Water vapour absorption in the clear atmosphere of a Neptune-sized exoplanet

Abstract

Transmission spectroscopy has so far detected atomic and molecular absorption in Jupiter-sized exoplanets, but intense efforts to measure molecular absorption in the atmospheres of smaller (Neptune-sized) planets during transits have revealed only featureless spectra1,2,3,4. From this it was concluded that the majority of small, warm planets evolve to sustain atmospheres with high mean molecular weights (little hydrogen), opaque clouds or scattering hazes, reducing our ability to observe the composition of these atmospheres1,2,3,4,5. Here we report observations of the transmission spectrum of the exoplanet HAT-P-11b (which has a radius about four times that of Earth) from the optical wavelength range to the infrared. We detected water vapour absorption at a wavelength of 1.4 micrometres. The amplitude of the water absorption (approximately 250 parts per million) indicates that the planetary atmosphere is predominantly clear down to an altitude corresponding to about 1 millibar, and sufficiently rich in hydrogen to have a large scale height (over which the atmospheric pressure varies by a factor of e). The spectrum is indicative of a planetary atmosphere in which the abundance of heavy elements is no greater than about 700 times the solar value. This is in good agreement with the core-accretion theory of planet formation, in which a gas giant planet acquires its atmosphere by accreting hydrogen-rich gas directly from the protoplanetary nebula onto a large rocky or icy core6.

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Figure 1: White-light transit curves and starspot crossing temperature estimates.
Figure 2: The transmission spectrum of HAT-P-11b.
Figure 3: Spectral retrieval results of our transmission spectrum.

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Acknowledgements

J.F., A.J. and N.E. acknowledge support from project IC120009 ‘Millennium Institute of Astrophysics (MAS)’ of the Millennium Science Initiative, Chilean Ministry of Economy; FONDECYT project 1130857; and BASAL CATA PFB-06. N.E. is supported by CONICYT-PCHA/Doctorado Nacional. We thank P. McCullough for his assistance in the planning and execution of our observations. We are grateful to I. Crossfield, L. Kreidberg and E. Agol for providing their open-source, Python code banks on their individual websites. We are also grateful for discussions with M. Line, J. Fortney and J. Moses about the nature of photochemistry and interior structures. We thank the ATLAS and PHOENIX teams for providing stellar models. We also thank the SciPy and NumPy associations for providing extensive and rigorous numerical routines for an assortment of mathematical and computational techniques.

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Authors

Contributions

J.F. led the data analysis for this project with contributions from D.D., H.K., N.E., A.J. and A.W. A.W. supplied Hubble spectral fitting routines and interpretations. N.E. and A.J. supplied Python routines for MCMC, wavelet and transit curve analyses specific to transiting exoplanets. D.D., H.K., N.E. and A.J. provided computational equipment and administration. D.D., N.M., H.K. and K.T. successfully applied for and provided data from Hubble. B.B. and N.M. provided atmospheric models and accompanying fits. B.B. performed atmospheric retrieval analysis and provided figures and interpretations. N.E. supplied stellar limb-darkening coefficients calculated from both ATLAS and PHOENIX models.

Corresponding author

Correspondence to Jonathan Fraine.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 HST white-light curve with exponential ramp effects.

The gaps resulted when HAT-P-11 was occulted by the Earth during Hubble’s 96 min orbit. We decorrelated the ramp effect by fitting an average, two-parameter (scale and amplitude) exponential function over time.

Extended Data Figure 2 An example of WFC3 scanning-mode observation spectral images.

a, Example spatial scan spectral image with the normalized summations in the dispersion (upper) and cross-dispersion or scanning (right) directions. b, Integrated spectrum (blue) and spectral template (red) before (top) and after (bottom) fitting; the amplitudes and colours are normalized to 1.0.

Extended Data Figure 3 Correlations between all fitted parameters for our HST WFC3 white-light curve.

We calculated the Pearson correlation coefficient over the posteriors of each parameter, and found the correlations to be small (<0.10 in magnitude), or in most cases negligible (<0.01 in magnitude). Blue represents regions of lesser posterior density and red represents regions of greater posterior density, with green and yellow in the middle.

Extended Data Figure 4 Wavelength-dependent transit light curves.

The coloured points are the wavelength light curves, ranging from blue (1.17 μm) to red (1.67 μm) with 18 nm spacing. The black lines represent the best-fit transit light curves over the wavelength range from 1.1 to 1.7 μm. The curves are shifted for display purposes only. The differential light curves were fitted with differential analytic transit curves to derive the planetary spectrum seen in Fig. 1. We added the white-light curve into the differential light curves to derive the data above.

Extended Data Figure 5 The distribution of Kepler starspot crossing anomalies.

We fitted a Gaussian profile to each of the 298 spot crossings seen during the 208 transits observed by Kepler. Here we show the distribution of starspot amplitudes, calculated as the height minus baseline of the fitted Gaussian profile in parts per million. The dashed lines represent the starspot crossing amplitudes observed during our four concurrent Spitzer observations. In particular, note that all four spot crossings with concurrent Spitzer observations are at the larger end of the distribution. In addition, the spot crossing on ut 7 July 2011, the largest spot crossing feature observed during our concurrent Spitzer observations, crossed a spot with ΔT ≈ 900 K.

Extended Data Figure 6 HAT-P-11 Kepler light curve for 4 yr of short cadence.

The variation in flux has a peak-to-peak modulation of 2%, consistent with the spot coverage inferred from our previous Kepler study12. The times of our Spitzer observations are marked with vertical blue lines. The time of our HST WFC3 observation, included in this analysis, are marked with a vertical red line.

Extended Data Table 1 Transit depths as a function of wavelength for Kepler, HST WFC3, Spitzer IRAC1 and Spitzer IRAC2
Extended Data Table 2 The system and planetary parameters of HAT-P-11b

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Fraine, J., Deming, D., Benneke, B. et al. Water vapour absorption in the clear atmosphere of a Neptune-sized exoplanet. Nature 513, 526–529 (2014). https://doi.org/10.1038/nature13785

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