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

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Abstract

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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Competing interests

The authors declare no competing interests.

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Acknowledgements

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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Supplementary Tables 1 and 2, Supplementary Figs. 1–17 and Supplementary references.

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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.