Detection of Ongoing Mass Loss from HD 63433c, a Young Mini-Neptune

We detect Lyman $\alpha$ absorption from the escaping atmosphere of HD 63433c, a $R=2.67 R_\oplus$, $P=20.5$ d mini Neptune orbiting young (440 Myr) solar analogue in Ursa Major Moving Group. Using HST/STIS, we measure transit depth $11.1 \pm 1.5$% blue wing and $8 3$% red. This signal is unlikely to be due stellar variability, but should confirmed by an upcoming second visit with HST. do not inner planet, smaller $R=2.15 R_\oplus$ on 7.1 orbit. use Keck/NIRSPEC place upper limit 0.5% helium for both planets. host star's X-ray spectrum FUV flux XMM-Newton, model outflow planets using 3D hydrodynamic code. provides reasonable match light curve line non-detection planet c, although it does explain tentative red or reproduce excess detail. Its predictions strong b are ruled out observations. predicts much shorter mass loss timescale b, suggesting that c fundamentally different: while latter still retains its hydrogen/helium envelope, former has likely lost primordial atmosphere.