Systematic Search for Rings around Kepler Planet Candidates: Constraints on Ring Size and Occurrence Rate

We perform a systematic search for rings around 168 Kepler planet candidates with sufficient signal-to-noise ratios that are selected from all the short-cadence data. We fit ringed and ringless models to their lightcurves, and compare the fitting results to search for the signatures of planetary rings. First, we identify 29 tentative systems, for which the ringed models exhibit statistically significant improvement over the ringless models. The lightcurves of those systems are individually examined, but we are not able to identify any candidate that indicates evidence for rings. In turn, we find out several mechanisms of false-positives that would produce ring-like signals, and the null detection enables us to place upper limits on the size of rings. Furthermore, assuming the tidal alignment between axes of the planetary rings and orbits, we conclude that the occurrence rate of rings larger than twice the planetary radius is less than 15 percent. Even though the majority of our targets are short-period planets, our null detection provides statistical and quantitative constraints on largely uncertain theoretical models of origin, formation, and evolution of planetary rings.