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A sub-Neptune exoplanet with a low-metallicity methane-depleted atmosphere and Mie-scattering clouds



With no analogues in the Solar System, the discovery of thousands of exoplanets with masses and radii intermediate between Earth and Neptune was one of the big surprises of exoplanet science. These super-Earths and sub-Neptunes probably represent the most common outcome of planet formation1,2. Mass and radius measurements indicate a diversity in bulk composition much wider than for gas giants3; however, direct spectroscopic detections of molecular absorption and constraints on the gas mixing ratios have largely remained limited to planets more massive than Neptune4,5,6. Here we analyse a combined Hubble/Spitzer Space Telescope dataset of 12 transits and 20 eclipses of the sub-Neptune exoplanet GJ 3470 b, whose mass of 12.6 M places it near the halfway point between previously studied Neptune-like exoplanets (22–23 M)5,6,7 and exoplanets known to have rocky densities (7 M)8. Obtained over many years, our dataset provides a robust detection of water absorption (>5σ) and a thermal emission detection from the lowest irradiated planet to date. We reveal a low-metallicity, hydrogen-dominated atmosphere similar to that of a gas giant, but strongly depleted in methane gas. The low metallicity (O/H = 0.2–18.0) sets important constraints on the potential planet formation processes at low masses as well as the subsequent accretion of solids. The low methane abundance indicates that methane is destroyed much more efficiently than previously predicted, suggesting that the CH4/CO transition curve has to be revisited for close-in planets. Finally, we also find a sharp drop in the cloud opacity at 2–3 µm, characteristic of Mie scattering, which enables narrow constraints on the cloud particle size and makes GJ 3470 b a key target for mid-infrared characterization with the James Webb Space Telescope.

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Data availability

The data presented in this work are publicly available in the Mikulski Archive for Space Telescope (https://archive.stsci.edu/hst/) and the Spitzer Heritage Archive (https://sha.ipac.caltech.edu/applications/Spitzer/SHA/).

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The authors declare no competing interests.

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  1. 1.

    Dressing, C. D. & Charbonneau, D. The occurrence of potentially habitable planets orbiting M dwarfs estimated from the full Kepler dataset and an empirical measurement of the detection sensitivity. Astrophys. J. 807, 45 (2015).

  2. 2.

    Fulton, B. J. & Petigura, E. A. The California-Kepler Survey VII. Precise planet radii leveraging Gaia DR2 reveal the stellar mass dependence of the planet radius gap. Astron. J. 156, 264 (2018).

  3. 3.

    Fulton, B. J. et al. The California-Kepler Survey. III. A gap in the radius distribution of small planets. Astron. J. 154, 109 (2017).

  4. 4.

    Kreidberg, L. et al. Clouds in the atmosphere of the super-Earth exoplanet GJ 1214b. Nature 505, 69–72 (2014).

  5. 5.

    Fraine, J. et al. Water vapour absorption in the clear atmosphere of a Neptune-sized exoplanet. Nature 513, 526–529 (2014).

  6. 6.

    Wakeford, H. R. et al. HAT-P-26b: a Neptune-mass exoplanet with a well-constrained heavy element abundance. Science 356, 628–631 (2017).

  7. 7.

    Stassun, K. G., Collins, K. A. & Gaudi, B. S. Accurate empirical radii and masses of planets and their host stars with Gaia parallaxes. Astron. J. 153, 136 (2017).

  8. 8.

    Ment, K. et al. A second planet with an Earth-like composition orbiting the nearby M dwarf LHS 1140. Astron. J. 157, 32 (2019).

  9. 9.

    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. Astron. Astrophys. 570, A89 (2014).

  10. 10.

    Knutson, H. A. et al. Hubble Space Telescope near-IR transmission spectroscopy of the super-Earth HD 97658b. Astrophys. J. 794, 155 (2014).

  11. 11.

    Benneke, B. & Seager, S. How to distinguish between cloudy mini-Neptunes and water/volatile-dominated super-Earths. Astrophys. J. 778, 153 (2013).

  12. 12.

    Nascimbeni, V. et al. The blue sky of GJ3470b: the atmosphere of a low-mass planet unveiled by ground-based photometry. Astron. Astrophys. 559, 32 (2013).

  13. 13.

    Chen, G. et al. The GTC exoplanet transit spectroscopy survey—V. A spectrally-resolved Rayleigh scattering slope in GJ 3470b. Astron. Astrophys. 600, A138 (2017).

  14. 14.

    Wakeford, H. R. et al. High-temperature condensate clouds in super-hot Jupiter atmospheres. Mon. Not. R. Astron. Soc. 464, 4247–4254 (2017).

  15. 15.

    Miller-Ricci Kempton, E., Zahnle, K. & Fortney, J. J. The atmospheric chemistry of GJ 1214b: photochemistry and clouds. Astrophys. J. 745, 3 (2012).

  16. 16.

    Moses, J. I. et al. Compositional diversity in the atmospheres of hot neptunes, with application to GJ 436b. Astrophys. J. 777, 34 (2013).

  17. 17.

    Morley, C. V. et al. Forward and inverse modeling of the emission and transmission spectrum of GJ 436b: investigating metal enrichment, tidal heating, and clouds. Astron. J. 153, 86 (2017).

  18. 18.

    Kreidberg, L., Line, M. R., Thorngren, D., Morley, C. V. & Stevenson, K. B. Water, high-altitude condensates, and possible methane depletion in the atmosphere of the warm super-neptune WASP-107b. Astrophys. J. Lett. 858, L6 (2018).

  19. 19.

    Lee, E. J. & Chiang, E. To cool is to accrete: analytic scalings for nebular accretion of planetary atmospheres. Astrophys. J. 811, 41 (2015).

  20. 20.

    Kosiarek, M. R. Bright opportunities for atmospheric characterization of small planets: masses and radii of K2-3 b, c, d and GJ3470 b from radial velocity measurements and Spitzer transits. Astron. J. 157, 97 (2019).

  21. 21.

    Moses, J. I., Madhusudhan, N., Visscher, C. & Freedman, R. S. Chemical consequences of the C/O ratio on hot Jupiters: examples from WASP-12b, CoRoT-2b, XO-1b, and HD 189733b. Astrophys. J. 763, 25 (2013).

  22. 22.

    Line, M. R., Vasisht, G., Chen, P., Angerhausen, D. & Yung, Y. L. Thermochemical and photochemical kinetics in cooler hydrogen-dominated extrasolar planets: a methane-poor GJ436b? Astrophys. J. 738, 32 (2011).

  23. 23.

    Hörst, S. M. et al. Haze production rates in super-Earth and mini-Neptune atmosphere experiments. Nat. Astron 2, 303–306 (2018).

  24. 24.

    He, C. et al. Laboratory simulations of haze formation in the atmospheres of super-Earths and mini-Neptunes: particle color and size distribution. Astrophys. J. 856, L3 (2018).

  25. 25.

    Ginzburg, S., Schlichting, H. E. & Sari, R. Super-Earth atmospheres: self-consistent gas accretion and retention. Astrophys. J. 825, 29 (2016).

  26. 26.

    Mayor, M. et al. The HARPS search for southern extra-solar planets XXXIV. Occurrence, mass distribution and orbital properties of super-Earths and Neptune-mass planets. Preprint at https://arxiv.org/abs/1109.2497 (2011).

  27. 27.

    Kreidberg, L. et al. A detection of water in the transmission spectrum of the hot Jupiter WASP-12b and implications for its atmospheric composition. Astrophys. J. 814, 66 (2015).

  28. 28.

    Rogers, L. A. & Seager, S. Three possible origins for the gas layer on GJ 1214b. Astrophys. J. 716, 1208–1216 (2010).

  29. 29.

    Mousis, O. et al. Determination of the minimum masses of heavy elements in the envelopes of jupiter and saturn. Astrophys. J. 696, 1348–1354 (2009).

  30. 30.

    Inamdar, N. K. & Schlichting, H. E. Stealing the gas: giant impacts and the large diversity in exoplanet densities. Astrophys. J. 817, L13 (2016).

  31. 31.

    Bourrier, V. et al. Hubble PanCET: an extended upper atmosphere of neutral hydrogen around the warm Neptune GJ 3470b. Astron. Astrophys. 620, A147 (2018).

  32. 32.

    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).

  33. 33.

    Tsiaras, A. et al. A new approach to analysing HST spatial scans: the transmission spectrum of HD 209458b. Astrophys. J. 832, 202 (2015).

  34. 34.

    Tsiaras, A. et al. A population study of gaseous exoplanets. Astron. J. 155, 156 (2018).

  35. 35.

    Demory, B.-O. et al. Spitzer observations of GJ 3470 b: a very low-density Neptune-size planet orbiting a metal-rich M dwarf. Astrophys. J. 768, 154 (2013).

  36. 36.

    Kammer, J. A. et al. Spitzer secondary eclipse observations of five cool gas giant planets and empirical trends in cool planet emission spectra. Astrophys. J. 810, 118 (2015).

  37. 37.

    Knutson, H. A. 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).

  38. 38.

    Lewis, N. K. et al. Orbital phase variations of the eccentric giant planet HAT-P-2b. Astrophys. J. 766, 95 (2013).

  39. 39.

    Benneke, B. et al. Spitzer observations confirm and rescue the habitable-zone super-Earth K2-18b for future characterization. Astrophys. J. 834, 187 (2017).

  40. 40.

    Berta, Z. K. et al. The flat transmission spectrum of the super-Earth GJ1214b from Wide Field Camera 3 on the Hubble Space Telescope. Astrophys. J. 747, 35 (2012).

  41. 41.

    Sing, D. K. et al. Determining atmospheric conditions at the terminator of the hot-Jupiter HD 209458b. Astrophys. J. 686, 667 (2008).

  42. 42.

    Nikolov, N. et al. Hubble Space Telescope hot Jupiter transmission spectral survey: a detection of Na and strong optical absorption in HAT-P-1b. Mon. Not. R. Astron. Soc. 437, 46–66 (2014).

  43. 43.

    Sing, D. K. et al. A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion. Nature 529, 59–62 (2016).

  44. 44.

    Wakeford, H. R. et al. HAT-P-26b: a Neptune-mass exoplanet with a well-constrained heavy element abundance. Science 356, 628–631 (2017).

  45. 45.

    Lothringer, J. D. et al. An HST/STIS optical transmission spectrum of warm Neptune GJ 436b. Astron. J. 155, 66 (2018).

  46. 46.

    Deming, D. et al. Spitzer secondary eclipses of the dense, modestly-irradiated, giant exoplanet HAT-P-20b using pixel-level decorrelation. Astrophys. J. 805, 132 (2015).

  47. 47.

    Kreidberg, L. Batman: basic transit model calculation in Python. Publ. Astron. Soc. Pacif. 127, 1161 (2015).

  48. 48.

    Mandel, K. & Agol, E. Analytic light curves for planetary transit searches. Astrophys. J. Lett. 580, L171 (2002).

  49. 49.

    Husser, T.-O. et al. A new extensive library of PHOENIX stellar atmospheres and synthetic spectra. Astron. Astrophys. 553, A6 (2013).

  50. 50.

    Parviainen, H. & Aigrain, S. ldtk: limb darkening toolkit. Mon. Not. R. Astron. Soc. 453, 3821–3826 (2015).

  51. 51.

    Biddle, L. I. et al. Warm ice giant GJ 3470b—II. Revised planetary and stellar parameters from optical to near-infrared transit photometry. Mon. Not. R. Astron. Soc. 443, 1810–1820 (2014).

  52. 52.

    Demory, B.-O. et al. Spitzer observations of GJ 3470 b: a very low-density Neptune-size planet orbiting a metal-rich M dwarf. Astrophys. J. 768, 154 (2013).

  53. 53.

    Foreman-Mackey, D., Hogg, D. W., Lang, D. & Goodman, J. emcee: the MCMC hammer. Publ. Astron. Soc. Pacif. 125, 306–312 (2013).

  54. 54.

    Goodman, J. & Weare, J. Ensemble samplers with affine invariance. Commun. Appl. Math. Comput. Sci. 5, 65–80 (2010).

  55. 55.

    Sing, D. K., Vidal‐Madjar, A., Désert, J. ‐M., Lecavelier des Etangs, A. & Ballester, G. Hubble Space Telescope STIS optical transit transmission spectra of the hot Jupiter HD 209458b. Astrophys. J. 686, 658–666 (2008).

  56. 56.

    Dragomir, D. et al. Rayleigh scattering in the atmosphere of the warm exo-Neptune GJ 3470b. Astrophys. J. 814, 102 (2015).

  57. 57.

    Fisher, C. & Heng, K. Retrieval analysis of 38 WFC3 transmission spectra and resolution of the normalization degeneracy. Mon. Not. R. Astron. Soc. 481, 4698–4727 (2018).

  58. 58.

    Benneke, B. & Seager, S. Atmospheric retrieval for super-Earths: uniquely constraining the atmospheric composition with transmission spectroscopy. Astrophys. J. 753, 100 (2012).

  59. 59.

    Knutson, H. A., Benneke, B., Deming, D. & Homeier, D. A featureless transmission spectrum for the Neptune-mass exoplanet GJ 436b. Nature 505, 66–68 (2014).

  60. 60.

    Benneke, B. Strict upper limits on the carbon-to-oxygen ratios of eight hot Jupiters from self-consistent atmospheric retrieval. Preprint at https://arxiv.org/abs/1504.07655 (2015).

  61. 61.

    Parmentier, V. & Guillot, T. A non-grey analytical model for irradiated atmospheres: I. Derivation. Astron. Astrophys. 562, A133 (2014).

  62. 62.

    Line, M. R. et al. A systematic retrieval analysis of secondary eclipse spectra. I. A comparison of atmospheric retrieval techniques. Astrophys. J. 775, 137 (2013).

  63. 63.

    Gao, P. & Benneke, B. Microphysics of KCl and ZnS clouds on GJ 1214 b. Astrophys. J. 863, 165 (2018).

  64. 64.

    Morley, C. V. et al. Quantitatively assessing the role of clouds in the transmission spectrum of GJ 1214b. Astrophys. J. 775, 33 (2013).

  65. 65.

    Morley, C. V. et al. Thermal emission and reflected light spectra of super Earths with flat transmission spectra. Astrophys. J. 815, 110 (2015).

  66. 66.

    Line, M. R., Knutson, H., Deming, D., Wilkins, A. & Desert, J.-M. A near-infrared transmission spectrum for the warm Saturn HAT-P-12b. Astrophys. J. 778, 183 (2013).

  67. 67.

    Tennyson, J. & Yurchenko, S. N. ExoMol: molecular line lists for exoplanet and other atmospheres. Mon. Not. R. Astron. Soc. 425, 21–33 (2012).

  68. 68.

    Rothman, L. S. et al. HITEMP, the high-temperature molecular spectroscopic database. J. Quant. Spectrosc. Radiat. Transf. 111, 2139–2150 (2010).

  69. 69.

    Swain, M. R., Line, M. R. & Deroo, P. On the detection of molecules in the atmosphere of HD 189733b using HST NICMOS transmission spectroscopy. Astrophys. J. 784, 133 (2014).

  70. 70.

    Waldmann, I. P. et al. τ-REx. II. Retrieval of emission spectra. Astrophys. J. 813, 13 (2015).

  71. 71.

    Line, M. R. et al. No thermal inversion and a solar water abundance for the hot Jupiter HD 209458b from HST/WFC3 spectroscopy. Astron. J. 152, 203 (2016).

  72. 72.

    Moses, J. I. et al. Disequilibrium carbon, oxygen, and nitrogen chemistry in the atmospheres of HD 189733b and HD 209458b. Astrophys. J. 737, 15 (2011).

  73. 73.

    Moses, J. I. et al. On the composition of young, directly imaged giant planets. Astrophys. J. 829, 66 (2016).

  74. 74.

    Bonfils, X. et al. A hot Uranus transiting the nearby M dwarf GJ 3470. Detected with HARPS velocimetry. Captured in transit with TRAPPIST photometry. Astron. Astrophys. 546, A27 (2012).

  75. 75.

    Crossfield, I. J. M., Barman, T., Hansen, B. M. S. & Howard, A. W. Warm ice giant GJ 3470b: I. A flat transmission spectrum indicates a hazy, low-methane, and/or metal-rich atmosphere. Astron. Astrophys. 559, A33 (2013).

  76. 76.

    Dragomir, D. et al. Rayleigh scattering in the atmosphere of the warm exo-Neptune GJ 3470b. Astrophys. J. 814, 102 (2015).

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This work is based on observations with the NASA/ESA HST, obtained at the Space Telescope Science Institute (STScI) operated by AURA, Inc. We received support for the analysis by NASA through grants under the HST-GO-13665 programme (PI B.B.). This work is also based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA (PIs H.A.K. and J.-M.D.). B.B. further acknowledges financial support by the Natural Sciences and Engineering Research Council (NSERC) of Canada and the Fond de Recherche Québécois—Nature et Technologie (FRQNT; Québec). J.M. acknowledges support from NASA grant NNX16AC64G, the Amsterdam Academic Alliance (AAA) Program, and the European Research Council (ERC) under the programme Exo-Atmos (grant agreement number 679633). D. Dragomir is a NASA Hubble Fellow and acknowledges support provided by NASA through Hubble Fellowship grant HST-HF2-51372.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy for NASA, under contract NAS5-26555.

Author information

B.B. led the data analysis of the HST and Spitzer transit data, with contributions from J.L., I.W. and H.A.K. L.K. and J.-M.D. performed independent analyses of the Spitzer transits and found consistent results. H.A.K. led the data analysis of the Spitzer secondary eclipse observations. J.M. provided the chemical kinetics atmosphere models. B.B. and C.M. provided the self-consistent atmospheric models and the atmospheric retrieval analysis. B.B. wrote the manuscript, with contributions from B.J.F., H.A.K. and J.M. All authors discussed the results and commented on the draft.

Competing interests

The authors declare no competing interests.

Correspondence to Björn Benneke.

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Fig. 1: Planet mass versus equilibrium temperature for known low-mass planets with signal-to-noise ratio >3 mass measurements.
Fig. 2: Transmission spectrum of GJ 3470 b.
Fig. 3: Thermal emission spectrum of GJ 3470 b.
Fig. 4: Constraints on gas composition and cloud properties in the atmosphere of GJ 3470 b.