“So,” he explained, “you basically have a mix, you have a ball of silicate rock, you put an ocean down, not too thick, and you have roughly the right temperature and atmosphere, then is it just a matter of time when life will arise?” Or would it come out differently? “Are we going to find, say ten or twenty examples of those”—a proposition that was looking better all the time, with Kepler already in orbit for more than a year and with at least one planet plausibly made of rock, not gas, discovered by Michel Mayor’s group a year or two earlier—“study them carefully, find that, although they look just like the Earth in terms of their properties, they just don’t have life for whatever reason?”
For all his excitement at the prospect of life, Charbonneau thinks either outcome is equally possible. “People push me on this,” he said, “and I really, honest to God, think it could go either way. When I teach, most of my students can’t imagine the latter case. They think that of course there will be life.” That doesn’t mean for a moment that he’s indifferent to the answer. He hopes life exists beyond Earth. “Even if they were completely foreign, I think we would feel less lonely, I think there would be this true loneliness if we found out that this was really it. And I think that does affect how people view how precious our planet is. Even though we could never go to those other places, I think that we would still view what we have differently if we knew that it was truly unique.”
These insights weren’t quite so fully formed, of course, when Charbonneau initially switched from cosmology to planet-hunting. But Bob Noyes’s scheme of looking for reflected light was instantly appealing. “I worked very hard on that,” said Charbonneau, “but we never did make a detection. It turns out that reflected light is a really hard problem. I always felt we were almost there, but it took Kepler to pull it off, six hundred million dollars and fifteen years later.” After this disappointing finish to the reflected-light project he went back to Noyes, who had become his adviser, to get advice on a research topic for his thesis, the grand finale to a graduate student’s career. “Bob told me it probably wasn’t the best bet to go into radial-velocity searches [like Marcy and Mayor were doing] because so many teams were so far ahead,” recalled Charbonneau. “Maybe I could try to confirm planets by looking for transits.” This made a lot of sense: It’s hard to imagine something other than a planet that could make a star wobble and make it dim, with exactly the same timing. Even so, while it was now 1999, at least a decade and a half after Bill Borucki had begun working seriously on what would become the Kepler mission, planetary transits were still something very few other astronomers were thinking about.
Confirming that a planet really existed was one reason to look for transits, but there was an even better one, which wasn’t fully appreciated at the time. When Marcy and Mayor found the radial-velocity signature of an orbiting planet, they could tell you how much time the planet took to complete one orbit, and they could tell you its mass—or its minimum mass, anyway. If you could see that a planet made a transit as well, that would confirm you were seeing the orbit precisely edge-on, so you would know that the minimum mass was also the actual mass. But you would also know the planet’s physical size. If it blocked, say, 2 percent of the star’s light, that meant the planet’s disk was 2 percent of the size of the star’s disk. With a little bit of high school geometry, you could calculate the planet’s volume. And once you had the mass and the volume, you knew the planet’s density.
Planetary scientists already know from our own solar system that planets come in different densities. Mercury, for example, is denser than Earth because it has a higher proportion of iron to rock than Earth does. Pluto, whether you call it a planet or a dwarf planet, is less dense than Earth because it’s made largely of ice. Saturn is even less dense, because most of its mass comes in the form of hydrogen and other gases, with only a little bit of rock down in its core. One of astronomy’s longest-standing fun facts is that if you could find a big enough bucket of water to put it in, Saturn would float.
Whether you’re interested in finding a place where life might exist, or simply in understanding how an alien planet or solar system formed, it’s crucial to know what the planet is made of. A planet with the mass of Earth bloated out to the size of Jupiter, to take an extreme and, in fact, physically impossible example, would be a wispy ball of dilute gas—not a good place to live. A planet the size of Earth but with a mass fifteen times as high would be so dense, and have such a crushing surface gravity, that probably nothing could live there either. For all of these reasons, the detection of planets where astronomers already had radial velocities would be incredibly valuable.
In 1999, when Dave Charbonneau was casting about for a thesis topic, Bill Borucki hadn’t yet gotten approval for Kepler. But a handful of other astronomers had begun thinking about transits. Tim Brown was not only thinking about them; he was actively searching. Brown was originally interested in studying the Sun, and by the 1980s was working at the National Center for Atmospheric Research (NCAR), in Boulder, Colorado. NCAR, which mostly concerns itself with climate, isn’t exactly a hotbed of astronomy, but since the Sun has a big effect on Earth’s climate, understanding how it works is perfectly in keeping with the lab’s mission. To give just one example, the Sun gets a little brighter overall when it has lots of sunspots, and dimmer when they almost disappear, in a regular boom-and-bust eleven-year cycle. During the 1600s and 1700s, though, sunspots pretty much disappeared entirely for an eighty-year stretch. At the same time, Europe experienced a period of unusual cold, known as the Little Ice Age. There’s good reason to think the cold spell was largely due to other factors, but even so, it’s useful to try to figure out how this and other changes in the Sun might affect the Earth—especially since some solar physicists think the Sun may now be entering another prolonged sunspot drought.
While he was at NCAR, Brown built a spectrograph to measure subtle pulsations in the solar surface so he could figure out what was going on inside. It could measure pulsations in other stars as well, and Brown realized that he might be able to use spectrography to look for the wobbles caused by orbiting planets. “Planets were always sort of an afterthought,” he told me during a telephone conversation during the summer of 2011. But he was aware that Geoff Marcy was looking for planets, and he knew about Michel Mayor and the Swiss group, and about the group in Canada. By the mid-1990s Brown had teamed up with a few other astronomers including Bob Noyes at the Harvard-Smithsonian Center for Astrophysics, or CfA—the umbrella organization, located within a single building complex in Cambridge, that includes the Harvard University astronomy department and the Smithsonian Astrophysical Observatory.
The Brown-Noyes group set up shop at the Smithsonian’s sixty-inch telescope on Mt. Hopkins, near Tucson, Arizona. They were mostly looking for pulsing stars, but they kept their eyes out for planets as well. “It’s fair to say that we didn’t get anywhere,” Brown told me in a phone conversation, “but it’s a curiosity that I do have two observations of 51 Pegasi that predated Michel Mayor’s discovery.” He made them in 1995, about six months before Mayor announced he’d found a planet. Brown looked at the data, saw what turned out to be the signature of 51 Peg b, and thought, “Aha, the spectrograph is probably misbehaving again.” After that, the team dabbled a bit more in looking for planet-induced wobbles, but, said Brown, “we were never seriously in that game, although we tried to be.”
But as early as 1992, he and his frequent collaborator Ron Gilliland, of the Space Telescope Science Institute, had already begun thinking about looking for planets by means of transits. It was mostly theoretical at first, since everyone still thought that other solar systems would resemble ours. If that was true, then the biggest, most easily detectable planets (like Jupiter) circle their stars in long, loping orbits. If they did transit, it would be only once every decade or more. Even then, the odds were long that they could spot such a transit: At a distance of hundreds of millions of miles from the star, the plane of a planet’s orbit would hav
e to be so precisely aligned with our line of sight that such a thing would happen only rarely. A huge planet orbiting right up against its star wouldn’t have to be aligned with nearly so much precision—but such a planet was beyond the imagination of most astronomers at the time. “But then in ’95,” said Brown, “when Mayor found 51 Peg in such a tight orbit, it became obvious that it was sensible to go looking. It was obvious to me, anyway. I had trouble convincing others.”
He and Gilliland had already convinced themselves, however, that they could achieve the necessary precision to detect transits, even without a huge telescope. Brown and Gilliland got together with Ted Dunham at Lowell Observatory—the same place where Percival Lowell had found “proof” that Mars was inhabited—to cobble together their first transit-search telescope. (Dunham would ultimately become the Science Team director for the Kepler Mission before leaving to work on a high-altitude infrared observatory.) By 1999, they had it built, and Brown set it up in a parking lot at NCAR to run it through some tests.
It was just at this moment when Bob Noyes, Brown’s old collaborator, was trying to help Dave Charbonneau find a thesis topic. Noyes knew about the transit-search project (Brown called it STARE, for STellar Astrophysics and Research on Exoplanets), and he suggested that Charbonneau head west to help out. “I bought a car,” recalled Charbonneau, “and I drove out to Boulder, and started working with Tim.” Before he went, he stopped to see Dave Latham. As Latham remembers it, “Dave ambushed me outside the door to my office as I was leaving one night and said, ‘I’m leaving for Boulder, and I need your advice on some good objects to look at.’ ”
Although Latham had let Michel Mayor and Tsevi Mazeh pick up the planet-hunting project and run with it a full decade earlier, he was still in touch with them. Latham knew, although it hadn’t been announced publicly, that Mayor and Mazeh had found a radial-velocity signal in a star called HD 209458. The size of the planet, and the nature of its orbit—assuming it was a planet, naturally—made it a prime candidate to transit across the face of the star. “Look at HD 209458,” Latham advised. “I’ll tell you exactly when to look.” This was in late August. Charbonneau forwarded Latham’s heads-up to Colorado, where, sure enough, Brown found the first transit. “I think it was 9/9/99, September 9, 1999,” said Charbonneau, as we walked from his class to his office. “That was the first recorded transit of an exoplanet in front of its star.” Charbonneau showed up in Boulder a couple of days later, in time for the second transit, on September 16.
But Charbonneau and Brown figured this out only in retrospect, a couple of months later. That’s because Brown had another search for transits going on at the same time, with his old partner Ron Gilliland. This one used the Hubble Space Telescope, in a sort of preview of the Kepler Mission. As Bill Borucki had realized ten years earlier, the chance of seeing a transit on a single random star (that is, one where Dave Latham hasn’t tipped you off first) is vanishingly small. The best way to search is to look at a field of stars all at once. With its small field of view, Hubble isn’t the ideal instrument to use, so Gilliland and Brown cheated a little: They focused on one of the hundreds of globular clusters that dot the central regions of the Milky Way.
Globular clusters are knots of up to a million stars—almost like miniature, spherical galaxies. The astronomers pointed the Hubble at a cluster called 47 Tucanae, and took data on about thirty-five thousand stars. 47 Tuc was much too far away for radial-velocity measurements, so there was no hope of getting densities for any planets they might find—but getting a sense of how many planets there might be in this big sample, and what sizes and orbits they came in, would still be useful. They predicted they might find seventeen transits. In the end, they found zero, perhaps because globular clusters contain mostly very old stars that have relatively little of the elements planets are made of.
Brown and Gilliland had gotten what Brown calls “a big pile of Space Telescope data” just as he and Charbonneau began doing their ground-based observations from the back parking lot. So they put aside the STARE data and began working their way through the Hubble data. Before they could get very far, Charbonneau got a call from back home. It was John Huchra, the director of graduate studies and former head of the Harvard astronomy department. Huchra was an unrepentant cosmologist—none of this planet stuff for him!—who had helped create the first large-scale maps of the universe in the eighties.
He was calling because Charbonneau had never gotten official permission to go west. Bob Noyes knew about the trip, but that wasn’t good enough. “Huchra said, ‘Dave, you’ve got to come back and you have to make a case for the science you’re doing. We’re really nervous about students going away and working with scientists outside of the CfA because we can’t supervise you.’ I think maybe he thought I’d gone out to Colorado to go skiing, I don’t know,” Charbonneau told me.
In any case, he rushed back to Cambridge. “I remember I went through a long defense—it was a couple of hours, at least.” About ten minutes of that was talking about why transits would be interesting; that part was pretty obvious. “The rest,” he said, “was what my thesis would say when I didn’t find a transiting planet, which was what they all assumed would happen.” So he spent the rest of the time talking about the things they believed he would find, like binary stars and pulsating stars, and all the good science he could wring out of those. “Of course,” Charbonneau continued, “the irony is that I already had the data that would show the transits of 209458, even though I didn’t know it yet.”
The thesis committee was satisfied and Charbonneau headed back to Boulder. In the end, it took two months of working on the Hubble data before they could get back to their parking lot project. At almost exactly the same time, though, Brown got wind that Geoff Marcy and a collaborator named Greg Henry, at Tennessee State, had detected a transit as well, although he didn’t know what star they were observing. “Geoff and I had an interesting phone conversation,” he recalled, “which amounted basically to ‘You show me yours and I’ll show you mine.’” It turned out to be HD 209458. Brown and Charbonneau didn’t get extra credit for the fact that their observations beat Marcy and Henry’s by two months, since they hadn’t analyzed them. The two teams published their discoveries in the same issue of the Astrophysical Journal the following year, and they’re generally given equal credit for the discovery (although some websites mention just one or the other; a news release published on the UC Berkeley website at the time, for example, mentions only Marcy’s group).
Credit aside, though, the observations marked another crucial step forward in the search for Earth-like planets. Four years earlier, Michel Mayor and Geoff Marcy and their teams had made a strong case that planets, or something that looked a lot like planets, orbited around Sun-like stars. With the discovery that at least one set of radial-velocity wobbles was matched by a series of transits, with precisely the same timing, it became impossible to doubt that many of them, if not most, had to be planets.
Moreover, the fact that the planet now known as HD 209458 b already had a known mass, thanks to Mayor and Mazeh, and that its size was now known from the amount of light it blocked during its transits, allowed the astronomers to calculate its density. It was surprisingly low: HD 209458 b turned out to be about 70 percent as massive as Jupiter, but about 35 percent bigger. With such a low density, it was clearly made mostly of gases, but what kinds, and in what percentages, and with what implications for the makeup of smaller, life-friendly worlds, were still unknown.
Before we finished our conversation, Charbonneau wanted to tell me one more thing. “When you defend your thesis,” he said, “you have to have an outside reader, in addition to members of your department. I picked Bill Borucki.” At the time, very few of the astrophysicists at Harvard knew much about Borucki, a government scientist who didn’t even have a Ph.D. But Charbonneau did. “I already knew that he was going to be PI of the Kepler Mission and I knew Kepler would find all of these planets, these Earth-like planets. Hopefull
y, anyway.”
Chapter 6
IMAGINING ALIEN ATMOSPHERES
When Dave Charbonneau arrived at Harvard, it turned out that he wasn’t the first Canadian from the University of Toronto to show up there intending to be a cosmologist. Two years earlier, a woman named Sara Seager had come to Harvard with exactly the same intention. They’d known each other back in Toronto, but she had been two years ahead. “That doesn’t seem like a lot now, but as you know, when you’re younger it’s a much bigger deal,” she told me one morning at her office at MIT. No one would ever describe the MIT campus as charming; it’s stark and soulless compared with leafy, ivy-covered, Georgian-brick Harvard a couple miles to the northwest. But Seager’s office is on an upper floor in the Green Building—a high-rise that is typically bleak on the outside, but with a view from the higher floors that plenty of Harvard professors would undoubtedly kill for. Her window looks down on the Charles River Basin, filled with sailboats and rowing shells for much of the spring, summer, and fall. Across the basin, the towers of downtown Boston loom only a mile or so away. Seager and Charbonneau had first met, she told me, when she was a senior and Charbonneau was a sophomore. “He was thinking of dropping out of physics,” she said, “and after I graduated he sent me this really nice letter thanking me for convincing him not to.”
When she first got to Harvard, Seager worked on the recombination of the universe. This is the time, about four hundred thousand years after the Big Bang, when the hot, dense universe cooled off enough for atoms to form out of subatomic particles. The light that burst free as a result of that event is what astronomers stumbled on in 1965, proving the Big Bang had happened, and what cosmologists use to try to figure out how matter in the universe was arranged at the time. Like Charbonneau, Seager was working on the most popular topic in astrophysics in the mid-1990s—just as it was starting, like the early universe itself, to cool off. “My most highly cited papers are still the ones I wrote in cosmology,” she said. “But that’s because there are more cosmologists to cite papers, just so you know.”
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