Oversize Orb: Puffy planet poses puzzle

Ron Cowen

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.

BIG! Planet HAT-P-1b closely orbits its orange-glowing parent star, while a partner star lies in the distance in this illustration. Inset compares Jupiter (left) with HAT-P-1b (right). D. Aguilar

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.

Bakos' team announced the finding Sept. 14, 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 Mt. 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 Mauna 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 Letters.

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.

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References:

Bakos, G.Á., R.W. Noyes, et al. In press. HAT-P-1b: A large-radius, low-density exoplanet transiting one member of a stellar binary. Astrophysical Journal. Abstract and preprint available at http://xxx.lanl.gov/abs/astro-ph/0609369.

McCullough, P.R., et al. 2006. A transiting planet of a sun-like star. Astrophysical Journal 648(Sept. 10):1228-1238. Abstract available at http://dx.doi.org/10.1086/505651. Preprint available at http://xxx.lanl.gov/abs/astro-ph/0605414.

O'Donovan, F.T., D. Charbonneau, et al. In press. TrES-2: The first transiting planet in the Kepler Field. Astrophysical Journal Letters. Abstract and preprint available at http://xxx.lanl.gov/abs/astro-ph/0609335.

Further Readings:

2006. Strange new planet baffles astronomers. Harvard-Smithsonian Center for Astrophysics press release. Sept. 14. Available at http://www.cfa.harvard.edu/press/pr0624.html.

2006. Massive planet found by astronomers using novel network of tiny telescopes. Harvard-Smithsonian Center for Astrophysics press release. Sept. 8. Available at http://cfa-www.harvard.edu/press/pr0623.html.

2006. Astronomers find Jupiter-sized transiting planet using innovative telescope network. Lowell Observatory press release. Sept. 8. Available at http://www.lowell.edu/press_room/releases/
recent_releases/TrES2.html.

For more information on the group of telescopes used to find the planet, go to http://cfa-www.harvard.edu/~gbakos/HAT/index.html.

A version of this article written for younger readers is available at Science News for Kids.

Sources:

Gáspár Bakos
Harvard-Smithsonian Center for Astrophysics
60 Garden Street
Cambridge, MA 02138

Adam Burrows
University of Arizona
Department of Astronomy
Steward Observatory
Tucson, AZ 85721

Alan P. Boss
Carnegie Institution of Washington
Department of Terrestrial Magnetism
5241 Broad Branch Road, N.W.
Washington, DC 20015-1305

David Charbonneau
Department of Astronomy
Harvard-Smithsonian Center for Astrophysics
60 Garden Street, MS 16
Cambridge, MA 02138

Peter R. McCullough
Space Telescope Science Institute
3700 San Martin Drive
Baltimore, MD 21218

Robert W. Noyes
Harvard-Smithsonian Center for Astrophysics
60 Garden Street
Cambridge, MA 02138