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JWST NIRSpec finds no clear signs of an atmosphere on TOI-1685 b

Fisher, Chloe E. ; Hooton, Matthew J. ; Gressier, Amélie ; Zgraggen, Merlin ; Tian, Meng ; Heng, Kevin ; Allen, Natalie H. ; Chatterjee, Richard D. ; Morris, Brett M. and Borsato, Nicholas W. LU orcid , et al. (2026) In Monthly Notices of the Royal Astronomical Society 545(4).
Abstract

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 James Webb Space Telescope (JWST)’s Near Infrared Spectrograph (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... (More)

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 James Webb Space Telescope (JWST)’s Near Infrared Spectrograph (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 ~5σ. Pure CO2, SO2, H2O, and CH4 atmospheres, or a mixed secondary atmosphere (CO+ CO2+ SO2) could explain the data ( ∆ ln Z< 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.

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organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
planets and satellites: terrestrial planets
in
Monthly Notices of the Royal Astronomical Society
volume
545
issue
4
article number
staf2187
publisher
Oxford University Press
external identifiers
  • scopus:105027293441
ISSN
0035-8711
DOI
10.1093/mnras/staf2187
language
English
LU publication?
yes
additional info
Publisher Copyright: © The Author(s) 2025. Published by Oxford University Press on behalf of Royal Astronomical Society.
id
01055483-ae75-4f16-8eb6-fe3e839ae4e5
date added to LUP
2026-03-09 13:27:50
date last changed
2026-03-09 13:28:53
@article{01055483-ae75-4f16-8eb6-fe3e839ae4e5,
  abstract     = {{<p>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 James Webb Space Telescope (JWST)’s Near Infrared Spectrograph (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 ~5σ. Pure CO<sub>2</sub>, SO<sub>2</sub>, H<sub>2</sub>O, and CH<sub>4</sub> atmospheres, or a mixed secondary atmosphere (CO+ CO<sub>2</sub>+ SO<sub>2</sub>) could explain the data ( ∆ ln Z&lt; 3). However, pure CH<sub>4</sub> atmospheres may be physically unlikely, and the pure H<sub>2</sub>O and CO<sub>2</sub> 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.</p>}},
  author       = {{Fisher, Chloe E. and Hooton, Matthew J. and Gressier, Amélie and Zgraggen, Merlin and Tian, Meng and Heng, Kevin and Allen, Natalie H. and Chatterjee, Richard D. and Morris, Brett M. and Borsato, Nicholas W. and Espinoza, Néstor and Kitzmann, Daniel and Meier, Tobias G. and Buchhave, Lars A. and Burgasser, Adam J. and Demory, Brice Olivier and Fortune, Mark and Hoeijmakers, H. Jens and Luque, Raphael and Meier Valdés, Erik A. and Mendonça, João M. and Prinoth, Bibiana and Rathcke, Alexander D. and Taylor, Jake}},
  issn         = {{0035-8711}},
  keywords     = {{planets and satellites: terrestrial planets}},
  language     = {{eng}},
  month        = {{02}},
  number       = {{4}},
  publisher    = {{Oxford University Press}},
  series       = {{Monthly Notices of the Royal Astronomical Society}},
  title        = {{JWST NIRSpec finds no clear signs of an atmosphere on TOI-1685 b}},
  url          = {{http://dx.doi.org/10.1093/mnras/staf2187}},
  doi          = {{10.1093/mnras/staf2187}},
  volume       = {{545}},
  year         = {{2026}},
}