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

• Published in: The Astrophysical journal 2022-12, Vol.941 (1), p.89
• Main Authors: Roy, Pierre-Alexis, Benneke, Björn, Piaulet, Caroline, Crossfield, Ian J. M., Kreidberg, Laura, Dragomir, Diana, Deming, Drake, Werner, Michael W., Parmentier, Vivien, Christiansen, Jessie L., Dressing, Courtney D., Kane, Stephen R., Morales, Farisa Y.
• Format: Article
• Language: English
• Subjects: Abundance; Albedo; Astrophysics; Brightness temperature; Bulk density; Clouds; Composition; Eclipses; Exoplanet atmospheres; Exoplanets; Extrasolar planets; General circulation models; High altitude; Hot Neptunes; Hydrogen; Likelihood ratio; Metallicity; Mini Neptunes; Neptune; Planetary mantles; Radiation; Simulation; Spectroscopy
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ac9f18

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 μ m channels, combined with a reanalysis of its interior composition. The measured eclipse depths (142 − 52 + 57 and 245 − 77 + 75 ppm) and brightness temperatures (1463 − 196 + 183 and 1484 − 202 + 180 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.