CHEOPS Geometric albedo measurements of benchmark hot Jupiters

By measuring the geometric albedo of a planet, the reflectivity of its atmosphere can be determined. This constitutes a vital piece of information when trying to characterise the nature of the planetary atmosphere. The albedo of a planet can be determined by measuring the drop in observed stellar flux when the planet is occulted by its host star during secondary eclipse.

We present observations of secondary eclipses of the gas giants HD189733b and HD209458b at optical wavelengths performed by the Characterising Exoplanets Satellite (CHEOPS). As both planets have moderate temperatures, the thermal contribution to their eclipse depth in the optical is small, making our observations uniquely sensitive to the reflectivity of the planetary atmospheres. Our data thus allow precise measurements of the planets’ geometric albedos, which we will present here. We will further discuss our findings and their implications in terms of the planetary atmospheric composition and the possible presence of aerosols. Finally, we compare our results with those of similar studies and discuss the implications of our findings in the context of future observations characterising gas giant atmospheres.