apjl The Astrophysical Journal Letters APJL Astrophys. J. Lett. 2041-8205 2041-8213 The American Astronomical Society apjlab7229 10.3847/2041-8213/ab7229 ab7229 AAS20413 The Interior and Atmosphere of the Habitable-zone Exoplanet K2-18b The Interior and Atmosphere of the Habitable-zone Exoplanet K2-18b Nikku Madhusudhan, Matthew C. Nixon, Luis Welbanks, Anjali A. A. Piette, and Richard A. Booth Madhusudhan Nikku 0000-0002-4869-000X Nixon Matthew C. 0000-0001-8236-5553 Welbanks Luis 0000-0003-0156-4564 Piette Anjali A. A. 0000-0002-4487-5533 Booth Richard A. 0000-0002-0364-937X Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK; nmadhu@ast.cam.ac.uk

Madhusudhan et al.

1 3 2020 27 02 2020 27 02 2020 891 1 L7 27 9 2019 28 1 2020 31 1 2020 31 01 2020 © 2020. The Author(s). Published by the American Astronomical Society. 2020 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. Abstract

Exoplanets orbiting M-dwarfs present a valuable opportunity for their detection and atmospheric characterization. This is evident from recent inferences of H2O in such atmospheres, including that of the habitable-zone exoplanet K2-18b. With a bulk density between Earth and Neptune, K2-18b may be expected to possess a H/He envelope. However, the extent of such an envelope and the thermodynamic conditions of the interior remain unexplored. In the present work, we investigate the atmospheric and interior properties of K2-18b based on its bulk properties and its atmospheric transmission spectrum. We constrain the atmosphere to be H2-rich with a H2O volume mixing ratio of 0.02%–14.8%, consistent with previous studies, and find a depletion of CH4 and NH3, indicating chemical disequilibrium. We do not conclusively detect clouds/hazes in the observable atmosphere. We use the bulk parameters and retrieved atmospheric properties to constrain the internal structure and thermodynamic conditions in the planet. The constraints on the interior allow multiple scenarios between rocky worlds with massive H/He envelopes and water worlds with thin envelopes. We constrain the mass fraction of the H/He envelope to be ≲6%; spanning ≲10−5 for a predominantly water world to ∼6% for a pure iron interior. The thermodynamic conditions at the surface of the H2O layer range from the supercritical to liquid phases, with a range of solutions allowing for habitable conditions on K2-18b. Our results demonstrate that the potential for habitable conditions is not necessarily restricted to Earth-like rocky exoplanets.

unified-astronomy-thesaurus http://astrothesaurus.org/uat/487 Exoplanet atmospheres 487 http://astrothesaurus.org/uat/1248 Planetary interior 1248 http://astrothesaurus.org/uat/2021 Exoplanet atmospheric composition 2021 http://astrothesaurus.org/uat/496 Exoplanet surface characteristics 496 http://astrothesaurus.org/uat/695 Habitable planets 695 http://astrothesaurus.org/uat/696 Habitable zone 696 Exoplanet atmospheres Planetary interior Exoplanet atmospheric composition Exoplanet surface characteristics Habitable planets Habitable zone Science and Technology Facilities Council, UK Gates Cambridge Trust Bill and Melinda Gates Foundation OPP1144 ccc 2041-8205/20/L7+6$33.00 crossmark yes