[astro-ph.EP] Sub-Neptunes are the most common short-period (P<100 d) planets known. A major goal for the field is to build a comprehensive understanding of their compositions and evolutionary histories, and an important tool for achieving this goal is metastable helium (He∗) transmission spectroscopy, which traces atmospheric escape.
In this paper, we report the detection of He∗ in the atmosphere of the sub-Neptune GJ 3090b at high spectral resolution. We observed two consecutive transits of this planet with Magellan II/WINERED, and detected peak He∗ absorptions during transit of 2.033±0.086% (23.6σ) and 1.619±0.122% (13.3σ), respectively.
The first transit was affected by a stellar flare near egress while the second transit was unaffected. The signal is variable in amplitude and deeper than anticipated by the JWST/NIRISS observations of Ahrer et al. (2025).
We do not observe a significant Doppler shift, and the signal has a FWHM of 25 km s−1, implicating photoevaporation. Our outflow modeling suggests that Zatm≲100× solar, as the observed He∗ amplitudes are challenging to achieve in a metal-rich atmosphere. To match the muted transmission spectra from JWST and VLT/CRIRES+, GJ 3090b’s lower atmosphere is likely covered by aerosols.
Finally, we found that GJ 3090b has an atmospheric lifetime of ∼300 Myr, much shorter than previously reported. This adolescent (∼1 Gyr) sub-Neptune is currently in a transformative phase of photoevaporative evolution.
Shreyas Vissapragada, Zifan Lin, Annabella Meech, Collin Cherubim, Leonardo A. Dos Santos, Aaron Householder, Krishna Kanumalla, Mercedes Lopez-Morales, Andrew McWilliam, William Misener, Ava Morrissey, Morgan Saidel, Jessica J. Spake, Johanna Teske, Nicole L. Wallack
Comments: 17 pages, 5 figures, 2 tables, submitted to ApJL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2609.14704 [astro-ph.EP] (or arXiv:2609.14704v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2609.14704
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Submission history
From: Shreyas Vissapragada
[v1] Sun, 13 Sep 2026 18:00:01 UTC (530 KB)
https://arxiv.org/abs/2609.14704
Astrobiology, exoplanet, astronomy,
