Horizontal and vertical exoplanet thermal structure from a JWST spectroscopic eclipse map

Abstract

Highly-irradiated giant exoplanets known as "ultra-hot Jupiters" are anticipated to exhibit large variations of atmospheric temperature and chemistry as a function of longitude, latitude, and altitude. Previous observations have hinted at these variations, but the existing data have been fundamentally restricted to probing hemisphere-integrated spectra, thereby providing only coarse information on atmospheric gradients. Here we present a spectroscopic eclipse map of an extrasolar planet, resolving the atmosphere in multiple dimensions simultaneously. We analyze a secondary eclipse of the ultra-hot Jupiter WASP-18b observed with the NIRISS instrument on JWST. The mapping reveals weaker longitudinal temperature gradients than were predicted by theoretical models, indicating the importance of hydrogen dissociation and/or nightside clouds in shaping global thermal emission. Additionally, we identify two thermally distinct regions of the planet's atmosphere: a "hotspot" surrounding the substellar point and a "ring" near the dayside limbs. The hotspot region shows a strongly inverted thermal structure due to the presence of optical absorbers and a water abundance marginally lower than the hemispheric average, in accordance with theoretical predictions. The ring region shows colder temperatures and poorly constrained chemical abundances. Similar future analyses will reveal three-dimensional thermal, chemical, and dynamical properties of a broad range of exoplanet atmospheres.