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

Journal name:
Nature
Volume:
513,
Pages:
526–529
Date published:
DOI:
doi:10.1038/nature13785
Received
Accepted
Published online

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.

At a glance

Figures

  1. White-light transit curves and starspot crossing temperature estimates.
    Figure 1: White-light transit curves and starspot crossing temperature estimates.

    a, Transit curves from the Hubble WFC3 and warm Spitzer, aligned in phase and shifted in flux for clarity. The four warm Spitzer transits at both 3.6 and 4.5 µm (ref. 9) are binned for illustration. Starspot crossings are seen as deviations near +0.5 h in the Kepler photometry (blue). b, We estimated the starspot temperatures by dividing the Spitzer transit residuals by the Kepler transit residuals (colours as in a). The dashed lines represent the photosphere-to-starspot temperatures for three stellar model atmospheres22. Water vapour has been detected in sunspots as cool as 3,000 K, corresponding to a contrast of ~1,800 K here14. There is essentially no starspot temperature that can produce sufficiently strong water absorption to mimic our result.

  2. The transmission spectrum of HAT-P-11b.
    Figure 2: The transmission spectrum of HAT-P-11b.

    a, Our WFC3 observations show transit depth variations in agreement with a hydrogen-dominated atmosphere. The coloured, solid lines23, 24 correspond to matching markers displayed in Fig. 3. The error bars represent the standard deviations over the uncertainty distributions. An atmosphere with a high mean molecular mass (dark blue line) is ruled out by our observations by >3σ. The WFC3 spectrum was allowed to shift, as a unit, over these uncertainties. Rs, stellar radius. b, Detailed view of our WFC3 spectrum. For the purposes of visually comparing the spectral significance, we shifted all of the models by 93 p.p.m. in the grey region in a and in b.

  3. Spectral retrieval results of our transmission spectrum.
    Figure 3: Spectral retrieval results of our transmission spectrum.

    The coloured regions indicate the probability density as a function of metallicity (relative to solar) and cloud-top pressure derived using our Bayesian atmospheric retrieval framework23, 24. Mean molecular weight was derived for a solar C/O ratio at 10 mbar. Black contours mark the 68%, 95% and 99.7% Bayesian credible regions. The depth of the observed water feature in the WFC3 spectrum required the presence of a large atmospheric scale height that can only be self-consistently obtained for an atmospheric metallicity below 700 times solar at 3σ (99.7%) confidence. The atmosphere is probably predominately cloud-free at least down to the 1 mbar level. We indicate the matching models plotted in Fig. 2 with coloured markers.

  4. HST white-light curve with exponential ramp effects.
    Extended Data Fig. 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.

  5. An example of WFC3 scanning-mode observation spectral images.
    Extended Data Fig. 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.

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

  7. Wavelength-dependent transit light curves.
    Extended Data Fig. 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.

  8. The distribution of Kepler starspot crossing anomalies.
    Extended Data Fig. 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.

  9. HAT-P-11 Kepler light curve for [sim]4 yr of short cadence.
    Extended Data Fig. 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.

Tables

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

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Author information

Affiliations

  1. Department of Astronomy, University of Maryland, College Park, Maryland 20742-2421, USA

    • Jonathan Fraine,
    • Drake Deming &
    • Ashlee Wilkins
  2. Instituto de Astrofísica, Pontificia Universidad Católica de Chile, 7820436 Macul, Santiago, Chile

    • Jonathan Fraine,
    • Andrés Jordán &
    • Néstor Espinoza
  3. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA

    • Jonathan Fraine,
    • Bjorn Benneke &
    • Heather Knutson
  4. NASA Astrobiology Institute’s Virtual Planetary Laboratory, Seattle, Washington 98195, USA

    • Drake Deming
  5. Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

    • Nikku Madhusudhan
  6. Department of Physics, ETH Zürich, 8049 Zürich, Switzerland

    • Kamen Todorov

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.

Competing financial interests

The authors declare no competing financial interests.

Corresponding author

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Author details

Extended data figures and tables

Extended Data Figures

  1. Extended Data Figure 1: HST white-light curve with exponential ramp effects. (175 KB)

    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.

  2. Extended Data Figure 2: An example of WFC3 scanning-mode observation spectral images. (198 KB)

    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.

  3. Extended Data Figure 3: Correlations between all fitted parameters for our HST WFC3 white-light curve. (281 KB)

    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.

  4. Extended Data Figure 4: Wavelength-dependent transit light curves. (926 KB)

    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.

  5. Extended Data Figure 5: The distribution of Kepler starspot crossing anomalies. (205 KB)

    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.

  6. Extended Data Figure 6: HAT-P-11 Kepler light curve for ~4 yr of short cadence. (142 KB)

    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 Tables

  1. Extended Data Table 1: Transit depths as a function of wavelength for Kepler, HST WFC3, Spitzer IRAC1 and Spitzer IRAC2 (492 KB)
  2. Extended Data Table 2: The system and planetary parameters of HAT-P-11b (232 KB)

Additional data