Probing the atmosphere of a sub-Jovian planet orbiting a cool dwarf

We derive the 0.01-mu m binned transmission spectrum, between 0.74 and 1.0 mu m, of WASP-80b from low-resolution spectra obtained with the Focal Reducer and low-dispersion Spectrograph 2 instrument attached to ESO's Very Large Telescope. The combination of the fact that WASP-80 is an active star, together with instrumental and telluric factors, introduces correlated noise in the observed transit light curves, which we treat quantitatively using Gaussian processes. Comparison of our results together with those from previous studies to theoretically calculated models reveals an equilibrium temperature in agreement with the previously measured value of 825 K, and a subsolar metallicity, as well as an atmosphere depleted of molecular species with absorption bands in the infrared (>> 5 sigma). Our transmission spectrum alone shows evidence for additional absorption from the potassium core and wing, whereby its presence is detected from analysis of narrow 0.003 mu m bin light curves (>> 5 sigma). Further observations with visible and near-ultraviolet filters will be required to expand this spectrum and provide more in-depth knowledge of the atmosphere. These detections are only made possible through an instrument-dependent baseline model and a careful analysis of systematics in the data.