“So, of course, they rejected the proposal,” he said. Again. NASA told him to go into the lab and build a test facility that proved it could be done. This time, at least, Borucki didn’t need to use his credit card to build the test facility. “They gave us five hundred thousand dollars. That’s nowhere near enough money, but Ames lent us another five hundred thousand dollars. Which was sort of scary because you have to pay five hundred thousand back. But it was progress.” That funding made it possible for Natalie Batalha to join the project. But it wasn’t just about following the money for her. “It’s more than that,” she said. “For me, this is such a profound quest. It’s exploration in the very fundamental source of the word, right? Human beings have that seed in them to always search for new horizons—humanity in general, not just scientists. I think about that a lot. It drives me and it motivates me, and it makes me particularly interested in this area. And for me just as a career in general, I couldn’t do anything that didn’t have some kind of profounder meaning. Perhaps scientists all have that in common as well. I don’t know.”
Natalie Batalha decided to become an astrophysicist after she took a freshman physics class and spent a summer at an observatory in Wyoming. For her old office mate Debra Fischer, it didn’t happen until she was in her late twenties; astrophysics was her second career. “I don’t like to tell anyone about the earlier part of my life,” she told me in her office at Yale. “It’s just—I don’t know why, because now it shouldn’t matter. I’m tenured at Yale, what difference does it make? But I got my first degree in nursing, it turns out.” This may not sound like such a terrible thing, but it’s possible, although at this point in her career highly unlikely, that some of her colleagues would take it as showing a lack of seriousness. Even Carl Sagan, an astrophysicist and a professor at Cornell, was looked at suspiciously, and even denied admission to the National Academy of Sciences, because he also wrote popular books, appeared frequently on The Tonight Show, and hosted the Cosmos series on PBS.
After Fischer got a degree in nursing at Iowa, she wound up at the hospital at Case Western Reserve University in Cleveland, Ohio. “The whole time,” she said, “I was much more fascinated with the instruments and the way things worked, the defibrillators and everything, and less focused on the poor sick people, actually.” Even so, she was so immersed in the world of health care that when she decided to take her next career step, the plan was to go to medical school. Her boyfriend at the time—now her husband—was a medical student at Case; when he went out to San Diego for his residency in internal medicine, Fischer enrolled at San Diego State University to fulfill her premed requirements. “It was kind of a luxury,” she said, “to go back to school and know that I could sort of play around a little bit. I had time to take a course on the history of jazz or classical music or art history.” She’d always loved mathematics, so she also took some math and physics and, just for fun, an astronomy class. “I realized that astronomy was this amazing study of everything,” she said, “with the insignificant exception of Earth. If you look at Earth compared to the universe it’s nothing. It seemed so exciting.”
By the time her husband moved on to a second residency, in cardiology, in San Francisco, Fischer had given up on medical school. She enrolled at San Francisco State University to do a master’s in physics. “They had just hired Geoff Marcy—I think it was 1984 or something like that. I went observing with him at Lick,” she said, “and just fell in love with the whole observatory and the process of looking out into the universe. It was a great opportunity.” The obvious next step was to apply to a Ph.D. program, but Fischer wasn’t so sure. “I thought, ‘I’m too old, I’m already thirty, I shouldn’t go back to school.’” So she taught some undergraduate physics courses at San Francisco State for a couple of years, but, she said, “I couldn’t stop thinking about going back.” She applied to several schools without a huge amount of confidence. “I remember Geoff saying, ‘It’s not up to you whether they accept you. It’s out of your hands. If they decide you’re too old, that’s the way it is.” Of the five schools she applied to, she got into four. She picked Santa Cruz, partly because it was the closest to San Francisco, where her husband was now a cardiologist, and where they now had a child. “Actually,” she said, “two children, by the time I started.”
Debra Fischer (Tony Rinaldo)
Like Natalie Batalha, Fischer worked on stars, not exoplanets. There simply weren’t enough groups working on the topic yet to provide grad students with good research projects, and money hadn’t yet poured into the field. Fischer’s thesis was on using the element lithium as a marker for stars’ ages. But at one point in 1997, she went to an International Astronomical Union symposium in Boston. Geoff Marcy had been there too. “It was a really exciting time,” she said, “because Geoff was talking about the first three exoplanets, which was all they had at the time.” She was sitting in economy, in a middle seat at the very back of the plane, and suddenly Geoff Marcy was standing there. “He’d come back from first class [so clearly some money was flowing], and he said, ‘I’ve been thinking. Paul Butler has this great opportunity to go start an exoplanet project at the Anglo-Australian Telescope, so we’ll need another person on my team. What do you think?’” As best as Fischer can recall, her answer was, “Are you kidding? This is so exciting! I will put my whole heart into this project.”
And she did, driving up to Lick late in the afternoon some eighty days a year, taking data all night, then driving back down for her day job, first as a postdoc and then as a research astronomer at Berkeley. “It would be absolutely insane,” she said, “but back then, every planet was a big deal.” It was an even bigger deal when the team found signs of a second planet around the star Upsilon Andromedae, where they had found one already. At this point, no star had yet been shown to have a second planet.
Fischer’s job was to take measurements of the star’s radial velocity over many different nights and try to see if they fit a curve. This was how Mayor and Marcy and Latham had been doing it for years. You can’t simply aim your telescope on a star and watch it move; you take a reading every so often and plot it on a graph. Today, the star is moving toward you at such and such a speed. Another day it’s moving away. Another day it isn’t doing much of anything. (This is an oversimplification: Stars are always drifting toward or away from Earth as they bobble along in their independent orbits around the core of the Milky Way. The motions Mayor and the others were looking for were on top of that constant, steady drift.)
Over time, those measurements should trace out a curve representing a repeating forward and backward motion—the signature of a planet. If you expect the curve to move leisurely up and down on your graph over years, you wouldn’t bother making a measurement every day. That’s how Marcy and Butler missed 51 Peg; a planet with a four-day orbit isn’t going to show any sort of obvious pattern if you look at it randomly once every few weeks or months.
If there are two planets orbiting the star, it gets more complicated. There are two radial-velocity curves now, with different strengths and periods, but they’re superimposed. Each measurement gives you the combined effects of both planets’ gravity on the star. Fischer appreciated this, but even so, she said, “it was horrible. It wasn’t coming out right at all.” Finally, she took the two curves that best fit the data, bad as they were, and subtracted them from the overall signal. When she did that, she recalled, “I saw the data doing this unbelievable extra sine curve. It sent chills down my spine. It looked like there was a third planet in there, and no one had expected it. I remember holding the plot,” she said, “and walking across the campus to Geoff’s office, thinking, ‘Look at this, there’s a planet with a 4-day orbit, there’s one with a 240-day orbit, and there’s one with a 2.5-year orbit, and they are big.” It was hard to imagine how such a system could be stable, with three huge, tightly packed planets tugging not just on their star but on one another. “It has completely changed our vision of how planets form, how much space they r
eally need, that sort of thing.”
At just about that time, Bob Noyes called up from Harvard to say he’d been looking at the same star, and thought he had enough data to confirm two planets. “I knew it wasn’t two planets, I knew it had to be three,” said Fischer, and she was really distressed when Marcy proposed that the two teams combine their data and publish a joint paper. Once they saw her analysis, she realized, the Harvard team would see the third planet too, and get some of the credit. “I look back,” she said, “and it’s silly to have been so disappointed. But I was.” Officially, the Harvard and Berkeley teams did get equal credit for discovering the first three-planet solar system beyond the Sun (a fourth may have now been found). Unofficially, without authority, and surely against their wishes if I should be foolish enough to ask permission, I hereby award full credit to the Berkeley group.
At the time, Marcy and Butler had been able to hone their technique to the point where they could measure a star’s motion to a precision of three meters per second. That was good enough to find Jupiter- and even Neptune-mass planets around other stars, but not good enough to find an Earth-mass planet, even in a very tight orbit. (Butler’s group in Australia would remain part of the Berkeley team, and when Butler later moved to the Carnegie Institution of Washington, the collaboration would continue.) The astronomers weren’t satisfied. They didn’t know how much better they could do, but they would try. “I remember,” said Fischer, “Steve Vogt, Paul Butler, Geoff Marcy, and I would always sign our e-mails ‘OMPSD,’ which stood for ‘one meter per second or death.’ Steve, especially, loved that. But it was comical. Because of course you could never get to one meter per second.”
Chapter 8
KEPLER APPROVED
When Bill Borucki got the million dollars that allowed him keep honing the Kepler concept, his job wasn’t just to convince NASA decision makers that his spacecraft would be stable, but also that it could reliably detect minute changes in light intensity. “They told us, ‘It’s probably going to take you several years to build it, because nobody’s ever done anything like it before,’” he told me. “So we went out and we bought up all of the invar we could find”—invar is an alloy of steel and nickel that expands and contracts very little when the temperature changes. The alloy, invented in 1896 by a Swiss metallurgist named Charles Édouard Guillaume, proved so useful for constructing high-precision scientific instruments that Guillaume won the Nobel Prize in 1920 as a result. Albert Einstein had to wait until 1921 for his.
Despite NASA’s pessimistic projection, said Borucki, “we got the design together in a few months, and then we got all the machine shops in the Bay Area building the parts for us. Got this whole thing built in a year, and got it debugged six months later. Then we had to figure out ways to simulate transits and measure them with high precision. We didn’t care about accuracy.”
For anyone but a scientist or an engineer, this probably sounds nutty, but it turns out to make perfect sense. Kepler didn’t need to measure how bright a given star is. The scientists didn’t care if this star is exactly as bright as that other star, or brighter or dimmer. They could afford to be inaccurate about that kind of measurement. All the satellite had to do was measure, with extremely high precision, the change in brightness when a planet passed in front of the star. “It’s very much like what Geoff Marcy does with radial velocities,” said Borucki. Marcy didn’t care about a star’s overall motion—it could be speeding toward the Earth or speeding away or sitting there like a bump on a log. All he cared about was the change in motion caused by an orbiting planet. “Geoff’s accuracy … well, I would hardly call it poor, but he considers it poor,” said Borucki. “But his relative precision is one part in a hundred million.”
So how hard would it be to achieve the kind of precision Borucki needed to find an Earth—a precision of between ten and one hundred parts per million, or a millionth to a ten-millionth of a percent? “Think about those holes in a metal plate that represent stars,” he said. “Now I slide a piece of clear glass in front of one of them. How much will the decrease in light be? For regular glass, it’s 4 percent. That’s way, way above ten or a hundred parts per million. So we take the glass and add an antireflective coating.” That makes less light bounce off the glass and more pass through. “Well, now it’s, you know, 1 percent or 0.5 percent. You’re still hundreds of times too high.”
No good. So in order to test the sensitivity of their detectors, they had to think of an entirely new way to create minuscule changes in light. David Koch, Borucki’s longtime collaborator, came up with one. “We drill laser holes in a steel plate,” said Borucki, “and we take a very fine wire and run it across the hole. Then we run a little current through the wire; the wire heats up and expands and it blocks more light.” They built it. It behaved perversely. “Of course,” he said, “when we applied the current, we got more light coming through, not less. Now, how can that possibly be?” It turned out that if the wire wasn’t absolutely straight to begin with, the expansion would make it bend. It was no longer covering the widest part of the round opening, so it was actually letting more light through. “So we had to go back and redrill the holes as squares.”
There was plenty more of this sort of thing, but by the time Natalie Batalha joined the group in 2000, Borucki and Koch had finished their experiments. They had convinced themselves that they could make precise (albeit inaccurate) measurements of the tiny changes in starlight that would signal the presence of a faraway, invisible world. They were writing up yet another proposal for submission to NASA’s Discovery program. But it wasn’t enough simply to prove that their space telescope would work properly; they also had to convince NASA that planetary transits were observable in principle. A planet would pass directly in front of its star only if the orbit were precisely edge-on. Since planetary orbits could come in any orientation at all, the Kepler team had to show that enough of them would line up correctly, purely by chance, to allow the spacecraft to find them in enough numbers to justify spending hundreds of millions of dollars on the mission.
Borucki had promised NASA that Kepler would measure the dimming of starlight with a precision nobody had ever achieved. To do that, however, the team needed a crucial piece of information. Say you measure the light curve of a transiting planet, and it dims the star’s light by 1 percent. That tells you that the planet’s diameter is 1 percent as big as the star’s. But how do you know how big the star is in the first place? Without knowing that information, you can’t learn anything.
Fortunately, Kepler can get the answer, by using a technique called astroseismology, which borrows its name from plain old seismology, the study of earthquakes. When an earthquake goes off, the shock propagates outward, but also downward. The downward shock waves travel toward the center of the Earth, then bounce when they run into a change in density—where the semi-molten rocky mantle meets the iron core, for example. Around the world, seismic stations pick up those bouncing shock waves, and seismologists use them to deduce the inner structure of the planet.
The Sun isn’t solid like the Earth; it’s a huge ball of gas. It’s so dense, however, that it acts something like a huge blob of incandescent Jell-O. Turbulence in its upper layers makes the entire blob vibrate, in many overlapping frequencies at once. Decades ago, solar astronomers began to study the vibrations in the Sun by looking for radial-velocity differences from one part of the Sun to another. These differences are caused by the rising and falling of its surface under the influence of the vibrations. The pattern of vibrations depends very sensitively on the Sun’s inner structure—on the temperature and pressure of its inner layers, mostly—but it also depends on the Sun’s size.
With Kepler, astronomers could start doing the same sort of analysis for stars, and since knowing a star’s size is crucial to calculating the size of a transiting planet, this was part of the Kepler Mission right from the start. As the project progressed through the 2000s, though, it began to run over budget. “NASA told us, ‘You’v
e got to cut things,’” said Borucki. “One of the things that came up for cutting was astroseismology.” So the Kepler team cut a deal with a coalition of European stellar astrophysicists. The Europeans would get access to the Kepler observations before they were released to the public, and the Kepler team would get the astroseismology readings it needed in return.
The team also had to give a good answer to another obvious problem. We know the Sun has sunspots—dark blotches caused by the Sun’s magnetic fields. We know that Sun-like stars have them too; they’re called starspots, unsurprisingly enough. So how would Kepler distinguish between a dark starspot on the surface cutting down on the brightness of a star and the dark silhouette of a planet passing in front if it? This was Batalha’s first assignment on the Kepler Mission. “I tackled the problem,” she said, “from a stellar populations perspective.” As a stellar astrophysicist, she knew that young stars tend to rotate quickly and have a lot of spots. That’s problematic in two ways: First, the spots can masquerade as planets. In older stars like the Sun, that’s not such a problem, since sunspots get fewer and a star rotates more slowly as it ages. The Sun rotates about once a month at its equator; a planetary transit should last only a few hours. It’s easy to tell the difference. So Batalha’s first job was to figure out what percentage of the Sun-like stars Kepler would look at might be too young to be trusted. “It came out to be about 25 to 30 percent of the sample,” she said. “We basically showed that there would be enough stars left over that you’d still be able to detect substantial numbers of Earth-size planets.”
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