Recent surveys have revealed that planets intermediate in size between Earth and Neptune (‘super-Earths’) are among the most common planets in the Galaxy1,2,3. Atmospheric studies are the next step towards developing a comprehensive understanding of this new class of object4,5,6. Much effort has been focused on using transmission spectroscopy to characterize the atmosphere of the super-Earth archetype GJ 1214b (refs 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17), but previous observations did not have sufficient precision to distinguish between two interpretations for the atmosphere. The planet’s atmosphere could be dominated by relatively heavy molecules, such as water (for example, a 100 per cent water vapour composition), or it could contain high-altitude clouds that obscure its lower layers. Here we report a measurement of the transmission spectrum of GJ 1214b at near-infrared wavelengths that definitively resolves this ambiguity. The data, obtained with the Hubble Space Telescope, are sufficiently precise to detect absorption features from a high mean-molecular-mass atmosphere. The observed spectrum, however, is featureless. We rule out cloud-free atmospheric models with compositions dominated by water, methane, carbon monoxide, nitrogen or carbon dioxide at greater than 5σ confidence. The planet’s atmosphere must contain clouds to be consistent with the data.
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This work is based on observations made with the NASA/ESA Hubble Space Telescope that were obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract number NAS 5-26555. These observations are associated with program GO-13021. Support for this work was provided by NASA through a grant from the Space Telescope Science Institute, the National Science Foundation through a Graduate Research Fellowship (to L.K.), the Alfred P. Sloan Foundation through a Sloan Research Fellowship (to J.L.B.), NASA through a Sagan Fellowship (to J.-M.D.), and the European Research Council (for D.H. under the European Community's Seventh Framework Programme, FP7/2007-2013 Grant Agreement number 247060).
The authors declare no competing financial interests.
The data used in this work can be accessed at the NASA Mikulski Archive for Space Telescopes (http://archive.stsci.edu).
Extended data figures and tables
Extended Data Figure 1 An example of a spatially scanned raw data frame. The exposure time was 88.4 s.
The dotted lines indicate the wavelength range over which we measure the transmission spectrum.
a, The raw broadband light curve. b, The broadband light curve corrected for systematics using the model-ramp technique (points) and the best-fit model (line). c, Residuals from the broadband light curve fit. d, The vector of systematics Z (see the Supplementary Information) used in the divide-white technique.
Extended Data Figure 4 The posterior distributions for the divide-white fit parameters for the 1.40-µm channel from the first transit observation.
The histograms represent the Markov chains for each parameter. The contour plots represent pairs of parameters, with lines indicating the 1σ, 2σ and 3σ confidence intervals for the distribution. The normalization constant is divided by its mean.
Extended Data Figure 5 Transit depths relative to the mean in 22 spectroscopic channels, for the 12 transits analysed.
The black error bars indicate the 1σ uncertainties determined by a Markov chain Monte Carlo fit.
Extended Data Figure 6 Fitted limb-darkening coefficients as a function of wavelength (black points) and theoretical predictions for stellar atmospheres with a range of temperatures (lines).
The uncertainties are 1σ confidence intervals from a Markov chain Monte Carlo fit. The temperature of GJ 1214 is estimated to be 3,250 K (ref. 22).
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Kreidberg, L., Bean, J., Désert, JM. et al. Clouds in the atmosphere of the super-Earth exoplanet GJ 1214b. Nature 505, 69–72 (2014). https://doi.org/10.1038/nature12888
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