On Atmospheric Retrievals of Exoplanets with Inhomogeneous Terminators

The complexity of atmospheric retrieval models is largely data-driven, and one-dimensional models have generally been considered adequate with current data quality. However, recent studies have suggested that using 1D models in retrievals can result in anomalously cool terminator temperatures and biased abundance estimates even with existing transmission spectra of hot Jupiters. Motivated by these claims and upcoming high-quality transmission spectra, we systematically explore the limitations of 1D models using synthetic and current observations. We use 1D models of varying complexity, both analytic and numerical, to revisit claims of biases when interpreting transmission spectra of hot Jupiters with inhomogeneous terminator compositions. Overall, we find the reported biases to be resulting from specific model assumptions rather than intrinsic limitations of 1D atmospheric models in retrieving current observations of asymmetric terminators. Additionally, we revise atmospheric retrievals of the hot Jupiter WASP-43b (T (eq) = 1440 K) and the ultra-hot Jupiter WASP-103b (T (eq) = 2484 K), for which previous studies inferred abnormally cool atmospheric temperatures. We retrieve temperatures consistent with expectations. We note, however, that in the limit of extreme terminator inhomogeneities and high data quality, some atmospheric inferences may conceivably be biased-although to a lesser extent than previously claimed. To address such cases, we implement a 2D retrieval framework for transmission spectra that allows accurate constraints on average atmospheric properties and provides insights into the spectral ranges where the imprints of atmospheric inhomogeneities are strongest. Our study highlights the need for careful considerations of model assumptions and data quality before attributing biases in retrieved estimates to unaccounted atmospheric inhomogeneities.

Automated summary

This study systematically explores the limitations of 1D models in atmospheric retrievals using synthetic and current observations. It finds that previous reported biases are due to specific model assumptions rather than intrinsic limitations of 1D atmospheric models. The study also revises atmospheric retrievals of two hot Jupiters and introduces a 2D retrieval framework for accurate inference of average atmospheric properties and insights into the impact of atmospheric inhomogeneities on spectral ranges.