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

TL;DR

This study used HST observations to search for neutral hydrogen escape from the 120 Myr old sub-Neptune HIP94235b, finding no significant hydrogen outflow and constraining the escape rate, which aligns with energy-limited models.

Contribution

First HST Ly-alpha observations of HIP94235b, providing constraints on hydrogen escape and insights into atmospheric evolution at this age.

Findings

No detectable neutral hydrogen escape observed.

Upper limit on escape rate is 10^13 g/s.

Planet likely to become a super-Earth in 1 Gyr.

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 million year (Myr) old phase of intense XUV irradiation. We present two observations of HIP94235 b using the Hubble Space Telescope's Space Telescope Imaging Spectrograph (HST/STIS) in the Ly-alpha wavelength region. We do not observe discernible differences across either the blue and red wings of the Ly-alpha 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 g/s, which remains consistent with energy-limited model predictions of 10^11 g/s. The Ly-alpha non-detection 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.