The anomalously large transiting planet around Boyajian's star: evidence for a circumplanetary ring

Abstract

A recent study reported a transiting giant-planet candidate with a mass of $9.4^{+4.9}_{-4.1}\,M_{\rm J}$ and a radius of $1.7\pm0.1\,R_{\rm J}$ on a wide orbit around Boyajian's star (KIC 8462852, also called Tabby's star), which is well known for its irregular, deep flux dips. The reported radius is puzzling, because cool giant planets of $\sim$10\,$M_{\rm Jup}$ are expected to have radii close to Jupiter's, so the companion appears $\sim$60--70\% larger than standard models allow. Here we explore the solution that the planet is actually ringed. We develop a fast and accurate code to compute the transit light curve due to a ringed planet, and apply it to the TESS light curve. The ring model is marginally favoured by the data by $Δχ^2>10$ and a difference in Bayesian information criterion (BIC) of $<2$. The preferred configuration is a Jupiter-sized ($1.1\,R_{\rm J}$) planet surrounded by an opaque, highly flattened ring extending to $3.1$--$4.4$ planetary radii with a nearly polar obliquity. While the ring geometry is uncertain, simple estimates suggest that a debris annulus extending out to the planet's Roche limit is needed to explain the large transit depth. The ring thus removes the radius anomaly altogether and is qualitatively consistent with the dusty, comet-shepherding environment inferred from the star's historic dimmings.