Exoplanets, -moons, -comets

A JWST Transmission Spectrum Of The Temperate Sub-Neptune TOI-732 c

By Keith Cowing

Status Report

astro-ph.EP

December 25, 2025

Filed under astro-ph.EP, atmosphere, exoplanet, imaging, JWST, M dwarf, NIRISS, spectoscopy, sub-Neptune, TOI-732, TOI-732 c

A JWST Transmission Spectrum Of The Temperate Sub-Neptune TOI-732 c — The transmission spectrum of TOI-732 c in the NIR. We show the spectrum and retrieved model fits for the one-offset retrieval, as discussed in Section 3.3. This model includes the 6 key CNO molecules (CH₄, CO₂, CO, H₂O, NH₃ and HCN) and the 2 species found to exhibit moderate preference across our retrieval cases, referred to as X, as discussed in Section 3.2. The data in orange and red show the NIRISS and NIRSpec spectra respectively, covering 0.9-2.8 µm and 2.8-5.2 µm. The retrievals were carried out using the native resolution data. For visual clarity, these are shown binned to resolutions of R ≈ 25 and R ≈ 55 for the NIRISS and NIRSpec data respectively. The NIRSpec spectrum has been vertically offset by the median retrieved value for the one-offset retrieval, by ≈ 112 ppm. In blue we show the median retrieved spectrum, with the lighter blue contours indicating the 1σ and 2σ intervals. The points in yellow show this median spectrum binned to the resolution of the observations. — astro-ph.EP

In recent years, JWST has facilitated detections of carbon-bearing molecules in the atmospheres of temperate sub-Neptunes orbiting M dwarfs, ushering in a new era in the characterization of this intriguing planetary regime.

We report the transmission spectrum of the temperate sub-Neptune TOI-732 c, observed with JWST NIRISS, NIRSpec G395H and MIRI LRS between 0.9-12 μm. The observations provide evidence for methane (CH₄) in a H₂-rich atmosphere, at a volume mixing ratio of ∼1%, and non-detection of NH₃ and HCN, along with nominal constraints on other prominent molecules H₂O, CO and CO₂, which are typically expected in H₂-rich atmospheres.

We conduct a comprehensive survey of 250 chemical species and find moderate to strong evidence (up to lnB∼5.9, 3.9σ) for additional absorption due to one or more complex molecules including higher-order hydrocarbons and/or sulfur-bearing molecules. The spectral features are strongly degenerate among these molecules and with methane, which we find at lnB=3.2−8.8 (up to 3.0−4.6σ) significance.

Two complex molecules are preferred with at least moderate evidence (lnB≳2.5) in both the near- and mid-infrared, while several others show such evidence in at least one of the two wavelength ranges. The preferred molecules are found in trace quantities on Earth, with no significant sources identified in other planetary atmospheres, requiring future work to assess their physical plausibility in this planet.

Future observations are required to resolve the degeneracies and place more robust constraints on these species. We highlight the need for further theoretical and experimental work to robustly characterize the atmospheric and internal composition of TOI-732 c and similar sub-Neptunes.

Frances E. Rigby, Nikku Madhusudhan, Subhajit Sarkar, Lorenzo Pica-Ciamarra, Måns Holmberg, Julianne I. Moses

Comments: Accepted for publication in ApJL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2512.15844 [astro-ph.EP] (or arXiv:2512.15844v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2512.15844
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Submission history
From: Madhusudhan Nikku
[v1] Wed, 17 Dec 2025 19:00:00 UTC (2,645 KB)
https://arxiv.org/abs/2512.15844

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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