The Persistent Thermal Anomalies in Rocky Worlds

Observing the dayside thermal emissions of rocky exoplanets provides essential insights into their compositions and the presence of atmospheres. Even though no conclusive evidence has been found for atmospheres on small rocky exoplanets orbiting M dwarfs, recent JWST observations identified puzzling thermal emission excesses. Some rocky exoplanets orbiting M dwarfs have dayside emission temperatures higher than the theoretical maximum temperatures, which are calculated assuming stellar irradiation as the sole energy source. Therefore, the observed thermal emission excesses imply that these planets may have internal heat sources. In this work, we simulate three possible internal planetary processes that may generate excess heat in addition to stellar irradiation: residual heating from formation, tidal heating, and induction heating due to interactions with the stellar magnetic field. We found that these mechanisms, even when combined, cannot explain the observed thermal emission excesses, nor can they account for a tentative positive trend in the brightness temperature scaling factor with irradiation temperature. Our results imply that planetary internal processes are unlikely to generate remotely detectable heat, so the observed thermal excesses, if astrophysical, are likely caused by stellar contamination, surface processes, geometric effect, or other internal processes not considered in this study. The ongoing JWST-HST Rocky Worlds Director's Discretionary Time Program and the upcoming Nancy Grace Roman Space Telescope will provide more insights into the thermal emission of rocky exoplanets.