Outflowing Helium from a Mature Mini-Neptune

We announce the detection of escaping helium from TOI 2134b, a mini-Neptune a few gigayears old. The average in-transit absorption spectrum shows a peak of 0.37% ± 0.05% and an equivalent width of W _avg = 3.3 ± 0.3 mÅ. Among all planets with helium detections, TOI 2134b is the only mature mini-Neptune, has the smallest helium signal, and experiences the lowest X-ray and ultraviolet (XUV) flux. Putting TOI 2134b in the context of all other helium detections, we report the detection of a strong (p = 3.0 × 10^-5) and theoretically expected correlation between F _XUV/ρ _XUV (proportional to the energy-limited mass-loss rate) and R _* W _avg (roughly proportional to the observationally inferred mass-loss rate). Here W _avg is the equivalent width of the helium absorption, and ρ _XUV is the density of the planet within the XUV photosphere, but the correlation is similarly strong if we use the optical photosphere. Having the lowest value on both axes, TOI 2134b anchors the relation. We encourage further observations to fill in the missing regions of this parameter space and improve estimates of F _XUV.