By Robert Naeye
Searching for planets around other stars is a high-stakes business. Every time astronomers find one of these so-called exoplanets, they add to humanity’s collective knowledge of what kinds of worlds exist out there. Better yet, each discovery brings us a wee bit closer to answering the ultimate question of whether we share the universe with other living beings. A lot of these discoveries have been made lately. Twenty years ago, astronomers didn’t know of a single planet outside our solar system. Today, the number stands at about 400, and the pace of discovery is about to accelerate.
Alpha Centauri
Courtesy of NASA/JPL/Space Science Institute
Although exoplanet hunters may not care to discuss it, their quest is also a race for the history books. Many milestone discoveries have generated headlines worldwide and will be celebrated in books and articles for decades or centuries to come.
The Most Prolific Planet-Hunting Team in History
Debra Fischer RI '10 knows this all too well. She’s been in this business for thirteen years, and her name will forever be associated with one of the most important milestones of all. In 1999, she did the computer data analysis that revealed that the star Upsilon Andromedae harbored not just one planet, as was previously known, but three. For the first time, astronomers knew for a fact that a star similar to our Sun was orbited by a family of planets. Suddenly, our solar system had cosmic company.
“Upsilon Andromedae was our first glimpse at another system of planets,” says Fischer, who recently moved from San Francisco State University to Yale, and is currently a fellow at the Radcliffe Institute for Advanced Study. “It was amazing to see three gas-giant planets packed inside the equivalent of Mars in our solar system. This prolific system showed us that planet formation was a robust process.”
For the first part of her career, Fischer was a key member of the most prolific planet-hunting team in history: the group founded by Geoff Marcy (now at the University of California at Berkeley) and Paul Butler (now at the Carnegie Institution in Washington, DC). This group has discovered about half of the 400 known exoplanets, and is renowned in the astronomical community for its scientific integrity. In a field littered with false alarms, the Marcy/Butler/Fischer group has never had to retract a single claimed planet discovery. “There are many planet-hunting teams, but the Marcy and Butler team commands a unique respect from their colleagues,” says exoplanet researcher Marc Kuchner of NASA’s Goddard Space Flight Center. “The feeling is that they don’t just find candidate planets, they find planets.”
Project Long Shot
Although Fischer still works with Marcy, she’s now leading her own research project in a competition to achieve yet another milestone: finding one or more planets around the star-system closest to the Sun, Alpha Centauri. “There’s a reason we call this Project Long Shot: It’s not going to be easy,” Fischer says. “Searching for planets around Alpha Centauri is the most speculative thing I have ever done.”
Our Milky Way Galaxy consists of some two hundred billion stars, most of them in multiple-star systems. Alpha Centauri contains two stars quite similar to the Sun (though one is a little bigger and brighter and the other a little smaller and dimmer). The stars orbit around each other every eighty years at a distance that averages about the same as Uranus’s distance from the Sun. The system also has a third member, a tiny, feeble star (a “red dwarf” in astronomy parlance) known as Proxima Centauri, which orbits the inner pair at such a wide distance that it takes at least a hundred thousand years to complete a single circuit. At a distance of 4.24 light-years (a light-year is the distance that light travels in one year, about 5.9 trillion miles), Proxima is the closest star to the Sun. Fischer and her colleagues are interested in the main pair, (Alpha Centauri A and B) because of their similarity to the Sun. These stars are 4.37 light-years from Earth.
To bag her elusive quarry, Fischer is employing the tried-and-true “wobble” technique with which astronomers have discovered a large majority of the 400 known exoplanets. As planets go around a star, they tug gravitationally on it, just as the star tugs on them. If a planet has enough mass, it causes the star to wobble, the way an Olympic hammer thrower will appear to wobble as he whirls around just before releasing the hammer. This wobble shows up as subtle shifts in the star’s spectrum, which modern instruments can record. When the star is moving toward Earth, its light shifts slightly toward the blue end of the spectrum, and when it’s moving away, the light shifts toward the red—much as an ambulance siren will appear to shift toward a high-frequency pitch when approaching and toward the bass when receding. This is the well-known Doppler effect.
Alpha Centauri A and B are among the brightest stars in our night sky, owing to their relative proximity, but they appear in the skies over the Southern Hemisphere. Fischer and her colleagues are using a telescope in the Chilean Andes that was on the verge of being mothballed. Their detector was built in the 1980s, but with $600,000 in federal stimulus funding from the National Science Foundation, they’re building a new instrument that will be ready around December 2010.
One reason the effort is considered a long shot is that many astronomers question whether planets could even form or survive around Alpha Centauri A or B. The two stars follow a highly elongated orbit, and the gravitational stirrings of either member could disrupt or even fling out planets around the other. But Fischer notes that several planets have already been found in similar double-star systems. “Never listen to theorists if you’re an observer,” she says, “because they would have told you that most of the planets we’ve found so far wouldn’t have been there.”
