Spin-Orbit Alignment of Planetary Systems (SOAPS): The case of 64 Kepler planets and planet candidates

The obliquity is the physical property that can be used to probe the history of formation and evolution of planetary systems describing their current orbital configuration. We study the exoplanetary architectures based on obliquity measurements in the line of sight for 34 stars observed by Kepler. These stars harbour a total of 63 transiting planets and planet candidates, with orbital periods ranging from 0.8 to 290 d, and planetary radii ranging from 0.3 to 12.5 R⊕, as measured from their Kepler transit light curves. We measured the obliquity in the line of sight through the implementation of an MCMC algorithm to find the most probable stellar inclination angle. To do so, we measured vsini employing Subaru HDS spectra (R∼160 000) combined with robust stellar rotational periods and stellar radii from the literature. As a result, we identify a dependence between planetary multiplicity and coplanarity, and high alignment of the orbits. We did not find evidence that candidate stellar companions modify the observed distribution of the obliquity in the line of sight. Our results do not support that alignment occurs preferably around relative cool stars through tidal interactions over hotter stars. Finally we explore the possible evolutionary pathways for the planetary systems in our sample to constrain mechanisms and scenarios that could explain the observed obliquities of our sample.