WINERED Detects a Strong Atmospheric Outflow on the Sub-Neptune GJ 3090b

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 \(2.033\pm0.086\%\) (23.6\(\sigma\)) and \(1.619\pm0.122\%\) (13.3\(\sigma\)), 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 \(Z_\mathrm{atm}\lesssim100\times\) 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 \(\sim300\) Myr, much shorter than previously reported. This adolescent (\(\sim1\) Gyr) sub-Neptune is currently in a transformative phase of photoevaporative evolution.

Recommended citation: Vissapragada, Shreyas and Lin, Zifan and Meech, Annabella et al. (2026). "WINERED Detects a Strong Atmospheric Outflow on the Sub-Neptune GJ 3090b." arXiv.
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