GJ 1214b and the Ks mystery


An outline of the paper: IRSF SIRIUS JHKs Simultaneous Transit Photometry of GJ1214b, by Narita et al.

The transmission spectrum of GJ 1214b has in previous papers by Berta et al. (2012) and Bean et al. (2011) amongst others, shown itself to be a flat, featureless spectrum. This has lead to theories that GJ 1214b either has an atmosphere with a large scale height obscured by high altitude haze or clouds, or a water dominated atmosphere, undetectable due a large mean molecular weight. For more background info see our previous post on the atmospheric chemistry of GJ 1214 b.

The only sign of a significant spectral absorption feature was presented in a paper by Croll et al. (2011) who found a larger effective radius in the Ks band (~2.15 microns). For this prominent spectroscopic feature to exist, the atmosphere would have to be H/He dominated with a massive scale height making the super-Earth more of a mini-Neptune. It was suggested in this paper and subsequent papers that this detection be remeasured, and remeasured it was.

Narita et al. remeasured the effective radius of GJ 1214 b in the Ks band and found it to be much shallower than what Croll et al. had measured, resulting in an inconsistency at the 4σ level. As mentioned in their paper, they find no good explanation for this discrepancy. It might be tempting to think that the measurement by Croll et al. is a statistical outlier, but as three separate transits events yielded a deeper transit in the Ks band compared to the J band, it is not that easy.

The GJ 1214b transmission spectrum in the region 1-2.5μm. The grey solid line represents the water-dominated atmosphere whilst the grey dotted line represents the hydrogen-dominated (Solar composition) atmosphere. The results by Narita et al. is shown in red, whilst the results from Croll et al. are shown in purple.

de Mooij et al. got a similar result to Croll et al. in the Ks band, but warn that their uncertainties are large due to systematic features in the light curves.

It is clear from the studies mentioned above that more observations in the Ks band is warranted, but as Narita et al. suggest, observations in the blue optical bands would be very useful. This is because the observations could help discern between the water-dominated atmosphere and a H/He + haze atmosphere, as the latter would show an increasing absorption towards the blue due to Rayleigh scattering. The main problem doing this, however, is that the M-star GJ1214 is emitting fewer photons towards bluer wavelengths, requiring longer observing times at expensive facilities.

For now, GJ1214 remains the ideal candidate for super-Earth atmospheric studies, being the only super-Earth within the parameter space were an atmospheric detection is possible. This is likely to be the case until surveys like the MEarth survey find another super-Earth or until the upcoming  Next Generation Transit Survey comes online, estimated to find many of them.

Featured image found here.


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Editor of Exoclimes.com. Observational exoplanet and brown dwarf astronomer studying the atmospheres of exoplanets. Interested in public outreach and conveying my interest in astronomy to others. Follow me on Twitter or Google+. (More)