Interplanetary magnetic field orientation and the magnetospheres of close-in exoplanets

Exoplanets should display strong interactions with the winds of their parent stars. Inspired by the physical processes observed on Venus, the authors study the interaction expected for an unmagnetized Earthlike planet located at 0.2 AU from its star. They use a realistic model for the planet’s exosphere and ionosphere that includes both the effects of stellar wind pick-up by the planet and the induced escape of the ionosphere for different orientations of the wind-advected magnetic field. The rate of loss from the planet’s atmosphere and the rate of pile-up of stellar wind are strongly influenced by the orientation of the interplanetary magnetic field relative to the wind direction, and appears greatest for parallel field and wind. If one also assumes that this angle dependence holds true in general, and if the stellar mass loss influences planetary evolution, then the angle between the interplanetary magnetic field and the stellar wind angle might also be important when relating observations of exoplanetary atmospheres to the formation of exoplanets.