Secondary Eclipse Detection and Thermal Emission Characterization of a Hot 2.9 Earth Radii Sub-Neptune

Secondary eclipse observations of exoplanets provide unique insight into the thermal emission of exoplanets and into their atmospheric composition. With few exceptions, however, eclipse detections have largely been limited to large planets with planetary radii greater than that of Neptune. Even for Neptune size exoplanets, we only have very few eclipse detections because of the hot Neptune desert, which defines the region of rare Neptune size planets with high equilibrium temperature (on close orbits) in the exoplanet population. Here, we report the detection of the secondary eclipses of TOI-824 b, a 2.9 Earth radii hot Neptune, at 3.6 and 4.5 µm with the Spitzer Space Telescope. With its rather small radius, TOI-824 b is the third smallest exoplanet for which we have detected an eclipse to date. This detection opens the door to further, more involved, thermal emission study and atmosphere characterization with the upcoming James Webb Space Telescope, which will allow for a wider wavelength coverage. We also present our atmospheric analysis of TOI-824 b based on our Spitzer infrared eclipse observations. To do so, we integrate the disequilibrium chemistry code VULCAN into the SCARLET atmospheric modeling framework to obtain self-consistent models of the atmosphere of TOI-824 b. This method allows us to properly model the chemistry and physics of this strongly irradiated exoplanet by considering photochemistry in the upper atmosphere.