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

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  • T. Nishioka
    Department of Earth and Planetary Science Graduate School of Science University of Tokyo Tokyo Japan
  • K. Seki
    Department of Earth and Planetary Science Graduate School of Science University of Tokyo Tokyo Japan
  • R. Sakata
    Department of Geophysics Graduate School of Science Tohoku University Sendai Japan
  • K. Yamamoto
    Department of Earth and Planetary Science Graduate School of Science University of Tokyo Tokyo Japan
  • N. Terada
    Department of Geophysics Graduate School of Science Tohoku University Sendai Japan
  • S. Sakai
    Department of Geophysics Graduate School of Science Tohoku University Sendai Japan
  • H. Shinagawa
    National Institute of Information and Communications Technology Koganei Japan
  • A. Nakayama
    Department of Physics College of Science Rikkyo University Tokyo Japan

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<jats:title>Abstract</jats:title><jats:p>TOI‐700 d is the first Earth‐sized planet in the habitable zone (HZ) discovered by the Transiting Exoplanet Survey Satellite. Here, we assess whether a Venus‐like exoplanet at the TOI‐700 d location could retain an atmosphere for a time comparable to the age of the host star based on multispecies magnetohydrodynamics simulations. We investigate the effects of X‐ray and EUV (XUV) radiation from the host star, the interplanetary magnetic field (IMF) orientation, and the planetary intrinsic magnetic field. In unmagnetized cases, major ion loss is caused by O<jats:sup>+</jats:sup> escape through a ring‐shaped region by the mass loading process after the ionization of the extended oxygen corona. As the IMF Parker spiral angle increases, the escape flux in the magnetotail shows stronger enhancement around the meridional current sheet, and the escape rate of molecular ions (<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/jgra57989-math-0001.png" xlink:title="urn:x-wiley:21699380:media:jgra57989:jgra57989-math-0001" /> and <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/jgra57989-math-0002.png" xlink:title="urn:x-wiley:21699380:media:jgra57989:jgra57989-math-0002" />) increases by an order of magnitude due to acceleration in the ionosphere by magnetic tension forces. In magnetized cases, the intrinsic magnetic field suppresses ion pickup loss from the neutral oxygen corona by deflecting the stellar wind and preventing ion pickup while promoting cusp‐origin escape from the lower ionosphere. These results suggest that the unmagnetized exoplanet would have difficulty retaining its atmosphere over a few billion years under extreme conditions where XUV is 30 times stronger than at the current Earth. However, the dipole intrinsic magnetic field of 1,000 nT at the equatorial surface reduces the escape rate and would help the exoplanet to retain its atmosphere even under strong XUV conditions.</jats:p>

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