Dumusque said that he and his colleagues observed Alpha Centauri B more than 450 times over several years to get a handle on its behavior. After subtracting away the star's orbital motions as well as the presumed effects of starspots and the like, what remained was a tiny fluctuation—an alternating speedup and slowdown, attributed to the gravitational pull of a low-mass planet on a 3.2-day orbit. The planet's tug makes a tiny contribution of about 50 centimeters per second to the velocity of Alpha Centauri B, which is currently moving about 20 kilometers per second with respect to Earth.
"This is a really tough detection to make," Fischer says. "I know that the Swiss team is extraordinarily careful. I have a lot of confidence in them." The Geneva group was the first to discover an exoplanet orbiting a sunlike star, back in 1995, and has since found more than 100 new worlds. Another leading planet hunter, Geoff Marcy of the University of California, Berkeley, says that the evidence for the new planet appears strong. "If the existence of the planet is correct, and I suspect it is, this is an historic discovery," Marcy wrote in an e-mail. "You can spit a watermelon seed to Alpha Centauri."
Fischer notes that she and her colleagues recently completed a major upgrade to their own spectrograph in Chile, and they are now working to determine whether their own limited data might be able to help confirm the existence of a planet around Alpha Centauri B. Meanwhile, astronomers around the globe will no doubt turn to our nearest star system with renewed interest in the hopes of finding more planets there, and especially planets that might harbor life. "This actually makes me very optimistic. I'm super excited about this result," Fischer says. "I wish it was me that found it."