Even casual skywatchers are familiar with Vega. The zero-magnitude star is the sky's fifth brightest, the jewel of the constellation Lyra, and a vertex of the Summer Triangle. Now it appears that Vega has another claim to fame: signs of a giant exoplanet orbiting it.

Vega looks bright because it is nearby, only 25 light-years away, and because it is hotter and larger than the Sun, putting out 50 times the Sun's light. Nearly two decades ago astronomers found far-infrared evidence of cold dust grains surrounded Vega. Observations in 1998 showed signs of structure in the dust. This week, at the American Astronomical Society meeting, two teams of astronomers announced that Vega's circumstellar dust is at least partly gathered into large clumps — in a characteristic shape that suggests the gravitational influence of a giant planet in an eccentric orbit.

Two separate teams made the observations using two telescopes half a world apart. The first group, led by David Koerner (University of Pennsylvania), used the Owens Valley Radio Observatory. The second, led by David Wilner (Harvard-Smithsonian Center for Astrophysics), used the Plateau de Bure Interferometer in the French Alps to resolve two knots in the circumstellar dust offset 60 and 75 astronomical units from Vega.

According to Wilner, the "features are naturally explained by a Jupiter-mass planet in an eccentric orbit around the star." The clumps, he says, are "trapped in resonances from a planet." Koerner adds that observations such as these are "starting to change the paradigm of what it means to be a disk."

While no planet has been seen, models by Wilner and his colleagues suggest that the semimajor axis of its orbit is most likely 30 a.u. (30 times the Earth's distance from the Sun). The simulations also place an upper limit to its mass. "If the planet is larger than 30 Jupiters, dust regions overlap and the resonances are destroyed," says Marc Kuchner, a theorist on Wilner's team.




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