Characterization of the regimes of hydrodynamic escape from low-mass exoplanets

Atmospheric hydrodynamic escape sculpts the population of low-mass close-in planets. However, distinguishing between the driving mechanisms responsible for the hydrodynamic escape of hydrogen-rich atmospheres is a complex task due to the involvement of many physical factors. Using simulations, I show that hydrodynamic escape can be driven solely by thermal energy deposited in the lower layers of the atmosphere, but only if the planet’s Jeans parameter is below 3. Otherwise, additional exogenous drivers are necessary. To characterize these drivers, an upgraded Jeans parameter that takes into account tidal forces is introduced. When the upgraded Jeans parameter falls below 3 or exceeds 6, atmospheric escape is primarily driven by tidal forces or extreme ultraviolet radiation from the host star, respectively. In the range 3 to 6, both factors can trigger the escape of the atmosphere. The upgraded Jeans parameter, which is closely related to the underlying physics, provides a concise method for categorizing the driving mechanisms of hydrodynamic escape. The results can also be applied to planetary evolution calculations. An updated Jeans parameter that includes tidal forces can distinguish the various driving forces, both exogenous and endogenous, of atmospheric escape from low-mass close-in exoplanets. Depending on its value, escape can be dominated by tidal forces, extreme ultraviolet stellar radiation or a combination of the two.