The TESS-Keck Survey. IV. A Retrograde, Polar Orbit for the Ultra-low-density, Hot Super-Neptune WASP-107b
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 $| \lambda | ={118}_{-19}^{+38}$ 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.
- Publication:
-
The Astronomical Journal
- Pub Date:
- March 2021
- DOI:
- 10.3847/1538-3881/abd177
- arXiv:
- arXiv:2101.09371
- Bibcode:
- 2021AJ....161..119R
- Keywords:
-
- Exoplanet dynamics;
- Orbit determination;
- Radial velocity;
- 490;
- 1175;
- 1332;
- Astrophysics - Earth and Planetary Astrophysics
- E-Print:
- 13 pages, 6 figures, to be published in The Astronomical Journal