Atmospheres, Climate, Weather

JWST NIRSpec Finds No Clear Signs Of An Atmosphere On TOI-1685 b

By Keith Cowing
Status Report
astro-ph.EP
December 18, 2025
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JWST NIRSpec Finds No Clear Signs Of An Atmosphere On TOI-1685 b
[LEFT] The light curves from the NIRSpec G395H observations of TOI-1685, with the five visits shown chronologically from left to right. Top panel: raw, undetrended light curves at wavelength resolution of 6.5 nm, which was used in our adopted fit. NRS1 is shown in blue colours and NRS2 is shown in red colours, with the darkening in the centre of each panel corresponding to the flux drop during the transit of TOI-1685 b. Middle panel: Raw, undetrended NRS1 and NRS2 white light curves for each visit. Unbinned data are shown in colour, with data in bins of 9 minutes shown in empty black markers, with error bars generally too small to be visible. Bottom panel: Light curves in 6 wavelength bins across each detector in the same structure as the middle panel, with the detectors separated by a dashed line. [RIGHT] The same light curves and layout but detrended (divided by the corresponding baseline model in each case). — astro-ph.EP

Determining the prevalence of atmospheres on terrestrial planets is a core objective in exoplanetary science. While M dwarf systems offer a promising opportunity, conclusive observations of terrestrial atmospheres have remained elusive, with many yielding flat transmission spectra.

We observe four transits of the hot terrestrial planet TOI-1685 b using JWST’s NIRSpec G395H instrument. Combining this with the transit from the previously-observed phase curve of the planet with the same instrument, we perform a detailed analysis to determine the possibility of an atmosphere on TOI-1685 b.

From our retrievals, the Bayesian evidence favours a simple flat line model, indicating no evidence for an atmosphere on TOI-1685 b, in line with results from the phase curve analysis. Our results show that hydrogen-dominated atmospheres can be confidently ruled out. For heavier, secondary atmospheres we find a lower limit on the mean molecular weight of ~10, at a significance of ~5 sigma.

Pure CO2, SO2, H2O, and CH4 atmospheres, or a mixed secondary atmosphere (CO+CO2+SO2) could explain the data (Delta lnZ < 3). However, pure CH4 atmospheres may be physically unlikely, and the pure H2O and CO2 cases require a high-altitude cloud, which could also be interpreted as a thin cloud-free atmosphere.

We discuss the theoretical possibility for different types of atmosphere on this planet, and consider the effects of atmospheric escape and stellar activity on the system. Though we find that TOI-1685 b is likely a bare rock, this study also highlights the challenges of detecting secondary atmospheres on rocky planets with JWST.

Chloe E. Fisher, Matthew J. Hooton, Amélie Gressier, Merlin Zgraggen, Meng Tian, Kevin Heng, Natalie H. Allen, Richard D. Chatterjee, Brett M. Morris, Nicholas W. Borsato, Néstor Espinoza, Daniel Kitzmann, Tobias G. Meier, Lars A. Buchhave, Adam J. Burgasser, Brice-Olivier Demory, Mark Fortune, H. Jens Hoeijmakers, Raphael Luque, Erik A. Meier Valdés, João M. Mendonça, Bibiana Prinoth, Alexander D. Rathcke, Jake Taylor

Comments: 18 pages, 14 figures, 3 tables; Accepted for publication in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2512.15338 [astro-ph.EP] (or arXiv:2512.15338v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2512.15338
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Submission history
From: Chloe Fisher
[v1] Wed, 17 Dec 2025 11:36:39 UTC (15,613 KB)
https://arxiv.org/abs/2512.15338

Astrobiology, exoplanet,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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