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Water Vapor and Clouds on the Habitable-zone Sub-Neptune Exoplanet K2-18b

Benneke, Björn and Wong, Ian and Piaulet, Caroline and Knutson, Heather A. and Lothringer, Joshua and Morley, Caroline V. and Crossfield, Ian J. M. and Gao, Peter and Greene, Thomas P. and Dressing, Courtney and Dragomir, Diana and Howard, Andrew W. and McCullough, Peter R. and Kempton, Eliza M.-R. and Fortney, Jonathan J. and Fraine, Jonathan (2019) Water Vapor and Clouds on the Habitable-zone Sub-Neptune Exoplanet K2-18b. Astrophysical Journal Letters, 887 (1). Art. No. L14. ISSN 2041-8213.

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Results from the Kepler mission indicate that the occurrence rate of small planets (<3 R⊕) in the habitable zone of nearby low-mass stars may be as high as 80%. Despite this abundance, probing the conditions and atmospheric properties on any habitable-zone planet is extremely difficult and has remained elusive to date. Here, we report the detection of water vapor and the likely presence of liquid and icy water clouds in the atmosphere of the 2.6 R ⊕ habitable-zone planet K2-18b. The simultaneous detection of water vapor and clouds in the mid-atmosphere of K2-18b is particularly intriguing because K2-18b receives virtually the same amount of total insolation from its host star (1368^(+114)_(-107) W m⁻²) as the Earth receives from the Sun (1361 W m⁻²), resulting in the right conditions for water vapor to condense and explain the detected clouds. In this study we observed nine transits of K2-18b using Hubble Space Telescope/WFC3 in order to achieve the necessary sensitivity to detect the water vapor, and we supplement this data set with Spitzer and K2 observations to obtain a broader wavelength coverage. While the thick hydrogen-dominated envelope we detect on K2-18b means that the planet is not a true Earth analog, our observations demonstrate that low-mass habitable-zone planets with the right conditions for liquid water are accessible with state-of-the-art telescopes.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Benneke, Björn0000-0001-5578-1498
Wong, Ian0000-0001-9665-8429
Knutson, Heather A.0000-0002-0822-3095
Lothringer, Joshua0000-0003-3667-8633
Morley, Caroline V.0000-0002-4404-0456
Crossfield, Ian J. M.0000-0002-1835-1891
Gao, Peter0000-0002-8518-9601
Greene, Thomas P.0000-0002-8963-8056
Dressing, Courtney0000-0001-8189-0233
Dragomir, Diana0000-0003-2313-467X
Howard, Andrew W.0000-0001-8638-0320
McCullough, Peter R.0000-0001-9165-9799
Kempton, Eliza M.-R.0000-0002-1337-9051
Fortney, Jonathan J.0000-0002-9843-4354
Fraine, Jonathan0000-0003-0910-5805
Alternate Title:Water Vapor on the Habitable-Zone Exoplanet K2-18b
Additional Information:© 2019 The American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2019 September 10; revised 2019 November 20; accepted 2019 November 21; published 2019 December 10. 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 and HST-GO-14682 programs (PI Benneke). 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 Benneke and Werner). B.B. further acknowledges financial supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada and the Fond de Recherche Québécois-Nature et Technologie (FRQNT; Québec). D.D. 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, Inc., for NASA, under contract NAS5-26555.
Group:Astronomy Department
Funding AgencyGrant Number
Natural Sciences and Engineering Research Council of Canada (NSERC)UNSPECIFIED
Fonds de recherche du Québec - Nature et technologies (FRQNT)UNSPECIFIED
NASA Hubble FellowshipHST-HF2-51372.001-A
Subject Keywords:Exoplanets; Mini Neptunes; Exoplanet atmospheres; Planetary atmospheres
Issue or Number:1
Classification Code:Unified Astronomy Thesaurus concepts: Exoplanets (498); Mini Neptunes (1063); Exoplanet atmospheres (487); Planetary atmospheres (1244)
Record Number:CaltechAUTHORS:20190916-075850280
Persistent URL:
Official Citation:Björn Benneke et al 2019 ApJL 887 L14
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:98644
Deposited By: Tony Diaz
Deposited On:16 Sep 2019 16:05
Last Modified:11 Dec 2019 03:05

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