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We measured the Rossiter-McLaughlin effect of WASP-107b during a singletransit with Keck/HIRES. We found the sky-projected inclination of WASP-107b'sorbit, relative to its host star's rotation axis, to be $|\lambda| ={118}^{+38}_{-19}$ degrees. This confirms the misaligned/polar orbit that waspreviously suggested from spot-crossing events and adds WASP-107b to thegrowing population of hot Neptunes in polar orbits around cool stars. WASP-107bis also the fourth such planet to have a known distant planetary companion. Weexamined several dynamical pathways by which this companion could have inducedsuch an obliquity in WASP-107b. We find that nodal precession and diskdispersal-driven tilting can both explain the current orbital geometry whileKozai-Lidov cycles are suppressed by general relativity. While each hypothesisrequires a mutual inclination between the two planets, nodal precessionrequires a much larger angle which for WASP-107 is on the threshold ofdetectability with future Gaia astrometric data. As nodal precession has nostellar type dependence, but disk dispersal-driven tilting does, distinguishingbetween these two models is best done on the population level. Finding andcharacterizing more extrasolar systems like WASP-107 will additionally helpdistinguish whether the distribution of hot-Neptune obliquities is a dichotomyof aligned and polar orbits or if we are uniformly sampling obliquities duringnodal precession cycles.

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