Mirror Earth (28 page)

Whether it might ever be named Vulcan in a formal sense is unclear. So far, the International Astronomical Union, whose job it is to assign official names to heavenly objects as opposed to the numbers and letters different catalogs use to list them, has not come up with any sort of exoplanet-naming convention. According to Geoff Marcy, his wife had an idea back in the 1990s, when he started finding his first planets. “She said I should call them Susan 1, Susan 2, and so on,” he told me. Although it should be obvious that she was kidding, it's probably a good idea to make it explicit: She was kidding. When 51 Peg b was discovered by Michel Mayor, a suggestion was floated that the planet be named Bellerophon, the name of the mythological Greek hero who tamed the flying horse Pegasus. One of the pulsar planets was unofficially named Methuselah (the planet was probably very old), and HD 209458 b, the first transiting planet, was called Osiris, after the Egyptian god, but these, too, were never considered official. I've rarely if ever heard them used by astronomers.

When the press conference rolled around the next morning, one more thought had evidently come to Batalha's mind. Or more likely, it wasn't a thought, but something subconscious that led her to speak of the planet in an unusual way. “Today,” she said to a group of reporters, anxious for the latest Kepler news, “we're announcing a new planet. She orbits her star …” Throughout her talk, Batalha referred to Kepler-10b as “she.” When it came time for questions, my hand shot up. “How,” I asked her, “did you determine the gender of the planet?” The room broke up and Batalha looked a little embarrassed. I was worried that I'd stepped over the line, even though I was obviously teasing, but Geoff Marcy, who had been up on the podium with her to provide commentary, assured me that I hadn't. “That was a great question!” he said to me after the press conference was over. “I was surprised because she was so consistent. I was like, okay, I heard it once, now I just heard it again … I don't know how much of a gender issue we still have in astronomy, since there are a lot of women at this meeting. But then,” he said, after thinking for a moment, “astronomy faculties are still mostly male, and the Kepler team is more male than female by a large margin. So the issue really hasn't gone away, and I think the men and women of Natalie's generation are still rightly sensitive to the issue.”

Kepler-10b was the closest thing to a Mirror Earth the Kepler team or anyone else had yet found, but it was only one of the four hundred planet candidates that had been held back the previous June. Just about three weeks after the Seattle meeting, the candidates were released at a press conference at NASA headquarters. When Bill Borucki began speaking before a
crowd of reporters, NASA officials, and TV cameras, however, he didn't just announce the four hundred; in the intervening half year, the team had vetted hundreds more. When the press conference was over, the number of planet candidates had leaped from 706 to more than 1,200. They broke down this way: 19 were larger than Jupiter; 165 were about the size of Jupiter; there were 662 Neptunes; 288 super-Earths; and, finally, 68 planets about the size of Earth, a few of which were a bit smaller than Earth itself. Slice it a different way, and you had 54 planets in their stars' habitable zones, 5 of which were approximately Earth-size (all the members of this last group were orbiting M-dwarfs, whose habitable zones were very close to the stars and where you could see the required minimum of three transits in just a few months).

This was the piñata Dennis Overbye was talking about in the
New York Times
, a harvest of planets so overwhelming nobody knew what to do with them all. If Kepler had been looking across the entire sky rather than at a tiny patch, Borucki said, it probably would have found about four hundred thousand planets. That's only in the nearby sky, not the Milky Way: Kepler is a powerful telescope, but not powerful enough to find planets much more than a few hundred light-years away. The Milky Way is about a thousand times bigger than that.

Kepler had found
potential
planets, Borucki emphasized; most of them would never be fully confirmed. Nevertheless, statistics based on the planets that had been confirmed suggested that about 90 percent of these objects were real—and these confirmed planets had come from only the first six months of Kepler observations. Having so many candidates brought the
Kepler scientists a long way toward their goal of calculating the frequency of Mirror Earths. The team would continue to try confirming as many planets as it could, of course, with every trick it could come up with. Kepler's primary goal was statistical, but finding real planets that could be studied, either with existing telescopes or with the more powerful telescopes that might be coming over the next decade, was a close second. The day after Bill Borucki's press conference, in fact, the cover of
Nature
featured an artist's rendering of the Kepler-11 system—the next one to be confirmed after Kepler-10b, Natalie Batalha's hot, rocky Vulcan. This was the six-planet system for which Daniel Fabrycky had created a simulation that so fascinated his children—five planets ranging in size from super-Earth all the way up to near Neptune size, all huddled up against their star. If you took five planets bigger than Earth and crammed them inside the orbit of Mercury, you'd have Kepler-11.

“I was the one who first saw this system in the data and said, ‘This is really interesting, we should put a lot of effort into it,'” Jack Lissauer, the Kepler scientist who served as lead author on the
Nature
paper, told me the day before it was formally released. That effort included a campaign to monitor the star itself very carefully, to be absolutely sure about its physical size, and to make calculations, based on transit-timing variations, to confirm the planets and get their masses—the job of Fabrycky, the paper's second author. (There were thirty-nine authors in all, including Eric Ford at number three; Bill Borucki, fourth; Geoff Marcy, sixth; Natalie Batalha, eleventh; Dave Charbonneau, sixteenth; Dave Latham,
thirtieth; and Dimitar Sasselov, thirty-seventh. It was a who's who of exoplanetology.)

What made the system so intriguing was partly the question of how it came to exist in the first place. All of the planets were presumably born farther out and migrated or were flung into their present positions. Packing them in so tightly, however, wasn't easy for theoretical models to do. Another surprise was that the planets all orbited in precisely the same plane. “This was also going to be tough to explain,” said Lissauer. “Our solar system is
approximately
flat, but given the chaotic process that gives rise to planetary systems, it's hard to see how this one came together. It's so unexpected, and we're getting so much information from this system,” he said. “I think this is the biggest thing in exoplanets since the discovery of 51 Peg b in 1995,” he continued. “My view is that a true Earth-mass, Earth-size planet in the habitable zone is the only thing that could be more interesting than this system. But until we find that, they'll all just be hot rocks.”

So far, the Kepler team had been talking about two categories of exoplanets. There were candidates, more than 1,200 of them as of early 2011. And there were confirmed planets—about 20, distributed around seven stars, and confirmed by either a radial-velocity signal or evidence from transit-timing variations. In May 2011, at the smaller, spring meeting of the American Astronomical Society, they began talking about another category: validated planets. These weren't quite confirmed, but had passed enough tests to rule out false positives that you could consider them planets anyway. The chances were just too small that they could be anything else.

One way to validate a Kepler planet was to observe a transit in infrared light, with the Spitzer Space Telescope—Dave Charbonneau's project under the Participating Scientist Program. The dip in light from an actual transit should look the same with Spitzer as it did with Kepler; an eclipsing binary should show a color shift. Another newer and more rigorous technique, however, didn't involve any observations at all. Instead, it relied on a computer-simulation technique called Blender, originally created by Harvard astrophysicist Guillermo Torres to rule out false positives in the OGLE survey. This was the ground-based survey originally designed to look for chunks of dark matter in the Milky Way, then redirected to look for the Einsteinian microlensing, or magnification, effect of distant solar systems on even more distant stars, then redirected again to look for transits.

As Bill Borucki had realized early on, a dip in starlight could be the signal of a planet, or it could be coming from a pair of eclipsing binary stars peeping over the shoulder of the target star—a blend of light from several stars, mimicking a transit. “OGLE,” Torres said at the meeting, which was in Boston, “looks at a very crowded field of stars, where the likelihood of a blend is much greater than with other surveys.” So he came up with Blender, which took all possible scenarios—a wide range of combinations of foreground and background stars of all brightnesses and sizes and relative distances from Earth—and used them to calculate how likely it was that the signal the telescope saw represented one of these combinations.

“When Kepler came along,” Torres said, “I realized that
they wouldn't be able to confirm most of their candidates.” He approached the team and proposed they use his simulation. “They were very enthusiastic,” he said, “and it's been very successful.” The first success was announced by his Harvard colleague Francois Fressin at the meeting. Using Blender, Fressin and Torres were able to validate the fact that Kepler-10b, Natalie Batalha's hot, rocky Vulcan, has a companion, 10c, just a little bigger, at about 2.2 times the radius of Earth, with a forty-five-day orbit. With Blender, said Torres, “we're fairly confident that what we're looking at is a planet.” By fairly confident, he explained, he meant that “we're looking for odds ratios of something like a thousand to one. That's when we call it a validated planet. It's not confirmed: There's a difference in the terminology here.”

Torres and his colleagues also used Blender to validate the sixth planet in the tightly packed, CD-flat Kepler-11 system. “We're now working on other candidates,” he said, “which have already been submitted for publication.” And they've used Blender to validate the existence of CoRoT-7b, the possibly rocky planet that was possibly found before Kepler-10b—although Torres, like everyone before him, couldn't nail down its mass. Could Blender turn out to be the only way to validate a true Mirror Earth in Kepler's list of candidates?

His straightforward answer: “Yes.”

If you ask me, it's best to arrive at the Jackson Lake Lodge, about thirty miles north of Jackson, Wyoming, at night. The lodge is just inside Grand Teton National Park, so you know there must be some pretty good scenery out there somewhere, but in the dark it's purely hypothetical—and unless there's a full Moon out, it's really, really dark. When you check in at the front desk, it feels like an ordinary hotel, aside from the western-themed artwork and the decorative wagon wheels and antlers sprinkled around the lobby. Most of the guest rooms aren't in the main building, but rather in a series of cottages fanning out along small, poorly lighted roads. If you haven't got a car, they hand you a flashlight so you won't get lost. The bears, the clerks promise, are very unlikely to eat you. Most of the cottages are nestled in stands of pine trees, so even in full daylight, as you walk back to the lodge for breakfast the next morning, your sense of nature may be limited to the clean air and woodsy smell.

But when you reenter the main lodge, walk past the front
desk, and go up a wide set of stairs, everything changes. You find yourself in an enormous room, probably one hundred feet long, eighty feet wide, and three stories tall, filled with couches and comfortable armchairs gathered in clusters. In the corners to your left and right are two enormous circular fireplaces, each about ten feet across. A little bit farther into the room, on the right, is a stuffed grizzly bear inside a glass case, rearing up on its hind legs with its front paws brandishing sharp, three-inch claws at just about the level of your eyes. Or, no, it isn't a grizzly after all. When you read the plaque next to the case, it turns out to be a brown bear, the grizzly's close cousin. Real grizzlies do roam the woods and grasslands around here, as do ordinary black bears. Brown bears don't: This monster was shot in Alaska. The ecological incongruity doesn't seem to matter to the tourists: They line up to pose with the bear. He towers over the tallest of them.

Imposing as they would be in any ordinary setting, though, you don't notice either the bear or the giant fireplaces at first. That's because right front of you, the western end of the room is a wall of windows framing a view so spectacular that it produces something like a physical shock. In the foreground, there's a broad expanse of flat, brushy, marshy land where you can sometimes see moose, elk, and the occasional grizzly. Beyond that is Jackson Lake itself, fifteen miles long and seven miles wide and filled with at least three species of trout along with countless other types of fish. And towering over it all, right in the center of the window, is rocky Mt. Moran, more than twelve thousand feet high, flanked by a series of less
lofty peaks of the Teton Range (Grand Teton, the highest mountain in the chain, is five or six miles to the southwest, but you can't see it from this perspective).

Over the course of my five-day visit, the mountain would take on all sorts of dramatically different looks, now glowing a warm reddish gold as it was lit by the rising Sun; now silhouetted against the slowly darkening western sky after sunset; now shrouded at the top by a violent thunderstorm, or at the bottom by a thick bank of fog hovering above the lake. Each of these, and a dozen more, was worthy of a postcard (and something like each, undoubtedly, appears on an actual postcard in one of the several gift shops inside the lodge).