Some like it hot

Astronomers detect exoplanet orbiting closer to its parent star than any other known planet

Normal 0 false false false MicrosoftInternetExplorer4 Talk about a star hugger.

Of the more than 300 planets found beyond the solar system, none lies closer to its parent star than the recently detected WASP-12b. Residing at just one-eighteenth the average distance that Mercury — the solar system’s innermost planet — orbits the sun, the sizzling planet is both the hottest planet known and the fattest.

Codiscoverer Leslie Hebb of the University of St. Andrews in Scotland and her colleagues estimate that the planet, which whips about its star in just 1.09 days, has an average temperature of 2,500 kelvins — nearly half that of the visible solar surface — and a diameter almost twice that of Jupiter, the solar system’s biggest planet. Blasted by its parent star, the close-in planet receives the highest dose of radiation of any known planet. Although this high intensity probably contributes to its puffiness, the extreme radiation can’t fully explain the body’s large size, Hebb’s team reported October 11 in Ithaca, N.Y., at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences.

The latest calculations offer new details about the planet; initial findings were presented by Hebb’s team at a meeting in Belfast, Northern Ireland, last April.

“Models of planet formation and evolution cannot fit [the planet’s] observed properties, especially its radius,” she says. “It is an extreme object that is pushing the boundaries of our understanding and forcing the theorists to improve their models.”

Hebb and her collaborators discovered the hot planet during an ongoing stellar survey known as SuperWASP, for Wide Angle Search for Planets. WASP uses two small telescopes, one in the Canary Islands and the other near Sutherland, South Africa, to hunt for planets that periodically pass in front of their parent star as seen from Earth, blocking a tiny but detectable amount of light during each transit.

The transits yield much more information, including the exact mass and radius of an extrasolar planet, than the more traditional search method, in which the presence of an orbiting planet is inferred because it pulls its parent star to and fro ever so slightly.

The intense stellar radiation striking WASP-12b, which weighs about 1.45 times the mass of Jupiter and lies about 1,000 light-years from Earth, may alter the planet’s atmosphere as well as its internal structure, Hebb says.

The impinging starlight may also be causing the atmosphere to evaporate at an increasing rate, comments Adam Burrows of Princeton University. Ultraviolet observations with a new spectrograph to be installed on the Hubble Space Telescope next year, as well as infrared observations now planned with NASA’s Spitzer Space Telescope could shed further light on the planet, he says.

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