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 \approx 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 indicate 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 co-planar 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 a isothermal Parker wind with a solar composition show that this non-detection is not constraining compared to escape rate predictions of \sim 0.1 M _ { \oplus } Gyr ^ { -1 } ; at such rates , at least several Gyr are required for a Neptune-like planet to evolve into a rocky super-Earth .