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

An analysis of transits of planets over starspots on the Sun-like star Kepler-30 shows that the orbits of the three planets are aligned with the stellar equator; this configuration is similar to that of our Solar System, and suggests that high obliquities are confined to systems that experienced disruptive dynamical interactions. Exoplanet trio in Solar-System-like orbits In our Solar System, the Sun's equator and the planets' orbital planes are almost in alignment. This probably reflects the way they formed, from a single spinning disk of gas. Many exoplanet systems do not display this arrangement, however, and isolated 'hot Jupiters' are often misaligned and even have a retrograde orbit. This paper reports an exoplanet system that features alignments similar to those in the Solar System. Analysis of planetary transits across starspots on the Sun-like star Kepler-30 shows that the orbits of its three planets are aligned with the stellar equator. These findings support the suggestion that high orbital tilts (obliquities) are confined to systems that have experienced dynamic interactions of the type that produce hot Jupiters and potentially rule out star–disk misalignments as a cause. 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 1 , magnetic interactions 2 or torques from neighbouring stars. Indeed, isolated ‘hot Jupiters’ are often misaligned and even orbiting retrograde 3 , 4 . Here we report an analysis of transits of planets over starspots 5 , 6 , 7 on the Sun-like star Kepler-30 (ref. 8 ), 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 impl