Constraints on sub-Neptune planet candidate KOI-972.01 via joint variability/gravity-darkening analysis

Abstract

We analyze Kepler photometry of transiting planet candidate KOI-972.01, accounting for both stellar variability and gravity darkening. KOI-972.01 stands out because of its small radius, less than that of Neptune, and because of its intermediate orbit period at 13.12 days, long enough to avoid significant tidal evolution, and thus it represents an underexplored exoplanet class. The parent star of KOI-972.01 is a rapidly rotating δ-Scuti variable, complicating transit lightcurve interpretation but also offering a potential independent source of stellar parameters. We measure the stellar rotation period (16.2 hr) by identifying the stellar rotation frequency and subsequently place a constraint on the stellar obliquity of no greater than 10°., but have difficulty isolating individual oscillation modes in the periodogram owing to time variation of the δ-Scuti oscillations. After subtracting the stellar oscillations, lightcurve fits place the transiting object radius at 3.07 ± 0.09 R⊕, but the shallow transit prevents useful constraints on the system’s spin–orbit alignment.