First Contact (20 page)

The technology and know-how for finding planets has exploded in
the past decade. It's growing at an ever-faster pace alongside, and to some extent because of, the exponentially increasing speed and power of computers, and that has instilled a broad confidence in the astronomy and astrobiology communities that the future will be full of discoveries, including planets the size and consistency of Earth. It also doesn't hurt that while the number of planet hunters could be counted on one hand when Butler started, he estimates there now are about a thousand. “We're just on the hairy edge of this,” he said regarding the discovery of those Earth-sized stars. “I'm convinced it's doable, and actually is beginning to get done by my group and two others”—the University of California at Berkeley group that Butler used to be part of and the Geneva Extrasolar Planet Search in Switzerland. This is not necessarily a majority view, because the technical challenges of finding Earth-sized planets remain daunting, and the best approaches now possible cannot definitively locate a planet the size of our ball of rock. But Butler says his team could find a planet the actual size of Earth right now if they had six full months of consistent observing time at a major telescope—a coup, however, that even he can't pull off.

Although the Hubble telescope has “imaged” a handful of distant planets from its perch above the atmosphere, virtually all the extrasolar planets identified have been discovered by astronomers and astrophysicists using ground-based telescopes. The process by which Butler and his colleagues find their extrasolar planets is both oddly simple and confoundingly complex: A big telescope (usually 4 to 10 meters in diameter) takes in billions of photons coming in at the speed of light from a selected star, bounces the light waves through a maze of mirrors that shape them in the desired way, and focuses the light through a narrow slit into a spectrometer, where several prisms and other optics divide the light into its spectral parts. That spectrum is then photographed at extremely high speed (by a camera cooled to –300 degrees Fahrenheit) and the image is embedded into a high-end CCD chip not dissimilar from the one that makes your digital camera work. The result is the production of a single number, which then gets massaged a little further and ultimately placed on a graph, which
captures the “wobble” of a sun caused by an exoplanet. At the AAT, Butler likes to say, 4 billion photons captured and worked over by his team of Americans and Australians become one data point. The process is then repeated scores, even hundreds of times to learn about the neighborhood of a single star.

The AAT was built when thick steel and heavy concrete were still in astronomical vogue, so it looks like a huge but comprehensible machine. The supposedly “simple” part of planet hunting is how and why those points on a graph can tell Butler whether a planet is orbiting the star he's observing. The key to Butler's team's technique is harnessing the Doppler shift, a phenomenon in physics that can be used to measure the speeding up or slowing down of just about everything. The Doppler shift was discovered and described by Austrian Christian Doppler in 1842. He, like many others, had been intrigued by the sound of a train whistle as it approached and then departed from a station it was passing by. The whistle, always the same in volume, nonetheless sounded different as the train approached, as it sped by, and as it left the area. That difference, Doppler concluded, was the result of a change in the frequency of the wavelengths, or of their pitch.

As scientists noted in the ensuing century and a half, the same effect occurred in waves of all sorts—with light, with X-rays, with radar and radio waves, and so on across the electromagnetic spectrum. Invariably, when a wave approached or receded from the observer's line of sound or sight, the perceived frequency changed in a measurable way. The trick was finding ways to capture that information and measure changes in wavelengths (or frequency) in relation to an observer, whether at a nearby train station, along a highway where police officers use radar guns based on the Doppler shift to identify speeders, or at observatories where astronomers were looking for ways to detect the minuscule changes in photon wavelengths that would be associated with distant stars that had planets orbiting around them.

Classical physics tells us that the gravitational force or pull of any and all extrasolar planets would have an effect on the suns they orbit. A star
with a planet around it would wobble ever so slightly in its own orbit because of the planet's pull. A star without exoplanets wouldn't wobble at all. The gravitational pull of Jupiter, for instance, makes our sun wobble around in a circle at a speed of almost forty feet per second. Astronomers have used their knowledge of Doppler shifts for more than a century to measure the movement of stars, but it wasn't until the 1980s that scientists—and especially a team from Canada that came very close to finding the first extrasolar planet—actively began using it for planet hunting, a technique called “precision Doppler velocity.” In effect, they captured and analyzed all those photons to determine the speed at which a star was moving toward or away from the Earth. When those single measurements were charted over a long period of time, they showed a straight line (a star with no planet-induced gravity tugging it one way or another) or an undulating line that told astronomers that the star was being tugged ever so slightly.

What Butler and his colleagues measure is akin to the speeding up or slowing down of a person walking, in a figurative sense, on the stars, which are roiling, gurgling balls of gas a million miles in diameter. If the speed of the star moving toward the Earth or away from it changes by as little as one meter per second due to the gravitational pull of an orbiting planet, Butler and his colleagues can detect this. That rate, one meter per second, is strolling speed, a not even particularly brisk walk, yet the planet hunters have used changes of that size embedded in light traveling to Earth from light-years away to actually prove that the planets are present. Not only that, they can determine as well the shape of the planet's orbit—circular or eccentric—from the same information.

At the AAT control room, all this translates into a pretty low-key affair that can involve as few as two people. A technical operator opens and closes the dome that protects the telescope, and solves a problem if one comes up; an astronomer who determines which stars to observe watches over the data as it comes in by computer, and decides to move on to another target when the “seeing”—the telescope's ability to take in enough photons from a star—starts to decline. The actual analyzing of information takes place in
computers back in Washington, D.C., although Butler can connect to them from Australia and has actually found several planets while sitting in his AAT living room.

Once the big dome is opened and the telescope is moved into place via gears that grind with a slow, hollow moan, computer screens light up and numbers and graphs appear. But the most exciting action is at the technician's console, where light from the star itself pours into a slit displayed on the screen and then is further directed between horizontal bars about a half-inch apart. The concentrated starlight jumps and dances between and sometimes outside the bars, movement you might expect from a star with a dynamic and always churning surface. But that's not really what's on display. The movement tracks how many of the star's photons are making it to the telescope and through its spectrometer, and any dancing outside the bars is considered bad. All this can now be corrected by computer, but not that long ago astronomy grad students would spend long hours with a joystick working to make sure the starlight remained within the narrow bars.

The weather was turning bad again and so Butler decreed it was time to drive down the hill to Coonabarabran, a three-pub town of some 2,500 people that works hard to both maintain a connection with the observatory and to benefit from its proximity. We headed for the Imperial Hotel pub, an old-school, dark-wood and tinted-glass establishment in just about every way, except for the wall-to-wall carpeting in azure blue with a star and planet motif, and a darts and video room called “Galaxy Games.” The technology, pace, and culture of both the observatory and Coona were definitely 1970s and '80s—with a lingering dose of mid-twentieth-century small-town America, or its Australian equivalent. That made it a fitting place to pick up the story of how a kid growing up in a Los Angeles police family came to be one of the world's premier planet hunters.

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Butler was born in 1960 and became interested in astronomy as a teenager. As is often the case with similarly minded kids, he wanted to build a
telescope. In his time off duty, his father, a thirty-year veteran of the force, used two-inch plumbing pipes to fashion the steel struts that supported the instrument, and accompanied him to an amateur telescope contest at California's Tehachapi Mountain. Butler developed his interest in astronomy the old-fashioned way—in a library. He sampled books from aisles of all sorts until he reached the astronomy section. It wasn't so much the science that first reeled him in, but rather the brilliance and inventiveness of the earliest astronomers like Tycho Brahe and Johannes Kepler. They're the ones who pretty much established astronomy as a science, where the goal became taking precise measurements, determining the level of error inherent in the calculations, and beginning the process of demystifying the cosmos. But just as eagerly and importantly, Butler read about the terrible fate of Giordano Bruno, a lapsed Italian monk and freethinker who, in the late 1500s, advanced the idea that planets orbited a universe full of moving distant stars—an idea that challenged conventional and ecclesiastic belief at the time. Bruno wasn't the only one proposing such things and he did not have the scientific instincts or knowledge of a Brahe, a Kepler, or a Galileo. But he definitely got on the wrong side of the Inquisition and was burned at the stake in Rome in 1600. One of his several heresies was the belief in and advocacy of the notion of “multiple worlds.”

Butler left home for San Francisco State University—a good school but hardly an academic powerhouse—and met a junior professor named Geoff Marcy. The teacher was getting nowhere in his work on the magnetic fields of stars while Butler was finishing up a bachelor's degree in chemistry and a master's in astrophysics. Marcy, as voluble as Butler was then reserved, recalls an epiphany in the shower, when he realized that to succeed in his chosen field, he had to address the kind of questions that he had cared about as a child—including whether planets orbited other suns. Butler's background in chemistry came in handy because to achieve the level of precision needed to identify extrasolar planets via the gravitational wobble of their suns, they had to find a precise, safe, and stable element to serve as an absorption cell as the light entered the spectrometer, a cylinder filled
with a gas that serves as a guidepost or measuring rod of the incoming spectrum. Previously, the Canadian team had used hydrogen fluoride as their standard, but the compound was both insufficiently precise and extremely toxic, requiring extreme and time-consuming care since the smallest exposure could be harmful or even fatal. Butler came up with the idea of using an absorption cell filled with iodine, a breakthrough that allowed the duo to move ahead more quickly with the task of increasing precision while avoiding the potentially lethal dangers of the hydrogen fluoride. Butler's iodine absorption cell became and remains the standard in the field.

What began as a collaboration became an obsession as Butler and Marcy tried to reach a level of precision in detecting those Doppler shifts that would finally allow for a planetary detection. When they started, they could detect motion if it reached 300 meters per second. But to be of any use, they had to bring that number down to a mere 3 meters. Today the goal is to get dependable measurements when the wobble consists of motion under 1 meter per second. “When we started in the eighties, we weren't thinking of aliens and life—we just wanted to find a damn planet. People have been thinking about extra solars since 1600, you know, when Bruno was burned at the stake for that. I wanted to help solve a scientific problem that could get people that upset, that still had such a huge power to affect people.” He and Marcy were two very smart kids who, despite their long distance from the Ivy Leagues, were determined to make a name for themselves. The process of refining their technique took eight years, saw them secretly commandeering time on colleagues' unused computers at night to process their data, and led them down blind alleys that ate up years at a time. “Basically, we had to swing for the fences or we would never get anywhere,” he says. Many aspire to be home run hitters but few succeed quite so spectacularly. As the discoveries rolled in, the two received about every prestigious award handed out in their field, including one from the National Academy of Sciences.

Like many important players in astrobiology, Butler didn't begin with any particular interest in extraterrestrial life, or any quixotic drive to find it. But, he says, “no question, we're not just totally disinterested robot scientists—we're
humans, right, and we want to know things that reflect on us, that tell us about what we are and where we came from. So we're interested in earths, we're interested in solar systems. These other planets that we've found have been shocking and amazing, but they bring even more to the center that ‘Just who the hell are we?' question. Where did we come from and how common are we? We just keep on coming back to looking for our kinds of earths and our kind of solar systems, and I imagine we'll continue until we find them.”

As I learned ten months later, Butler wasn't just speculating about finding “our kinds of earths.” Seated halfway around the world beside excited and smiling officials of the National Science Foundation outside of Washington, Butler and his research partner of fifteen years, astronomer Steven Vogt of the University of California at Santa Cruz, announced they had detected a planet only three to four times larger than Earth in an apparently habitable zone. Based on eleven years of data collected at the Keck Observatory, they concluded that planet Gliese 581G, an astronomically close 20 light-years (or 117 trillion miles) from our solar system, had the mass to hold an atmosphere and was at a proper distance from its sun to hold liquid water. They both said it was plausible that life could exist on the planet but also that Gliese 581G is definitely no Earth clone, since it always faces its sun just as one side of the moon always faces Earth. Still, they said the regions where full-time sun shaded into full-time dark were large enough to support living systems, if a variety of other conditions were met. “This is our first Goldilocks planet—just the right size and the right distance from its sun,” said an ebullient Butler, who had reluctantly broken his no-long-pants rule in warm weather for the event.