An outline of the paper: Probing potassium in the atmosphere of HD 80606b with tunable filter transit spectrophotometry from the Gran Telescopio Canarias by Colón, K et al.
HD 80606 b is an exoplanet on a extremely eccentric orbit (e = 0.93) with a long orbital period. In fact HD 80606 b is the only long-period transiting planet observed orbiting around a bright target (V=9 mag star). The highly eccentric orbit causes temperature changes of about ~700 K occurring over a 6 hour period. This makes HD 80606 b a special case worth studying in detail.
Taking the first steps in characterizing the atmosphere of HD 80606 b, Knicole Colón and her team have made an attempt at determining the level of potassium absorption in the atmosphere of HD 80606 b. To do this they used tunable filters together with the GTC 10.4 m telescope (imaged below). Potassium is generally predicted to be the second strongest absorber for hot Jupiters within a certain range of atmospheric temperatures. Sodium was not probed with the GTC as the blue filter has not been commissioned yet. More on the GTC here.
Using tunable filters, the wavelength region around the core of the potassium line and slightly towards the blue was explored. No potassium line core was found. Not too surprising as the models by Fortney et al. (2010) don’t predict a significant line core feature due to a low equilibrium temperature of HD 80606 b (~500 K) during transit. The lack of the potassium line core is attributed to potassium having condensed into clouds lower in the atmosphere (due to sufficiently rapid cooling) before the time of transit. The authors do make note of having found a large change in apparent radius with wavelength. They explain that this is likely caused by potassium being present in high-speed winds driven of the exoplanets exosphere where the higher temperatures and lower pressures lead to the observed differences in radii.
The paper is rich in detail about why they observe an increasing apparent radius with wavelength instead of a potassium line core. The effect of stellar spots are considered as a possible explanation for this increase but as they repeatedly state throughout the paper, follow up observations are needed.
Spin-orbit misalignment in the HD 80606 planetary system Pont, F et al. 2009