On the behaviour of spin–orbit connection of exoplanets

Abstract

Star–planet interactions play, among other things, a crucial role in planetary orbital configurations by circularizing orbits, aligning the star and planet spin and synchronizing stellar rotation with orbital motions. This is especially true for innermost giant planets, which can be schematized as binary systems with a very large mass ratio. Despite a few examples where spin–orbit synchronization has been obtained, there is no demographic study on synchronous regimes in those systems yet. Here we use a sample of 1,055 stars with innermost planet companions to show the existence of three observational loci of star–planet synchronization regimes. Two of them have dominant fractions of subsynchronous and supersynchronous star–planet systems, and a third less populated regime of potentially synchronized systems. No synchronous star–planet system with a period higher than 40 days has been detected yet. This landscape is different from eclipsing binary systems, most of which are synchronized. We suggest that planets in a stable asynchronous spin state belonging to star–planet systems in a supersynchronized regime offer the most favourable conditions for habitability. An analysis of 1,055 planets around main sequence stars identifies three subsamples of star–planet synchronization: subsynchronized, dominant for periods shorter than 6.2 days; supersynchronized, for periods longer than 13.5 days; and a transitional regime in between. Synchronized systems are a minority, contrary to eclipsing binaries.