Searching for Neutral Hydrogen Escape from the 120 Myr Old Sub-Neptune HIP94235b using HST

Abstract HIP94235 b, a 120 Myr old sub-Neptune, provides us the unique opportunity to study mass loss at a pivotal stage of the system’s evolution: the end of a 100 Myr old phase of intense XUV irradiation. We present two observations of HIP94235 b using the Hubble Space Telescope’s Space Telescope Imaging Spectrograph in the Ly α wavelength region. We do not observe discernible differences across either the blue and red wings of the Ly α line profile in and out of transit, and report no significant detection of outflowing neutral hydrogen around the planet. We constrain the rate of neutral hydrogen escaping HIP94235 b to an upper limit of 10 13 gs −1 , which remains consistent with energy-limited model predictions of 10 11 gs −1 . The Ly α nondetection is likely due to the extremely short photoionization timescale of the neutral hydrogen escaping the planet’s atmosphere. This timescale, approximately 15 minutes, is significantly shorter than that of any other planets with STIS observations. Through energy-limited mass loss models, we anticipate that HIP94235 b will transition into a super-Earth within a timescale of 1 Gyr.