Is the hot, dense sub-Neptune TOI-824b an exposed Neptune mantle? Spitzer detection of the hot day side and reanalysis of the interior composition

• Published in: arXiv.org
• Main Authors: Pierre-Alexis, Roy, Benneke, Björn, Piaulet, Caroline, Crossfield, Ian J M, Kreidberg, Laura, Dragomir, Diana, Drake Deming, Werner, Michael W, Parmentier, Vivien, Christiansen, Jessie L, Dressing, Courtney D, Kane, Stephen R, Morales, Farisa Y
• Format: Paper Journal Article
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 21.11.2022
• Subjects: Abundance; Brightness temperature; Bulk density; Composition; Eclipses; Extrasolar planets; High altitude; Hydrogen; Likelihood ratio; Metallicity; Neptune; Physics - Earth and Planetary Astrophysics; Planetary mantles
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2211.11793

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 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-824b (\(2.93\,R_\oplus\) and \(18.47\,M_\oplus\)) 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-824b's hot day side via Spitzer secondary eclipse observations in the \(3.6\) and \(4.5\,\mathrm{\mu m}\) 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-824b 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 day-side temperature is then naturally explained by a high-metallicity envelope re-emitting the bulk of the incoming radiation from the day side. TOI-824b'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 GCM simulation to a solar-metallicity GCM simulation with a likelihood ratio of 7:1. The new insights into TOI-824b'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.