Mirror Earth (18 page)

In the case of Wolszczan and Frail's discovery, the awkward part has to do with the star they were looking at. It's a pulsar, the collapsed, super-dense nugget left over after a star has died in the titanic explosion known as a supernova—an explosion so bright that for a few days it can outshine the rest of the stars
in the galaxy combined. For a really big star, the leftover is sometimes a black hole. For something smaller, it's a chunk of matter just a few miles across, but more massive than the Sun. Just a teaspoonful of neutron star would weigh something like ten million tons. Some neutron stars rotate hundreds of times per second, and of these, some send out bursts of radio energy as they spin. When these incredibly rapid, precise blips were first picked up by radio astronomers in 1968, nobody had a clue what they were. They were briefly nicknamed LGMs, for Little Green Men, since at first nobody could think of a natural process that could cause such a precise, rapidly repeating signal.

What Wolszczan and Frail noticed was that the blips from a pulsar called PSR 1257+12 would vary slightly, coming closer and closer together, then farther apart, then closer. The best explanation was that something was pulling the pulsar toward, then away from the Earth. As the pulsar moves toward us, each blip of radio energy has just a little less distance to travel than the one before, so they come closer together in time. As it moves away, the blips have to travel a bit farther each time. The timing suggested two planets, each more or less the size of the Earth. It was crazy enough that Wolszczan and Frail held off on announcing the discovery, mindful of Richard Feynman's dictum that “you must not fool yourself—and you are the easiest person to fool.” But while they were reanalyzing their data, a colleague named Andrew Lyne announced a planet around a
different
pulsar. Lyne's discovery was published in the prestigious journal
Nature
with great fanfare. Wolszczan and Frail figured they'd blown it. Maybe they'd been too cautious.

As it turned out, they hadn't been. A few months after his astonishing discovery had made headlines around the world, Lyne was preparing a triumphal talk about it for the upcoming meeting of the American Astronomical Society. Working late one night, he realized to his horror that he'd left out a crucial step in analyzing his observations. He knew, instantly and instinctively, what would happen when he redid the analysis. Sure enough, he told me shortly afterward, “the planet disappeared.”

It would have been bad for Lyne's reputation if someone else had discovered the error before he did. It would have been even worse if he had insisted he was right after everyone else realized he was wrong. But Lyne found the mistake himself, and gave his talk as scheduled, but with a very different theme than he'd planned. The audience was utterly silent as he explained his mistake—and then, when Lyne finished, hundreds of astronomers gave him a standing ovation that lasted more than a minute. At the time, John Bahcall, Sara Seager's old adviser, was the society's president. After the talk, Bahcall came up to me and said, “I want you to know that Andrew Lyne's talk was the most honorable thing I've ever seen. A good scientist is ruthlessly honest with him-or herself, and that's what you've just witnessed.”

Lyne's pulsar planet wasn't real, but Wolszczan and Frail's, it turned out, were. If the search for a Mirror Earth is ultimately a search for life, this discovery doesn't fit anywhere. Life couldn't possibly exist here. Planets couldn't survive a supernova explosion, so they must have formed afterward, out of some of the debris. But the neighborhood of a tiny, radiation-spitting
neutron star would be about the most hostile environment possible. So the pulsar worlds are at once the first planets ever found and off at a tangent that makes them irrelevant to the search for a Mirror Earth. Exoplaneteers invariably mention the pulsar worlds when they talk about the history of exoplanet science, but don't talk about them much at all when discussing the current state of the research.

The pulsar planets were pretty much completely unexpected. So were the hot Jupiters Marcy and Mayor began to find in 1995. At that time, the theory of planetary formation was based on the assumption that our own solar system was probably typical. It wasn't obvious why this should be true, but it was even less obvious why it shouldn't be. The idea was that a cloud of interstellar gas and dust collapsed, less than five billion years ago, spinning faster and faster as it got smaller and flattening out into a disk. The dense core of the disk formed into the Sun, whose heat drove the lighter material, including hydrogen, helium, and water vapor, outward toward the edges and left only the heavier rocky material closer in. The rocky stuff formed into the Earth, Venus, Mars, and Mercury. Farther out, the rocky material congealed as well, but there was more of it. If you mentally divide the disk into bands, the bands farther out are much bigger around, so they have more total stuff—more rocky material, plus the extra gas pushed outward by the newborn Sun. The rocky matter would have congealed into big, rocky planets whose gravity then started vacuuming up the lighter gases. The result is worlds like Jupiter and Saturn, with solid cores shrouded in massive, thick atmospheres.

A Jupiter or a Saturn couldn't form close in, however, because there wouldn't be enough rock to form its massive core, and there wouldn't be enough gas to make the atmosphere. When 51 Peg b showed up, it was something like the situation when an elementary particle called the muon was discovered in 1936. No theorist had predicted such a particle, and the Columbia physicist Isidor Isaac Rabi responded by asking, rhetorically, “Who ordered
that?
” as though the muon were an exotic dish delivered unexpectedly from a Chinese restaurant. At least one theorist, however—Doug Lin, of the University of California, Santa Cruz—had predicted hot Jupiters. He'd argued, even before the discovery of 51 Peg, that in some cases a Jupiter-size planet could spiral inward, pushed toward its star by the gravitational effects of gas remaining in the disk. But most of his colleagues either didn't know about it or thought it was just a clever theoretical exercise. Suddenly, in the fall of 1995, it was a plausible explanation for a discovery that had come completely out of left field.

A pulsar planet could never support life. Neither could a hot Jupiter, not only because it's hot but also because any solid surface would be buried under the crushing weight of a thick atmosphere. For that reason, a cold Jupiter like ours wouldn't be likely to be habitable either, nor would a lukewarm Jupiter. If Lin's theory was right, however, the very existence of a hot Jupiter might rule out life anywhere in its solar system. If such a massive planet had spiraled inward, any smaller, Earth-like planet it met along the way would probably have been flung out of its stable orbit, and maybe even out of the system altogether. Hot Jupiters are the easiest planets to find, whether
you're using radial velocities (they're big and close in, so they have the greatest possible leverage for yanking around their parent stars) or transits (a big planet blots out more light than a smaller one, and if it's in a tight orbit, it's more likely to pass directly in front of the star, purely by chance). The fact that so many early exoplanet discoveries were hot Jupiters could well be a biased sample. The first things you find are the easiest things
to
find. They're the low-hanging fruit that you can grab from the tree with the least possible effort.

If hot Jupiters are the rule, on the other hand, the odds of finding a Mirror Earth could be depressingly low. Within a few years after it launched, Kepler would presumably have an answer to this question. But the exoplaneteers weren't going to sit around waiting. “The probability of success is difficult to estimate,” Philip Morrison and Giuseppe Cocconi had written in their
Nature
paper on SETI forty years earlier. “But if we never search, the chance of success is zero.” They were talking about the search for extraterrestrial radio signals, but the same principle applied here. So while Borucki, Batalha, and the others on the Kepler team kept pushing ahead on building the spacecraft and the pipeline of software and human analysis that would turn raw observations into discoveries, everyone else in the business kept pushing on their own projects. No one could do better than Kepler—but everyone wanted to steal just a little bit of the mission's thunder.

“Of course you could never get to one meter per second.” In 1999, when Debra Fischer codiscovered the second and third planet around Upsilon Andromedae, she was making what seemed to be a reasonable statement. Geoff Marcy had spent years struggling to convince other astronomers that his entire life's work wasn't a waste of time. Even after he and the Swiss team led by Michel Mayor began finding planets, their colleagues tried to argue that these weren't really planets, but something else. And even after it had become clear that they were planets and
not
something else, Marcy, Mayor, and the others who searched for wobbling stars—Bill Cochran and Artie Hatzes, of the University of Texas, for example, who had been looking for planets since the early 1990s; Gregory Henry, of Tennessee State University; George Gatewood, of the University of Pittsburgh; and many more—had to deal with the argument that they'd never be able to make measurements good enough to find planets anywhere near as small as Earth.

The problem was partly the precision of their instruments. “If you took a metal ruler a couple of inches long,” Steve Vogt
once told me, “and then stood it on end, the amount it would shrink due to gravity is the kind of effect you're trying to measure. If you picked it up, the expansion due to heat from your hand is a hundred times
more
than the effect you're looking for. And you've got to measure that.” Marcy's iodine cell was one way to try to achieve maximum precision. Mayor's super-stable spectrograph was another. Both the California and the Swiss team had kept refining their instruments to push their precision even higher.

But they faced another problem as well. “I've been active in this field for twelve years now,” Dave Charbonneau told me in 2010, “and I remember several times when people like Geoff would explain how they had worked so hard and improved their precision. And the naysayers would say—this is a good and important process in science, I want to make clear—the naysayers would say, ‘I think you've hit the intrinsic limit of stars.' Stars have jitter, it is not a matter of having a better instrument, it is that the basic instability of stars isn't going to let you go below … and they would say a number. And that number would change over time and get lower and lower. Initially that number was five meters per second. ‘You can't do better than five meters per second.' But the number kept going down.”

It's true that all stars vibrate at some level, which makes the wobble from a planet much harder to tease out. “We know that some stars are intrinsically noisy,” said Charbonneau, “and it would be very difficult to do these kinds of measurements. But there may be a healthy subset, maybe 15 percent, maybe 10 percent, that are extremely stable at a level of ten centimeters a second, which is where you have to get to detect
an Earth. There's a lot of stars out there, so it could be that there's a sufficient number for finding Earth-like planets.” This assumes that the spectrograph builders can create a device sensitive enough to detect them.

By the end of the 2000s, they hadn't built such a device, but they were getting closer. Michel Mayor's team in particular had built a spectrometer they called HARPS, for High Accuracy Radial Velocity Planet Searcher. In 2003, they installed it on a 3.6-meter-diameter telescope at the European Southern Observatory, at La Silla, in Chile. And even though it had been obvious to Fischer ten years earlier that you couldn't get there, HARPS was so stable, and so thoroughly well understood by the astronomers, that it had come all the way down to the one-meter-per-second barrier, and then broken it. Mayor was now down to half a meter per second.

This still wasn't sensitive enough to find a Mirror Earth, but it let the Europeans make a number of important discoveries through the decade, including several multiplanet systems. Perhaps the most intriguing of these, and ultimately the most controversial, made its debut in 2005. At first, the HARPS team was convinced only that they'd found one planet orbiting a star called Gliese 581 every 5.4 days. True to convention, the new planet was named Gliese 581 b. It was a hot Neptune, around seventeen times the mass of the Earth. But it wasn't all that hot, because Gliese 581, like many of the stars in the Gliese catalog, is an M-dwarf. Gliese stars are all relatively close to the Earth, and since M-dwarfs are the most common type of star in the Milky Way, it's not surprising that they're overrepresented
in any catalog that simply samples everything within a given volume of space.

Since a bright, distant star and a dim, nearby star can look pretty much the same, Gliese created his catalog of nearby stars by looking for evidence of parallax—the apparent change of position of a nearby object when you look at it from a new point of view. You can do your own experiment to see how it works. Hold up one finger about six inches in front of your nose and close one eye. Then open that eye and close the other. As you alternate between one eye and the other, your finger appears to jump back and forth against the background. It does that because your eyes are a couple inches apart, so each has a different perspective.

Astronomers can see stars jump back and forth too, by looking at them at one time of year, then looking six months later, when the Earth has traveled through half of its orbit and is now on the other side of the Sun, about 186,000 million miles away from where it was. The nearest stars will appear to move against the background of more distant stars, even though they haven't really moved, because our perspective has changed (the more distant stars don't move, just as that tree in the distance didn't move when you blinked your eyes, because the change in perspective is small compared to how far away the distant objects are).