A low stellar obliquity for WASP-47, a compact multiplanet system with a hot Jupiter and an ultra-short period planet

TL;DR

This study measures the stellar obliquity of WASP-47, a unique system with a hot Jupiter and close planetary companions, finding it to be nearly aligned, which informs theories of planetary system formation.

Contribution

First measurement of the Rossiter-McLaughlin effect for WASP-47, revealing a low stellar obliquity in a system with a hot Jupiter and multiple planets.

Findings

WASP-47's stellar obliquity is approximately 0° with a ±24° uncertainty.

Most compact multiplanet systems have low stellar obliquities.

Kepler-56 is a notable exception with a high obliquity.

Abstract

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 $\lambda = 0^\circ \pm 24^\circ$. We can firmly exclude a retrograde orbit for WASP-47b, 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$^\circ$, remains the only clear counterexample.