Image: HR 8799b
G. Bacon  /  NASA / ESA / STScI
The extrasolar planet HR 8799b, shown in this artist's conception, orbits a young star 130 light-years from Earth.
updated 9/1/2010 4:39:20 PM ET 2010-09-01T20:39:20

Astronomers peering at a large alien planet have found it to be covered by an atmosphere unlike that of any extrasolar planet yet seen.

The planet, known as HR 8799b, is about seven times the mass of Jupiter and was discovered in 2008. Since then, astronomers have found little hint of the methane expected in the planet's atmosphere and evidence that it is hotter and cloudier than previously thought.

The discovery suggests that young gas giants may be cloudier than previously imagined. HR 8799b is among a half-dozen exoplanets that have been directly imaged with telescopes, researchers said.

"We are at a point where not only can we directly image planets around other stars, but we can begin to study the properties of their atmospheres in detail," said the study's lead author Brendan Bowler, a graduate student at the University of Hawaii, in a statement. "Direct spectroscopy of exoplanets is the future of this field."

HR 8799b is one of three gas giants known to orbit the star HR 8799, about 130 light-years from Earth. The multi-planet system was discovered in 2008.

Planet's weird atmosphere
Astronomers used the Keck Observatory in Hawaii to study the spectrum of light emitted by HR 8799b.

Methane can be used as a sort of planetary thermometer, so the astronomers looked for the gas's signature in the light spectrum. But they found that HR 8799b's atmosphere contains little to none of the stuff.

Based on the new data, as well as past observations and models, the researchers estimate that the coolest possible temperature for HR 8799b is about 1,700 degrees Fahrenheit (927 degrees Celsius).

This is about 750 degrees F or so (417 degrees C) warmer than the planet was expected to be, assuming its atmosphere is moderately dusty or cloudy, researchers said. But the temperature makes sense if HR 8799b is much dustier or cloudier than current models predict, they added.

The researchers now suspect HR 8799b may be a very cloudy place, cloudier than expected.

The planet could also serve as a model for the atmospheres of other extrasolar gas giants, the researchers said. Astronomers first detected an extrasolar planet's atmosphere less than 10 years ago.

"Direct studies of extrasolar planets are just in their infancy," said study co-author Michael Liu, also of the University of Hawaii. "But even at this early stage, we are learning they are a different beast than objects we have known about previously."

The research will be detailed in the Astrophysical Journal later this year.

Dim alien planets
The planets around HR 8799 are incredibly faint, about 100,000 times dimmer than their parent star.

To obtain the spectrum of HR 8799b, the team relied on the adaptive optics system of the Keck II Telescope to make an ultra-sharp image of the star over many hours. Adaptive optics change the shape of a telescope's mirror to filter out the blurring effects of Earth's atmosphere.

They then used the Keck facility instrument called OSIRIS, a special kind of spectrograph, to separate the spectrum of the planet from the light of its parent star.

"Adaptive optics systems on Keck and other large ground-based telescopes make sharper images than even the Hubble Space Telescope," said study co-author Trent Dupuy, also of the University of Hawaii. "With adaptive optics, we are learning an incredible amount about objects that are smaller than the lowest-mass stars and larger than the most massive gas-giant planets in our solar system."

More than 400 extrasolar planets have been discovered to date; only six have been directly imaged. Three of these are around HR 8799, researchers said.

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