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A super-earth with a
steamy atmosphere?

Posted: 01 December 2010

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Using ESO's Very Large Telescope (VLT) astronomers have, for the first time, analysed the atmosphere of a ‘super-earth’ exoplanet.

GL 1214b, which has a radius 2.6 times that of Earth and a mass 6.5 times greater, was first discovered in 2009 using the HARPS instrument on ESO's 3.6 metre telescope in Chile. Initial observations hinted that the planet hosted an atmosphere rich in either hydrogen, steam, or clouds and hazes.

Astronomers have probed the atmosphere of a super-earth type planet for the first time, narrowing down its make-up to either an atmosphere rich in steam, or one with clouds and hazes abound. Image: Paul A. Kempton.

Now, new measurements led by Jacob Bean of the Harvard–Smithsonian Center for Astrophysics rule out the hydrogen-rich atmosphere. "This is the first super-Earth to have its atmosphere analysed," he says. "We’ve reached a real milestone on the road toward characterising these worlds."

Orbiting its star at a distance of just two million kilometres, some seventy times closer than the Earth orbits the Sun, the measurements were made as the planet transited in front of the star once every 38 hours. As the starlight passed through the planet's atmosphere, its fingerprint constituents were measured using the FORS instrument on the VLT.

Determining the atmospheric composition, whether it hosts a steamy atmosphere or one filled with clouds and hazes, will reveal how the planet formed and evolved. Bean tells Astronomy Now, "If further observations confirmed the presence of clouds, then that would indicate either a small 'mini-Neptune' type planet that formed beyond the star's snow line and migrated inward, accreting a substantial amount of gas from the original disc surrounding the young star. Alternatively, such an atmosphere could have been outgassed during the formation of a massive terrestrial planet."

On the other hand, if further observations confirm the steam atmosphere, then although the planet still probably formed beyond the ice line and migrated inward, it would not have accreted gas from the star's disc. "This indicates a late time of formation, or it lost a significant amount of its atmosphere due to strong irradiation from its host star," says Bean.

Although termed a super-earth, with temperatures upwards of 1,000 degrees centigrade and pressures hundreds to thousands of times what we have at sea level on Earth, GL 1214b would unlikely have a solid surface. "A planet with a steam atmosphere would not really have a surface, sort of like Jupiter and the other Solar System gas giants," describes Bean. "As you move down through the atmosphere the hot water vapor would transition directly to an exotic form of matter known as a super critical fluid."

The team plan to make follow-up observations in longer wavelength infrared light to distinguish between the two possible atmospheric scenarios.

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