ON THE ORBIT OF THE SHORT-PERIOD EXOPLANET WASP-19b

WASP-19b has the shortest orbital period of any known exoplanet, orbiting at only 1.2 times the Roche tidal radius. By observing the Rossiter–McLaughlin effect we show that WASP-19b's orbit is aligned, with λ = 4 6 ± 5 2. Using, in addition, a spectroscopic v sin I and the observed rotation period we conclude that the obliquity, ψ, is less than 20°. Further, the eccentricity of the orbit is less than 0.02. We argue that hot Jupiters with orbital periods as short as that of WASP-19b are two orders of magnitude less common than hot Jupiters at the 3–4 day "pileup." We discuss the evolution of WASP-19b's orbit and argue that most likely it was first moved to near twice the Roche limit by third-body interactions, and has since spiralled inward to its current location under tidal decay. This is compatible with a stellar tidal-dissipation quality factor, Q ' * , of order 10 7 .