An outline of the paper: The Flat Transmission Spectrum of the Super-Earth GJ1214b from Wide Field Camera 3 on the Hubble Space Telescope by Berta et al.
Of the observational results that have been published this year, the set of multiwavelength lightcurves for GJ 1214 b presented by Zachory Berta and collaborators has been one of the highlights (Figure 1). The measurements were made in the near infrared using the Wide Field Camera 3 (WFC3) on HST, and are the first from this instrument to be published for a transiting exoplanet.
GJ1214 b itself has received a lot of attention since it was discovered in 2009. For one thing, its radius is only 2.7 times larger than the Earth’s, while its mass is 6.5 times that of the Earth. This raises the possibility that it could be a rocky planet, albeit one with an extended atmosphere much thicker than the Earth’s. What’s more, the favourable planet-to-star radius ratio () makes the atmosphere easier to detect.
Despite the accumulation of data for GJ 1214 b (see Figure 2 and Further Reading below), it has proven difficult to devise a realistic model for the planetary atmosphere that simultaneously explains all of the published results. This is largely due to a deep transit measured by Croll et al (2011) in the Ks band (~2.2 microns) relative to the transit depth they measured in the J band (~1.3 microns), which those authors suggested could be due to CH4 or H2O absorption. The detection of such a feature would imply a large scale height for the atmosphere, so we’d expect to see molecular absorption features at other wavelengths as well. However, most of the observations made so far have failed to detect evidence for any such features.
The transmission spectrum that Berta et al uncover in the WFC3 data across the 1.1-1.7 micron range is very flat, which can be seen most clearly in the bottom panel of Figure 1. This lack of detected absorption features adds to the burgeoning evidence that GJ 1214 b has either an atmosphere with a low scale height dominated by heavy molecules such as water, or a hydrogen-dominated atmosphere with a high altitude cloud deck that mutes the absorption features in the transmission spectrum. Berta et al argue that the former is more likely, as there is no known cloud or haze species that could account for the uniform absorption across the whole wavelength range.
While more work is needed to clarify our understanding of GJ 1214 b’s atmosphere, this study has introduced the community to the potential of WFC3 as an effective tool for exoplanet atmosphere studies. In particular, the authors managed to convincingly remove correlated systematics from their data, which is something that has caused a few headaches in the past with other instruments. Over the next few years we’ll find out if WFC3 can routinely produce datasets as reliable as this one.
Feature Image: An astronaut at work on the final servicing mission to HST in 2009, during which WFC3 was installed. Taken from NASA.