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Main Authors: Challener, Ryan C., Mansfield, Megan Weiner, Cubillos, Patricio E., Piette, Anjali A. A., Coulombe, Louis-Philippe, Beltz, Hayley, Blecic, Jasmina, Rauscher, Emily, Bean, Jacob L., Benneke, Björn, Kempton, Eliza M. -R., Harrington, Joseph, Komacek, Thaddeus D., Parmentier, Vivien, Casewell, S. L., Iro, Nicolas, Mancini, Luigi, Nixon, Matthew C., Radica, Michael, Steinrueck, Maria E., Welbanks, Luis, Batalha, Natalie M., Caceres, Claudio, Crossfield, Ian J. M., Crouzet, Nicolas, Désert, Jean-Michel, Molaverdikhani, Karan, Nikolov, Nikolay K., Palle, Enric, Rackham, Benjamin V., Schlawin, Everett, Sing, David K., Stevenson, Kevin B., Tan, Xianyu, Turner, Jake D., Zhang, Xi
Format: Preprint
Published: 2025
Subjects:
Earth and Planetary Astrophysics
Online Access:https://arxiv.org/abs/2510.24708
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Table of Contents:
  • 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.

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