Role of Planetary Radius on Atmospheric Escape of Rocky Exoplanets

Abstract

Large-scale characterization of exoplanetary atmospheres is on the horizon, thereby making it possible in the future to extract their statistical properties. In this context, by using a well-validated model in the solar system, we carry out 3D magnetohydrodynamic simulations to compute nonthermal atmospheric ion escape rates of unmagnetized rocky exoplanets as a function of their radius based on fixed stellar radiation and wind conditions. We find that the atmospheric escape rate is, unexpectedly and strikingly, a nonmonotonic function of the planetary radius R and that it evinces a maximum at R ∼ 0.7 R ⊕. This novel nonmonotonic behavior may arise from an intricate trade-off between the cross-sectional area of a planet (which increases with size, boosting escape rates) and its associated escape velocity (which also increases with size but diminishes escape rates). Our results could guide forthcoming observations because worlds with certain values of R (such as R ∼ 0.7 R ⊕) might exhibit comparatively higher escape rates when all other factors are constant.