Warm Saturns: On the Nature of Rings around Extrasolar Planets that Reside Inside the Ice Line

Abstract: We discuss the nature of rings that may exist around extrasolar planets. Taking the general properties of rings around the gas giants in the Solar System, we infer the likely properties of rings around exoplanets that reside inside the ice line. Due to their proximity to their host star, rings around such exoplanets must primarily consist of rocky materials. However, we find that despite the higher densities of rock compared to ice, most of the observed extrasolar planets with reliable radii measurements have sufficiently large Roche radii to support rings. For the currently known transiting extrasolar planets, Poynting-Robertson drag is not effective in significantly altering the dynamics of individual ring particles over a time span of $10^8$ years provided that they exceed about 1 m in size. In addition, we show that significantly smaller ring particles can exist in optically thick rings, for which we find typical ring lifetimes ranging from a few times $10^6$ to a few times $10^9$ years. Most interestingly, we find that many of the rings could have nontrivial Laplacian planes due to the increased effects of the orbital quadrupole caused by the exoplanets' proximity to their host star, allowing a constraint on the $J_2$ of extrasolar planets from ring observations. This is particular exciting, since a planet's $J_2$ reveals information about its interior structure. Furthermore, measurements of an exoplanet's oblateness and of its $J_2$, from warped rings, would together place limits on its spin period. Based on the constraints that we have derived for extrasolar rings, we anticipate that the best candidates for ring detections will come from transit observations by the Kepler spacecraft of extrasolar planets with semi-major axes $\sim 0.1$ AU and larger.