A New Window into Escaping Exoplanet Atmospheres: 10830 \AA\ Line of Metastable Helium.

Observational evidence for escaping exoplanet atmospheres has been obtained for a few exoplanets to date. It comes from strong transit signals detected in the ultraviolet, most notably in the wings of the hydrogen Lyman-$\alpha$ (Ly$\alpha$) line. However, the core of the Ly$\alpha$ line is often heavily affected by interstellar absorption and geocoronal emission, limiting the information about the atmosphere that can be extracted from that part of the spectrum. Transit observations in atomic lines that are (a) sensitive enough to trace the rarefied exosphere gas, and (b) do not suffer from significant extinction by the interstellar medium (ISM), could enable more detailed observations and thus provide better constraints on theoretical models of escaping atmospheres. The absorption line of metastable helium at 10830 \AA\ could satisfy both of these conditions for some exoplanets. We develop a simple 1D model of escaping planetary atmospheres containing hydrogen and helium. We use it to calculate the density profile of helium in the 2 $^3$S metastable excited state, and the expected in-transit absorption at 10830 \AA\ for two exoplanets known to have escaping atmospheres. Our results indicate that exoplanets similar to GJ 436b and HD 209458b should exhibit enhanced transit depths at 10830 \AA, reaching $\sim 10\%$ and $\sim 4\%$ in the line core, respectively.