Searching for Helium Escape and a Low-density Atmosphere Around the 120 Myr Old Sub-Neptune HIP94235b Using CRIRES+

Abstract

Atmospheric mass loss is thought to induce the bimodality in the small planet population as we observe it today. Observationally, active mass loss can be traced by excess absorption in spectral lines of lighter species, such as the hydrogen Ly α line and the metastable helium triplet. We search for helium escape from the young (120 Myr old) sub-Neptune (3 R _⊕ ) HIP94235b. We obtained two transit observations of HIP94235b using the CRyogenic InfraRed Echelle Spectrograph on the Very Large Telescope. We find no evidence for escaping helium across both visits, allowing us to place a mass loss rate upper limit of 10 ^11 g s ^−1 , based on 1D Parker wind models. Additionally, we search for molecular spectral features in the planet’s transmission spectrum, and cross correlate our observations with high-resolution template spectra for H _2 O, the dominating molecule in the Y band. We detect no significant absorption. We demonstrate that some atmosphere models at 10× solar metallicity would have been retrievable if present. Through the null detection of neutral hydrogen and helium escape, we conclude the atmosphere of HIP94235b likely lacks a large hydrogen-helium envelope. This is consistent with the expectation of small planet photoevaporation models, which suggest most planets lose their primordial hydrogen-helium envelopes within 100 Myr of evolution.

Keywords

• Observational astronomy
• Exoplanet astronomy
• Exoplanet atmospheres
• Exoplanet atmospheric evolution