EU-funded researchers have successfully completed the first analysis of the atmosphere of a super-earth, an extrasolar planet with a mass higher than Earth's. The so-called planet GJ 1214b was discovered in 2009 and this latest research confirms initial findings by astronomers in Chile that the planet had an atmosphere. They said the study, presented in the journal Nature, was 'a milestone on the road towards characterising these worlds'. The research was partially funded by a Marie Curie International Incoming Fellow grant of the EU's Seventh Framework Programme (FP7).
The planet GJ 1214b was discovered last year using the HARPS (High Accuracy Radial velocity Planet Searcher) spectrograph on the European Southern Observatory's (ESO's) 3.6-metre telescope in Chile. Initial findings suggested that this planet had an atmosphere, and this has been studied in detail and confirmed by an international team of astronomers led by Jacob Bean from the Harvard–Smithsonian Center for Astrophysics in the US. They used the UV FOcal Reducer and low dispersion Spectrograph (FORS) instrument on ESO's Very Large Telescope (VLT) to carry out this research.
'This is the first super-Earth to have its atmosphere analysed,' Dr Bean said. 'We've reached a real milestone on the road toward characterising these worlds.'
GJ 1214b has a radius of about 2.6 times that of the Earth and is about 6.5 times as massive, putting it into the class of exoplanets known as super-Earths — there are now over 500 confirmed exoplanets. Its host star lies about 40 light-years from Earth in the constellation of Ophiuchus or the Serpent Bearer. It is a faint star; if GJ 1214 were seen at the same distance from us as our Sun, it would appear 300 times fainter. However, it is also small, which means that the size of the planet is large compared to the stellar disc, making it relatively easy to study. The planet travels across the disc of its parent star once every 38 hours as it orbits at a distance of only 2 million kilometres — about 70 times closer than the Earth orbits the Sun.
To study the atmosphere, the team observed the light coming from the star as the planet passed in front of it. During these transits, some of the starlight passes through the planet's atmosphere and, depending on the chemical composition and weather on the planet, specific wavelengths of light are absorbed. The team then compared these precise new measurements with what they would expect to see for several possible atmospheric compositions.
Before the new observations, astronomers had suggested three possible atmospheres for GJ 1214b. First, that the planet was shrouded by water, which, given the close proximity to the star, would be in the form of steam. The second possibility was that this was a rocky world with an atmosphere consisting mostly of hydrogen, but with high clouds or hazes obscuring the view. The third option was that this exoplanet was like a mini-Neptune, with a small rocky core and a deep hydrogen-rich atmosphere.
The new measurements did not show the telltale signs of hydrogen and so the researchers ruled out the third option. The atmosphere is either rich in steam, or it is blanketed by clouds or hazes, similar to those seen in the atmospheres of Venus and Titan in our Solar System, which hide the signature of hydrogen.
'Although we can't 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,' said Dr Bean. 'Follow-up observations in longer wavelength infrared light are now needed to determine which of these atmospheres exists on GJ 1214b.'
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