Observing Atmospheric Escape in Sub-Jovian Worlds with JWST

Hydrodynamic atmospheric escape is considered an important process that shapes the evolution of sub-Jovian exoplanets, particularly those with short orbital periods. The metastable He line in near-infrared at $1.083$ $\mu$m a reliable tracer hot advantage being observable from ground. However, observing escaping planets has remained challenging due to systematic effects and telluric contamination present ground-based data. With successful launch operations JWST, we now have access extremely stable high-precision spectrographs space. Here predict observability JWST two representative previously well-studied warm Neptunes, GJ 436 b ($T_{\rm eq} = 687~{\rm K}$, $R_{\rm p} 0.37~{\rm R_J}$) 1214 588~{\rm 0.25~{\rm R_J}$). Our simulated observations for demonstrate single transit NIRSpec/G140H sensitive mass loss rates are orders magnitude lower than what detectable exercise show best configuration observe relatively weak outflows Neptunes NIRSpec/G140H, NIRSpec/G140M NIRISS/SOSS less optimal. Since none these instrument configurations can spectrally resolve planetary absorption, conclude 1D isothermal Parker-wind approximation may not be sufficient interpreting such observations. More sophisticated models critical breaking degeneracy between outflow temperature mass-loss rate measurements He.