The Asymmetric Limbs of HD 209458 b Observed with JWST NIRCam F322W2/F444W
7 affiliations
- 1University of Bristol
- 2Max Planck Institute for Astronomy (MPIA)
- 3University of Exeter
- 4Imperial College London
- 5University of Bern
- 6The University of Dublin
- 7University of Chicago
Abstract
We present a reanalysis of JWST NIRCam F322W2/F444W transit observations of the hot Jupiter HD 209458 b to extract morning and evening transmission spectra. To determine whether asymmetries in the transit light curves arise from atmospheric structure, we refine the HD 209458 system parameters through a reanalysis of archival transit and radial velocity data. We then extract limb-resolved spectra using two independent light-curve modeling frameworks, Harmonica and catwoman , which yield consistent results. We interpret the transmission spectra using both 1D and 1.5D (i.e., retrievals in which each limb of the planet is modeled independently) atmospheric retrievals. From our 1.5D free-chemistry retrievals, we find a strong preference ( Δ ln Z = 2.81 ± 0.49 ) for a model in which the morning and evening limbs are retrieved independently over one in which they share atmospheric parameters, despite the additional free parameters this requires. This asymmetry is most pronounced in the retrieved cloud properties, consistent with 3D atmospheric circulation models that predict longitudinally varying cloud distributions, and to a lesser extent in the CH ₄ and CO abundances. In contrast, the H ₂ O and CO ₂ abundances and the isothermal temperature structure remain largely consistent between limbs. Similarly, our 1.5D equilibrium chemistry retrieval reveals a uniform composition between the limbs, alongside a marginally hotter evening limb relative to the morning limb. While the retrieved temperatures, chemical abundances, metallicities, and carbon-to-oxygen ratios (C/O) are uniform across both limbs, they exhibit significant differences from our 1D retrieval results, highlighting the importance of multidimensional modeling in mitigating 1D retrieval biases.
Related
- A Terrestrial Gamma-ray Flash with an Asymmetric Footprint Observed at the Pierre Auger Observatory
- Recovering the Morning and Evening Limbs of WASP-94Ab from Its Limb-Averaged JWST Transmission Spectrum
- Model-Based Retrieval of Four Years of JWST/NIRCam Wavefronts
- geko : A tool for modelling galaxy kinematics and morphology in JWST /NIRCam slitless spectroscopic observations
- Increasing Sensitivity to Trailed Solar System Objects in Archival JWST NIRCam Imaging with Group Differencing
BibTeX
@article{maguire2026asymmetric,
title = {{The Asymmetric Limbs of HD 209458 b Observed with JWST NIRCam F322W2/F444W}},
author = {Cathal Maguire and Eva-Maria Ahrer and Charlotte Fairman and Hannah R. Wakeford and Duncan A. Christie and James Kirk and Elspeth K.H. Lee and Evert Nasedkin and Michael Radica},
journal = {The Astrophysical Journal Letters},
year = {2026},
month = oct,
doi = {10.3847/2041-8213/aeac49},
url = {https://iopscience.iop.org/article/10.3847/2041-8213/aeac49},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - The Asymmetric Limbs of HD 209458 b Observed with JWST NIRCam F322W2/F444W
AU - Cathal Maguire
AU - Eva-Maria Ahrer
AU - Charlotte Fairman
AU - Hannah R. Wakeford
AU - Duncan A. Christie
AU - James Kirk
AU - Elspeth K.H. Lee
AU - Evert Nasedkin
AU - Michael Radica
PY - 2026
DA - 2026/10/12
JO - The Astrophysical Journal Letters
DO - 10.3847/2041-8213/aeac49
UR - https://iopscience.iop.org/article/10.3847/2041-8213/aeac49
AB - We present a reanalysis of JWST NIRCam F322W2/F444W transit observations of the hot Jupiter HD 209458 b to extract morning and evening transmission spectra. To determine whether asymmetries in the transit light curves arise from atmospheric structure, we refine the HD 209458 system parameters through a reanalysis of archival transit and radial velocity data. We then extract limb-resolved spectra using two independent light-curve modeling frameworks, Harmonica and catwoman , which yield consistent results. We interpret the transmission spectra using both 1D and 1.5D (i.e., retrievals in which each limb of the planet is modeled independently) atmospheric retrievals. From our 1.5D free-chemistry retrievals, we find a strong preference ( Δ ln Z = 2.81 ± 0.49 ) for a model in which the morning and evening limbs are retrieved independently over one in which they share atmospheric parameters, despite the additional free parameters this requires. This asymmetry is most pronounced in the retrieved cloud properties, consistent with 3D atmospheric circulation models that predict longitudinally varying cloud distributions, and to a lesser extent in the CH ₄ and CO abundances. In contrast, the H ₂ O and CO ₂ abundances and the isothermal temperature structure remain largely consistent between limbs. Similarly, our 1.5D equilibrium chemistry retrieval reveals a uniform composition between the limbs, alongside a marginally hotter evening limb relative to the morning limb. While the retrieved temperatures, chemical abundances, metallicities, and carbon-to-oxygen ratios (C/O) are uniform across both limbs, they exhibit significant differences from our 1D retrieval results, highlighting the importance of multidimensional modeling in mitigating 1D retrieval biases.
SN - 2041-8205
SN - 2041-8213
ER -