Zodiacal exoplanets in time - XI. The orbit and radiation environment of the young M dwarf-hosted planet K2-25b
Kuzuhara, M.; Tamura, M.; Kudo, T.; France, K.; Konishi, M.; Hirano, T.; Hodapp, K. W.; Gaidos, E.; Krishnamurthy, V.; Harakawa, H.; Jacobson, S.; Kotani, T.; Nishikawa, J.; Omiya, M.; Serizawa, T.; Ueda, A.; Vievard, S.; Wilson, D. J.; Rockcliffe, K.; Newton, E.; Feiden, G.; Ishizuka, M.; Kurokawa, T.
United States, Japan
Abstract
M dwarf stars are high-priority targets for searches for Earth-size and potentially Earth-like planets, but their planetary systems may form and evolve in very different circumstellar environments than those of solar-type stars. To explore the evolution of these systems, we obtained transit spectroscopy and photometry of the Neptune-size planet orbiting the ≈650-Myr-old Hyades M dwarf K2-25. An analysis of the variation in spectral line shape induced by the Doppler 'shadow' of the planet indicates that the planet's orbit is closely aligned with the stellar equator ($\lambda =-1.7_{-3.7}^{+5.8}$ deg), and that an eccentric orbit found by previous work could arise from perturbations by another planet on a coplanar orbit. We detect no significant variation in the depth of the He I line at 1083 nm during transit. A model of atmospheric escape as an isothermal Parker wind with a solar composition shows that this non-detection is not constraining compared to escape rate predictions of ~0.1 M⊕ Gyr-1; at such rates, at least several Gyr are required for a Neptune-like planet to evolve into a rocky super-Earth.