Ever Elusive Exospheres: One Probable Detection and Two Nondetections of H α Transits in Young Systems

Abstract Gaps in the exoplanet population, such as the Neptunian Desert, point to the importance of mass loss in sculpting the radii of close-in exoplanets. Young planets (<500 Myr) offer the opportunity to detect such mass loss while it is still strong, and to test models of the underlying physical processes. We search for evidence of an H α transit in high-resolution spectra of three young planets, HD 63433 b (400 Myr), DS Tuc A b (45 Myr), and HIP 67522 b (17 Myr) using HARPS-N, Magellan-PFS, and CHIRON, respectively. We validate our method by testing it on several photospheric lines less impacted by stellar variability. We find no evidence of a transit signal for HD 63433 b and DS Tuc A b (3 σ limits of 0.9% and 0.3%, respectively). For HIP 67522 b, we detect significant excess absorption (3.44% ± 0.28%) aligned with the transit time, narrow compared to the stellar line, and blueshifted from the stellar rest frame. In combination, these suggest the signal is from the planet. However, stellar variation in the H α line over the course of the observations is comparable in size to the transit signature and the duration is shorter than the photometric transit, so this detection requires confirmation. Our findings, and other recent results in the literature, suggest that planets younger than 50 Myr are more favorable for the detection of atmospheric escape with H α observations, though older populations might still show escape in other diagnostics.