The TESS-Keck Survey. IV. A Retrograde, Polar Orbit for the Ultra-low-density, Hot Super-Neptune WASP-107b

March 2021 • 2021AJ....161..119R

Authors • Rubenzahl, Ryan A. • Dai, Fei • Howard, Andrew W. • Chontos, Ashley • Giacalone, Steven • Lubin, Jack • Rosenthal, Lee J. • Isaacson, Howard • Batalha, Natalie M. • Crossfield, Ian J. M. • Dressing, Courtney • Fulton, Benjamin • Huber, Daniel • Kane, Stephen R. • Petigura, Erik A. • Robertson, Paul • Roy, Arpita • Weiss, Lauren M. • Beard, Corey • Hill, Michelle L. • Mayo, Andrew • Mocnik, Teo • Murphy, Joseph M. Akana • Scarsdale, Nicholas

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.


IPAC Authors


Benjamin Fulton

Assistant Scientist