Title: A cautionary tale: limitations of a brightness-based spectroscopic approach to chromatic exoplanet radii,
Abstract: Determining wavelength-dependent exoplanet radii measurements is an excellent
way to probe the composition of exoplanet atmospheres. In light of this, Borsa
et al. (2016) sought to develop a technique to obtain such measurements by
comparing ground-based transmission spectra to the expected brightness
variations during an exoplanet transit. However, we demonstrate herein that
this is not possible due to the transit light curve normalisation necessary to
remove the effects of the Earth's atmosphere on the ground-based observations.
This is because the recoverable exoplanet radius is set by the planet-to-star
radius ratio within the transit light curve; we demonstrate this both
analytically and with simulated planet transits, as well as through a
reanalysis of the HD 189733b data.