[astro-ph.EP] Thousands of sub-Neptunes have been discovered mainly through space-based surveys such as Kepler and TESS. Their bulk compositions and internal structures are thought to reflect their formation and evolutionary pathways, and atmospheric observations provide constraints on these processes.
The near-infrared helium triplet is a potential tracer of extended, escaping H/He atmospheres. Recent models that include geometric effects suggest that planets orbiting nearby late M dwarfs may offer favorable conditions for detecting this signal. Nevertheless, helium has been reported for only three planets around M dwarfs to date.
We conducted high-resolution transmission spectroscopy of three sub-Neptunes (TOI-2136b, TOI-654b, and LP 791-18c) and a super-Earth (TOI-1634b) orbiting M dwarfs with the InfraRed Doppler (IRD) spectrograph on the Subaru Telescope.
We find no statistically significant helium absorption in any target; accordingly, we derive 95% confidence upper limits on the helium line depth of 1.36%, 0.60%, 2.07%, and 3.00%, and on the equivalent width of 7.3, 2.1, 7.4, and 9.1 mÅ, for TOI-2136b, TOI-1634b, TOI-654b, and LP 791-18c, respectively.
We further explored constraints on the upper-atmospheric temperature and mass-loss rate by comparing these results with isothermal Parker-wind models. While we have compared with self-consistent ATES models of primordial H/He atmospheres spanning a range of assumed X-ray luminosities, changes in the assumed XUV flux do not appear to account for the non-detections.
The results suggest that these planets have metal-enriched H/He primary atmospheres or non-primordial atmospheres, such as water-rich envelopes. Future observations of other absorption lines, such as Lyman-α, H-α, and H2O, may provide further constraints on these atmospheres.
Kiyoe Kawauchi, Norio Narita, Yuichi Ito, Akifumi Nakayama, Teruyuki Hirano, Masayuki Kuzuhara, Mayuko Mori, Izuru Fukuda, Akihiko Fukui, Yuya Hayashi, Yasunori Hori, Masahiro Ikoma, Kai Ikuta, Taiki Kagetani, Yugo Kawai, Tadahiro Kimura, Vigneshwaran Krishnamurthy, Huan-Yu Teng, Noriharu Watanabe, Yujie Zou, Hiroki Harakawa, Klaus Hodapp, Tomoyuki Kudo, Takashi Kurokawa, Jun Nishikawa, Masashi Omiya, Motohide Tamura, Akitoshi Ueda, Sébastien Vievard
Comments: 15 pages, 10 figures, accepted for publication in PASJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2608.19030 [astro-ph.EP] (or arXiv:2608.19030v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2608.19030
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Related DOI:
https://doi.org/10.1093/pasj/psag104
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Submission history
From: Kiyoe Kawauchi
[v1] Wed, 19 Aug 2026 15:24:23 UTC (512 KB)
https://arxiv.org/abs/2608.19030
Astrobiology, exoplanet,
