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

December 2022 • 2022ApJ...941...89R

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.

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 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 ${}_{-52}^{+57}$ 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.


IPAC Authors


Jessie Christiansen

Associate Scientist