Observing Atmospheric Escape in Sub-Jovian Worlds with JWST
Leonardo A. Dos Santos, Munazza K. Alam, Néstor Espinoza, Shreyas Vissapragada
Abstract
Abstract 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 μ m 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.