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

Benneke, Björn and Knutson, Heather A. and Lothringer, Joshua and Crossfield, Ian J. M. and Moses, Julianne I. and Morley, Caroline and Kreidberg, Laura and Fulton, Benjamin J. and Dragomir, Diana and Howard, Andrew W. and Wong, Ian and Désert, Jean-Michel and McCullough, Peter R. and Kempton, Eliza M.-R. and Fortney, Jonathan and Gilliland, Ronald and Deming, Drake and Kammer, Joshua (2019) A sub-Neptune exoplanet with a low-metallicity methane-depleted atmosphere and Mie-scattering clouds. Nature Astronomy, 3 (9). pp. 813-821. ISSN 2397-3366. https://resolver.caltech.edu/CaltechAUTHORS:20190424-140933918

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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 formation. Mass and radius measurements indicate a diversity in bulk composition much wider than for gas giants; however, direct spectroscopic detections of molecular absorption and constraints on the gas mixing ratios have largely remained limited to planets more massive than Neptune. 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⊕) and exoplanets known to have rocky densities (7 M⊕). 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 CH_4/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.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1038/s41550-019-0800-5DOIArticle
https://rdcu.be/bJjoVPublisherFree ReadCube access
ORCID:
AuthorORCID
Benneke, Björn0000-0001-5578-1498
Knutson, Heather A.0000-0002-0822-3095
Lothringer, Joshua0000-0003-3667-8633
Crossfield, Ian J. M.0000-0002-1835-1891
Moses, Julianne I.0000-0002-8837-0035
Morley, Caroline0000-0002-4404-0456
Kreidberg, Laura0000-0003-0514-1147
Fulton, Benjamin J.0000-0003-3504-5316
Dragomir, Diana0000-0003-2313-467X
Howard, Andrew W.0000-0001-8638-0320
Wong, Ian0000-0001-9665-8429
Désert, Jean-Michel0000-0002-0875-8401
McCullough, Peter R.0000-0001-9165-9799
Kempton, Eliza M.-R.0000-0002-1337-9051
Fortney, Jonathan0000-0002-9843-4354
Gilliland, Ronald0000-0002-1554-5578
Deming, Drake0000-0001-5727-4094
Kammer, Joshua0000-0002-3441-3757
Additional Information:© 2019 Springer Nature Publishing AG. Received 27 December 2018; Accepted 01 May 2019; Published 01 July 2019. 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. 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/). Author Contributions: 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. The authors declare no competing interests.
Funders:
Funding AgencyGrant Number
NASAGO-13665
NASA/JPL/CaltechUNSPECIFIED
Natural Sciences and Engineering Research Council of Canada (NSERC)UNSPECIFIED
Fonds de recherche du Québec – Nature et technologies (FRQNT)UNSPECIFIED
NASANNX16AC64G
Amsterdam Academic Alliance (AAA)UNSPECIFIED
European Research Council (ERC)679633
NASA Hubble FellowshipHST-HF2-51372.001-A
NASANAS5-26555
Subject Keywords:Exoplanets
Issue or Number:9
Record Number:CaltechAUTHORS:20190424-140933918
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20190424-140933918
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:94933
Collection:CaltechAUTHORS
Deposited By: George Porter
Deposited On:08 Jul 2019 18:15
Last Modified:06 Nov 2019 18:02

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