Oversize orb: puffy planet poses puzzle.

Astronomers have discovered what may be the largest planet yet found--an orb that's 36 percent wider than Jupiter and that circles a nearby star. Researchers say that they're baffled by the giant extrasolar body, which has the lowest density of any known planet.

Half as massive as Jupiter and residing 450 light-years from Earth, the planet is just one-twentieth the distance from its parent star that Earth is from the sun. But the planet's presence in this hot zone isn't enough to explain the orb's low density, about one-quarter that of water, says codiscoverer Robert Noyes of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Many other extrasolar planets lie even closer to their stars, but they aren't nearly as puffy.

"We have a bit of a puzzle" says Noyes. Planet models can't explain the large radius.

The discovery team, led by Gaspar Bakos of Harvard-Smithsonian, could detect the object because it periodically passes directly between its parent star, the fainter member of a double-star system called ADS 16402, and Earth. During each transit, which lasts about 2 hours, the planet blocks 1.5 percent of the star's light from reaching Earth.

At press time, Bakos' team was scheduled to announce the finding at the Smithsonian Institution in Washington, D.C.

The new object, known as HAT-P-1b, was found by astronomers using six small robotic telescopes. Four of the telescopes are at the Whipple Observatory atop Mr. Hopkins in Arizona, while the other two are atop Hawaii's Mauna Kea. The network scans star fields as large as the Big Dipper. Follow-up observations with the large Keck 1 and Subaru telescopes on Manna Kea confirmed the finding and provided additional data.

Although astronomers know of some 200 extrasolar planets, most have been found indirectly, by measuring the small amount of wobble that a planet induces in the motion of its parent star. But the wobble method provides only a planet's maximum mass. In contrast, researchers can detect the exact mass and size of extrasolar planets observed in transit across their stars, as HAT-P-1b'S were. HAT-P-1b is the twelfth planet discovered with the transit method.

Only one other transiting planet, HD 209458b, has a density nearly as low as that of HAT-P-1b, and some researchers had regarded HD 209458b as a fluke. Such supergiant Jupiters now have to be taken seriously as a class, says theorist Adam Burrows of the University of Arizona in Tucson.

Alan Boss of the Carnegie Institution of Washington (D.C.) says that "puffy hot Jupiters do not appear to be as rare as one might have been able to argue before this discovery."

Two other teams have also recently found planets transiting nearby stars, but those planets' sizes and densities are less unusual than those of HAT-P-1b. Weighing 1.4 times as much as Jupiter, the planet TrES-2 lies 500 light-years from Earth, David Charbonneau of Harvard-Smithsonian and his colleagues report in an upcoming Astrophysical Journal.

Another newly identified planet, XO-1b, is nine-tenths of Jupiter's mass and about 650 light-years away, Peter R. McCullough of the Space Telescope Science Institute in Baltimore and his colleagues report in the Sept. 10 Astrophysical Journal.