'Milestone' in bid to sniff atmosphere of a 'super Earth' light-years away

Some 40 light years from Earth, well within the sun's galactic "neighborhood," a planet nearly three times the size of Earth orbits a dim, low-mass star much smaller than the sun.

Recent observations indicated that the planet, a so-called "super Earth," has an atmosphere, and scientists Wednesday were applauding reports of the first measurements aimed at determining the kind of atmosphere the planet has – measurements that were taken despite the planet's distance, the star's dimness, and its relatively small size. It was also the first time ever that measurements have been taken of a super Earth's atmosphere.

At this stage, "process of elimination" is the operative phrase. In the results reported Wednesday, a team of astronomers led by Jacob Bean at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., use their data to rule out one of three broad types of atmosphere that others had suggested the planet could have.

"Even with these new measurements, we can't say yet what that atmosphere is made of," said Dr. Bean. Still, the ability to rule out one possibility represents "a milestone on the road toward characterizing these worlds," he added in a prepared statement.

Atmosphere indicated by planet's low density

Last December, another team of astronomers announced that they'd discovered the planet, dubbed GL 1214b. Data they gathered watching the star's light dim and brighten as the planet crossed in front of it indicated a planet with nearly seven times Earth's mass. Yet when the team estimated the planet's density, it was too low for a planet of its size and mass if the planet was made entirely from rock and water ice.

Modeling results suggested that the planet itself was smaller than the observed radius, with the rest of the volume the team measured taken up with an atmosphere, perhaps consisting of steam. Other models pointed to an atmosphere made largely of hydrogen and helium gas. Still others suggested a haze- or cloud-shrouded atmosphere like Venus's or like the atmosphere blanketing Saturn's moon Titan.

The results reported Wednesday in the journal Nature suggest that scientists can drop the hydrogen-helium possibility.

Interest in characterizing the atmospheres of extrasolar planets – particularly those beginning to approximate Earth's size and mass – stems not only from a desire to fine tune the technologies that allow for such studies. A planet's atmosphere also can yield clues about its history.

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