2D Retrieval Frameworks for Hot Jupiter Phase Curves

TL;DR

This paper introduces a 2D retrieval framework for hot Jupiter phase curves, improving atmospheric property inferences by capturing phase-dependent geometry, and demonstrates its advantages over traditional 1D models through simulations and real data application.

Contribution

The paper develops a novel 2D retrieval approach for exoplanet phase curves, addressing biases in 1D models and enhancing accuracy in atmospheric characterization.

Findings

2D models reduce biases in molecular abundance estimates.

Joint fitting of all phases improves abundance precision by a factor of 2.

2D retrievals are strongly favored for JWST data.

Abstract

Spectroscopic phase curves provide unique access to the three-dimensional properties of transiting exoplanet atmospheres. However, a modeling framework must be developed to deliver accurate inferences of atmospheric properties for these complex data sets. Here, we develop an approach to retrieve temperature structures and molecular abundances from phase curve spectra at any orbital phase. In the context of a representative hot Jupiter with a large day-night temperature contrast, we examine the biases in typical one-dimensional (1D) retrievals as a function of orbital phase/geometry, compared to two-dimensional (2D) models that appropriately capture the disk-integrated phase geometry. We guide our intuition by applying our new framework on a simulated HST+Spitzer phase curve data set in which the "truth" is known, followed by an application to the spectroscopic phase curve of the canonical hot Jupiter, WASP-43b. We also demonstrate the retrieval framework on simulated JWST phase curve observations. We apply our new geometric framework to a joint-fit of all spectroscopic phases, assuming longitudinal molecular abundance homogeneity, resulting in an a factor of 2 improvement in abundances precision when compared to individual phase constraints. With a 1D retrieval model on simulated HST+Spitzer data, we find strongly biased molecular abundances for CH$_4$ and CO$_2$ at most orbital phases. With 2D, the day and night profiles retrieved from WASP-43b remain consistent throughout the orbit. JWST retrievals show that a 2D model is strongly favored at all orbital phases. Based on our new 2D retrieval implementation, we provide recommendations on when 1D models are appropriate and when more complex phase geometries involving multiple TP profiles are required to obtain an unbiased view of tidally locked planetary atmospheres.