A featureless transmission spectrum for the Neptune-mass exoplanet GJ 436b

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Abstract

GJ 436b is a warm—approximately 800 kelvin—exoplanet that periodically eclipses its low-mass (half the mass of the Sun) host star, and is one of the few Neptune-mass planets that is amenable to detailed characterization. Previous observations1,2,3 have indicated that its atmosphere has a ratio of methane to carbon monoxide that is 105 times smaller than predicted by models for hydrogen-dominated atmospheres at these temperatures4,5. A recent study proposed that this unusual chemistry could be explained if the planet’s atmosphere is significantly enhanced in elements heavier than hydrogen and helium6. Here we report observations of GJ 436b’s atmosphere obtained during transit. The data indicate that the planet’s transmission spectrum is featureless, ruling out cloud-free, hydrogen-dominated atmosphere models with an extremely high significance of 48σ. The measured spectrum is consistent with either a layer of high cloud located at a pressure level of approximately one millibar or with a relatively hydrogen-poor (three per cent hydrogen and helium mass fraction) atmospheric composition7,8,9.

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Figure 1: White-light transit curves for the four individual visits.
Figure 2: Averaged transmission spectrum for GJ 436b.
Figure 3: Joint constraints on cloud-top pressure versus atmospheric metallicity.

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Acknowledgements

We thank P. McCullough for his assistance in the planning and executing of these observations. We are also grateful to J. Moses, M. Line and N. Nettelmann for conversations on the nature of high-metallicity atmospheres as well as discussions of specific interior and atmosphere models for GJ 436b. D.H. has received support from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013 grant agreement number 247060).

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Authors

Contributions

H.A.K. carried out the data analysis for this project with input from D.D. B.B. provided the planetary atmosphere models and accompanying fits, and D.H. supplied the PHOENIX atmosphere models used to calculate the stellar limb-darkening coefficients.

Corresponding author

Correspondence to Heather A. Knutson.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1

Representative raw image from the 29 November 2012 ut observation, showing the scanned spectrum.

Extended Data Figure 2 Raw white-light photometry for the four individual transits.

Data are vertically offset for clarity. Transits shown were obtained on the following dates (from top to bottom): 26 October 2012 ut, 29 November 2012 ut, 10 December 2012 ut and 2 January 2013 ut. The raw fluxes are shown as filled black circles, and the best-fit solutions for the instrumental effects and transit light curves are shown as filled red circles.

Extended Data Figure 3 White-light residuals.

Data are vertically offset for clarity. Transit residuals shown were obtained on the following dates (from top to bottom): 26 October 2012 ut, 29 November 2012 ut, 10 December 2012 ut and 2 January 2013 ut. The difference between the white-light fluxes and best-fit model solutions are shown as filled black circles.

Extended Data Figure 4 Individual transmission spectra for each of the four visits.

Transmission spectra are shown as filled circles, with colours indicating the date of the observations: 26 October 2012 ut (dark blue), 29 November 2012 ut (light blue), 10 December 2012 ut (yellow) and 2 January 2013 ut (red). This plot shows the errors in the measured transit depths, but does not include the additional systematic errors from the limb-darkening models.

Extended Data Figure 5 Observed minus calculated transit times using the new best-fit ephemeris.

The solid line denotes observed minus calculated equal to zero. Transit times from this paper are plotted as filled stars, and previously published observations are shown as filled circles, with 1σ uncertainties overplotted for both. The colour of the points denotes the wavelength of the observations (blue for visible, red for infrared). Transits shown include all previously published observations for this planet3,39,40,41,42,43,44,45. The figure is adapted from figure 8 of our previous study3.

Extended Data Figure 6 Comparison to published transit depths for GJ 436b.

Filled black circles show previously published transit depths3,42,43,45,46, with 1σ uncertainties overplotted. The white-light transit depths from our WFC3 observations are plotted as black stars. We show three models for comparison, including a solar-metallicity cloud-free model (red line), a hydrogen-poor 1,900 × solar model (blue line), and a solar metallicity model with optically thick clouds at 1 mbar (grey line). As we discuss in ref. 3, the apparent variations in transit depth at different epochs could plausibly be explained by the occultation of active regions on the surface of the star. If correct, this would make broadband photometry collected at different epochs unreliable for the purpose of constraining the planet’s transmission spectrum. p.p.m., parts per million.

Extended Data Figure 7 Stellar activity versus time.

Filled black circles show the measured emission levels in the Ca ii H and K line cores from Keck HIgh Resolution Echelle Spectrometer (HIRES) spectroscopy of GJ 436 (refs 3 and 46); these are parameterized using the SHK index, where larger values indicate increased stellar activity. Vertical lines mark the approximate dates of the six most recent Spitzer transit observations (dashed line), as well as the four HST transits presented in this paper (solid line).

Extended Data Figure 8 Averaged stellar spectrum versus PHOENIX model atmospheres.

Spectra are averaged over each HST visit and then normalized using the sensitivity curve for that visit. These spectra are plotted as dark blue (October), light blue (November), yellow (December), and red (January) lines. For comparison we show two PHOENIX stellar atmosphere models with effective temperatures of 3,500 K and log(g) = 5.0 (black line, where g is surface gravity) and 3,350 K and log(g) = 4.8 (grey line) binned to the same pixel resolution as our data. These data include an additional component of instrumental broadening that smoothes out the sharp spectral features visible in the model spectra.

Extended Data Figure 9 Joint constraints on cloud-top pressure versus atmospheric metallicity for an oxygen-rich and a carbon-rich atmosphere.

a, C/O = 0.3; b, C/O = 1.0. The models shown in Fig. 3 assume a solar C/O ratio of 0.5. The coloured shading indicates the normalized probability density as a function of cloud-top pressure and metallicity derived from a variation of the Bayesian retrieval methods. We vary the amount of metals in the atmosphere (defined as elements heavier than H and He) linearly using the scaling factor shown on the lower x axes. The black contours show the 68%, 95% and 99.7% Bayesian credible regions.

Extended Data Table 1 Averaged differential transit depths

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Knutson, H., Benneke, B., Deming, D. et al. A featureless transmission spectrum for the Neptune-mass exoplanet GJ 436b. Nature 505, 66–68 (2014). https://doi.org/10.1038/nature12887

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