Study of Atmospheric Ion Escape From Exoplanet TOI-700 d: Venus Analogs

Abstract: The recent discovery of Earth-sized planets in the habitable zones (HZ) of cool stars (M dwarfs) has focused attention on whether liquid water and life exist on these planets. These planets are exposed to stronger X-ray and EUV (XUV) radiation than the Solar system terrestrial planets because the X-ray to bolometric luminosity ratio of M dwarfs is substantially larger than Sun-like stars. TOI-700 system is one such target, with an Earth-sized planet (TOI-700 d) in the HZ of the M2 star. Using a multi-species MHD model, we assess whether a Venus-like exoplanet could retain an atmosphere for a time comparable to the age of the host star, specifically considering TOI-700 d as a representative exoplanet in the HZ surrounded by inactive M dwarfs. In this study, the effect of stellar XUV and magnetic fields on ion escape is investigated. The results suggest that unmagnetized TOI-700 d would have difficulty retaining its atmosphere over a few billion years under strong XUV conditions above 30 times the current Earth. However, an intrinsic dipole magnetic field of 1000 nT at the equatorial surface drastically reduces the escape rate and helps the exoplanet to retain its atmosphere. It means an intrinsic magnetic field plays a crucial role in retaining an atmosphere of exoplanets in the HZ around M dwarfs.