The Role of Extended-Source Geometry and Diurnal Cycles on Exoplanetary Thermal Baselines: Reconciling the Brightness Temperature of 55 Cancri e

Abstract

This study augments the geometry-based InstellCa code that calculates accurate irradiance values to InstellCa-2.0, which computes the longitude averaged instellation over a given latitude on the planet along with its thermal profile. The estimation of instellation is performed considering the planet as a 3D body that rotates with respect to the star with a varying zenith angle of the star across the diurnal cycle. We focus on the implications of irradiation geometry on a specific class of ultra-short period (USP) rocky planets that may exhibit asynchronous rotation. This serves as a specialised case study to demonstrate how rotation and proximity to host-stars can affect the estimated brightness temperatures of bare rocky worlds. 55 Cancri e is selected specifically to demonstrate the importance of this effect as it has been earlier hypothesised to exhibit asynchronous rotation and also has a debated scientific discourse about the existence of an atmosphere on the planet. The results show excellent agreement of the hemisphere-averaged brightness temperature calculated through the geometric model using the bare rocky planet and Bond albedo (A_B = 0.3) assumption, with the highly precise JWST MIRI brightness temperature estimate for 55 Cancri e (1796 K). This naturally offers an explanation of the observed thermal imprint, which elegantly reconciles previous atmospheric models that have been suggested for the planet.