Mirror Earth (2 page)

To qualify as a Mirror Earth, a planet presumably had to be more or less the size of the original—big enough to hold on to an atmosphere, but not so big that the atmosphere would be
crushingly dense. It had to be made mostly of rock, like Earth: A planet that was the right size but mostly made of water or gas presumably wouldn't be hospitable. It had to orbit within the so-called habitable zone surrounding its star, the region where heat from the star was just enough to let water exist in liquid form, rather than purely as ice or water vapor. That's because water is a necessary ingredient for all known forms of life on Earth, and the same rule would presumably apply to a Mirror Earth as well.

Finding a world that met all these requirements wouldn't happen in a single stroke—in part because the features that distinguish a Mirror Earth from some other kind of planet require a different sort of observation, and in part because the technologies and the search strategies that would ultimately lead to that discovery had to be invented and tested and perfected and funded over periods that lasted years. Nobody could really predict when that discovery would finally happen; all they could do was press to find smaller and smaller worlds; figure out ways to estimate their composition, and sample their atmospheres, and calculate whether they were common or rare in the Milky Way.

In 1995, the idea of finding a Mirror Earth was still just a dream. Today, thanks to a dramatic series of advances in both technology and in the ways astronomers are conducting their searches, they're on the verge of doing just that. This book describes how it all unfolded.

Bill Borucki was starting to look a little impatient. He had just hosted a press conference at which he and several other astronomers had talked about the latest results from NASA's Kepler space telescope. The event was over, and now it was time for lunch. There was really no time to lose. The press conference was held as part of the February 2011 meeting of the American Association for the Advancement of Science—the biggest general-science meeting of the year, hosted by the biggest scientific society in the world. It was taking place at the Washington, D.C., convention center, which consists of two huge concrete buildings straddling M Street, just north of the capital's main tourist area.

The convention center is so big that the cavernous basement of one of the buildings can hold major sporting events, and while some of the most eminent scientists in the world were on the upper floors pondering the deepest mysteries of the universe, a regional volleyball tournament was happening down below. Five hundred or so teenagers had been playing
at least a dozen side-by-side, simultaneous matches, rotating on and off the courts in a continuous stream since early that morning.

Now, five hundred ravenous young people, along with parents, friends, and coaches, were streaming out to the dining area, where their warm-up suits and flushed cheeks contrasted sharply with the rumpled suits and relatively pasty faces of graduate students, professors, and research scientists, including not a few Nobel Prize winners. Neither group would have a wonderful experience (a review on
Yelp.com
read, in part, “One Starbucks for a whole convention, you're crazy. Hence the Disneyland-esque line we waited in … The food court is also pretty bad. A cabbage salad with chicken, ewww…”).

Borucki might not have been aware of this impending dining disaster, but his impatience had a more immediate and obvious explanation. Seth Borenstein, the lead science reporter for the Associated Press, had missed the press conference, and he needed to catch up. “Sorry, no story today” are words that would never issue from Borenstein's lips. Unfortunately, there wasn't any actual news. It's not that the Kepler Mission itself wasn't newsworthy. Since its launch in the spring of 2009, Kepler has utterly revolutionized the search for planets circling stars beyond the Sun. Before the mid-1990s, the search had limped along for decades without success. Since the first planet was discovered in 1995, planet hunters had identified between four hundred and five hundred of these alien worlds (they're technically known as “extrasolar planets,” or simply “exoplanets”), one by painstaking one. The number isn't exact
because not everyone agrees that all of the discoveries are real.

Then Kepler came along. It was clear from the start that if the spacecraft worked as it was supposed to, it would blow the competition out of the water. This was true for several reasons. To start with, Kepler orbits high above Earth's atmosphere, like the Hubble Space Telescope, but even farther out in deep space. As a result, the atmosphere's blurring effect—the same thing that makes the stars seem to twinkle—isn't a problem. Another huge advantage is that Kepler doesn't look at one star at a time. It looks at more than 156,000 of them all at once, in a patch of the northern sky that lies between the constellations Lyra and Cygnus. And it keeps looking at those same 156,000 stars continuously, around the clock, day after day, month after month, year after year. That's impossible from the ground: When the Sun rises, the stars vanish. Since one night's observing isn't nearly enough to find a planet around a distant star, you have to keep returning to a given star many times, over many nights, to get any information worth using. Kepler doesn't return to any of its 156,000 stars because it never leaves them in the first place.

If Kepler were a general-purpose telescope—even one like the Hubble—it wouldn't be permitted to linger on a single patch of sky indefinitely. Most telescopes are used to study all sorts of cosmic phenomena, from distant galaxies to exploding stars to black holes. If you're looking for planets with a general-purpose telescope, you might get to use it for a few nights at most before the next astronomer in line gets her turn. If you're using the Hubble, which is vastly oversubscribed, you get more
like a few hours. Kepler, by contrast, was built to look at only one tiny patch of sky for its entire working lifetime. It will never avert its gaze from these 156,000 stars.

By the time Borenstein, the AP reporter, had cornered Borucki, Kepler had proven itself to be technologically perfect, or pretty close to it. It had been staring at its target stars for nearly two years. Only the first six months' worth of observations had been fully processed, though. That's how long it takes for the Kepler team's computers to pore through the terabytes of electronic data beamed down from the spacecraft, letting custom-written algorithms flag the tiny changes in starlight that might (or might not) betray the presence of a planet, weeding out false positives—things that look like planets but aren't. If a potential planet passes all these tests, that's still not good enough. The software has to pick up the planet's signal not once, not twice, but at least three separate times for it to make the cut. Usually, though, since the signal is often very faint, it takes a lot more than three sightings: some run into the hundreds. And even then, a dozen or so mission scientists look at each of what they call KOIs—Kepler Objects of Interest. These are something like the persons of interest law-enforcement types talk about in criminal investigations. They're not being charged … yet. But you shouldn't be at all surprised if they end up being indicted.

The reason Borucki hadn't announced any new results at the Washington press conference was that he'd already presented everything he had just two weeks earlier, at a press conference at NASA headquarters, in Washington. There was plenty to say: In just the first six months' worth of observations,
Kepler had come up with no fewer than 1,235 possible planets, about 90 percent of which were almost certainly real. Kepler had barely warmed up, and it had identified at least twice as many planets as all the astronomers in the world had found in the previous sixteen years. “Astronomers have cracked the Milky Way like a piñata,” Dennis Overbye wrote in the
New York Times
, “and planets are now pouring out so fast that they do not know what to do with them all.”

After he gave reporters the number of new planets—or, rather, “planet candidates,” in the very careful language Kepler scientists prefer to use—Borucki explained, like a pollster projecting the outcome of an election based on just a small sample of voters, that Kepler is looking at only about 1/400th of the sky. If the spacecraft had been able to monitor the whole, he said (or if NASA had sent up four hundred identical Keplers, pointing in all different directions), they'd be talking not about twelve hundred planets, but about the more than four hundred thousand the probe would undoubtedly have seen.

It was a terrific story—two weeks earlier. The talk at the American Association for the Advancement of Science meeting was pretty much just a replay. For a science reporter—especially a reporter for the Associated Press, where late-breaking news is a specialty—that just wasn't good enough. Borenstein couldn't write a story that said, in essence, “The Kepler results announced two weeks ago are still true.” So, while the editor of
Discover
magazine and I stood by, looking on in comradely amusement, Borenstein kept pushing the Kepler team leader to say something new. Borucki was clearly reluctant to be pushed.

“Okay, so if I understand you correctly,” the reporter asked, “you said you've found 1,235 planet candidates, right?”

“That's right,” said Borucki. He'd said this two weeks earlier. Borenstein knew it. But like a prosecutor in a courtroom, Borenstein was building his case.

“And of those, fifty-four are in the habitable zone of their stars?”

“Yes, that's correct.”

Again, this news was two weeks old, but it was really important. The habitable zone is the orbital band surrounding a star where the temperature allows water to exist as a liquid rather than as ice or vapor. It's sometimes called the Goldilocks Zone, even by astronomers—even on NASA's website—since like the porridge in the fairy tale, it's not too hot and not too cold, but just right. Biologists have long insisted that water is essential for life, because nutrients can dissolve in it easily, to be distributed to every part of an organism. That's what blood does for most animals, and blood is mostly water. Life on Earth wouldn't be possible, says the conventional wisdom, if most of our water boiled off into the atmosphere or froze solid. Earth is inhabited because we live within the Sun's habitable zone.

If you're interested in finding life on other worlds—and that's what just about every scientist who hunts for planets is ultimately looking for—planets in the habitable zone are what you want. Planets about the size of Earth in the habitable zone are even better. The question of whether life exists beyond Earth is one of the oldest mysteries of nature, dating back at least to the ancient Greeks, and probably even further. At some
times in history, the notion of alien life has been considered heretical; at others, learned men took it as a given that planets, both within and outside the solar system, were home to intelligent beings. The eighteenth-century astronomer William Herschel, who discovered the planet Uranus, was convinced that even the Sun was inhabited (he had a handy explanation for why the Sun creatures weren't incinerated).

Kepler isn't capable of answering the question of whether life exists on other worlds, but it can take the first step by finding an Earth-like planet, a Mirror Earth, where life could be thriving, at least in principle. Kepler was designed with several scientific objectives in mind, but number one on the list that appears on the mission website is this: “Determine the abundance of terrestrial [that is, Earth-size] and larger planets in or near the habitable zone of a wide variety of stars.”

“So, as I understand it,” continued Borenstein, pressing his interrogation, “there are about 300 billion stars in the Milky Way. If you're looking at 150,000 stars, and found 1,200 with planets, and 54 of those in the habitable zone … that means …” The reporter stared at the ceiling, wheels turning in his mind. Borucki looked on, politely. “… there should be something like 50 billion planets in our galaxy, right? And 500 million should be in the habitable zone.”

Borucki thought about that for a moment. “Yes, that sounds right,” he answered.

Two weeks earlier, Borucki had talked about a hypothetical four hundred thousand planets that could be detectable from Earth. Now, under intense, though friendly, questioning, he was admitting to five hundred million overall. Borenstein had
his story. Later that day, the Knight Science Journalism Tracker, a blog that aggregates and reviews science stories, described it this way:

Seth says 50 billion planets, minimum, in Milky Way. Nobody said that at the press conference. Minor consternation ensued among other reporters after he filed. How'd he get that angle? Explanation: Seth missed the press conference. Saw Borucki afterward talking with a few reporters including Michael Lemonick of
Time
. “Just a nice chat where you riff together,” Borenstein says. Borucki says one in two stars has planets, Seth says let's do the math, Borucki complies and double checks, and that's why it can pay to be there in the flesh.

This remark about the flesh may well have been a dig at media outlets that are no longer willing to pay for reporters to go to conferences. The author of the blog post, Charles Petit, covered science for the
San Francisco Chronicle
for years, and takes a dim view of how his profession has been downsized. Still, the calculation done by the AP reporter was so simple and obvious that Borucki could easily have done it for the other reporters who were present, and for the hundreds more who get NASA press releases by e-mail. He could have done it for the press conference two weeks earlier.

The fact that he hadn't says a lot about Bill Borucki. Some astronomers are showmen—Neil deGrasse Tyson is a good example. Tyson, the director of the Hayden Planetarium in New York, is a serious and highly respected scientist, but he's
also a frequent guest of both Jon Stewart and Stephen Colbert on Comedy Central. When he taught at Princeton (he had a part-time appointment to the faculty there for many years), his dynamic lectures drew students by the hundreds. Tyson is a tall, powerfully built man in his forties, a former athlete and dancer who once told me that “in high school, I was a nerd, but a nerd who could kick your butt.” I once saw him at an astronomy conference striding across the hotel lobby in a form-fitting black workout outfit, complete with weightlifting gloves—a ninja astronomer heading for the gym. Every eye in the place followed him.