Temporal albedo variability in the phase curve of KELT-1b

Abstract: The dayside brightness spectrum of a highly irradiated transiting brown dwarf KELT-1b has been shown to be challenging to explain with the current brown dwarf atmosphere models. The spectrum has been measured from observations spanning ten years and covering high-precision secondary eclipses and phase curves from space in blue-visible (CHaracterising ExOPlanet Satellite, CHEOPS), red-visible (Transiting Exoplanet Survey Satellite, TESS), and near-infrared (Spitzer), as well as secondary eclipse observations in near-infrared from the ground. First, the dayside of KELT-1b was observed to be brighter in the TESS passband than expected based on earlier near-infrared phase curve observations with Spitzer, and recently, the dayside was observed to be extremely dark in the CHEOPS passband. While several theories have been proposed to reconcile the discrepancy between the TESS and Spitzer bands, explaining the difference between the largely overlapping CHEOPS and TESS bands has proven to be more difficult. Here, I model the TESS photometry from Sector 17 together with the new TESS photometry from Sector 57 and show that the discrepancies in KELT-1b's dayside brightness spectrum are best explained by temporal variability in KELT-1b's albedo. This variability is most likely due to changing silicate cloud coverage on the brown dwarf's dayside, that is, weather.