A Low Stellar Obliquity for Wasp-47, a Compact Multiplanet System with a Hot Jupiter and an Ultra-short Period Planet

We have detected the Rossiter-Mclaughlin effect during a transit of WASP-47b, the only known hot Jupiter with close planetary companions. By combining our spectroscopic observations with Kepler photometry, we show that the projected stellar obliquity is λ = 0± 24. We can firmly exclude a retrograde orbit for WASP47b, and rule out strongly misaligned prograde orbits. Low obliquities have also been found for most of the other compact multiplanet systems that have been investigated. The Kepler-56 system, with two close-in gas giants transiting their subgiant host star with an obliquity of at least 45, remains the only clear counterexample. Subject headings: planetary systems — planets and satellites: formation — planet-star interactions — stars: rotation