Orbital misalignment of the Neptune-mass exoplanet GJ 436b with the spin of its cool star

TL;DR

This study reveals that the Neptune-mass exoplanet GJ 436b has a highly inclined, eccentric orbit around its cool star, suggesting complex dynamical history possibly involving unseen companions and challenging assumptions about star-planet alignment.

Contribution

It introduces a novel method to measure the three-dimensional orbit of a planet around a cool star, revealing a significant orbital misalignment.

Findings

GJ 436b's orbit is nearly perpendicular to the star's equator.

Eccentric and misaligned orbits around cool stars may indicate unseen companions.

The planet's orbit suggests a history of dynamical interactions like Kozai migration.

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.