During graduate school and on into his postdoctoral fellowship, Geoff Marcy had gotten very good at finding and measuring spectral lines. So he went with his strength and decided to look for planets this way. With astrometry, you have to be looking down on an alien solar system from above to see the star moving from side to side (“down” and “above” don’t really have any meaning in space—you could just as easily say “up from below,” and you’d be equally inaccurate, but it’s such an instinctive way to describe it that astronomers talk this way anyway, and nonscientists understand instantly what they’re talking about).
Marcy wanted to look for that same motion, but from an edge-on perspective. He wanted to catch planets tugging their stars toward Earth (just the tiniest bit), then away. Since the red-shifting and blue-shifting of spectral lines betrays that sort of motion, that’s what he proposed to look for.
The only problem with this idea, Marcy soon learned, was that it was nearly impossible. When cosmologists use shifting spectral lines to measure the speed of galaxies racing apart as the universe expands, they’re looking at objects moving at many thousands of miles per hour. They take a spectrum from a galaxy’s collective starlight, lay it next to a reference spectrum from a motionless object—the Sun, for example, or a laboratory reference lamp that generates an artificial spectrum—and see how far a given line has shifted. There’s some imprecision in the process, but if they’re off by a couple of thousand mph or so, that’s plenty accurate enough. They don’t have any need to improve their precision.
But the back-and-forth motion Jupiter causes in the Sun is a piddling 28 mph or so. The greatest expert in measuring cosmological redshifts would fail utterly to detect it. So Marcy tried tightening up the procedure in every way he could think of to make it more precise. He succeeded up to a point: He managed to get his accuracy down to about 450 mph. But since he was trying to find distant Jupiters, this wasn’t nearly good enough. No matter how careful he was, the act of moving the telescope from star to reference lamp changed his measurement system enough to make the measurement unreliable. Imagine you wanted to measure the length of two different objects—two bricks, say—with high precision. You’d be smart not to use two different rulers, since one of them might be just a little bit off. But if you wanted to be really precise, it’s a problem even if you use just one ruler—moving the ruler from one brick to the other could change things. The second brick could be in a slightly warmer place, so the ruler might expand just an infinitesimal amount. Or you might hold the ruler in a slightly different way, so it would sag under gravity differently, distorting its shape. These are absurdly small changes, but if you really needed absolute accuracy, they could make a difference. The best way to make absolutely sure you’re measuring things exactly the same way is to measure them at exactly the same time.
Marcy decided to do just that. He’d measure a star’s spectrum and a reference spectrum all at once, so he didn’t have to move anything. He might have figured out a way to do it, but it turned out that he didn’t have to. A Canadian postdoc named Bruce Campbell, at the University of British Columbia, had come up with a solution a half decade or so earlier. Working with a colleague named Gordon Walker, Campbell had realized that you could take a gas whose spectral absorption lines were thoroughly understood, put it in a glass container, and let starlight pass through the gas on its way to the spectrometer. The reference spectrum and the real spectrum would be measured at exactly the same time with exactly the same instrument.
It worked. Campbell and Walker were able to measure the wobbling of stars to an accuracy of plus or minus 30 mph or so. That still wasn’t precise enough to find an alien Jupiter, however, since the measurement error was as big as the signal you’d be looking for. Beyond that, Campbell and Walker had settled on hydrogen fluoride gas for their reference—well understood, but also corrosive, explosive, and horribly toxic. Marcy needed a better gas, and he also needed a collaborator for this project, which was growing increasingly complicated.
By now, Marcy was on the faculty at San Francisco State University. After asking around a bit, he learned of a recent San Francisco State graduate who had joint degrees in physics and chemistry, and who also had a strong interest in astronomy. Paul Butler was still at the university, working on a master’s in physics and looking for a research topic. When Marcy approached him, Butler was intrigued. Like Marcy, Butler was drawn to research with long odds but a potentially huge payoff. And he loved the challenge of trying to make measurements more precise than anyone had ever been able to pull off.
The only downside was that Paul Butler had a somewhat rough personality. He divided the world into good guys and bad guys, and the bad guys included some of the world’s most eminent scientists. These would eventually include, once he signed on with Geoff Marcy, many of their professional colleagues. He thought nothing of describing senior astronomers at places like Caltech and Cornell and especially Harvard as evil, or cowardly, or even mentally ill. He would say these things openly, and later on, when Marcy and Butler finally began making discoveries and getting some public recognition, he would sometimes even say them to reporters.
Nevertheless, Butler was very good at his job. He spent more than a year hanging out with chemists, trying out one element or compound after another, looking for the ideal reference gas to use for finding planets. Ultimately, he settled on iodine. It was not only safe, but it also had an enormous number of spectral absorption lines that spanned the visible spectrum all the way from red to violet. That would give each measurement plenty of cross-checks. The lines created by hydrogen fluoride, by contrast, were not only fewer in number, but they also bunched up in a small part of the visible-light spectrum.
So Marcy and Butler built what they called an “iodine cell” to attach to the Hamilton Spectrograph at Lick Observatory near San Jose, and began using a small telescope to take data on relatively bright, nearby Sun-like stars, looking for wobbles. They didn’t have the software yet to analyze their observations; the spectrum of iodine was so horribly messy that they couldn’t disentangle its spectrum on their images from the spectra of the stars. But Butler was also a talented software writer, so while they continued to take unreadable measurements, he worked on code that might someday make sense of them.
In the end, it took him six years. “It’s my Rembrandt,” Butler told me in 1996. “It’s as close to great art as I’ll ever get.” Even then, however, Marcy and Butler could get to a precision of only twelve meters per second—still not good enough to find an alien Jupiter. The Hamilton Spectrograph, built by Steven Vogt, Geoff Marcy’s thesis adviser in grad school at Santa Cruz, was now the limiting factor. Vogt had to upgrade the device, and then Butler had to rewrite his software to account for the upgrade. Finally, Geoff Marcy, Paul Butler, and Steven Vogt, their new collaborator, were able to measure the wobbles of stars to within an astonishing and unprecedented three meters per second. They could find a planet like Jupiter.
Chapter 3
HOT JUPITERS: WHO ORDERED THOSE?
After Geoff Marcy and Paul Butler had all the kinks worked out in their hardware and software, the only thing standing in their way was time. It takes Jupiter eleven years to orbit the Sun just once. If you were an alien astronomer looking toward our solar system using an iodine cell and the Hamilton Spectrograph, it would take you eleven years to watch the Sun move toward you, then away, then back in a single orbital cycle. And if you were a really careful alien astronomer, who didn’t want to risk the embarrassment of making a discovery that turned out to be wrong, you’d want to see not just one, but at least two or three cycles to convince yourself you really were seeing a planet and not, say, some weird pulsation of the star itself.
Geoff Marcy knew very well that astronomers had fooled themselves about planets before. The best-known example was the “discovery” of planets around a nearby star known as Barnard’s Star, by Swarthmore College astronomer Peter van de Kamp in the 1960s. What van de Kamp thought was a side
-to-side motion in the star, caused by a planet, was actually a change in his telescope—a minuscule repositioning of a lens when the telescope was refurbished. The slight change in focal length made Barnard’s Star appear to move, and van de Kamp interpreted the motion as the tug of a planet. When the mistake was discovered in the 1970s, van de Kamp compounded the problem by being slow to acknowledge it (in fact, it’s not clear that he ever did).
To be sure they could claim a planet discovery with confidence, Marcy, Butler, and Vogt would have to wait a long time to confirm that any wobble they spotted matched the signature of a planet. They’d also have to convince other astronomers that their complicated apparatus and their complicated software—which may have been Butler’s Rembrandt, but which was so complicated that it would take years to analyze observations of a single star—really was capable of doing what they claimed. Some of their colleagues at other universities literally laughed at them when they heard a description of the research. Many of the laughers ended up on Butler’s growing enemies list.
Near the top of that list was an astronomer named David Latham. Latham has been at Harvard since the late 1960s; he got his Ph.D. there in 1970. When I arrived as a freshman in the fall of 1971, he was the deputy instructor of a hugely popular undergraduate course called The Astronomical Perspective, taught by the astronomer and historian of science Owen Gingerich. I remember looking down from the back row of a steeply angled lecture hall and seeing Latham, a young, skinny, nondescript guy, standing off to the side as Gingerich lectured. Forty years later, I sat in his office at the Harvard-Smithsonian Center for Astrophysics—the CfA, if you want to sound like an insider—listening to an older guy, less skinny, with grayer hair, wearing a jacket and tie (he hadn’t worn them as a teaching assistant in 1971, and most astronomers don’t wear them now unless they’re getting a major award). He was still teaching the course I’d taken so many years earlier, but it now had a different name, and he was now the lead instructor, since Gingerich had retired. Latham was still pretty nondescript, but now it was in an avuncular way—low key, companionable, easy to get along with. You wouldn’t immediately guess, nor would you have guessed in 1971, that he was and is a competitive motocross racer (a photo of Latham kicking up dust on his dirt bike appears on his Harvard homepage) and a hockey player. It’s less surprising to learn that he’s a wine connoisseur.
About midway between his appearance at the front of that lecture hall in 1971 and our conversation in 2011, Dave Latham had become a planet hunter too. Originally, he was largely interested in cosmology, the study of the origin and evolution of the universe. The Big Bang had become the dominant theory of the universe in the mid-1960s, after lingering around the edges of science for many decades, but there were all sorts of profound unanswered questions remaining. When did the universe begin? What happened during the first moments after the Big Bang? What is the universe made of? What will its ultimate fate be?
During the 1970s, it had also became apparent that the universe was pervaded by some sort of mysterious, invisible substance, at first known as the “missing mass” and later called “dark matter” (that mystery still hasn’t been solved). The dark matter would have had a powerful influence on how the cosmos evolved, so astronomers wanted to understand how galaxies were spread throughout the universe. Were they sprinkled evenly or were they assembled into some sort of pattern that hinted at how much dark matter there was and how it was distributed? (We now know they’re in patterns that resemble Swiss cheese, which helps to rule out some models of cosmic evolution.)
Latham was involved in some of the early surveys that tried to understand galaxy distribution. The galaxy surveys, operated out of the Smithsonian’s telescopes on Mt. Hopkins, near Tucson, Arizona, used spectrographs. But galaxies are too faint to see when the Moon is bright in the sky. Latham hated to see the instruments sit idle, so when the Moon sidelined the galaxy surveys, he began looking at stars in the Milky Way to see if they wobbled. He wasn’t interested in planets at this point, but in binary stars—pairs of stars that orbit each other. These are actually more common than single stars in the Milky Way. The Sun is unusual in wandering through the galaxy alone. “We were looking to see how frequent binaries were,” said Latham, “and what their characteristics were. By the eighties we were mass-producing radial velocities [the technical term for motion toward and away from the observer]. We had our errors down to maybe five hundred meters per second,” which is about a thousand miles per hour.
Then, in 1984, an Israeli astronomer named Tsevi Mazeh contacted Latham. Mazeh was interested in radial-velocity measurements too. He had just been out in Santa Cruz, California, consulting with Steve Vogt. Geoff Marcy had barely begun thinking about planets at this point. Mazeh was on his way back to Tel Aviv, but he stopped in Massachusetts en route. “He had this idea,” said Latham, “to use my radial-velocity instrument to search for giant planets.” Latham patiently explained to Mazeh why this wouldn’t work, that even a planet like Jupiter would pull on its star too weakly to show up, and that it would take more than a decade to see a single orbit in any case. “No,” said Mazeh, “I mean giant planets with very short periods.”
You could certainly find those: A short orbital period means you can watch several orbits go by without waiting forever. The planet Mercury orbits the Sun once every eighty-eight days. If a giant planet hugged its star the way Mercury does the Sun, you could see four full orbits in just under a year. Beyond that, a planet’s gravitational effect on its star is more powerful the closer it orbits—it has more leverage—so the observations don’t have to be nearly as delicate. That was all true, acknowledged Latham, but giant planets don’t orbit that close to stars. They certainly don’t in our solar system, and most planetary theorists were confident they couldn’t exist at all.
“Maybe,” said Mazeh, “the theorists are wrong.”
“He seemed like a nice fellow,” recalled Latham, “so I agreed to work with him.” To boost their chances further, Mazeh suggested they look at M-dwarfs, red stars that are at most half as massive as the Sun. M-dwarfs make up about 70 percent of the stars in the Milky Way, but they’re too dim to see with the naked eye. The fact that they’re such lightweights means that a giant planet orbiting an M-dwarf would make the star wobble even more, making it that much easier to detect. “We also decided to look at some Sun-like stars,” said Latham. “On the night of March 31, 1988,” he continued, “I was working up the observations on one of them, a star called HD 114762.”
At this point, readers should be warned that the naming conventions for stars are complicated enough to make your head hurt. The HD in the name of the star Dave Latham was looking at signals that it appears in the Henry Draper catalog, a list of more than 350,000 of the brightest stars visible from Earth, classified according to features in their spectra. (Draper, a medical doctor and amateur astronomer, took some of the earliest photographs of a star’s spectrum. The catalog named in his honor was assembled by Harvard astronomer Edward Pickering, using funds donated by Draper’s widow in the 1880s.)
But that’s just one of many star catalogs. Another is the Gliese catalog of the closest, rather than the brightest, stars. The German astronomer Wilhelm Gliese put it together in 1957. “Gliese names,” Geoff Marcy told me, “are kind of nice, because they tell you right away it’s a nearby star even if you don’t know anything else. But the HD catalog is more widely used.” And then there are the Hipparcos and the Tycho catalogs, gathered by the European Space Agency’s Hipparcos satellite in the 1990s (the satellite’s mission was to map the positions of millions of stars; the Hipparcos catalog has very high precision, the Tycho catalog is based on somewhat less careful measurements with the same satellite).
It doesn’t end there. Many stars are also identified by the constellation they’re part of. Alpha Centauri is the brightest star in the southern constellation Centaurus (alpha is the first letter in the Greek alphabet). If a constellation has more than twenty-two stars, you run out o
f Greek letters, so you go to numbers. The constellation Pegasus has a star known as Alpha Pegasi, and it also has one called 1 Pegasi. Then there are stars with given names—Vega, Sirius, Aldebaran, Betelgeuse, Polaris, Arcturus, and more. These stars are so bright and prominent in the night sky that ancient Greek and Arab sky watchers thought of them as old friends, with distinct personalities. Finally, when astronomers are conducting an organized search, they’ll sometimes create their own catalogs—the Kepler catalog, the Wide Angle Search for Planets (WASP) catalog, the Hungarian Automated Telescope (HAT) catalog, and so on. According to one astronomer I spoke with, this is partly to make sure the credit for a planet’s discovery goes prominently to the search team.
The one catalog astronomers don’t pay attention to is the one compiled by the International Star Registry. This is the company that lets you “name a star after someone.” The ads say, quite truthfully, that the names will be recorded in book form in the Library of Congress. But that’s true for any book, whether it’s an erudite volume of history or a trashy romance novel. The company makes clear on its website that astronomers don’t consult the book.
Even when you’re talking about stars identified by their numbers in a single catalog—in this case, the Henry Draper catalog—the names begin to swim before your eyes. Unless you’re a planet hunter, that is. Then they’re as distinctive as the names of your children. “Exactly!” said Debra Fischer, a Yale astronomer, when I proposed this analogy. “You don’t know HD 209458? These names are burned into my memory. Someday I will have Alzheimer’s, but I will remember these stars.” For the record, Fischer tends to rely on the Hipparcos catalog, because it has more than two million stars; because it notes their positions and distance from Earth with exquisite accuracy; and because all of the information is online, making it easy to access.
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