The transmission spectrum of the Super-Earth GJ 1214 b


A summary of the paper: The Optical and Near-Infrared Transmission Spectrum of the Super-Earth Planet GJ1214b: Further Evidence for a Metal-Rich Atmosphere, by Bean et al.

Super-Earths are a class of exoplanets with masses between 1-10 M_{\oplus}. The study of Super-Earths are of great interest as there is no planet in this mass and size regime in our solar system. Describing their bulk compositions and atmospheres is a great challenge as in this mass and radius regime the theoretical models are degenerate. Different combinations of mass and radius can provide equally plausible solutions for the bulk composition of the planet.

GJ 1214 b for example has a mass of 6.55 \pm 0.98M_{\oplus} and a radius of 2.68 \pm 0.13 R_{\oplus} which puts it smack in the middle of the degenerate region of parameter space. The only thing which can be said with a high level of confidence is that GJ 1214 b is too large to be composed solely of solid material.

Fortunately, it is not all grim. There is a way to break this degeneracy by obtaining constraints on the planet’s atmospheric composition, by measuring the spectrum of the planet’s atmosphere.

Using transmission spectroscopy, Bean et al. found the spectrum of GJ 1214 b to be featureless. This is indicative of an atmosphere with a small scale height. A small scale height suggests that a cloud-free hydrogen-dominated atmosphere is unlikely. What is more likely is that GJ 1214 b either has an atmosphere of at least 70% H_2O by mass or has optically thick high altitude clouds or hazes. Thus it is plausible that the planet is either composed primarily of water ice with a gas envelope dominated by water vapour or that it is a true Super Earth composed of purely rocky material. To further narrow down the possibilities, a more accurate determination of the radius of the planet is needed. This in turn requires a more accurate determination of the stars radius as the planet’s radius is measured relative to the stars radius.

A paper by Croll et al. (2011)  finds an absorption feature in the Ks band (near-IR), which would imply that the spectrum is not flat. This 4\sigma detection is indicative of GJ 1214 b having a large scale-height atmosphere. The strong detection in the Ks band was not confirmed by Bean et al. who observed at the same wavelengths.

Various transmission spectra of GJ 1214 b indicative of a relatively flat transmission spectrum. The coloured lines are high resolution theoretical models. The filled points are broadband measurements from different instruments. See figure. 12 in Bean et al. 2011.

From my understanding the hazy atmosphere seems like a reasonable explanation, although more results will have to confirm this theory. It could be that the divergent measurements result from systematic errors frequently found in near-IR data. After all we are looking for tiny signals using noise prone instruments.

Further Reading:

The nature of the atmosphere of the transiting super-earth GJ 1214b
Three Possible Origins for the Gas Layer on GJ 1214b


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