Evidence for Morning-to-Evening Limb Asymmetry on the Cool Low-Density Exoplanet WASP-107b

TL;DR

This study presents evidence of morning-to-evening limb asymmetry in the atmosphere of the cool exoplanet WASP-107b using JWST data, challenging expectations that such asymmetry is limited to ultra-hot planets.

Contribution

First detection of limb asymmetry in a cool exoplanet, expanding understanding of atmospheric heterogeneity beyond ultra-hot giants.

Findings

WASP-107b shows a ~100 K temperature difference between limbs.

The evening limb is hotter than the morning limb.

Results suggest limb asymmetry may be common in diverse exoplanet atmospheres.

Abstract

The atmospheric properties of hot exoplanets are expected to be different between the morning and the evening limb due to global atmospheric circulation. Ground-based observations at high spectral resolution have detected this limb asymmetry in several ultra-hot (>2000 K) exoplanets, but the prevalence of the phenomenon in the broader exoplanetary population remains unexplored. Here we use JWST/NIRCam transmission spectra between 2.5 and 4.0 $\mu$m to find evidence of limb asymmetry on exoplanet WASP-107 b. With its equilibrium temperature of 770 K and low density of 0.126 gm c$^{-3}$, WASP-107 b probes a very different regime compared to ultra-hot giant planets and was not expected to exhibit substantial spatial heterogeneity according to atmospheric models. We infer instead a morning-evening temperature difference on the order of 100 K with a hotter evening limb. Further observations on other cooler exoplanets are needed to determine whether WASP-107 b is an outlier or the models underestimate the presence of limb asymmetry in exoplanets.