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.
This is a preview of subscription content, access via your institution
Subscribe to this journal
Receive 51 print issues and online access
$199.00 per year
only $3.90 per issue
Rent or buy this article
Prices vary by article type
Prices may be subject to local taxes which are calculated during checkout
Knutson, H. et al. A featureless transmission spectrum for the Neptune-mass exoplanet GJ 436b. Nature 505, 66–68 (2014)
Kreidberg, L. et al. Clouds in the atmosphere of the super-Earth exoplanet GJ 1214b. Nature 505, 69–72 (2014)
Knutson, H. et al. Hubble Space Telescope near-IR transmission spectroscopy of the super-Earth HD 97658b. Preprint at http://arxiv.org/abs/1403.4602 (2014)
Ehrenreich, D. et al. Near-infrared transmission spectrum of the warm-Uranus GJ 3470b with the Wide Field Camera-3 on the Hubble Space Telescope. Preprint at http://arxiv.org/abs/1405.1056v3 (2014)
Moses, J. et al. Compositional diversity in the atmospheres of hot Neptunes, with application to GJ 436b. Astrophys. J. 777, 34 (2013)
D’Angelo, G., Durisen, R. H. & Lissauer, J. J. in Exoplanets (ed. Seager, S. ) 319–346 (Univ. Arizona Press, 2010)
Bakos, G. et al. HAT-P-11b: a super-Neptune planet transiting a bright K star in the Kepler field. Astrophys. J. 710, 1724–1745 (2010)
Deming, D. et al. Infrared transmission spectroscopy of the exoplanets HD 209458b and XO-1b using the Wide Field Camera-3 on the Hubble Space Telescope. Astrophys. J. 774, 95 (2013)
Fazio, G. et al. The Infrared Array Camera (IRAC) for the Spitzer Space Telescope. Astrophys. J. 154, 10 (2004)
Borucki, W. et al. Kepler Planet-Detection Mission: introduction and first results. Science 327, 977–980 (2010)
Knutson, H., Howard, A. & Isaacson, H. A correlation between stellar activity and hot Jupiter emission spectra. Astrophys. J. 720, 1569–1576 (2010)
Deming, D. et al. Kepler and ground-based transits of the exo-Neptune HAT-P-11b. Astrophys. J. 740, 33 (2011)
Sanchis-Ojeda, R. & Winn, J. Starspots, spin-orbit misalignment, and active latitudes in the HAT-P-11 exoplanetary system. Astrophys. J. 743, 61 (2011)
Bernath, P. Water in sunspots and stars. Int. Astron. Union Symp. 12, 70 (2002)
Berta, Z. et al. The GJ1214 super-Earth System: stellar variability, new transits, and a search for additional planets. Astrophys. J. 736, 12 (2011)
Fraine, J. D. et al. Spitzer transits of the super-Earth GJ1214b and implications for its atmosphere. Astrophys. J. 765, 127 (2013)
Sing, D. et al. Hubble Space Telescope transmission spectroscopy of the exoplanet HD 189733b: high-altitude atmospheric haze in the optical and near-ultraviolet with STIS. Mon. Not. R. Astron. Soc. 416, 1443–1455 (2011)
Patil, A., Huard, D. & Fonnesbeck, C. PyMC: Bayesian stochastic modelling in Python. J. Stat. Softw. 35, 4–85 (2010)
Ford, E. Quantifying the uncertainty in the orbits of extrasolar planets. Astron. J. 129, 1706–1717 (2005)
Ford, E. Improving the efficiency of Markov chain Monte Carlo for analyzing the orbits of extrasolar planets. Astrophys. J. 642, 505–522 (2006)
Castelli, F. & Kurucz, R. New grids of ATLAS9 model atmospheres. Preprint at http://arxiv.org/abs/astro-ph/0405087 (2004)
Husser, T.-O. et al. A new extensive library of PHOENIX stellar atmospheres and synthetic spectra. Astron. Astrophys. 553, A6 (2013)
Benneke, B. & Seager, S. Atmospheric retrieval for super-Earths: uniquely constraining the atmospheric composition with transmission spectroscopy. Astrophys. J. 753, 100 (2012)
Benneke, B. & Seager, S. How to distinguish between cloudy mini-Neptunes and water/volatile-dominated super-Earths. Astrophys. J. 778, 153 (2013)
Madhusudhan, N. et al. A high C/O ratio and weak thermal inversion in the atmosphere of exoplanet WASP-12b. Nature 469, 64–67 (2011)
Madhusudhan, N. C/O ratio as a dimension for characterizing exoplanetary atmospheres. Astrophys. J. 758, 36 (2012)
Rogers, L. & Seager, S. A framework for quantifying the degeneracies of exoplanet interior compositions. Astrophys. J. 712, 974–991 (2010)
Fortney, J. et al. A framework for characterizing the atmospheres of low-mass low-density transiting planets. Astrophys. J. 775, 80 (2013)
Chiang, E. & Laughlin, G. The minimum-mass extrasolar nebula: in situ formation of close-in super-Earths. Mon. Not. R. Astron. Soc. 431, 3444–3455 (2013)
Hu, R. & Seager, S. Photochemistry in terrestrial exoplanet atmospheres. III. Photochemistry and thermochemistry in thick atmospheres on super Earths and mini Neptunes. Astrophys. J. 784, 63 (2014)
McCullough, P. M. & MacKenty, J. Considerations for Using Spatial Scans with WFC3. Instrum. Sci. Rep. WFC3 2012–8 (Space Telescope Science Institute, 2012)
Wilkins, A. et al. The emergent 1.1-1.7 μm spectrum of the exoplanet Corot-2b as measured using the Hubble Space Telescope. Astrophys. J. 783, 113 (2014)
Claret, A. A new non-linear limb-darkening law for LTE stellar atmosphere models. Calculations for −5.0 ≤ log[M/H] ≤ +1, 2000 K ≤ Teff ≤ 50000 K at several surface gravities. Astron. Astrophys. 363, 1081–1190 (2000)
Oliphant, T. Python for scientific computing. Comput. Sci. Eng. 9, 10–20 (2007)
Rajan, A. et al. WFC3 Data Handbook 2011 (Space Telescope Science Institute, 2011)
Mandel, K. & Agol, E. Analytic light curves for planetary transit searches. Astrophys. J. 580, L171 (2002)
Mandell, A. et al. Exoplanet transit spectroscopy using WFC3: Wasp-12 b, Wasp-17 b, and Wasp-19 b. Astrophys. J. 779, 128 (2013)
Kass, R. & Raftery, A. Bayes factors. J. Am. Stat. Assoc. 90, 773–795 (1995)
Carter, J. A. & Winn, J. N. Parameter estimation from time-series data with correlated errors: a wavelet-based method and its application to transit light curves. Astrophys. J. 704, 51–67 (2009)
Lewis, N. et al. Orbital phase variations of the eccentric giant planet HAT-P-2b. Astrophys. J. 766, 95 (2013)
Knutson, A. et al. The 3.6-8.0 μm broadband emission spectrum of HD 209458b: evidence for an atmospheric temperature inversion. Astrophys. J. 673, 526–531 (2008)
Ballard, S. et al. A search for a sub-Earth-sized companion to GJ 436 and a novel method to calibrate warm Spitzer IRAC observations. Publ. Astron. Soc. Pacif. 122, 1341–1352 (2010)
Knutson, H. et al. 3.6 and 4.5 μm phase curves and evidence for non-equilibrium chemistry in the atmosphere of extrasolar planet HD 189733b. Astrophys. J. 754, 22 (2012)
Todorov, K. et al. Warm Spitzer observations of three hot exoplanets: XO-4b, HAT-P-6b, and HAT-P-8b. Astrophys. J. 746, 111 (2012)
Eastman, J., Siverd, R. & Gaudi, S. Achieving better than 1 minute accuracy in the heliocentric and barycentric Julian dates. Publ. Astron. Soc. Pacif. 122, 935–946 (2010)
Knutson, H. et al. Friends of hot Jupiters. I. A radial velocity search for massive, long-period companions to close-in gas giant planets. Astrophys. J. 785, 126 (2014)
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.
The authors declare no competing financial interests.
Extended data figures and tables
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.
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.
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.
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.
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.
About this article
Cite this article
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
This article is cited by
Nature Astronomy (2021)
Nature Astronomy (2021)
Nature Astronomy (2020)
Nature Astronomy (2020)
A sub-Neptune exoplanet with a low-metallicity methane-depleted atmosphere and Mie-scattering clouds
Nature Astronomy (2019)