that Mayor didn’t want to flout the rules. There were reporters at the Florence conference who begged him for interviews. He politely refused, so they went ahead and announced the discovery without quoting the man who had made it.
In California, meanwhile, Geoff Marcy and Paul Butler began hearing about Mayor’s find, first from colleagues who had been at the meeting, and then from reporters who were desperate to find an expert they could talk to. It seemed obvious to Marcy that Mayor had made a mistake. The sort of planet he was describing couldn’t possibly exist. The theorists said so. Besides, Marcy couldn’t possibly be scooped: He and Butler had been working tirelessly to find planets for six years now. How could someone else just stumble onto the discovery?
Still, he wasn’t going to say Mayor was wrong without being absolutely sure. Astonishing things often turn out to be false—but not always. In 1989, for example, two chemists from the University of Utah claimed they’d discovered “cold fusion,” an inexpensive source of potentially limitless clean energy. I called Rob Goldston, the director of the Princeton Plasma Physics Laboratory, who was struggling to create fusion in a multibillion-dollar installation owned by the Department of Energy. “I don’t know the details of the experiment,” he told me, “so I can’t make any definitive statement.” “But,” he continued, making it clear as diplomatically as possible how he really felt about the claim, “if it’s true, it means that everything we’ve learned about nuclear physics over the past fifty years is false.” In other words, Goldston was almost certain the chemists were wrong, but knew that an absolute statement might come back to bite him.
Marcy and Butler were convinced Mayor must be wrong. But maybe everything they knew about planets was false. Either way, they were all set up to find out for themselves. If Mayor’s instruments could see this “planet,” theirs could too. To find a planet in a four-day orbit, you have to look at least once a day; Marcy and Butler had never bothered to look at any star more often than once every few months, because the wobbles they were looking for would play out over years. So they went up to Lick Observatory, where they’d already been approved for four nights on the 120-inch reflecting telescope. As the data streamed in from 51 Pegasi, they would immediately funnel it into their computers for processing, taking more data all the while.
After a couple of days, they knew Mayor was right. They’d been scooped by someone with a less sensitive instrument. Arguably, they’d been scooped by Dave Latham back in 1989 as well, but Latham’s discovery had never been accepted as a planet, even by Latham himself. Ultimately, all the strikes against Latham’s object would be eliminated as astronomers began to understand how strange planets really could be. In hindsight, Geoff Marcy now gives Dave Latham credit for the very first planet orbiting a Sun-like star.
Michel Mayor couldn’t talk to reporters, but Marcy and Butler were under no such obligation, since they didn’t have a paper about to come out. Reporters couldn’t talk to Mayor, so they descended on the Californians. And while Mayor had made the discovery, Marcy and Butler could have made it. If luck had been on their side, they inevitably would have. So it was a sort of poetic justice that Marcy and Butler were the ones who ended up being lavished with public recognition. Geoff Marcy also realized that if they’d known how comparatively easy the universe would make it to find planets, they might not have worked so hard to make the Hamilton Spectrograph so precise. Their ignorance had put them behind, but it had given them an edge on future discoveries.
In fact, the discoveries had already been made, even if Marcy and Butler didn’t know it. The astronomers had been searching for wobbles for many months now, returning over and over to a list of 120 stars. But they hadn’t bothered analyzing the data, since they thought no star would show a perceptible wobble over so short a time. The information was just sitting in storage on magnetic tapes. Now that they knew such a thing was possible, however, they realized the signals of planets might be on those unread tapes. Butler was “insane,” he later told me, to find out what was on them. He got his hands on some relatively fast computers and began feverishly processing data around the clock.
Within less than two months, Butler managed to tease out the signal of a planet orbiting the star 47 Ursae Majoris, in the Big Dipper. Then he found another, orbiting 70 Virginis, in the constellation Virgo. Neither of them was as crazy as 51 Peg b, as astronomers were now nicknaming it, but they were still closer to their stars than Jupiter is to the Sun. They were still a little crazy. 70 Virginis b, in particular, had an unusually eccentric, egg-shaped orbit. That was one of the strikes Dave Latham had listed against HD 114762.
Nevertheless, Geoff Marcy arranged to give a talk on their new planets at the winter meeting of the American Astronomical Society, which was coming up in a few weeks. Unlike Michel Mayor a few months earlier, he and Paul Butler didn’t have a paper ready for publication, so they weren’t bound to avoid reporters. They gave a heads-up to the society’s press officer, an astronomer named Steve Maran, who immediately scheduled a press conference. He wouldn’t say in advance what it would be about, although he told me privately, and undoubtedly told other reporters as well, that I’d be crazy to miss it.
Despite the veil of secrecy, however, word got out that Marcy would have something important to say, and when he finally did, he spoke with a theatrical flair that I would later recognize to be his trademark. “After the discovery of 51 Pegasi b,” he said, “everyone wondered if it was a one-in-a-million observation. The answer is … no. Planets aren’t rare after all.” He went on to describe 47 UMa b and 70 Vir b. He also pointed out that the latter orbited in the habitable zone of its star, the region where water could exist in liquid form, the necessary ingredient for life. And while 70 Vir b itself was too big to support living beings (it’s about six times as massive as Jupiter), it might have habitable moons. This was pure speculation, but it was a bold enough statement to get the discovery on the cover of Time and into headlines and news broadcasts around the world.
What all those viewers and readers didn’t realize was that Marcy and Butler’s announcement marked an enormous change in the way scientists would think about extraterrestrial life from that moment forward. For about two thousand years, philosophers and scientists had actively debated the question of whether life exists beyond the Earth. From the Renaissance on, it was widely believed that the answer was yes. But since the early 1900s, when astronomer Percival Lowell convinced himself that he could see canals and other evidence of life on Mars, thinking about and looking for life on other planets had been considered something of a fringe idea in science. The UFO craze that started in the 1950s didn’t help.
In principle, scientists thought it was plausible that life existed elsewhere in the universe, and a few even tried looking for it—Frank Drake, who began the formal Search for Extraterrestrial Intelligence in 1961, for example, and a few NASA scientists who designed biology-sensing experiments for the Viking Mars landers in the 1970s. But no one even knew for sure that there were any planets beyond the solar system for life to exist on, and it was clear to most astronomers that planets were just too difficult to find with existing technology.
Suddenly, because Marcy had ignored the conventional wisdom, and because Mayor had gotten lucky, it was clear that this simply wasn’t true. It’s something like what happened when the British athlete Roger Bannister ran the world’s first sub-four-minute mile in 1954. Until he did it, many people thought it was impossible. Once Bannister showed otherwise, plenty of runners found that they could break four minutes as well. Those first three planet discoveries, considered impossible by many scientists, forced the entire field of astronomy to shift its perspective. Men and women who had gone to graduate school planning to study other topics changed direction and decided to look for planets instead.
Senior astronomers who had spent their careers thinking about the Big Bang or the formation of galaxies did the same.
This influx of brainpower and of funding from NASA and other agencies led in turn to ingenious new ways to search, which turned up dozens, then scores, then hundreds of new planets, in such a bewildering array of sizes, shapes, and orbits that astronomers are still arguing, a decade and a half later, about how solar systems form and evolve. Even so, by the time Bill Borucki’s lunch was delayed by an AP reporter’s nagging questions in early 2011, no one had yet found a true twin of Earth—the likeliest place, given the admittedly little we know about extraterrestrial biology, where life might actually be found.
It wouldn’t be long, though. And while many of the astronomers working on the Kepler project had been inspired to go into planet-hunting by Geoff Marcy’s and Michel Mayor’s extraordinary discoveries in the 1990s—and even though Marcy himself would sign on as a project scientist with the Kepler Mission before the spacecraft launched—neither Marcy nor Mayor might end up playing a direct role in that discovery.
Chapter 4
AN ANCIENT QUESTION
When Bill Borucki and Geoff Marcy set out independently to find worlds orbiting other stars, other astronomers were dubious only because it seemed obvious that astronomical technology wasn’t yet sophisticated enough to find them. They had no doubt that such worlds existed. The almost universal attitude among astronomers, and among most other scientists who stopped to think about the question, was, “How could there not be planets around other stars?” The argument was both numerical and Copernican. The Copernican part refers to Nicolaus Copernicus, who showed in the 1500s that the Earth was not, as European philosophers believed, at the center of the universe. That discovery has now been generalized to argue that the Earth, and the Sun, and the solar system, and even the Milky Way galaxy, aren’t special in any way that matters to the universe. If planets exist in our solar system, the Copernican principle suggests they probably exist around at least some other stars as well.
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