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

Abstract

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 ($2.93R_{\oplus }$ and $18.47M_{\oplus }$) 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 $4.5\mu \mathrm{m}$ 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-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.

Accepted for publication in The Astrophysical Journal, 14 pages, 5 figures

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