Hot Rocks Survey VI: An anomalously hot dayside for the rocky planet GJ 357 b

@misc{zgraggen2026rocks,
  title = {{Hot Rocks Survey VI: An anomalously hot dayside for the rocky planet GJ 357 b}},
  author = {Merlin Zgraggen and Brice-Olivier Demory and Hannah Diamond-Lowe and João M. Mendonça and Erik Meier Valdés and Mark Fortune and Kathryn D. Jones and Daniel Kitzmann and Millie Smith and Natalie H. Allen and Prune C. August and Amélie Gressier and Måns Holmberg and Jegug Ih and Lars A. Buchhave and Néstor Espinoza and Neale P. Gibson and Kevin Heng},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.30552},
  eprint = {2609.30552},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.30552},
}

TY  - UNPB
TI  - Hot Rocks Survey VI: An anomalously hot dayside for the rocky planet GJ 357 b
AU  - Merlin Zgraggen
AU  - Brice-Olivier Demory
AU  - Hannah Diamond-Lowe
AU  - João M. Mendonça
AU  - Erik Meier Valdés
AU  - Mark Fortune
AU  - Kathryn D. Jones
AU  - Daniel Kitzmann
AU  - Millie Smith
AU  - Natalie H. Allen
AU  - Prune C. August
AU  - Amélie Gressier
AU  - Måns Holmberg
AU  - Jegug Ih
AU  - Lars A. Buchhave
AU  - Néstor Espinoza
AU  - Neale P. Gibson
AU  - Kevin Heng
PY  - 2026
DA  - 2026/09/24
PB  - arXiv
DO  - 10.48550/arxiv.2609.30552
UR  - https://arxiv.org/abs/2609.30552
AB  - 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 ± 12.7,ppm, corresponding to a dayside brightness temperature at 15,μm of T_b,15,μm=923^+39_−38,K. The inferred brightness temperature substantially exceeds the zero-Bond-albedo, no-redistribution, disk-integrated bolometric dayside reference temperature of 686 ± 14,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_2O, 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.
ER  -