[astro-ph.EP] 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 2.7–5.3μm, and combine the new spectrum with archival HST STIS and WFC3 observations for a 0.3–5.3μm 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 log10M/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.
Reza Ashtari, Stephen P. Schmidt, Guangwei Fu, Avinash Verma, David K. Sing, Kevin B. Stevenson, Jayesh Goyal, Katherine A. Bennett, Joshua D. Lothringer, Jacob Lustig-Yaeger, Sagnick Mukherjee, Carlos Gascón, Natalie H. Allen, Patrick McCreery, Le-Chris Wang, Mei Ting Mak, Kristin S. Sotzen, Lakeisha M. Ramos Rosado, N. J. Mayne
Comments: 25 pages, 11 figures, 5 tables. Accepted to The Astronomical Journal; final revision pending
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2608.28538 [astro-ph.EP] (or arXiv:2608.28538v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2608.28538
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Submission history
From: Reza Ashtari
[v1] Fri, 28 Aug 2026 17:13:07 UTC (13,413 KB)
https://arxiv.org/abs/2608.28538
Astrobiology, exoplanet,
