Title: Orbital misalignment of the Neptune-mass exoplanet GJ 436b with the spin of its cool star,
Abstract: The angle between the spin of a star and its planets' orbital planes traces
the history of the planetary system. Exoplanets orbiting close to cool stars
are expected to be on circular, aligned orbits because of strong tidal
interactions with the stellar convective envelope. Spin-orbit alignment can be
measured when the planet transits its star, but such ground-based spectroscopic
measurements are challenging for cool, slowly-rotating stars. Here we report
the characterization of a planet three-dimensional trajectory around an M dwarf
star, derived by mapping the spectrum of the stellar photosphere along the
chord transited by the planet. We find that the eccentric orbit of the
Neptune-mass exoplanet GJ 436b is nearly perpendicular to the stellar equator.
Both eccentricity and misalignment, surprising around a cool star, can result
from dynamical interactions (via Kozai migration) with a yet-undetected outer
companion. This inward migration of GJ 436b could have triggered the
atmospheric escape that now sustains its giant exosphere. Eccentric, misaligned
exoplanets orbiting close to cool stars might thus hint at the presence of
unseen perturbers and illustrate the diversity of orbital architectures seen in
exoplanetary systems.