Outflowing helium from a mature mini-Neptune

Abstract: We announce the detection of escaping helium from TOI 2134b, a mini-Neptune a few Gyr old. The average in-transit absorption spectrum shows a peak of 0.37 +- 0.05% and an equivalent width of $W_{\rm avg}=3.3 \pm 0.3$ m$\r{A}$. Among all planets with helium detections, TOI 2134b is the only mature mini-Neptune, has the smallest helium signal, and experiences the lowest XUV flux. Putting TOI 2134b in the context of all other helium detections, we report the detection of a strong (p=3.0e-5) and theoretically expected correlation between $F_{\rm XUV}/\rho_{\rm XUV}$ (proportional to the energy-limited mass loss rate) and $R_* W_{\rm avg}$ (roughly proportional to the observationally inferred mass loss rate). Here, $W_{\rm avg}$ is the equivalent width of the helium absorption and $\rho_{\rm XUV}$ is the density of the planet within the XUV photosphere, but the correlation is similarly strong if we use the optical photosphere. TOI 2134b anchors the relation, having the lowest value on both axes. We encourage further observations to fill in missing regions of this parameter space and improve estimates of $F_{\rm XUV}$.