Zodiacal Exoplanets in Time. XI. The Orbit and Radiation Environment of the Young M Dwarf-Hosted Planet K2-25b

TL;DR

This study investigates the orbit and atmospheric escape of the young Neptune-sized exoplanet K2-25b orbiting an M dwarf, revealing a closely aligned orbit and placing constraints on atmospheric escape rates.

Contribution

It provides the first detailed analysis of the orbit and atmospheric escape environment of K2-25b, a young Neptune-sized exoplanet around an M dwarf, using transit spectroscopy and photometry.

Findings

Orbit is closely aligned with stellar equator (lambda ≈ -1.7°)

No significant He I line variation detected during transit

Atmospheric escape rate consistent with ~0.1 Mearth/Gyr, implying long timescales for evolution

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 indicate that the planet's orbit is closely aligned with the stellar equator (lambda = -1.7+5.8/-3.7 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 ~0.1 Mearth/Gyr; at such rates, at least several Gyr are required for a Neptune-like planet to evolve into a rocky super-Earth.