Observing atmospheric escape in sub-Jovian worlds with JWST

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 $1.083$ $\mu$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 ($T_{\mathrm{eq}}=687\mathrm{K}$, $R_{\mathrm{p}}=0.37{\mathrm{R}}_{\mathrm{J}}$) and GJ 1214 b ($T_{\mathrm{eq}}=588\mathrm{K}$, $R_{\mathrm{p}}=0.25{\mathrm{R}}_{\mathrm{J}}$). 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.

14 pages, 7 figures, under review at AAS Journals; this version follows the first round of revision. Feedback from the community is welcome

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