Astronomers have uncovered the first evidence of a planet orbiting two stars at once. If confirmed by other researchers, the planet would not just be an astronomical novelty; Its detection, reported on the e-print server at the Los Alamos National Laboratory (see astro-ph/9908038), would also be a triumph for a new and potentially powerful technique for finding planets around other stars.
Extrasolar planets are too dim to be seen directly. The several dozen detected so far have betrayed themselves by tugging their parent stars back and forth, creating a small, oscillating signal in the starlight--a technique sensitive only to massive, Jupiter-like planets. But there's another, potentially more sensitive, way to find planets. Every concentration of mass--star or planet--bends space-time. So when a dim star in our galaxy passes almost directly between Earth and a second star, the gravitational field of the intervening "lens" star bends and magnifies light from the background star, a process called gravitational microlensing. By observing tens of millions of different stars every night, several search teams are now regularly observing the tell-tale stellar brightening.
When a single star acts as the lens, the background star simply brightens and fades. But soon after astronomers in the Massive Compact Halo Object (MACHO) survey team discovered MACHO-97-BLG on 19 June 1997, they realized the object's flickering was too complex to result from a single-star lens. The Microlensing Planet Search collaboration began monitoring the 100-day-long event in collaboration with MACHO and the Global Microlensing Alert Network. At first astronomers thought they might have detected a planet around a single star somewhere in our galaxy. But in the end it took a model that included a binary star and a third object--a planet weighing three times as much as Jupiter and orbiting about 7 astronomical units (1 AU is equal to the separation of the Earth and sun) from the two stars--to account for the fluctuating light.
"It's a pretty nifty result," says fellow planet-hunter Geoff Marcy of the University of California, Berkeley. "But the architecture of the system is quite odd," which makes him wonder whether the planet is real. He and other astronomers say the spinning binary, just 1.8 AU apart, could create "gravitational chaos" near the hypothesized planet, perhaps flinging it from the system. Not necessarily, says Harvard astrophysicist Matt Holman, who has used a computer to simulate how a planet around a binary star would behave over millions of orbits. "Most likely it is stable," says Holman.