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Is the Hot, Dense Sub-Neptune TOI-824 b an Exposed Neptune Mantle? Spitzer Detection of the Hot Dayside and Reanalysis of the Interior Composition

Roy, Pierre-Alexis and Benneke, Björn and Piaulet, Caroline and Crossfield, Ian J. M. and Kreidberg, Laura and Dragomir, Diana and Deming, Drake and Werner, Michael W. and Parmentier, Vivien and Christiansen, Jessie L. and Dressing, Courtney D. and Kane, Stephen R. and Morales, Farisa Y. (2022) Is the Hot, Dense Sub-Neptune TOI-824 b an Exposed Neptune Mantle? Spitzer Detection of the Hot Dayside and Reanalysis of the Interior Composition. Astrophysical Journal, 941 (1). Art. No. 89. ISSN 0004-637X. doi:10.3847/1538-4357/ac9f18.

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The Kepler and TESS missions revealed a remarkable abundance of sub-Neptune exoplanets. Despite this abundance, our understanding of the nature and compositional diversity of sub-Neptunes remains limited, to a large part because atmospheric studies via transmission spectroscopy almost exclusively have aimed for low-density sub-Neptunes, and even those were often affected by high-altitude clouds. The recent TESS discovery of the hot, dense TOI-824 b (2.93 R_⊕ and 18.47 M_⊕) opens a new window into sub-Neptune science by enabling the study of a dense sub-Neptune via secondary eclipses. Here, we present the detection of TOI-824 b's hot dayside via Spitzer secondary-eclipse observations in the 3.6 and 4.5 μm channels, combined with a reanalysis of its interior composition. The measured eclipse depths (142⁺⁵⁷₋₅₂ and 245⁺⁷⁵₋₇₇ ppm) and brightness temperatures (1463⁺¹⁸³₋₁₉₆ and 1484⁺¹⁸⁰₋₂₀₂ K) indicate a poor heat redistribution (f < 0.49) and a low Bond albedo (A_B < 0.26). We conclude that TOI-824 b could be an "exposed Neptune mantle": a planet with a Neptune-like water-rich interior that never accreted a hydrogen envelope or that subsequently lost it. The hot dayside temperature is then naturally explained by a high-metallicity envelope reemitting the bulk of the incoming radiation from the dayside. TOI-824 b's density is also consistent with a massive rocky core that accreted up to 1% of hydrogen, but the observed eclipse depths favor our high-metallicity general circulation model (GCM) simulation to a solar-metallicity GCM simulation with a likelihood ratio of 7:1. The new insights into TOI-824 b's nature suggest that the sub-Neptune population may be more diverse than previously thought, with some of the dense hot sub-Neptunes potentially not hosting a hydrogen-rich envelope as generally assumed for sub-Neptunes.

Item Type:Article
Related URLs:
URLURL TypeDescription
Roy, Pierre-Alexis0000-0001-6809-3520
Benneke, Björn0000-0001-5578-1498
Piaulet, Caroline0000-0002-2875-917X
Crossfield, Ian J. M.0000-0002-1835-1891
Kreidberg, Laura0000-0003-0514-1147
Dragomir, Diana0000-0003-2313-467X
Deming, Drake0000-0001-5727-4094
Werner, Michael W.0000-0003-4990-189X
Parmentier, Vivien0000-0001-9521-6258
Christiansen, Jessie L.0000-0002-8035-4778
Dressing, Courtney D.0000-0001-8189-0233
Kane, Stephen R.0000-0002-7084-0529
Morales, Farisa Y.0000-0001-9414-3851
Additional Information:We wish to thank the reviewer for the provided comments, which greatly enhanced the manuscript. We further thank L.-P. Coulombe, S. Delisle and S. Pelletier for their insightful comments and ideas on the multiple iterations of the paper. P.-A.R., B.B., and C.P. acknowledge financial support by the Natural Sciences and Engineering Research Council (NSERC) of Canada, and the Fond de Recherche Québécois Nature et Technologie (FRQNT, Québec). P.-A.R. and C.P. further acknowledge financial support from the University of Montreal, and the NSERC CREATE Technologies for Exo-Planetary Science (TEPS) program. D.D. acknowledges support from the TESS Guest Investigator Program grant No. 80NSSC19K1727 and NASA Exoplanet Research Program grant No. 18-2XRP18_2-0136. This work is based on observations made with the Spitzer Space Telescope, which was operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Natural Sciences and Engineering Research Council of Canada (NSERC)569321
Fonds de recherche du Québec - Nature et technologies (FRQNT)291038
University of MontrealUNSPECIFIED
Issue or Number:1
Record Number:CaltechAUTHORS:20230117-371388100.28
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:118834
Deposited By: Research Services Depository
Deposited On:10 Feb 2023 20:55
Last Modified:10 Feb 2023 20:55

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