The TESS-Keck Survey IV: A Retrograde, Polar Orbit for the Ultra-Low-Density, Hot Super-Neptune WASP-107b

TL;DR

This study measures the orbit of WASP-107b, revealing a polar orbit and exploring dynamical mechanisms that could have caused this misalignment, contributing to understanding planetary migration and orbital evolution.

Contribution

It provides the first measurement of WASP-107b's sky-projected obliquity and discusses potential dynamical pathways for its polar orbit, expanding knowledge of hot Neptune system architectures.

Findings

WASP-107b has a highly misaligned, polar orbit.

Nodal precession and disk dispersal are plausible causes of the obliquity.

Future Gaia data could distinguish between dynamical models.

Abstract

We measured the Rossiter-McLaughlin effect of WASP-107b during a single transit with Keck/HIRES. We found the sky-projected inclination of WASP-107b's orbit, relative to its host star's rotation axis, to be $|\lambda| = {118}^{+38}_{-19}$ degrees. This confirms the misaligned/polar orbit that was previously suggested from spot-crossing events and adds WASP-107b to the growing population of hot Neptunes in polar orbits around cool stars. WASP-107b is also the fourth such planet to have a known distant planetary companion. We examined several dynamical pathways by which this companion could have induced such an obliquity in WASP-107b. We find that nodal precession and disk dispersal-driven tilting can both explain the current orbital geometry while Kozai-Lidov cycles are suppressed by general relativity. While each hypothesis requires a mutual inclination between the two planets, nodal precession requires a much larger angle which for WASP-107 is on the threshold of detectability with future Gaia astrometric data. As nodal precession has no stellar type dependence, but disk dispersal-driven tilting does, distinguishing between these two models is best done on the population level. Finding and characterizing more extrasolar systems like WASP-107 will additionally help distinguish whether the distribution of hot-Neptune obliquities is a dichotomy of aligned and polar orbits or if we are uniformly sampling obliquities during nodal precession cycles.