Contributed Talk - Splinter Exoplanets

Wednesday, 23 September 2020, 17:20   (virtual room D)

Role of the impact parameter in exoplanet transmission spectroscopy

X. Alexoudi, M. Mallonn, E. Keles, K. Poppenhäger, C. von Essen, K. G. Strassmeier
Leibniz Institute for Astrophysics Potsdam (AIP)

Even though transmission spectroscopy is one of the key – methods to understand exoplanetary atmospheres, the faint planetary signals do not always favor those processes. In the literature, there are reported discrepancies regarding the atmospheric characterization of individual targets and with our work, we aim to assess those inconsistencies and propose an explanation for their occurrence. More specifically, we investigated the dependence of the transmission spectra on deviations of the orbital parameters, that define the impact parameter of the star-planet system, and how an uncertainty on those parameters translates into an uncertainty on the planetary spectral slope. Our results showed a wavelength-dependent offset, driven by the limb darkening effects of the host star, that produces a steeper spectral slope with an increasing impact parameter value. As a consequence, this “impact parameter degeneracy” can transform flat spectra into sloped spectra, and vice versa. We discuss the application of the degeneracy on a sample of reported discrepancies from the literature. The “impact parameter degeneracy” cannot explain fully the inconsistencies, except the case of HAT-P-12 b, as there are probably other sources of errors to consider. We conclude that transmission spectra can be hard to interpret due to limitations in applying a precise impact parameter value for the star-planet system.