A cautionary tale: limitations of a brightness-based spectroscopic approach to chromatic exoplanet radii

TL;DR

This paper shows that a brightness-based spectroscopic method for measuring wavelength-dependent exoplanet radii is fundamentally limited by transit light curve normalization, invalidating previous approaches that relied on this technique.

Contribution

It analytically and empirically demonstrates the limitations of a brightness-based spectroscopic approach for exoplanet radius measurements, challenging prior methods.

Findings

Brightness-based method cannot reliably measure wavelength-dependent radii

Transit light curve normalization removes the spectral signal

Reanalysis of HD 189733b data confirms the limitation

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.