Alignment of the stellar spin with the orbits of a three-planet system

TL;DR

This study finds that the three-planet system Kepler-30 has planets aligned with its stellar equator, supporting the idea that high obliquities in exoplanets result from disruptive dynamical interactions rather than star-disk misalignments.

Contribution

It provides observational evidence that multi-planet systems can be well-aligned, contrasting with hot Jupiters, and suggests dynamical interactions cause high obliquities.

Findings

Kepler-30 planets are aligned with stellar equator

Planets are aligned with each other within a few degrees

Supports dynamical interactions as cause of hot Jupiter misalignments

Abstract

The Sun's equator and the planets' orbital planes are nearly aligned, which is presumably a consequence of their formation from a single spinning gaseous disk. For exoplanetary systems this well-aligned configuration is not guaranteed: dynamical interactions may tilt planetary orbits, or stars may be misaligned with the protoplanetary disk through chaotic accretion, magnetic interactions or torques from neighbouring stars. Indeed, isolated 'hot Jupiters' are often misaligned and even orbiting retrograde. Here we report an analysis of transits of planets over starspots on the Sun-like star Kepler-30, and show that the orbits of its three planets are aligned with the stellar equator. Furthermore, the orbits are aligned with one another to within a few degrees. This configuration is similar to that of our Solar System, and contrasts with the isolated hot Jupiters. The orderly alignment seen in the Kepler-30 system suggests that high obliquities are confined to systems that experienced disruptive dynamical interactions. Should this be corroborated by observations of other coplanar multi-planet systems, then star-disk misalignments would be ruled out as the explanation for the high obliquities of hot Jupiters, and dynamical interactions would be implicated as the origin of hot Jupiters.