Mirror Earth (3 page)

That makes Bill Borucki the anti-Tyson. He's in his early seventies, below average in height, with thinning hair and wire-rimmed glasses. He speaks softly where Tyson booms, and he pauses before he answers a question, where Tyson fires rapidly. Borucki wouldn't do well on
The Colbert Report
. His talks are generally delivered in a thoughtful, measured tone, without laugh lines or oratorical fireworks. Bill Borucki would look ridiculous in a black workout suit. Wearing a coat and tie, as he was when Borenstein cornered him after the Washington press conference, he could be mistaken for an accountant. In the more relaxed atmosphere of a recent astronomy conference, he ambled through the Washington State Convention Center in Seattle in a green windbreaker, looking like someone Hollywood might have cast as the clerk at an old-fashioned hardware store. Tyson grew up on New York's Upper West Side; he has an undergraduate degree in physics from Harvard and a Ph.D. from Columbia. Borucki grew up in Delavan, Wisconsin, in the space-happy 1950s. He built model rockets and, as head of the high school science club, organized the construction of a transmitter to contact UFOs. His undergraduate degree in physics comes from the University of Wisconsin, in Madison, and he has no Ph.D. at all—just a master's in physics, which he also got at Madison, in 1962.

Bill Borucki
(Courtesy of NASA)

Borucki's temperament is cautious enough that any calculation he might do for a press conference would tend to involve as little speculation as possible. When he came up with
the figure of four hundred thousand planets across the entire sky, he was talking about planets that Kepler could in principle have detected if it was pointed in their direction. When Borenstein came up with five hundred million, he was talking about planets anywhere in the Milky Way, most of which couldn't be found by Kepler, or, for that matter, by any other telescope that could conceivably be built.

Beyond that, it would be easy for the general public to leap to premature conclusions about what Kepler had actually found. The fifty-four planets Borucki had announced in the habitable zones of their stars weren't necessarily anything like Earth. Many of them were much bigger—as big as Neptune or even Jupiter—and therefore not a place you could easily imagine finding life. Also, they didn't orbit stars like the Sun. They orbited red dwarf stars, which are significantly smaller and dimmer, and whose planets might not be good places for life to take hold. Sixty-eight Earth-size planets he'd also talked about, conversely, weren't in their stars' habitable zones, so there wasn't much chance of life there either.

In other words, the true story was a little complicated and a little subtle. There's often a tension between the reporter's need to have the most exciting story possible and the scientist's need to avoid too much hype. Borenstein walked that line like a tightrope. The story that came out of the Borucki-Borenstein encounter was absolutely accurate, but while it might have seemed otherwise to a lay reader, there was no new science in it—just a small victory for a reporter in his respectful but relentless tug-of-war with a scientist.

Talking to the press wasn't a problem Bill Borucki had to
deal with much in the early part of his career. His master's degree was enough to get him a job in the early sixties at the Ames Research Center, at Moffett Field, near Mountain View, California. His first assignment there: to help design heat shields to protect space capsules from burning up as they reentered Earth's atmosphere. The only breaking news on heat shields happened when Mission Control feared that the shield protecting John Glenn's
Friendship 7
capsule had worked its way loose during his first orbital flight, in 1962. If it had fallen off he would have been incinerated—but it wasn't loose after all. Even if it had fallen off, the heat shield itself wouldn't have been to blame.

During a visit to Ames in October 2010, I suggested to Borucki that the Kepler project was probably a lot more exciting than this first project must have been.

“Oh, my God,” he said, his eyebrows rising with either dismay or pity, or maybe a little of both. “You don't know anything about heat shields, do you?”

“But … planets orbiting other stars,” I protested feebly. “It's something the human race has dreamed of for thousands of years …”

“But imagine this reentry vehicle coming in,” he protested back, “heating the shock wave in front of it to many thousands of degrees hotter than the surface of the Sun. That is interesting! Hotter than the surface of the Sun, and we've got to calculate the heat radiation on this shield—otherwise the astronauts die on their way back. That's a pretty impressive thing to work on.” It was also a challenge: At the time, nobody had a very good idea about how to calculate what
happens to air when you heat it up to tens of thousands of degrees. So Borucki and his colleagues began studying lightning, which does exactly that—taking images of the flashes and analyzing the light for evidence of what was happening to air molecules at these temperatures.

At the end of the 1960s, Borucki left the exciting world of heat shields. “After the day we were successful getting to the Moon,” he said, “I moved over to the theoretical studies group at Ames.” It wasn't as drastic a move as you might imagine, however, because he was still studying lightning. Except this time, the lightning was on Jupiter. Scientists using radio telescopes had detected bursts of static coming from the giant planet and suspected that lightning was the cause. Borucki and his colleagues wanted to understand how lightning on Jupiter might differ from lightning on Earth.

At first they were stuck with building laboratory experiments to simulate Jovian lightning, since at that point nobody had gotten a close look at the planet itself. They created miniature Jupiter atmospheres inside what amounted to huge test tubes. Then they fired lasers into them, which triggered electric sparks. And then they studied the flashes, just as they'd done with real lightning a few years earlier. In both cases, they had to build detectors that could measure changes in light with extraordinary precision. “You're trying to understand the fundamental measurements, you're trying to understand their time dependence, you're building photometers that people generally don't build that are running on the nanosecond level,” said Borucki. When space probes finally detected the real thing during flybys of Jupiter in the late 1970s, the work
Borucki had done helped planetary scientists understand what was happening on the giant planet itself.

At about the same time Borucki was trying to understand lightning on Jupiter and the nature of the atmosphere that gave rise to it, he said, “there were seminars here at Ames. People would come and talk about future projects, going to Mars and finding life and things like that. Those were very inspirational, and I began to think about whether what I knew could help solve the problem of whether there's life in the galaxy.”

His experience with model rocketry notwithstanding, Borucki didn't know much about going to Mars or designing experiments to look for life. He didn't know much about SETI, the Search for Extraterrestrial Intelligence, which his colleague Frank Drake had been working on since the early 1960s. Drake's idea was to listen with radio telescopes for the broadcasts, deliberate or inadvertent, that might be coming from alien civilizations. SETI is what Jodie Foster was doing in the movie
Contact
, although, naturally, with a little more drama and romance than the real thing.

In thinking about the search for life on other worlds, Drake had quickly realized that if alien civilizations really existed, they probably needed planets to live on. Not just any planets: They needed, as far as anyone knew, planets that were at least vaguely Earth-like, orbiting in their stars' habitable zones. In the 1960s nobody knew how many of these there might be in the Milky Way, if any at all. But Drake went ahead on the assumption that they must exist, given the vastness of the Milky Way. By the 1980s, astronomers still didn't know. They didn't even know if planets of any sort existed, Earth-like or not. A
tiny handful of planets had been “discovered,” starting in the 1960s, but every one of them had been undiscovered later on. The original detections had simply been mistakes.

Borucki thought maybe he could do better. He knew there was no way to see planets around other stars directly. Everyone knew they would be too faint, hidden in their star's much brighter glare. The astronomers who had made those earlier, false detections had used another method entirely. They'd looked at the stars themselves, hoping to see them wobble in place as the gravity of an orbiting planet tugged them just a tiny bit—first one way, then the other, as the planet circled from one side of the star to the other. This technique is known as astrometry, meaning “star measurement.” The motion would be tiny, thus very difficult to see, because a planet is tiny compared with a star. It's not a huge surprise that those early discoveries were mistakes.

Partly as a result of these false detections, and partly because any sort of detection was impossible with any existing telescope, the search for planets around other stars had become a scientific backwater. There wasn't much point in looking until someone—NASA was the obvious choice—provided astronomers with powerful new instruments. If you can't see a tiny motion with an ordinary telescope, went the reasoning, build a gigantic telescope—or, since that's really expensive and impractical, use a group of smaller telescopes at one time to simulate a single, huge one. “People had sketched out systems that might do this,” said Borucki, but it turned out to be a lot more difficult and expensive than anyone thought. (It wasn't until 2009 that two astronomers, using technology far more
sophisticated than anything available in the 1980s, finally announced they'd found the first planet ever discovered with astrometry. NASA put out a press release with the headline PLANET-HUNTING METHOD SUCCEEDS AT LAST. But it hadn't. That discovery, like the earlier ones, is now widely considered to have been a mistake.)

Borucki had no expertise in measuring the positions of stars in the sky, but he did know how to measure light. “I said to myself, ‘Here's another way to do it. It's a rather simple way. It doesn't require the kind of extreme equipment you need for astrometry.'” So in the summer of 1983, he sat down with a colleague named Audrey Summers and wrote a paper titled “The Photometric Method of Detecting Other Planetary Systems.”

The following year, their paper was published in the planetary-science journal
Icarus
. The idea was simple enough: If everything happened to be lined up in just the right way, a planet orbiting a distant star would pass right between the star and the Earth once a year. That's the planet's year, not ours, since “year” really means “one orbit.” A year on Venus lasts 224 days, so if an alien Bill Borucki were looking toward our solar system from just the right angle and saw Venus passing by, he'd see it a second time 224 days—one year—later. If he spotted Mars, “one year later” would be 686 days; and for Mercury, it would be 88 days.

If you were watching a star when one of these crossings took place (they're officially known as “transits”) you'd see the star dim just a little, as the planet blocked some of the starlight. If you plotted the star's brightness over time, the dimming
would show up as a dip in an otherwise more or less steady glow. If it really were a transiting planet, the dip would repeat, like clockwork, every time the planet came around again on its orbit. The plot is known as a light curve; the regularity and appearance of the dip (V-shaped? U-shaped?) should tell the astronomers whether they were seeing a planet or something else.

That's what Borucki and Summers proposed to look for. They realized that you'd have to be looking at a distant solar system precisely edge-on to see this happen. Solar systems are tilted in random ways as seen from Earth, so only a fraction of them would be lined up the right way. In order to stand a chance of spotting even a single transit, therefore, you'd have to monitor hundreds, or even thousands, of stars at once. The easiest planets to see would obviously be the biggest ones, since they block the most light. But big planets like Jupiter orbit pretty far from their stars, as far as anyone knew at the time. Jupiter's year is about eleven Earth years long. So if your telescope was powerful enough to see only the dimming of a big planet, you'd have to wait a long time to see anything at all, even in an edge-on solar system. If you're looking at many hundreds of stars at once, a few score should be edge-on, and in some fraction of those, a Jupiter should be just about to make its transit. “Based on the stated assumptions,” wrote Borucki and Summers, “a detection rate of one planet per year of observation appears possible.”

This “simple way” did, however, require light detectors of unprecedented precision. If a Jupiter-size planet transited in front of a Sun-like star, the star's light should dim by
about 1 percent, or one part in a hundred. At the time, when astronomers talked about high-precision photometry—that is to say, brightness measurements—they were talking about a sensitivity of one part in ten, or ten times less sensitive. “The first thing we had to do,” said Borucki, “was to show we could build photometers with the necessary precision.”