Recent work on the characterization of small exoplanets has allowed us to accumulate growing evidence that sub-Neptunes with radii greater than ∼2.5 R⊕ often host H2/He-dominated atmospheres both from measurements of their low bulk densities and from direct detections of their low mean molecular mass atmospheres. However, the smaller sub-Neptunes in the 1.5–2.2 R⊕ size regime are much less understood and often have bulk densities that can be explained either by the H2/He-rich scenario or by a volatile-dominated composition known as the "water world" scenario. Here we report the detection of water vapor in the transmission spectrum of the 1.96 ± 0.08 R⊕ sub-Neptune GJ 9827 d obtained with the Hubble Space Telescope (HST). We observe 11 HST Wide Field Camera 3 transits of GJ 9827 d and find an absorption feature at 1.4 μm in its transit spectrum, which is best explained (at 3.39σ) by the presence of water in GJ 9827 d's atmosphere. We further show that this feature cannot be caused by unocculted starspots during the transits by combining an analysis of the K2 photometry and transit light source effect retrievals. We reveal that the water absorption feature can be similarly well explained by a small amount of water vapor in a cloudy H2/He atmosphere or a water vapor envelope on GJ 9827 d. Given that recent studies have inferred an important mass-loss rate (>0.5 M⊕ Gyr−1) for GJ 9827 d, making it unlikely to retain a H-dominated envelope, our findings highlight GJ 9827 d as a promising water world candidate that could host a volatile-dominated atmosphere. This water detection also makes GJ 9827 d the smallest exoplanet with an atmospheric molecular detection to date.
Water Absorption in the Transmission Spectrum of the Water World Candidate GJ 9827 d
- Creators
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Roy, Pierre-Alexis1
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Benneke, Björn1
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Piaulet, Caroline1
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Gully-Santiago, Michael A.2
- Crossfield, Ian J. M.3
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Morley, Caroline V.2
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Kreidberg, Laura4
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Mikal-Evans, Thomas4
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Brande, Jonathan3
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Delisle, Simon1
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Greene, Thomas P.5
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Hardegree-Ullman, Kevin K.6
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Barman, Travis6
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Christiansen, Jessie L.7
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Dragomir, Diana8
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Fortney, Jonathan J.9
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Howard, Andrew W.10
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Kosiarek, Molly R.9
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Lothringer, Joshua D.11
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1.
University of Montreal
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2.
The University of Texas at Austin
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3.
University of Kansas
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4.
Max Planck Institute for Astronomy
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5.
Ames Research Center
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6.
University of Arizona
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7.
Infrared Processing and Analysis Center
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8.
University of New Mexico
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9.
University of California, Santa Cruz
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10.
California Institute of Technology
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11.
Utah Valley University
Abstract
Copyright and License
© 2023. The Author(s). Published by the American Astronomical Society.
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Acknowledgement
All of the data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute. The specific observations analyzed can be accessed via 10.17909/dvqh-2r48.
We wish to thank the reviewer for the insightful comments that enhanced the quality of our manuscript. P.-A.R. further thanks L. Bazinet, L.-P. Coulombe, and S. Pelletier for their help, comments, and ideas throughout the multiple iterations of this work.
This work is based on observations with the NASA/ESA HST, obtained at the Space Telescope Science Institute (STScI) operated by AURA, Inc. P.-A.R., B.B., and C.P. acknowledge financial support from the Natural Sciences and Engineering Research Council (NSERC) of Canada. P.-A.R. and C.P. further acknowledge support from the University of Montreal and the Trottier Institute for Exoplanets (iREx). B.B. also acknowledges financial support from the Fond de Recherche Québécois-Nature et Technologie (FRQNT; Québec).
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Additional details
- Natural Sciences and Engineering Research Council
- Université de Montréal
- Fonds de Recherche du Québec – Nature et Technologies
- Accepted
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2023-07-29
- Available
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2023-09-12Published
- Caltech groups
- Astronomy Department, Infrared Processing and Analysis Center (IPAC), Division of Physics, Mathematics and Astronomy (PMA)
- Publication Status
- Published