The Exoplanet Next-Door: Astronomers Discover World in Nearest Star System

The small planet orbiting Alpha Centauri B, just four light-years away, is too hot for life but may have habitable neighbors
Exoplanet orbiting nearby star Alpha Centauri B

ESO/L. Calcada

As humankind's radio and television broadcasts leak out into space and propagate across the galaxy at the speed of light, it is tempting to speculate on who might be listening in—and what they might know about us. A hypothetical civilization 50 light-years away, should they possess the technology to receive and decipher our stray radio waves, would witness a planet enmeshed in the Cuban Missile crisis, the sights and sounds of 1962 only just having reached their detectors.

An alien civilization inhabiting the closest star system to the sun, on the other hand, would be much more in touch with modern Earth life. Extraterrestrials there could be following the final months of the 2008 U.S. presidential contest between Sens. John McCain and Barack Obama—without, we hope, their having rendered too harsh a judgment of humanity based all those negative political ads.

Until recently, though, no planets were known to orbit any of the trio of stars—Proxima Centauri, Alpha Centauri A and Alpha Centauri B—nearest the sun. Now a team of European researchers has reinvigorated hopes that our solar system indeed has nearby neighbors. In a paper published online October 17 in Nature, the group reported the discovery of an extrasolar planet orbiting Alpha Centauri B, just 4.3 light-years away. (Scientific American is part of Nature Publishing Group.)

"Alpha Centauri B is of course a very special case," study co-author Stéphane Udry of the Observatory of Geneva in Switzerland said in a teleconference with reporters. "It's our next-door neighbor." The second-nearest known exoplanet, Epsilon Eridani b, is more than twice as distant.

"To me this is the find of the decade," says astronomer Debra Fischer of Yale University, who did not contribute to the new study. "Look at it—it’s the nearest star system."

But the newfound world, which by convention will receive the awkward designation Alpha Centauri B b, is more than a statistical novelty. It ought to be a rocky world like Earth—its mass may be as small as 1.13 times that of our planet. And although the world is far too hot to support life as we know it, its presence suggests that additional terrestrial planets may exist in more temperate orbits within the same planetary system.

"These rocky planets tend to come in bunches," Fischer says. "It's a good bet, then, that there are additional rocky planets around Alpha Centauri B. These are going to be sitting further out and could be right in the habitable zone."

Of course, that supposition depends on the validity of the exoplanetary discovery. The claimed finding is just on the edge of what is detectable with the best astronomical instruments and has yet to be confirmed by other astronomers.

Udry and his colleagues found the putative planet by tracking Doppler shifts in the starlight from Alpha Centauri B—the so-called radial velocity method of planet searching. Orbiting planets exert a gravitational pull on their host stars; some accelerate the star toward or away from Earth ever so slightly. The Geneva team's HARPS spectrograph, mounted on a 3.6-meter telescope at La Silla Observatory in Chile, is the world’s premier instrument for measuring small changes in stellar radial velocities.

The smaller the planet, the smaller the gravitational tug. And at roughly Earth mass, the planet orbiting Alpha Centauri B ranks as one of the smallest known. In order to extract its minuscule signal, the researchers needed to carefully monitor and characterize the star's long-term activity. Much like the sun, where magnetic sunspots appear in bunches and then all but vanish as they follow an 11-year cycle, Alpha Centauri B also demonstrates a cyclical pattern of stellar activity. Magnetic spots can change the convective flow of plasma within a star, which in turn impacts its measured velocity with respect to Earth. (A star with onrushing plasma appears to be moving more quickly toward Earth than does a star with receding plasma.) "Here on Alpha Centauri B we have a magnetic cycle of about eight years," lead study author Xavier Dumusque of the Geneva Observatory said in the teleconference. "During different activity phases, what we have is that more and more spots will appear on your star, and that will perturb your radial velocities."

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."

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