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.
Cite
@article{arxiv.2510.24708,
title = {Horizontal and vertical exoplanet thermal structure from a JWST spectroscopic eclipse map},
author = {Ryan C. Challener and Megan Weiner Mansfield and Patricio E. Cubillos and Anjali A. A. Piette and Louis-Philippe Coulombe and Hayley Beltz and Jasmina Blecic and Emily Rauscher and Jacob L. Bean and Björn Benneke and Eliza M. -R. Kempton and Joseph Harrington and Thaddeus D. Komacek and Vivien Parmentier and S. L. Casewell and Nicolas Iro and Luigi Mancini and Matthew C. Nixon and Michael Radica and Maria E. Steinrueck and Luis Welbanks and Natalie M. Batalha and Claudio Caceres and Ian J. M. Crossfield and Nicolas Crouzet and Jean-Michel Désert and Karan Molaverdikhani and Nikolay K. Nikolov and Enric Palle and Benjamin V. Rackham and Everett Schlawin and David K. Sing and Kevin B. Stevenson and Xianyu Tan and Jake D. Turner and Xi Zhang},
journal= {arXiv preprint arXiv:2510.24708},
year = {2025}
}
Comments
Published in Nature Astronomy. First two authors contributed equally