Retrieval analysis of 38 WFC3 transmission spectra and resolution of the normalisation degeneracy

TL;DR

This study reanalyzes 38 WFC3 exoplanet transmission spectra, demonstrating that the normalization degeneracy can be partially resolved with WFC3 data alone and fully with JWST-like spectra, revealing insights into atmospheric compositions.

Contribution

It provides a comprehensive hierarchical retrieval analysis of WFC3 spectra, exploring the normalization degeneracy and the impact of cloud and temperature models on atmospheric characterization.

Findings

Normalization degeneracy can be partially broken with WFC3 data alone.

JWST-like spectra can fully resolve the normalization degeneracy.

Most spectra are consistent with simple isothermal, grey cloud models.

Abstract

A comprehensive analysis of 38 previously published Wide Field Camera 3 (WFC3) transmission spectra is performed using a hierarchy of nested-sampling retrievals: with versus without clouds, grey versus non-grey clouds, isothermal versus non-isothermal transit chords and with water, hydrogen cyanide and/or ammonia. We revisit the "normalisation degeneracy": the relative abundances of molecules are degenerate at the order-of-magnitude level with the absolute normalisation of the transmission spectrum. Using a suite of mock retrievals, we demonstrate that the normalisation degeneracy may be partially broken using WFC3 data alone, even in the absence of optical/visible data and without appealing to the presence of patchy clouds, although lower limits to the mixing ratios may be prior-dominated depending on the measurement uncertainties. With James Webb Space Telescope-like spectral resolutions, the normalisation degeneracy may be completely broken from infrared spectra alone. We find no trend in the retrieved water abundances across nearly two orders of magnitude in exoplanet mass and a factor of 5 in retrieved temperature (about 500 to 2500 K). We further show that there is a general lack of strong Bayesian evidence to support interpretations of non-grey over grey clouds (only for WASP-69b and WASP-76b) and non-isothermal over isothermal atmospheres (no objects). 35 out of 38 WFC3 transmission spectra are well-fitted by an isothermal transit chord with grey clouds and water only, while 8 are adequately explained by flat lines. Generally, the cloud composition is unconstrained.