Water Vapor and Clouds on the Habitable-zone Sub-Neptune Exoplanet K2-18b
Abstract
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.
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.Attached Files
Published - Benneke_2019_ApJL_887_L14.pdf
Submitted - 1909.04642.pdf
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Additional details
- Alternative title
- Water Vapor on the Habitable-Zone Exoplanet K2-18b
- Eprint ID
- 98644
- Resolver ID
- CaltechAUTHORS:20190916-075850280
- HST-GO-13665
- NASA
- HST-GO-14682
- NASA
- NASA/JPL/Caltech
- Natural Sciences and Engineering Research Council of Canada (NSERC)
- Fonds de recherche du Québec - Nature et technologies (FRQNT)
- HST-HF2-51372.001-A
- NASA Hubble Fellowship
- NAS5-26555
- NASA
- Created
-
2019-09-16Created from EPrint's datestamp field
- Updated
-
2021-11-16Created from EPrint's last_modified field
- Caltech groups
- Astronomy Department, Division of Geological and Planetary Sciences