Warm Saturns: On the Nature of Rings around Extrasolar Planets That Reside inside the Ice Line

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 radius 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 yr 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 yr. 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 particularly exciting, since a planet's J_2 reveals information about its interior structure. Furthermore, measurements of an exoplanet's J_2 from warped rings and of its oblateness 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 semimajor axes ~0.1 AU and larger