Helium escape signatures are generally strongest during younger ages but this age dependence is lost in the diversity of observed exoplanets

Abstract

Highly irradiated exoplanets undergo extreme hydrodynamic atmospheric escape, due to their high level of received XUV flux. Over their lifetime, this escape varies significantly, making evolution studies essential for interpreting the growing number of observations of escaping planetary atmospheres. In a previous work, we modelled this evolving escape, alongside one of its observable tracers, the helium triplet transit signature at 1083nm. Using hydrodynamic and ray-tracing models, we demonstrated that atmospheric escape and the corresponding He 1083nm signature are stronger at younger ages, for a 0.3$~M_\text{J}$ gas-giant. Yet, the current literature includes several young (<1Gyr) planets with weak or non-detections in He 1083nm. To understand this apparent discrepancy, we now perform detailed modelling for many of these systems. The resulting He 1083nm predictions align relatively well with the observations. From our two studies, we conclude that for any given planet, stronger atmospheric escape during younger ages produces deeper He 1083nm absorption. However, for a population of exoplanets, the relation between younger ages and stronger He absorptions is lost to the broad diversity of their various other system parameters. Accordingly, for the current sample of young, 1083nm-observed exoplanets, alternative trends take precedence. One such trend is that planets with deeper geometrical transits exhibit more favourable detections. Our modelling also agrees with the strong empirical trend in the literature between $ EW \cdot R_{*}^{2}$ and $F_{\text{xuv}} \cdot R_{\text{pl}}^2 / \Phi_{g}$. Additionally, we show that the coupling between the lower and upper atmospheres is necessary for a robust prediction of the 1083nm signature.