2023AJ....165..244D

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