The spin-orbit angles of the transiting exoplanets WASP-1b, WASP-24b, WASP-38b and HAT-P-8b from Rossiter-McLaughlin observations

We present observations of the Rossiter-McLaughlin effect for the transiting exoplanet systems WASP-1, WASP-24, WASP-38 and HAT-P-8, and deduce the orientations of the planetary orbits with respect to the host stars’ rotation axes. The planets WASP-24b, WASP-38b and HAT-P-8b appear to move in prograde orbits and be well aligned, having sky-projected spin orbit angles consistent with zero: λ = −4.7± 4.0, λ = 15 ◦ −43 and λ = −9.7 ◦ −7.7 , respectively. The host stars have Teff < 6250 K and conform with the trend of cooler stars having low obliquities. WASP-38b is a massive planet on a moderately long period, eccentric orbit so may be expected to have a misaligned orbit given the high obliquities measured in similar systems. However, we find no evidence for a large spinorbit angle. By contrast, WASP-1b joins the growing number of misaligned systems and has an almost polar orbit, λ = −79.0 ◦ −4.3 . It is neither very massive, eccentric nor orbiting a hot host star, and therefore does not share the properties of many other misaligned systems.