The Well-aligned Orbit of Wasp-84b: Evidence for Disk Migration of a Hot Jupiter

We report the sky-projected orbital obliquity (spin-orbit angle) of WASP-84 b, a 0.69{{M}[SUB]Jup[/SUB]} planet in an 8.52 day orbit around a G9V/K0V star, to be λ = -0.3 ± 1.7°. We obtain a true obliquity of ψ = 17.3 ± 7.7° from a measurement of the inclination of the stellar spin axis with respect to the sky plane. Due to the young age and the weak tidal forcing of the system, we suggest that the orbit of WASP-84b is unlikely to have both realigned and circularized from the misaligned and/or eccentric orbit likely to have arisen from high-eccentricity migration. Therefore we conclude that the planet probably migrated via interaction with the protoplanetary disk. This would make it the first “hot Jupiter” (P\lt 10 d) to have been shown to have migrated via this pathway. Further, we argue that the distribution of obliquities for planets orbiting cool stars (T[SUB]eff[/SUB] < 6250 K) suggests that high-eccentricity migration is an important pathway for the formation of short-orbit, giant planets. Based on observations made with the HARPS-North spectrograph on the 3.6 m Telescopio Nazionale Galileo under OPTICON program 2013 B/069, the HARPS spectrograph on the ESO 3.6 m telescope under program 090.C-0540, and the RISE photometer on the 2.0 m Liverpool Telescope under programs PL12B13 and PL14A11. The photometric time-series and radial-velocity data used in this work are available at the CDS.