Weak evidence for variable occultation depth of 55 Cnc e with TESS

Abstract

Context.55 Cnc e is in a 0.73 day orbit transiting a Sun-like star. It has been observed that the occultation depth of this super-Earth, with a mass of 8M⊕and radius of 2R⊕, changes significantly over time at mid-infrared wavelengths. Observations withSpitzermeasured a change in its day-side brightness temperature of 1200 K, possibly driven by volcanic activity, magnetic star-planet interaction, or the presence of a circumstellar torus of dust.Aims.Previous evidence for the variability in occultation was in the infrared range. Here we aim to explore if the variability also exists in the optical range.Methods.The Transiting Exoplanet Survey Satellite (TESS) observed 55 Cnc during sectors 21, 44, and 46. We carefully detrended the data and fitted a transit and occultation model for each sector in a Markov chain Monte Carlo (MCMC) routine. In a later stage, we used the leave-one-out (LOO) cross-validation statistic to compare with a model of constant occultation for the complete set and a model with no occultation.Results.We report an occultation depth of 8 ± 2.5 ppm for the complete set of TESS observations. In particular, we measured a depth of 15 ± 4 ppm for sector 21, while for sector 44 we detected no occultation. In sector 46 we measured a weak occultation of 8 ± 5 ppm. The occultation depth varies from one sector to the next between 1.6 and 3.4σsignificance. We derived the possible contribution on reflected light and thermal emission and set an upper limit on the geometric albedo. Based on our model comparison, the presence of an occultation is favoured considerably over no occultation, and the model with varying occultation across sectors takes most of the statistical weight.Conclusions.Our analysis confirms a detection of the occultation in TESS. Moreover, our results weakly lean towards a varying occultation depth between each sector, while the transit depth is constant across visits.