by Marc Kaufman
But our first night at the telescope was cloud covered—actually, was socked in and packing knock-you-down wind from a cyclone on the eastern coast—so instead of observing, we talked. Butler was eager to explain exactly why the AAT remains such a godsend even though significantly more sophisticated telescopes are available, and his discoveries about the planets orbiting the star called 61 Virginis were exhibit A. The research was published in 2009 and represents the teasing out of one of the first three-planet solar systems orbiting a sunlike star.
“Look at this curve,” he said, pointing at a computer screen full of initially indecipherable but nonetheless elegant graphs, the kind that allow him and other planet hunters to determine that a distant planet is present. Specifically, he motioned to the chart labeled “61 Vir,” which happens to be one of the closest bright stars to Earth and one that is visible to the naked eye. “We’d been observing that star for years, and now we were seeing something. But we had to pull out the signal, and it was very complicated.” Astronomers will one day be able to routinely see or “image” distant planets using considerably more sophisticated telescopes than those available today, but for now most planet hunting involves indirect measurements of the effects of extrasolar planets on their suns.
First Butler and his team found signs of a planet the size of five Earths orbiting 61 Vir in a breakneck four days. “But after a while the pattern changed—something that doesn’t happen unless there’s a good reason. We suspected there was another planet, and that one turned out to be Neptune-sized with a thirty-eight-day orbit.” But still something was off, and Butler wouldn’t know what it was until he and his team observed for several more weeks. What they ultimately found was the signature of not one planet but of three: one orbiting closely, one at 38 days to circle its sun, and then another at 125 days. This only became clear because Butler had weeks of time on the telescope—forty-seven straight nights, a run that would be impossible at the bigger and more sophisticated observatories in Chile and Hawaii. And all those nights of observing allowed his team to put enough dots on its chart so it could read the complicated message being sent by the planets.
When astronomers began detecting exoplanets in the mid-1990s, it was very big news that landed a story about Butler and his colleague, Geoff Marcy, on the cover of Time magazine. The implications were as exciting as the discoveries themselves: If planets were found to orbit tens of billions of stars in the Milky Way alone, then it seemed entirely plausible that some contained liquid water, nutrients including carbon, and an atmosphere to keep out the most damaging cosmic and solar rays. In other words, the basic conditions for life as we know it. And who knows, some of those planets could well be home to life as we don’t know it, based on different chemicals and conditions. Suddenly the prospects for extraterrestrial existence increased dramatically. It was no coincidence that the emergence of astrobiology as a respected and, soon after, a hot field of research occurred in the mid- and late 1990s—right as it became clear that the discovery of the first extrasolar planet, 51 Pegasi, would be followed by many more. NASA started its formal Astrobiology Program in 1998 with these exoplanet discoveries, as well as that controversial announcement of signatures of life in a Martian meteorite, very much in mind.
But as more planets were found, it became clear that many, and probably most, were strikingly different than what almost all astronomers and planetary scientists expected, what Butler calls the “Everything You Know Is Wrong” phase of extrasolar planet research, borrowing from the Firesign Theatre comedy team of the 1970s and one of its iconic acts. The consensus of the astronomy community had been that distant solar systems would be similar to ours, that the known physics and dynamics of star and solar system creation required a certain kind of arrangement. Yet huge Jupiter-like planets were discovered revolving extraordinarily close to their suns. In fact, planets with highly eccentric orbits were found to be far more common than the near-circular orbits of planets in our solar system. Even more unexpectedly, solar systems were found that were somewhat like ours but with seemingly impossible variations—for instance, with a circular-orbiting Jupiter in what is considered the roughly “right place” in relation to its sun, along with an eccentrically orbiting and even larger Jupiter in the inner solar system region where rocky planets are supposed to live. As it turns out, Butler said, having our solar system as a model “can be worse than having a sample of zero because it leads you down one road and you don’t imagine the others.” But because of research like Butler’s, the field of planet hunting has abandoned its previous assumptions and now is working hard to make sense of the new reality that solar systems structured like our own are a distinct minority.
None of these discoveries were, or are, particularly good in terms of the search for extraterrestrial life. But they’re not the final story at all; rather, they’re scientific waystations along the path to detecting the Earthlike planets virtually all astronomers believe exist, and an introduction to the kinds of solar systems to avoid if finding habitable zones and distant biology is your goal.
For instance, a consensus exists within the astronomy community that to have any chance of supporting life, a solar system needs a huge Jupiter- or Saturn-sized planet (300 and 100 times more massive than Earth, respectively) in roughly the locations where they sit in our solar system. That’s because the gravitational force of the giant planets serves to pull in and destroy asteroids and other celestial bodies that might otherwise head into the “habitable” zone and smash the small rocky planets to bits. This is why in astronomical circles Jupiter is often called our protective “big brother” or “big bouncer.” But if Jupiters and Saturns in many other solar systems are close in to their suns, or otherwise in what is considered the wrong place, then they can offer no protection at all. The question of eccentric orbits is perhaps even more unsettling. A planet that swings very close to and then very far from its sun will almost assuredly experience temperature swings too extreme to support life. We know that living things can exist in very hot and very cold environments, but the same organisms probably can’t exist in both. In addition, the gravitational force of a large planet with a strongly eccentric orbit would most likely kick any smaller planets out of their solar system and into space. Nothing personal—it’s just gravitational physics at work. “Solar systems with really eccentric orbits are about the worst place to look for life,” Butler says.
So while Butler and his colleagues continue their two-decade search to detect and characterize extrasolar planets, the new and most important questions in the field have changed and become quite a bit more complicated and ambitious. With Miles Davis and John Coltrane as the backdrop to his thinking, Butler described the two goals that he hoped to help achieve before his time as a peripatetic, globe-hopping astronomer comes to an end. Like his stargazing colleagues everywhere, Butler speaks in a language that can often seem mysterious and impregnable—throwing out references to laws of physics, categories of stars and planets, and modes of measurement that are foreign to the uninitiated. The concepts behind them, however, usually make a pleasing, even elegant kind of sense:
“Overall, what we’re trying to find is solar system analogues because we’d like to know how common or rare the architecture of our solar system is. What are the systems that have Jupiters and Saturns beyond four or five AUs [astronomical units, or the distance from the sun to the Earth]? What are the systems in circular orbits? Those are the signposts for us—systems with noneccentric orbits and with big brothers to shield the smaller inner planets. That’s where you’ll find habitable zones with the potential for life. When we find them we want to go back and stare at them hard and look for the Earths and other inner planets [that] should be there.” He said it would probably take another ten years of planet hunting to get a good representative census of solar system architectures, and that the percentage of systems like ours might be as low as 5 to 15 percent—a perhaps disappointing number until you recall that there are trillions of trillio
ns of stars out there.
“Second, we need to know about habitable planets, how common or rare they might be. Right now our best guess is that rocky planets like Earth and Mars in zones where life could theoretically exist are present in most solar systems, but we really don’t know and could be all wrong. Finding them will be hard because of the ways we look for planets, and so it may seem that any clear understanding is way off.” But people are making progress, he said, and right now his team can find planets only five times the size of Earth in habitable zones—that is, positions in relation to their suns where they are likely to be rocky planets with liquid water—around M dwarf stars. M dwarf stars, or red dwarfs, are the most prevalent in the sky. They are about half the size of our sun and produce far less energy, making it theoretically possible for habitable planets to orbit in close, where current planet-hunting technology can better detect them.
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.
• • •
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.”