Wide Field Camera 3 and the Atmosphere of GJ 1214 b

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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.

Figure 1. A very neat visualisation of the WFC3 transmission spectrum for GJ 1214 b between 1.1-1.7 microns. The top panel shows the lightcurves with time since mid-transit running along the vertical axis and wavelength along the horizontal axis. Three transits were observed at different times, and these are indicated by the different colours. The bottom panel shows the measured transit depths as a function of wavelength, with different colours again corresponding to the different transits, and the black circles indicating the value obtained from combining all three transits. Adapted from Berta et al (2012).

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 (R_p/R_s \sim 0.1) 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.

Figure 2. The transmission spectrum for GJ 1214 b. The red line shows the expected transmission spectrum if GJ 1214 b had a solar composition atmosphere, which agrees with the J and Ks band measurements of Croll et al (2011), but provides a terrible fit to other data sets. The blue line shows the expected transmission spectrum if GJ 1214 b has an atmosphere dominated by H2O. This model provides a better fit to all data except for the Ks band measurement of Croll et al (2011). Adapted from Berta et al (2012).

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.

Further Reading:

Bean et al, 2010, Nature, 468, 669

Bean et al, 2011, ApJ, 743, 92

Berta et al, 2011, ApJ, 736, 12

Charbonneau et al, 2009, Nature, 462, 891

Croll et al, 2011, ApJ, 736, 78

Crossfield et al, 2011, ApJ, 736, 132

de Mooij et al, 2012, A&A, 538, A46

Désert et al, 2011, ApJ, 731, L40

Howe & Burrows, 2012, submitted to ApJ

Kundurthy et al, 2011, ApJ, 731, 123

Nettelmann et al, 2011, ApJ, 733, 2

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About Author

I'm a PhD student at the University of Oxford. My work focuses on transiting exoplanets and, in particular, what we can learn about the atmospheres of these systems. A large part of this involves getting a better handle on the various instrumental systematics that contaminate the small signals we're trying to measure, and devising methods to remove them from the data. I'm also investigating ways of correcting for the effect of star spots on planetary transmission and emission spectroscopy measurements. My supervisor is Suzanne Aigrain.