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

Abstract

We derive the 0.01 µm binned transmission spectrum, between 0.74 and 1.0 µm, of WASP-80b from low resolution spectra obtained with the FORS2 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 825K, and a sub-solar metallicity, as well as an atmosphere depleted of molecular species with absorption bands in the IR ( 5σ). 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 µm bin light curves ( 5σ). Further observations with visible and near-UV 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.

The GTC exoplanet transit spectroscopy survey

“…Especially, we do not detect significant Na or K absorption, and, our results do not agree with the detection of potassium by Sedaghati et al (2017).…”

“…Both of our divide-by-white approaches are consistent with each other, as are the transmission spectra estimated separately from the two observing runs (as discussed below in Sects. 7.3 and 7.4), which leads us to believe that the strong K I signal reported by Sedaghati et al (2017) is due to systematics, even though the analysis described in their paper seems rigorous in all standards.…”

“…This very likely places the planet into the pL class (no temperature inversion) in the classification by Fortney et al (2008). The main spectroscopic features in the visible passband for pL class planets are expected to be from Rayleigh scattering and K I and Na I resonance doublet absorption, of which K I absorption detection was recently claimed by Sedaghati et al (2017) based on transmission spectroscopy analysis carried out with the FORS2 spectrograph installed in the VLT.…”

“…7.6). Sedaghati et al (2017) reported of a detection of strong K I absorption in WASP-80b's atmosphere based on transmission spectroscopy with the FORS2 spectrograph. Our results disagree with the reported K I detection, as shown in Fig.…”

Distinguishing the albedo of exoplanets from stellar activity

“…From the planetary primary transit, it is possible to derive the transmission spectrum of a planet: different transit depths in different wavelengths imply that the planetary radius varies with the wavelength (Burrows 2014;Kirk et al 2017;Sedaghati et al 2017), and they imply different interactions of the atmosphere with the stellar flux. Significant peaks at certain wavelengths are justified with stronger absorptions by certain molecules (e.g., Howe & Burrows 2012;Kreidberg et al 2015;Deming et al 2013;Charbonneau et al 2008).…”