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Last Updated: Thursday, 11 January 2007, 16:50 GMT
Supersonic gales rage on planets
By Paul Rincon
Science reporter, BBC News, Seattle

Artist's impression of giant planet. Image: Nasa/JPL-Caltech/R. Hurt (SSC)
The three giant planets are orbiting very close in to their stars
Strong winds of 14,000 km/h (9,000mph) may gust on three "gas giant" planets outside our Solar System, astronomers have told a US science meeting.

Each planet orbits very close to its "Sun"; one side of each world always faces the star, while the other side remains in permanent darkness.

Despite this, temperatures across these planets are uniform - and blazing hot.

The team believes these winds are mixing the atmospheres, balancing temperatures across the planets.

Eric Agol, a co-author of the study, presented his findings at a meeting of the American Astronomical Society (AAS) in Seattle.

The planets, gas giants similar in size to Jupiter, orbit stars about the same size as our Sun. They lie less than 150 light-years from Earth.

They orbit within about five million miles (eight million kilometres) of their stars, far less than Mercury's distance from the Sun.

Both sides now

It had been supposed that planets orbiting so close to their stars, with one side in constant daylight and the other in permanent darkness, might exhibit sharp temperature differences between the day and night sides.

If heat from the parent star is carried to the dark side, then the overall temperature would be lowered somewhat
Eric Agol
Dr Agol and his colleagues Nicholas Cowan and David Charbonneau measured infrared radiation from each of the planetary systems at eight different positions in their orbits late in 2005.

Using the Spitzer Space Telescope, they measured the thermal brightness of the systems when the planets' day sides faced Earth.

They also measured this property when the night sides faced our planet, and at various stages between.

The researchers found no infrared brightness variations in any of these planetary systems.

This suggests there are no big differences in temperatures on the day and night sides; instead, the planets appear to have constant temperatures of about 925C (1,700F).

"If heat from the parent star is carried to the dark side, then the overall temperature would be lowered somewhat because the heat is being distributed across the planet," said Dr Agol.

"Some theorists believe that supersonic winds are responsible for re-circulating the heat."

The three extrasolar planets are: 51 Pegasi, about 50 light-years from the Sun; HD179949b, about 100 light-years in distance; and HD209458b, which is about 147 light-years away.


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