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

Abstract

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${}^{\ast }$) transmission spectroscopy, which traces atmospheric escape. In this paper, we report the detection of He${}^{\ast }$ 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${}^{\ast }$ absorptions during transit of $2.033\pm 0.086\%$ (23.6$\sigma$) and $1.619\pm 0.122\%$ (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 $Z_{\mathrm{atm}}\lesssim 100\times$ solar, as the observed He${}^{\ast }$ 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 $\sim 300$ Myr, much shorter than previously reported. This adolescent ($\sim 1$ Gyr) sub-Neptune is currently in a transformative phase of photoevaporative evolution.

17 pages, 5 figures, 2 tables, submitted to ApJL