Mirror Earth (19 page)

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 about 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 where 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).

But other calculations—by other astronomers, and even by
some of the CoRoT scientists themselves—came up with densities ranging from much lower than Earth's to much higher. “I think I can say this without prejudice,” Geoff Marcy told me. “Everybody agrees now that its mass is very poorly known. In fact, it's so poorly known that's its existence has been called into question. If I had to bet, I'd bet that something is orbiting with that period, but I don't know what that something is. And what its density might be is very hard to know.”

The clearest illustration I've seen of this point appears on a slide used by several different astronomers at talks over the years. It's a plot of planet size on one axis and mass on the other. The chart shows the points where Earth, Neptune, and other solar system planets fall. But it also shows a line, passing through each of those points, where planets of similar composition, but greater or smaller mass, would lie. If a rocky planet like Earth were four times as massive as our home world, say, the plot shows how physically large it would be. Not a whole lot larger, it turns out, because a planet's mass increases much faster than its radius. One reason is that a more massive planet would squeeze itself tighter under gravity than Earth does. A rocky super-Earth would be denser than the actual Earth. But the more important reason is simple geometry: If you double the radius of a sphere, you increase its volume eightfold. Triple the radius, and the volume goes up by a factor of twenty-seven.

You can also take an actual exoplanet whose size and density you know and see where it fits on the plot: If it falls on the “Earth” line, it's almost certainly rocky. If not—depending on how far off it is, it might not be. If the planet's mass or size
isn't known precisely, however, its density won't show up as a dot, but rather as a blob that covers the range of sizes and densities it might have. When you plot all of the possible combinations of size and mass for CoRoT-7b, based on all of the different estimates by different teams of astronomers, you get a blob that just barely edges over the Earth line. So it's possible that CoRoT-7b is a rocky planet. But there's a very real possibility that it isn't.

Whatever it's made of, CoRoT-7b is almost certainly a super-Earth—bigger than our home planet, but not as big as Neptune. It's not the first one ever found; a handful had already been discovered via microlensing, radial-velocity searches, and pulsar timing. CoRoT-7b is the first one, however, that could be described as even plausibly Earth-like in composition. It was the same sort of planet Dave Charbonneau had begun to search for with his ground-based MEarth project, and the same sort that Kepler should find easily. “There aren't any super-Earths in the solar system,” Dave Latham told me during a visit to Cambridge. “There's a big gap. But,” he said, smiling at what the CoRoT team had found, “old Mother Nature knows how to make them. It's a really nice discovery.”

Despite the ambiguity about its mass, CoRoT-7b had taken the first step into the super-Earth era in exoplanetology. Later that same year—after Kepler's launch, but before the probe had announced any discoveries—the MEarth Project took the second. In May 2009 Dave Charbonneau was in a fancy hotel in Washington, D.C., getting dressed. He'd gotten a prize from the National Science Foundation—the Alan T. Waterman Award, given to an outstanding scientist under the age of
thirty-five—and he was getting ready for the award dinner over at the State Department. “I don't swim in these D.C. waters,” he told me later. “I'm a Boston academic and these people talk differently and dress differently. They put us up at the Ritz-Carlton, and I'm getting into a tuxedo, and I'm kind of nervous about what I'm going to say.” He decided to check his e-mail, partly as a diversion and partly because he was missing the weekly MEarth meeting, when team members caught each other up on what they were doing.