![The best-fit chemical equilibrium models to our co-added data performed with three different retrieval suites. For all chemical equilibrium fits, the models produced poor fits to several features on the spectrum, namely near 3 µm and between 4 − 5 µm region. For the 4.3 micron region, the differences between features are due to a combination of differences in the retrieval framework (namely prior constraints) and chemical networks used (FastChem vs GGChem) for each retrieval suite. -- [astro-ph.EP]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/08/GEMS-JWST.png?fit=1200%2C677&ssl=1)
[astro-ph.EP] HATS-6 b is one of several recently discovered Giant Exoplanets orbiting M-dwarf Stars (GEMS) and is part of a JWST survey that aims to compare bulk and atmospheric properties of these rare planets against their FGK star counterparts. HATS-6 b is a warm (Teq∼700 K), Saturn-mass (Mp∼0.3 MJ), Jupiter-radius (Rp∼1 RJ) planet that transits its star every ∼ 3 days.
In this study, we present the transmission spectrum of HATS-6 b obtained with two transits using the PRISM mode of JWST Near Infrared Spectrograph (NIRSpec), spanning a wavelength range of 0.6−5.3 um.
Analyzing these JWST observations using an iterative approach between forward modeling and free chemistry retrievals, we derive a low metallicity (log[M/H]=−1.99+0.2−0.2) sub-solar C/O (log[C/O]=−0.46+0.2−0.2) atmosphere, and find strong evidence for H2O, CH4, and NH3 at volume mixing ratios (in log[X]) of −4.88+0.25−0.24, −5.38+0.18−0.19, and −6.03+0.18−0.19, respectively.
We consistently retrieve a significantly lower Teq than predicted from the orbital configuration of HATS-6 b, which was impervious to any data reduction and retrieval choices, suggesting a non-zero bond albedo.
Our planetary interior models retrieve bulk metallicities three orders of magnitude larger than our retrieved atmospheric metallicity, also suggesting that the atmosphere is not well-mixed. We find an excess feature around 3 um, and expand on possible explanations for this, such as the presence of HCN or hydrocarbons like C2H4.
Yet, due to the degeneracies present for hydrocarbon features in this wavelength region, we do not draw any conclusions about the excess feature and instead encourage further observations and follow-up of this intriguing target.
Giannina Guzmán Caloca, Caleb I. Cañas, Nicole L. Wallack, Erin M. May, Shang-Min Tsai, Simon Müller, Ravit Helled, Shubham Kanodia, Jacob Lustig-Yaeger, Knicole D. Colón, Ian Czekala, Megan Delamer, Peter Gao, Te Han, Jessica Libby-Roberts, Suvrath Mahadevan, Anjali A. A. Piette, Guðmundur Stefánsson, Kevin B. Stevenson
Comments: 33 pages excluding appendices, 33 figures, 8 tables, 10 appendices, accepted for publication in AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2608.16990 [astro-ph.EP] (or arXiv:2608.16990v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2608.16990
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Submission history
From: Giannina Guzman Caloca
[v1] Mon, 17 Aug 2026 18:00:07 UTC (15,599 KB)
https://arxiv.org/abs/2608.16990
Astrobiology, Astronomy, Exoplanet,
