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 420, and the pace of discovery is about to accelerate.
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 13 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 the Edward, Frances, and Shirley B. Daniels Fellow at the Radcliffe Institute. “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 80 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 astronomical parlance) known as Proxima Centauri, which orbits the inner pair at such a wide distance that it takes at least 100,000 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 420 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.”
Competing for Interstellar Destinations
A European team is also hunting for Alpha Centauri planets, using a bigger telescope and a better detector. This group recently announced 32 new planets, several times scooping Fischer’s team. Over the years, the American group founded by Marcy and Butler has had somewhat testy relations with their European counterparts, as the two teams have outdone each other and aimed for glory. But Fischer points out a bright side to the competition: If either group finds Alpha Centauri planets, the other can provide independent confirmation.
There’s also the problem of the stars themselves. Just as on the Sun, giant bubbles of gas rise and fall in the upper layers of Alpha Centauri A and B, and their motion can mimic the subtle spectral shifts induced by orbiting planets. Fischer’s group will have to use sophisticated mathematical analysis to tease out the periodic signals caused by these orbiting planets, and that will require hundreds of thousands of measurements taken over four or five years. Project Long Shot began in January 2009 and has already taken more than 50,000 measurements. “I’m estimating 60 percent odds that everyone will be happy with the results,” says astrophysicist Greg Laughlin of the University of California at Santa Cruz.
If Fischer’s team, or its European competitors, finds planets around Alpha Centauri, they will be logical targets for humanity’s first true interstellar mission. Although five NASA spacecraft are currently on trajectories that will escape the solar system, they are moving very slowly relative to the speed of light and will have fallen silent tens of thousands of years before encountering any star. But with Alpha Centauri less than five light-years away, it’s conceivable that our descendants may decide to target this system, especially if it proves to have planets.
Kepler Blows the Roof Off
Dimitar D. Sasselov AM ’03, a Bulgarian-born astronomer who is now a leading exoplanet researcher at the Harvard-Smithsonian Center for Astrophysics—and a faculty associate of the Institute’s Academic Engagement Programs—suggests that one day scientists may build miniature probes the size of cell phones and accelerate them toward Alpha Centauri. Using giant sails pushed by the solar wind and perhaps ultra-high-powered lasers, such craft might achieve 10 percent the speed of light, and thus reach their destination within the lifetime of the people who built them. “My son is now 15, and he’s planning to go to MIT to study robotic engineering, so maybe someday he can take over this project,” jokes Fischer.
By the time humanity is able to launch bona fide interstellar probes, the exoplanet count will be in the thousands. On March 6, 2009, NASA launched Kepler, a new space telescope devoted to finding exoplanets by watching them pass in front of their host stars. By monitoring 100,000 stars with an ultrasensitive telescope and detectors, Kepler is likely to add hundreds of planets in the next few years alone—including the first Earth-size planets in the habitable zones of their host stars—and thus will blow the roof off the field. But as Fischer notes, “Kepler never would have gotten off the ground without these earlier discoveries.”