MHD Modeling Atmospheric Ion Escape from a Mars-like Exoplanet Orbiting TRAPPIST-1*

More planets are known to orbit M dwarf stars than any other type of star in our galaxy, in part because they are the most numerous stellar type. Habitable planets are of particular interest around these stars. However, the much closer-in habitable zone for an M dwarf creates a harsh plasma and magnetic environment for the planets, potentially causing massive atmospheric escape through various channels. An important channel for oxygen is ion escape, where charged particles in the ionosphere are accelerated to their escape velocities. A global planetary magnetic field plays a crucial role in the interactions between stellar wind plasma and the atmosphere, but its long-assumed shielding effect on the atmosphere has been challenged in recent years, raising the question of its necessity for habitability. In this study, we explore the habitability of a Mars-like planet around the ultracool M dwarf, TRAPPIST-1, by investigating the ion escape from its atmosphere. We perform a steady-state simulation using a multispecies single-fluid magnetohydrodynamic (MHD) model with the photoionization frequency of each dominant neutral species in the Martian atmosphere and upstream stellar wind conditions at TRAPPIST-1g. In addition to unmagnetized and weakly magnetized cases, we equip Mars with a strong planetary dipole field of 5000 nT at eight different tilt angles with respect to the +z axis to study the change in the ion loss rate. The simulation results suggest a total escape rate that is two to three orders of magnitude higher than for present-day Mars in our solar system. The escape rate shows a symmetric relation about the dipole tilt angle of 90 degrees.