A Clearer View of HAT-P-1 b: JWST NIRSpec G395H Reveals Water, Carbon Dioxide, and Possibly Hydrogen Sulfide
Abstract
As part of JWST's Exoplanet Grand Tour Survey, we use panchromatic transmission spectroscopy to connect HAT-P-1 b's previously studied optical and near-infrared atmosphere to the longer-wavelength molecular bands accessible with JWST. We present JWST NIRSpec G395H transmission spectroscopy of the hot Jupiter HAT-P-1 b over 2.7--5.3~μm, and combine the new spectrum with archival HST STIS and WFC3 observations for a 0.3--5.3~μm atmospheric analysis. We independently reduce the JWST data with the Eureka!, FIREFLy, and Tswift pipelines, finding mutually consistent transmission spectra across the G395H bandpass. Atmospheric retrievals yield strong evidence for H2O and CO2 with Bayes factors of log10BH2O=8.9 and log10BCO2=52.3, while providing tentative evidence for H2S (log10BH2S=1.4). The joint H2O and CO2 constraints favor an atmosphere near chemical equilibrium, with log10 M/H=0.99+0.19−0.14, corresponding to ∼10× Solar or ∼9× relative to the near-solar metallicity host star, and a 3σ upper limit of C/O <0.52. Because H2O and CO2 provide a metallicity comparatively insensitive to vertical mixing in this temperature regime, their combined detection suggests the composition is dominated by bulk enrichment rather than strong disequilibrium transport. We find no significant evidence for clouds; instead, the persistence of molecular structure across the spectrum argues against strong cloud muting. The tentative H2S signal, if confirmed, would further suggest limited photochemical processing at the pressures probed. Together, the molecular inventory, enriched metallicity, and low C/O ratio point to an oxygen-rich atmosphere and establish HAT-P-1 b as a benchmark for comparative studies of hot-Jupiter atmospheric composition.
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BibTeX
@misc{ashtari2026clearer,
title = {{A Clearer View of HAT-P-1 b: JWST NIRSpec G395H Reveals Water, Carbon Dioxide, and Possibly Hydrogen Sulfide}},
author = {Reza Ashtari and Stephen P. Schmidt and Guangwei Fu and Avinash Verma and David K. Sing and Kevin B. Stevenson and Jayesh Goyal and Katherine A. Bennett and Joshua D. Lothringer and Jacob Lustig-Yaeger and Sagnick Mukherjee and Carlos Gascón and Natalie H. Allen and Patrick McCreery and Le-Chris Wang and Mei Ting Mak and Kristin S. Sotzen and Lakeisha M. Ramos Rosado and N. J. Mayne},
howpublished = {arXiv},
year = {2026},
month = aug,
doi = {10.48550/arxiv.2608.28538},
eprint = {2608.28538},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.28538},
note = {CC0},
}
RIS
TY - UNPB
TI - A Clearer View of HAT-P-1 b: JWST NIRSpec G395H Reveals Water, Carbon Dioxide, and Possibly Hydrogen Sulfide
AU - Reza Ashtari
AU - Stephen P. Schmidt
AU - Guangwei Fu
AU - Avinash Verma
AU - David K. Sing
AU - Kevin B. Stevenson
AU - Jayesh Goyal
AU - Katherine A. Bennett
AU - Joshua D. Lothringer
AU - Jacob Lustig-Yaeger
AU - Sagnick Mukherjee
AU - Carlos Gascón
AU - Natalie H. Allen
AU - Patrick McCreery
AU - Le-Chris Wang
AU - Mei Ting Mak
AU - Kristin S. Sotzen
AU - Lakeisha M. Ramos Rosado
AU - N. J. Mayne
PY - 2026
DA - 2026/08/28
PB - arXiv
DO - 10.48550/arxiv.2608.28538
UR - https://arxiv.org/abs/2608.28538
AB - As part of JWST's Exoplanet Grand Tour Survey, we use panchromatic transmission spectroscopy to connect HAT-P-1 b's previously studied optical and near-infrared atmosphere to the longer-wavelength molecular bands accessible with JWST. We present JWST NIRSpec G395H transmission spectroscopy of the hot Jupiter HAT-P-1 b over 2.7--5.3~μm, and combine the new spectrum with archival HST STIS and WFC3 observations for a 0.3--5.3~μm atmospheric analysis. We independently reduce the JWST data with the Eureka!, FIREFLy, and Tswift pipelines, finding mutually consistent transmission spectra across the G395H bandpass. Atmospheric retrievals yield strong evidence for H_2O and CO_2 with Bayes factors of log_10B_H_2O=8.9 and log_10B_CO_2=52.3, while providing tentative evidence for H_2S (log_10B_H_2S=1.4). The joint H_2O and CO_2 constraints favor an atmosphere near chemical equilibrium, with log_10 M/H=0.99^+0.19_−0.14, corresponding to ∼10× Solar or ∼9× relative to the near-solar metallicity host star, and a 3σ upper limit of C/O <0.52. Because H_2O and CO_2 provide a metallicity comparatively insensitive to vertical mixing in this temperature regime, their combined detection suggests the composition is dominated by bulk enrichment rather than strong disequilibrium transport. We find no significant evidence for clouds; instead, the persistence of molecular structure across the spectrum argues against strong cloud muting. The tentative H_2S signal, if confirmed, would further suggest limited photochemical processing at the pressures probed. Together, the molecular inventory, enriched metallicity, and low C/O ratio point to an oxygen-rich atmosphere and establish HAT-P-1 b as a benchmark for comparative studies of hot-Jupiter atmospheric composition.
ER -