Nonthermal Atmospheric Escape on the Kepler-11 “Super-Earths” Driven by Stellar Wind Sputtering

Jeans escape or hydrodynamic escape is believed to dominate atmospheric loss for hot Jupiters. However, nonthermal mechanisms likely contribute substantially on hydrogen-rich “super-Earths” with relatively cold and extended atmospheres. This study is devoted to investigating the role of stellar wind sputtering on Kepler-11b–f, with the aid of Monte Carlo test particle calculations. Such a mechanism is widely known to be important on many solar system bodies, but its impact has never been evaluated rigorously on any exoplanet. Our calculations reveal complicated variations of the H sputtering yield with the stellar wind inclination angle and neutral heating efficiency at different Kepler-11 planets. Further calculations suggest the H loss rates on Kepler-11b–f to be 2.8 × 10 6 –1.5 × 10 8 g s −1 , driven by stellar wind sputtering. The H loss rate obtained here is compared to those driven by other mechanisms, indicating that the importance of stellar wind sputtering is highly variable and this mechanism could induce a large atmospheric loss rate comparable to the blow-off rate driven by stellar far-ultraviolet radiation, in particular for planets with relatively low gravities.