Astronomers have probed the atmosphere of a super-Earth for the first time
A crude spectrum has been obtained for the atmosphere of a super-Earth orbiting a dim red dwarf star 40 light years away. The planet鈥檚 upper atmosphere is apparently dominated by steam or cloudy haze.
The star, Gliese 1214 (GJ 1214) in Ophiuchus, is 300 times dimmer than the sun. Its planet was discovered in 2009 when the detected the planet鈥檚 silhouette periodically dimming the star. The planet has 6.5 Earth masses, as determined later by the star鈥檚 gravitational wobbles, and it circles the little star very closely in just 38 hours. The transits reveal the planet鈥檚 diameter to be 2.6 times Earth鈥檚 鈥� making its average density very low, only about a third of Earth鈥檚 density.
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Using one of the European Southern Observatory鈥檚 8.2-metre Very Large Telescope reflectors, a team of astronomers detected a telltale absorption spectrum caused by a tiny fraction of the star鈥檚 light filtering through the planet鈥檚 atmosphere during each transit. The spectrum was featureless, indicating that the upper atmosphere either consists mostly of water vapor or is dominated by high-altitude clouds or haze.
鈥淭his is the first super-Earth to have its atmosphere analysed. We鈥檝e reached a real milestone on the road toward characterising these worlds,鈥� said team leader Jacob Bean of the Harvard鈥揝mithsonian Center for Astrophysics in a .
Before this observation, astronomers had suggested three possible atmospheres for Gliese 1214b. The planet could be shrouded by water 鈥� which, given its high temperature so close to the star (200 潞C), would be in the form of steam. Or it could be a rocky world with an atmosphere of mostly hydrogen obscured by high clouds or hazes. Or it might be a mini-Neptune, with a small rocky core and a deep hydrogen-rich atmosphere, the upper part of which would be clear.
The measurements clearly show no sign of hydrogen and thus rule out the third option. So the atmosphere is either rich in steam or blanketed by clouds or hazes. The planet鈥檚 low density, meanwhile, indicates that it鈥檚 a waterworld.
鈥淎lthough we can鈥檛 yet say exactly what that atmosphere is made of, it is an exciting step forward to be able to narrow down the options for such a distant world to either steamy or hazy,鈥� says Bean. 鈥淔ollowup observations in longer-wavelength infrared light are needed to determine which of these atmospheres exists on Gliese 1214b.鈥�
Journal reference: (DOI: 10.1038/nature09596)
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