Mirror Earth (30 page)

Thanks to Kepler's unprecedented observations of Sun-like stars, however, the effort spearheaded by Ron Gilliland, the team now knew that the Sun is unusually quiet. Most Sun-like stars are about twice as noisy, which makes it just that much harder to tease out the signal of an Earth-size transiting planet. You can do it, but you need as many transits as possible, so the regular, repeating signal of a planet begins to stand out against the random flicker of noise. For an Earth-size planet in the habitable zone of a Sun-like star, transits come along only once a year. With a mission funded for only three and a half years, that's probably not enough to find a Mirror Earth. A few months after the Wyoming conference, the team would be petitioning NASA to extend the mission for another three and a half years. It's not as though the satellite would fall out of the sky, but it takes money to keep the analysis going.

That's why Natalie Batalha was so worried about the upcoming budget review by NASA, which was now only four months away. The Kepler team would do its best to convince agency officials that they simply had to have funding for an extended mission. But the Hubble Space Telescope was also up for renewed funding. In a budget situation that appeared to be getting worse by the month, there was a reasonable chance that the most successful exoplanetology search in history would be cut off before it could fulfill its mission. In the end, NASA would announce in April 2012 that Kepler, Hubble, and seven other missions under consideration would all be allowed to continue.

The day after the Boston meeting of the American Astronomical Society ended in May 2011, Sara Seager hosted a small and exclusive workshop at MIT to talk about the next forty years of exoplanet science. It was already clear to exoplaneteers that the wish list of telescopes and other instruments they'd been counting on to let them identify Mirror Earths and search these new worlds for life wasn't going to happen on any sort of reasonable schedule. In the face of that disappointment, how, Seager wanted to know, did her colleagues see the next four decades playing out?

The workshop took place on the top floor of a modern gem of a building at MIT that serves as headquarters for the Media Lab, an interdisciplinary laboratory that looks at how humans do and will interact with emerging and future technologies. As the guests trickled in and helped themselves to coffee, tea, and pastries, Seager urged everyone to take a quick stroll out onto an enormous balcony, with a wide view of the Charles River Basin and the Boston skyline beyond. And then, after a few minutes of milling around—pretty much everyone there
knew everyone else, so there was more catching up than introductions—everyone moved into a small auditorium where Seager explained how things would work, and why she had convened the workshop.

She began by talking about cicadas, which are insects with a somewhat bizarre lifestyle. In the larval stage, cicadas lurk underground for years—thirteen in some places, seventeen in others—and then they emerge, all at once. They come out and feed on tree branches and leaves, and they hum. Millions upon millions of bugs fill the air with a loud, whirring noise that sounds like some sort of heavy machinery. It goes on for about three weeks. Then they mate and die. The new larvae burrow back into the ground, the bodies of millions of dead insects litter the ground, and all goes quiet for another thirteen or seventeen years. “I really want all of you to see that some day,” she said.

The cicadas, Seager continued, “remind me a little bit of exoplanets, because, just like Earths, they may be rare, few, and far between but if you know where to look, they're everywhere.” She recalled a conversation she and Paul Butler had had a few years earlier. “We were looking at a photograph of a cicada,” she said, “and Paul told me, ‘You know, the next time the cicadas return, we will be old.' And I went, ‘Me, I'm never going to be old.' That's what I thought at the time. I was thirty-two, and I'd never thought about being old. TPF was always going to happen, I would always be young when TPF happened.' “

In just two months, Seager said, she'd be turning forty. “So I convened my friends here to come and tell me what they
think should happen in the next forty years.” The question was especially important to think about now, however, with TPF and SIM on the far back burner and the Webb telescope in financial trouble, Congress slashing budgets left and right, and word of Kepler's problem with stellar noise already spreading quietly around the exoplanetology community. She was also concerned that the field had lost some of the exciting, groundbreaking, out-on-the-edge feeling that Paul Butler and Geoff Marcy and Dave Charbonneau and Bill Borucki and the rest had brought to it. “I ask all of you to think,” she said, “about how to continue the wave of exciting discoveries.”

With that, the talks began. They were limited to ten minutes each, just enough time to convey a few big ideas in the most general sort of way, followed by questions. Dave Charbonneau got up to talk about the fact that a remarkable 246 M-dwarf stars lie within just ten parsecs, or thirty-two light-years, of Earth. They have distinct personalities, he said, “and so what I'd like to advocate for today is that we need to know and understand every single one of these stars.” Based on the still-evolving eta-sub-Earth numbers coming from Kepler and from the radial-velocity surveys, he said, “the answer appears to be that we are guaranteed that there are bodies at the right distance from those M-stars [to be habitable]. We have no idea if they're inhabited but they're a good place to start looking.”

Geoff Marcy was more downbeat: He talked about his frustration with all the canceled exoplanetology missions. Dave Latham talked about a new generation of gigantic ground-based telescopes—at a minimum, three times as big as the Keck, the
largest in existence, with twenty-seven times the collecting area—armed with equally powerful spectroscopes that will pick up the radial-velocity signals of Mirror Earths by the dozens, perhaps as soon as the early 2020s. Matt Mountain, director of the Space Telescope Science Institute, made the case that the Webb telescope, along with a starshade, would be a more powerful planet-hunting telescope than anyone appreciated.

There were more than a dozen speakers in all, and it would be impossible to reproduce all of the ideas that were tossed around, especially in the Q&A sessions that followed the talks. One speaker made an especially powerful impression, however, and she wasn't an all-star of exoplanetology by a long shot. Her name was Rebecca Jensen-Clem; she was an MIT junior who had taken a class with Seager in which the students developed a satellite that was slated for a NASA launch sometime in 2012. It was a rectangular box, four inches by four inches by twelve inches, crammed with a small telescope and all sorts of electronics, and its job would be to look for transits—not around 150,000 stars, like Kepler, but around a single star. If it worked, this “ExoPlanetSat” could be mass-produced so quickly and cheaply that it could largely eliminate the need for full-blown space missions.

This wasn't a grander version of today's grandest technology: It was a complete rethinking of planet-hunting, and it was exactly this sort of innovation, Seager said during the Q&A, that could transform exoplanetology over the next forty years. “I hope,” Jensen-Clem ended by saying, “that more students in the next forty years will have the opportunity to work with
scientists and engineers as we did in ExoPlanetSat. I have every confidence that forty years from now, students will be able to come up to their professors and say, ‘I don't remember a time when Earth was the only habitable planet.' ”

And shortly after that, it was time to wrap things up. “Thank you,” said Seager, taking the microphone one last time. “I'm going to finish with one remark and then a couple of other things. I want to tell you the origin of this picture.” It was an image of two children pointing up and silhouetted against the glow of sunset, which itself was reflected in a pond. The children were hers—two boys, the older named Maxwell Solstice and the younger, Alexander Orion. “Does anybody recognize this setting?” she asked. “This is actually out at Walden Pond. It's almost impossible to take a picture of children, so you know how I actually got them to point?” Up in the darkening sky, she explained, outside the frame of the picture, was the crescent Moon, with a brilliant, sparkling gem—the planet Jupiter—sitting almost right between the points of the crescent. “They were just having a hysterical laugh because they're like, ‘Oh, the Moon is eating Jupiter.'” Today, she said, pointing to one of the children, “that little one there, he's six years old and he walks around saying, ‘I'm going to be an astronaut. I'm going to travel to the Earth-like planets that you find.'” And with that, the workshop was over. It was followed by a reception, and then a dinner in a conference room high up in the Green Building, to celebrate Seager's birthday.

Before everyone sat down to dinner, Seager had one last surprise. She led us to a tiny elevator where, five or six at a time, we rode up to the building's roof. It's the tallest building
in Cambridge; the view was spectacular in all directions, as the exoplaneteers walked around, drinks in hand, catching up with one another and talking mostly about other things besides their research. Geoff Marcy decided to climb a ladder that led up into one of the white radar domes—the landmarks I used to see from down on the Charles Basin when I was a student. He couldn't get inside, which was probably just as well, but he did pose for some pictures.

The Charles River Basin itself was filled with sailboats on this warm, late-spring evening, the sailors unaware that the world's greatest exoplaneteers were looking down on them. Off in the west, the Sun was setting over Walden Pond, more or less—it was out in that direction somewhere. Although Seager had planned everything perfectly, she hadn't managed to re-create the Moon-Jupiter pairing that had fascinated her sons. Nobody seemed to mind. In small groups, everyone headed back downstairs, to toast Sara Seager's fortieth, and to talk, not only about what the program might be for a follow-up meeting forty years hence, but about the odds for finding a true Mirror Earth sometime in the next year. It had been just fifteen years since Geoff Marcy, holding forth at the far end of the dinner table, had been among the first scientists ever to detect an exoplanet—a giant, searingly hot ball of gas, where no living thing could possibly survive.

Now, thanks to the relentless efforts of Marcy and Charbonneau and Borucki and Batalha and dozens of other exoplaneteers around the world, scientists had cataloged hundreds upon hundreds of distant worlds, pushing to ever smaller and more hospitable and Earth-like planets all the time. They had
found the first truly rocky planets. They knew that the Milky Way has more Neptune-size worlds than it has Jupiters, and more super-Earths than Neptunes, and almost certainly more Earths than super-Earths. They knew that planets are common, not just orbiting Sun-like stars, but orbiting the far more numerous red dwarf stars as well. They knew that with any sort of luck at all, the magic combination of factors—an Earth-size, rocky world, orbiting in the habitable zone of its parent star—would be found very, very soon. It might even be lurking in Kepler's vast data files, or in the data from observing runs by Michel Mayor's team, or in one of Marcy's data sets, just waiting to be analyzed and introduced to the world.

As they looked forward into the future from the vantage point of 2011, these superstars of exoplanetology knew that they couldn't really imagine where the field might stand forty years in the future. But they could easily see that a Mirror Earth might well be identified and confirmed within months rather than years. By the time Sara Seager turned forty-one, there was every chance that astronomers would be echoing my father's last words by proclaiming: “A Mirror Earth goes around a distant star—
that's
what's going on!”

As is always the case with a work of nonfiction, my determination to write about exoplaneteers and their search for a Mirror Earth wouldn't have gone very far without the help of a long list of people. At the top is Cynthia Cannell, a terrific literary agent, but also a warm, thoughtful, and enormously supportive person. My editor, Jacqueline Johnson, has been a pleasure to work with. She didn't necessarily consider every word of my manuscript to be like a drop of precious gold, but that's a good thing: Like most writers, I need a smart, tasteful editor to help me weed out passages of writerly self-indulgence and keep the words flowing smoothly and clearly. I've had good editors and bad, and Jackie is one of the very best.

I obviously wouldn't have gotten very far without the help of dozens of exoplaneteers, including (in no particular order) Geoff Marcy, Debra Fischer, Natalie Batalha, Dave Latham, Sara Seager, Bill Borucki, Dave Charbonneau, Eric Ford, and many, many more. These scientists are among the busiest people I know, but they took the time to talk to me at great length,
and in many cases more than once, to explain the work they were doing, and how and what they were finding, and why it mattered so much. Dan Fabrycky deserves special thanks: He read through the entire manuscript (before Jackie Johnson helped me work out the kinks!) to make sure I wouldn't make too many egregious errors.

I also want to thank my students at Princeton, Columbia, Johns Hopkins, and New York University, past and present, whose energy and intellectual curiosity invariably help shift my brain into high gear when it's idling unproductively in neutral. And I especially appreciate the willingness of two good friends and colleagues, Emily Elert and Alyson Kenward, who read parts of the manuscript and made helpful suggestions. Finally, as always, I thank my wife, Eileen, for being unfailingly supportive, even when this project intruded on family life. I owe her that trip to Vietnam, or India, or wherever she wants to go.