Study of Atmospheric Ion Escape from Exoplanet TOI-700 d

Terrestrial planets around cool stars (M dwarfs) in the habitable zone (HZ) are relatively easy to observe because of frequent transits and their relatively large ratio of cross-sectional areas for the planet and the star. Therefore, they are important observational targets for seeking signs of life. However, close-in exoplanets around M dwarfs are expected to experience strong ion loss because the X-ray to bolometric luminosity ratio of M dwarfs is substantially larger than Sun-like stars (e.g., Ribas et al., A&A, 2017; Peacock et al., ApJ, 2019).

We focused here on exoplanet TOI-700 d, which was discovered in January 2020 (Gilbert et al., AJ, 2020; Rodriguez et al., AJ, 2020). This is the first Earth-sized planet in the HZ discovered by the Transiting Exoplanet Survey Satellite (TESS). In this study, we assessed the feasibility for atmospheric retention on TOI-700 d using multi-species MHD simulations model, REPPU-Planets (e.g., Terada et al., JGR, 2009; Sakata et al., JGR, 2022). We consider effects of the X-ray and EUV (XUV) radiation, the interplanetary magnetic field (IMF) orientation, and the intrinsic magnetic field. The similar stellar wind conditions to previous studies (Cohen et al., ApJ, 2020; Dong et al., ApJL, 2020) are used in the simulations.

The results suggest that unmagnetized TOI-700 d will be difficult to retain its atmosphere over a few billion years under strong XUV condition above 25 times of the current Earth. However, the intrinsic dipole magnetic field of 1000 nT at equatorial surface drastically reduces the escape rate and helps the exoplanet to retain its atmosphere. O+ ions contribute most of the total ion loss regardless of input parameters. In unmagnetized cases, O+ escape through the ring-shaped region by the pickup process of the extended oxygen corona. As IMF Parker spiral angle increases from 4 to 45 degrees, the escape rate of molecular ions (O2+ and CO2+) increases by an order of magnitude due to acceleration of polar ionospheric plasma by the magnetic tension force, and peaks of the escape flux are confined to the meridional current sheet. In magnetized cases, the intrinsic magnetic field increases the escape rate of molecular ions by promoting outflow from the lower ionosphere but decreases the O+ escape rate by deflecting the stellar wind and preventing ion pickup.