Year: 2026
Type: preprint
Abstract: As part of the JWST Hot Rocks Survey, we analyzed a JWST/MIRI F1500W secondary-eclipse observation of GJ 357 b, a rocky exoplanet orbiting a nearby M dwarf. We also performed a new global analysis of the GJ 357 system, combining TESS photometry, a JWST/NIRSpec transit observation, the MIRI secondary eclipse, radial velocities, and Gaia parallax measurements to obtain a self-consistent set of stellar and planetary parameters. We measure an occultation depth of $200.5 \pm 12.7,\mathrm{ppm}$, corresponding to a dayside brightness temperature at $15,μ\mathrm{m}$ of $T_{\mathrm{b},15,μ\mathrm{m}}=923^{+39}_{-38},\mathrm{K}$. The inferred brightness temperature substantially exceeds the zero-Bond-albedo, no-redistribution, disk-integrated bolometric dayside reference temperature of $686 \pm 14,\mathrm{K}$. The measured eclipse depth differs from the tested zero-albedo, grey airless-surface model by $5.2σ$ and is also inconsistent with the high-mean-molecular-weight atmospheric models considered here, including pure CO$_2$, pure H$_2$O, and N$_2$ with 100 ppm CO$_2$. Possible explanations for the excess emission in the MIRI/F1500W band include wavelength-dependent surface emissivity, localized hot regions, or atmospheric temperature structures and opacity sources not included in the tested models. Follow-up mid-infrared eclipse spectroscopy would constrain the shape of the dayside emission spectrum and help test the proposed surface and atmospheric explanations.
Source: arXiv (Cornell University)
Authors: Zgraggen, Merlin, Demory, Brice-Olivier, Diamond-Lowe, Hannah, Mendonça, João M., Valdés, Erik Meier, Fortune, Mark, Jones, Kathryn D., Kitzmann, Daniel, Smith, Millie, Allen, Natalie H., August, Prune C., Gressier, Amélie, Holmberg, Måns, Ih, Jegug, Buchhave, Lars A., Espinoza, Néstor, Gibson, Neale P., Heng, Kevin
Institutions: University of Bern, ETH Zurich, Institute for Particle Physics and Astrophysics, Center for Space and Habitability, Space Telescope Science Institute, University of Southampton, University of Oxford, Macquarie University, Johns Hopkins University, DTU Space, Technical University of Denmark, Université de Montréal, Trinity College Dublin
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Funders: National Science Foundation, National Aeronautics and Space Administration, Space Telescope Science Institute, Science Foundation Ireland, Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung, HORIZON EUROPE Framework Programme, Staatssekretariat für Bildung, Forschung und Innovation, Agencia Estatal de Investigación
OpenAlex ID: W7214662957