The exoplanet GJ 1214b is a “super-Earth” – i.e. a planet with a mass intermediate between rocky planets, like Earth and Venus, and gaseous planets, like Uranus and Neptune. It has a mass of 6.5 Earth masses, a radius of 2.7 Earth radii, an orbital period of 1.5 days, and transits the nearby M-dwarf star GJ 1214.. It is the only super-Earth amenable to atmospheric characterization, thanks to its location close to the Sun (the “GJ” in the name stands for the Gliese-Jahreiss catalogue of nearby stars) and to the tiny size of its parent star (20% of the size of the Sun) which make the transit signal large.
What is the atmosphere of GJ1214b made of?
The main component of the atmosphere of giant planets, in the Solar System and around other stars, is hydrogen. Rocky planets like Earth are not heavy enough to retain hydrogen, their atmosphere consists of heavier molecules, like carbon dioxide, nitrogen or water.
Super-Earths are especially interesting for comparative planetology, because the dominant component of their atmosphere can be any of these molecules. With no example in the Solar System, we are venturing into completely unexplored territory.
With the size and mass of GJ 1214b, hydrogen or water vapour are expected to be the two plausible options . Thus the planet could be the first known case of an atmosphere mainly composed of water vapour, gradually compressed into a super-critical fluid in the deeper layers.
Transit spectroscopy offers a way to distinguish the two cases: a hydrogen-dominated atmosphere would be much more extended (because the H2 molecule is 9 times lighter than H2O), causing a much larger signal in the transmission spectrum.
The story so far
The study of GJ1214b in the past five years has been as full of plot twists and turns as a good detective story. The jury has been swayed repeatedly one way or another, and cries of “water!” been heard repeatedly. In the end the verdict has come in an unexpected form.
The first transit spectroscopy of GJ1214b in 2009  failed to show the large features expected from a cloud-free hydrogen/helium atmosphere. This was interpreted as evidence for a heavier atmosphere, probably made of water vapour.
A subsequent measurement showed a clear feature near 2 microns , which together with a marginal detection of Rayleigh diffusion in the UV , implied a hydrogen atmosphere after all.
However, precise data from the Hubble Space telescope in 2012  did not see any feature, which cast a strong doubt on the previous detections. Only a heavy-molecule atmosphere could suppress the atmospheric features in that way.
Further measurements with the Subaru giant telescope  showed that indeed the previous feature detections at 2 microns and in the UV were not robust. This was taken as solid confirmation of the water-vapour scenario – and got the press excited about “water-worlds”.
Finally, at the end of 2013, a massive investment with the Hubble Space Telescope – 60 orbits of the Hubble measuring 15 separate transits  – established that the spectral features were not only too small to allow a cloud-free hydrogen atmosphere, but even to small for a clean water-vapour atmosphere!
The only explanation left was one that had been entertained from the start but only as an alternate scenario: that clouds and hazes at high altitude in the atmosphere of GJ1214b were obscuring our view in transit.
The bottom line of what we have learnt about the atmosphere of GJ 1214b with this considerable investment of Hubble, Spitzer, VLT, Keck observations is this:
The transit spectrum of GJ 1214b is absolutely flat
i.e. we see no signature whatsoever of the atmosphere in transmission, no variations with size of the planet’s shadow as a function of wavelength.
The presence of obscuring clouds and hazes high in the atmosphere of GJ 1214b seems to be the only reasonable explanation for the flat transmission spectrum.
What are the clouds of GJ 1214b made of?
The atmosphere of GJ 1214b is much cooler than that of hot Jupiters (400-500 K compared to 1000-3000 K), so that the silicates, iron and metallic oxide clouds present on hot Jupiters cannot form. Still the atmosphere of GJ 1214b is too hot for water clouds to form. The remain possible constituents include organic hazes produced by the photolysis of methane – the kind of hazes that shroud Titan in an orange veil – or grains from the condensation of potassium chloride (KCl) and zinc sulphide (ZnS), two common compounds that condense at the right temperatures. Present models struggle though to explain how such clouds could form at very high altitude.
Thus the GJ 1214b story is “to be continued”. The next chapter will probably not be written until the advent of the James Webb Space Telescope, which will be able to probe deeper into the transit spectrum (see previous post), identify the nature of the clouds and maybe peek below them into the deeper atmosphere.
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