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.
Chapter 10
KEPLER SCOOPED
“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).
With a little bit of trigonometry, astronomers can use the change in viewing angle and the distance Earth has moved to calculate how far away the jumping, nearby stars are. Parallax is such an important tool for astronomers that they’ve invented a unit called the parallax-second, or parsec. It’s how far away an object has to be in order to (appear to) move by just one second of arc, or 1/3600 degree, as the Earth makes half a revolution around the Sun. A parsec is a
bout 3.26 light-years—and, in fact, astronomers almost always talk in terms of parsecs, or kiloparsecs, or megaparsecs, not light-years. The reason light-year is a more familiar term to most people is that parsecs take too long to explain, so astronomers convert for us.
Michel Mayor knew as well as Dave Charbonneau did that a planet would be easier to spot around an M-dwarf than around a bigger, Sun-like star, and that the habitable zone around the cooler M-dwarf would be closer in to the star. Mayor wasn’t about to launch a radial-velocity version of Charbonneau’s MEarth project (which in any case didn’t even exist in 2005). He had enough to do already. But it wasn’t crazy to include some M-dwarfs in the HARPS search. Gliese 581, an M-dwarf a little over twenty light-years away from Earth, or about six parsecs, turned out to be a gold mine. Once HARPS had found one planet orbiting the star, it made sense to keep watching. Since it was first, 581 b was by definition the easiest planet to spot in the system, but it wasn’t necessarily the only one. The other planets, if they were there, might not be Earth-like, but a system with two or three or more worlds would at the very least help theorists understand how planetary systems form and evolve.
It took two more years of monitoring, but in 2007 those continued observations began to pay off. Mayor’s team announced they’d found two more planets around Gliese 581. One was a world they’d already suspected was there. Labeled Gliese 581 c, it was, they said, a minimum of 5.6 times as massive as Earth, which meant it was probably too small to have sucked in a smothering blanket of gases the way Neptune has in our own solar system. It might well be a rocky planet like Earth. It might even have oceans. The planet orbited once every thirteen days or so—quite possibly within the habitable zone of this cool, dim star. The planet’s surface temperature, the astronomers calculated, was between 0° and 40° Celsius, or from just freezing to well below boiling. Water, if oceans existed, could be liquid. If Gliese 581 c wasn’t quite a Mirror Earth, it was getting awfully close. “On the treasure map of the universe,” team member Xavier Delfosse, of the University of Grenoble, said at the time, “one would be tempted to mark this planet with an X.”
But in this case, it would have been wise to resist the temptation. How warm a planet is depends not just on how much energy it gets from its star, but also on what happens to the energy once it arrives. In our solar system, Venus is much hotter than Earth, and Mars is much colder. That’s only partly because Venus is closer to the Sun, however, and Mars is farther away. It also has to do with their atmospheres. Venus is surrounded by a thick blanket of carbon dioxide, a heat-trapping greenhouse gas, which drives the surface temperature up to around 900° Fahrenheit, hot enough to melt lead. Mars has such a thin atmosphere that it retains very little heat; at best, the temperature rises into the twenties Fahrenheit. Billions of years ago, before Mars’s relatively weak gravity let much of its original atmosphere escape, the surface was warmer, and liquid water flowed freely on the surface. We know this because orbiting spacecraft have seen unmistakable evidence of ancient river channels and lake beds, and because the Mars rovers Spirit and Opportunity have found minerals on the surface that almost certainly formed in the presence of water. When climate scientists ran their computer models on Gliese 581 c, they decided it wasn’t likely to be habitable after all. Assuming it had an atmosphere, the planet was probably more of a Venus than an Earth, with a runaway greenhouse effect that would probably long since have sterilized it of any life that might have tried to take hold.
But there was a third world in the system as well, called 581 d, and over the next few years, the Swiss team would find still another, 581 e, and maybe 581 f (though they couldn’t confirm this one). The Swiss had the best instruments—even their archrival Geoff Marcy admitted this—and they had been observing this star longer than anyone else; it’s not surprising that they were the ones who kept finding new planets. But nobody gets to reserve a star to themselves, and while the Swiss had the best spectrograph, the instrument Steve Vogt had built for Geoff Marcy’s team wasn’t far behind. Once Mayor’s rivals in the United States realized what a rich hunting ground Gliese 581 was, they began taking their own radial-velocity measurements as well. Maybe they could find a planet Mayor had missed.
Back in Europe, meanwhile, the European Space Agency had decided to beat Kepler into space with its own space-based transit mission. Just as Michel Mayor and his colleagues had no monopoly on Gliese 581, Bill Borucki had no monopoly on looking for transits from above Earth’s atmosphere. Mounting a competing mission as sophisticated and powerful as Kepler wouldn’t be worth the trouble, since it couldn’t be done significantly faster than Kepler, and it would be an expensive duplication of effort. So what the European Space Agency did was build a satellite less sophisticated and less powerful than Kepler, and get it into orbit as fast as possible. The satellite, named CoRoT (for COnvection ROtation et Transits planétaires) would be able to find planets only in relatively tight orbits around Sun-like stars, and it wouldn’t be sensitive enough to find a planet as small as Earth. It could, however, find planets just a few times bigger.
In February 2009, just weeks before Kepler launched, it did. The parent star was known as TYC 4799-1733-1, TYC being the abbreviation for the Tycho star catalog. When the planet was spotted making a transit, CoRoT astronomers renamed the star CoRoT-7—they were free, after all, to create their own catalog, and the name would remind people that their satellite had made the discovery. They calculated that the planet, CoRoT-7b, was a bit less than twice the size of Earth. If that was correct, it would be, without question, the smallest exoplanet ever found. Like all transiting planets, it was also in the ideal edge-on orbit that would let radial-velocity instruments figure out its mass. Now that they knew where to point, Mayor’s HARPS team swung their Chile-based telescope toward the star and began taking measurements.
Unfortunately, there was a complication. CoRoT-7 is similar to the Sun, but much younger—only about 1.5 billion years old, compared with our own star’s 5 billion or so. Adolescent stars, like adolescent humans, don’t always have the clearest skin. As Natalie Batalha had told me, they’re prone to star-spots, and CoRoT-7 is no exception. This turns out not to be such a problem for measuring transits. Since the star rotates once every twenty-three days, the darkening caused by the starspots lasts much too long to be confused with a transiting planet. But spots can confound radial-velocity measurements. Depending on where the spot is at a given time, it can blot out part of the star’s leading edge—the part that’s rotating toward you—thus making it seem like the star as a whole isn’t moving toward you as fast as it really is. Or it can blot out part of the trailing edge, with the opposite effect. CoRoT-7 is not the sort of quiet, middle-aged star that radial-velocity searchers liked to deal with.
Eventually, after a total of seventy hours’ worth of observing time spread over the next several months, the HARPS astronomers managed to tease out a signal. “The longest set of HARPS measurements ever made,” read a press release issued in September 2009, “has firmly established the nature of the smallest and fastest-orbiting exoplanet known, CoRoT-7b, revealing its mass as five times that of Earth’s. Combined with CoRoT-7b’s known radius, which is less than twice that of our terrestrial home, this tells us that the exoplanet’s density is quite similar to the Earth’s, suggesting a solid, rocky world.” CoRoT-7b’s composition might well be similar to Earth’s, but its orbit is not. With a “year” lasting just a little more than twenty hours, it’s more than twenty times closer to CoRoT-7 than Mercury is to the Sun, and has a surface temperature of between 3,300° and 4,700° Fahrenheit. If the planet was rocky, the surface might well be a sea of molten lava.
But that didn’t take away from the potential importance of the discovery. Ever since Michel Mayor found 51 Peg b in 1995, exoplaneteers had been pushing toward smaller and smaller worlds, inching closer and closer to a true Mirror Earth. Nobody doubted that Earth-size, rocky worlds were out there, but it was possible that everyone was wrong. A universe whe
re hot Jupiters could exist, in utter defiance of conventional astronomical wisdom, might also be a universe where rocky planets were vanishingly rare. Finding even one outside our own solar system would imply that they weren’t rare at all. CoRoT-7b itself couldn’t support life, but now it was clear that other small, rocky worlds must be relatively common, and some would surely turn out to be habitable. This was a major step forward.
It was, that is, if CoRoT-7b was truly made of rock. There’s always some uncertainty in every astronomical measurement, because no measuring instrument—no telescope, no spectrograph, not even the best in the world—is perfect. A turbulent, spotty star just makes it worse. A longer series of observations can help, because turbulence is more or less random, while the back-and-forth radial-velocity tug a planet imposes on a star is like clockwork. Over time, that regular signal can build up to stand out from the visual noise caused by stellar turbulence. But the more noise there is, the more uncertain the signal will be, even with lots of observations. The CoRoT team’s best calculation put the planet’s density at about 5.6 grams per cubic centimeter, about the same as Earth’s (water, by contrast, weighs one gram per cubic centimeter).
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