Heaven and earths
Astronomer Geoff Marcy leads the search for planets outside our solar system. Published in California Monthly magazine.
This is the story of the making of a planet. This is also the story of the making of a planet-hunter.
Billions of years ago, out of the primordial clouds of hydrogen gas, a star was born. For its first few million years, it was a fireball of hydrogen atoms crashing together to make helium. Time passed, and the star began to make heavier elements in its fiery core–carbon, nitrogen, oxygen, and finally iron. All of its hydrogen fuel used up, the star collapsed under its own weight, creating a cosmic shockwave that blew it apart. The flash of light and energy from this supernova was for a brief time so intense it was visible in distant galaxies.
Time passed. The gas and dust blasted into space by this magnificent explosion began to condense again. In the center of this swirling cloud, a new star was born, surrounded by a disk of stellar fallout. More time passed. Heavier particles in the disk began to clump together, sweeping up everything in their path. Some of these young planets grew huge, with rocky cores and thick atmospheres of gas; others were smaller, mostly rock with just a thin atmosphere. They jockeyed for position in a cosmic game of musical chairs until nine remained, calmly orbiting the central star as if they had done so forever. One smallish, rocky planet with an atmosphere not too thick and not too thin found a place for itself 93 million miles from the sun–not so far away that it was icy cold, not so near that it was searing hot, but just right for life to flourish.
Forty-eight years ago, on the above-mentioned planet, a boy was born. Thirteen years passed. His parents bought him a map of the solar system, planting the seeds of interest in astronomy. When they bought him a small telescope, his passion for the subject grew. Thirteen more years passed and the young man, Geoff Marcy, had earned a Ph.D. in astronomy and astrophysics and was embarked on his own research. But pressures were building in Marcy’s own core. He thought that everyone around him had better ideas; he felt like a failure and wasn’t even sure he wanted to be an astronomer. Marcy’s world imploded; he gave up his dream of being a professional research astronomer and instead took a teaching position at San Francisco State University.
Then, in a sudden bright flash one morning in 1983, Marcy decided to take on a research project that appealed to him at a gut level–to hell with the established scientific community. The project: to find planets around other stars. Time passed. A student of his, Paul Butler, was drawn to Marcy’s ambitious project and asked to join him in his quest. More time passed. Then, in 1995, the first planet outside this solar system was discovered by a team in Switzerland. The planet hunt quickly gathered momentum, and Marcy and Butler found more and more planets around other stars. Marcy and his postdoctoral researcher Debra Fischer came to Berkeley, and conditions were just right for the world’s most formidable planet-hunting team to flourish.
There are nine planets around our Sun, itself just one of 200 billion stars in the galaxy. The question of whether there might be planets around those stars, and whether there might be life on those planets, has kept astronomers wondering for hundreds of years. Before 1995, it was anyone’s guess, but since then Marcy and his team of planet-hunters have found more that 70 so-called extrasolar planets. Another 30 have been found by other groups, but Marcy’s is recognized as the world’s best at finding these elusive objects.
Every day Geoff Marcy gets e-mails and letters from “fans” wanting to know more about the search for extrasolar planets. When children write, they often want to know what the planets look like, what color they are. But Marcy has to admit he has no idea. “The bad news is that we can’t actually see the planets around other stars–they’re just too faint and, moreover, they’re lost in the glare of the host star,” he explains. Even a planet the size of Jupiter, the largest in our solar system, would reflect just one-billionth as much light as its host star puts out. How does he get around the problem? “It’s simple. We watch the star, not the planet.”
Nearly four hundred years ago, Galileo proved that the Sun is fixed at the center of the solar system, circled by the Earth and the rest of the planets. And yet, says Marcy, it moves. Just as the gravitational pull of the Sun on the planets causes them to revolve around it, the planets themselves also tug on the Sun, causing it too to trace a path–albeit a small, slow one–around a common center. From afar, our Sun would appear to “wobble” periodically back and forth.
It was the former chair of the Berkeley astronomy department, Otto Struve, who first realized in 1952 that there might be a way of actually observing the wobbles caused by planets around distant stars. Though the wobble itself would be imperceptibly small, Struve reasoned that astronomers could analyze the spectrum of their starlight and make use of the Doppler Effect. If a star is moving away from us, Struve noted, the wavelength of the starlight will be stretched, making it appear reddish; if the star is moving towards us, the wavelength will be squeezed, making it appear more blue. “That’s really all there is to it,” says Marcy. “The problem is, the effect is tiny.”
The change in the wavelength of light astronomers would have to detect is one part in a hundred million, which is why, more than 30 years after Struve described how it could be done, no one had actually found an extrasolar planet. In fact, by 1983, when Marcy first decided to try to find one himself, most astronomers thought it was impossible. “When I would tell people, especially other professional astrophysicists, that I was working on a technique to find planets, they would often look down at their shoes, scuffle their feet a bit, and change the subject. It was not considered ‘proper science,'” he recalls.
In the mid-1980s, Marcy and Butler were two of a handful of astronomers around the world who braved professional scorn to look for planets around other stars. One group, at the University of British Columbia, was trying to pass starlight through hydrogen fluoride gas, which absorbs certain wavelengths of light, creating darkened “absorption lines” that act as handy points of reference. The trouble was that hydrogen fluoride gas is poisonous, deadly at one part in a million, and will corrode your lungs before you’ve even realized that you’ve stopped breathing. “Our great innovation was to use a nonlethal gas,” says Marcy.
By 1986, Marcy and Butler had developed a glass cell filled with iodine gas that would absorb the starlight they were collecting from the telescope at Lick Observatory on Mount Hamilton. “Iodine has lots of advantages–there are more absorption lines, it has better calibration, and it’s entirely safe,” Marcy explains. “The iodine molecules are of course not wobbling like the star is, so while the absoption lines in the stellar spectrum shift back and forth, telling us the speed of the star, the iodine absorption lines don’t. We use this as the ruler against which to measure the Doppler Effect.”
From the start, Marcy and Butler realized that they had little hope of detecting a planet as small as Earth and set their sights instead on detecting large distant planets. Our own Jupiter, which is 317 times the mass of the Earth, causes the Sun to trace a small circle at about 12 meters per second–bicycling speed. To detect a Jupiter-sized planet around a distant star, says Marcy, they would have to discern whether the star is bicycling, either towards them or away from them. For nine years they collected data from Lick and wrote the thousands of lines of computer code needed to analyze it. They were in for the long haul–Jupiter itself takes 12 years to make one revolution around the Sun. So they were taken completely by surprise when, on October 12, 1995, a group from Geneva, Switzerland scooped them with the announcement of the first extrasolar planet, orbiting the star 51 Pegasi. Marcy and Butler dropped everything, rushed to Lick, and within a week had confirmed the observation using their own methods. Incredibly, this large planet, half the size of Jupiter, completed one orbit in just four days. No one imagined that such a planet would even exist–the Swiss group had stumbled upon it while looking for failed stars called brown dwarfs.
Using the iodine technique they had been developing for almost a decade, Marcy and Butler found planet after extrasolar planet. So far, they have found more than two-thirds of the 100 or so planets known to exist outside our solar system. (The rival Swiss group has found most of the rest.) “We’re lucky for reasons that we never could have anticipated,” Marcy reflects. “We’re simply fortunate that nature makes these close-in planets that allowed us to pick them off like cherries on the lowest branches on the tree.” (The closer a planet is to its star, the more strongly it tugs, making it easier to detect.) Their success has brought them worldwide attention and recognition, and not just among their colleagues in astronomy. In 1996, Marcy and Butler were featured in a cover story in Time magazine, and two years ago Marcy discussed planet-hunting on the Late Show with David Letterman.
Last June, Marcy’s group found the first Jupiter-like planet orbiting 55 Cancri, one of the original set of stars they began observing in 1987. The planet is several times larger than our own Jupiter, but it orbits at about the same distance, taking 13 years to go once around its star. They expect to find plenty more of these familiar-looking solar systems in the next few years. “We’ve only been looking at about 50 stars long enough to observe a Jupiter,” says Debra Fischer, a postdoctoral researcher who has been working with Marcy since 1997. “That number has jumped to 1,200, but for the bulk of stars we’ve only been looking for about four years. Our solar system isn’t so weird that there’s nothing else like it. In fact, I think there are just a range of endless possibilities. We discovered the easy-to-find systems, and now we’re looking at systems more like our own.”
The most exciting result, says Fischer, was finding the first multiple-planet system. Marcy had noticed that it seemed like there might be a second planet around the star Upsilon Andromedae (additional planets cause a star to have multiple wobbles, creating a curlicue pattern) and asked Fischer to analyze the data. To her amazement, she found not one but two more planets around the star. “At that moment, chills ran down my spine,” she recalls. Marcy agrees that these multiple-planet systems are the most intriguing: “There’s an enormous amount of beauty in seeing a system that is like a giant machine; but it’s also beautiful in a human sense, because we ourselves live in a planetary system. Not just a star with one planet, but with nine.”
Most astronomers believe that planets like those in our own solar system are a common byproduct of the formation of stars like our Sun, and Marcy estimates that there may be rocky, Earth-like planets around as many as 70 percent of the stars in our galaxy.
In the years since his crisis of confidence, life has treated Marcy well. His success in finding planets not only brought him a professorhip at Berkeley, it also brought him access to some of the world’s best telescopes. Apart from UC’s Lick Observatory, his group also uses the world’s largest telescope, at the Keck Observatory on top of Hawaii’s Mauna Kea volcano, as well as the Anglo-Australian Telescope in Australia and the Las Campanas Observatory in Chile. Marcy’s team is also spread far and wide–Paul Butler now works at the Carnegie Institute in Washington, D.C.; Steve Vogt of UC Santa Cruz has joined the group; and there are collaborators across the globe. Observing and analyzing all this data keeps them busy seven days a week, sometimes until midnight, but it’s clearly a labor of love. “Paul and I still don’t have children; we’ve dedicated our lives to this. We’re living the lives of possessed artists or composers,” says Marcy. (It’s not all work and no play for Marcy, who is an avid tennis player. “To me tennis comes first, the research comes second,” he says.)
And he doesn’t plan to rest until he has found the holy grail of planet-hunters–a planet like our own. Last year, he secured funding for the world’s first dedicated planet-hunting telescope, which will be built at Lick by the summer of 2004. It will look at target stars 365 nights a year and will be fitted with a state-of-the-art spectrometer that should allow the astronomers to detect planets just ten times the mass of the Earth. Unlike the gas giant planets found so far, these small planets could be rocky, terrestrial planets that orbit in the “habitable” zone, where it is not too hot and not too cold. These “Goldilocks” planets are just right for liquid water to collect and so, perhaps, for life to emerge. Marcy’s group is also heavily involved in plans for NASA’s Space Interferometry Mission, a space-borne telescope scheduled to launch in 2010 that will directly observe the wobble of a star as it is orbited by a planet. It should be able to detect planets just a few times the mass of the Earth.
NASA plans to follow up ten years later with an even more ambitious mission, the Terrestrial Planet Finder, which will attempt to analyze the light reflected by a planet itself to determine the chemical constituents of its atmostphere. “You don’t have to be a scientist to realize that if you see oxygen and water floating around in the atmosphere, that smells like the kind of conditions that might be right for life on the surface of the planet,” says Marcy.
While finding planets is exciting, it is the question of life on other planets that drives Marcy. “Are we one in a million, one in a billion, or are we one in ten? We don’t know.” To help answer that question, two years ago he set up the Center for Integrative Planetary Science at Berkeley. He has dreams that this will one day blossom into the world’s first fully fledged extrasolar planet institute, where biologists, geophysicists, and astronomers will come together to work on the problem.
Nowadays, when Marcy tells people about his research, they don’t look at their shoes or scuffle their feet. In fact, they’re more likely to say: “Oh yeah! I read about that.” The search for planets around other stars seems to have captured the public imagination. Recently, a “night of planet-hunting with Geoff Marcy” at the Keck Observatory was sold on eBay for $16,000 (the money will go to the Astronomical Society of the Pacific). And when Congressman Jerry Lewis of Riverside heard that Marcy and his team were trying to build their dedicated planet-hunting telescope, he proposed it as a line item on the 2002 federal budget, and it had no trouble being voted through. “People just get it,” says Marcy, who shares their fascination. “I think it’s astonishing, at any age, that the universe makes these beautiful and colorful constructs that float in the blackness of space.”
Some compare the search for planets to the great explorations of Columbus or Magellan. While Marcy is humbled by such comparisons, he believes that his work is part of the destiny of our species. “We’re on the beginning steps of a voyage into our Milky Way galaxy. It might take thousands of years for humanity to complete that voyage, but we’re doing the early reconnaissance of new worlds.”