  Planetary companions to the source stars of a caustic-crossing binary
microlensing events can be detected via the deviation from the parent light
curves created when the caustic magnifies the star light reflecting off the
atmosphere or surface of the planets. The magnitude of the deviation is delta_p
e_p rho_p^{-1/2}, where e_p is the fraction of starlight reflected by the
planet and rho_p is the angular radius of the planet in units of angular
Einstein ring radius. Due to the extraordinarily high resolution achieved
during the caustic crossing, the detailed shapes of these perturbations are
sensitive to fine structures on and around the planets. We consider the
signatures of rings, satellites, and atmospheric features on caustic-crossing
microlensing light curves. We find that, for reasonable assumptions, rings
produce deviations of order 10% delta_p, whereas satellites, spots, and zonal
bands produce deviations of order 1% delta_p. We consider the detectability of
these features using current and future telescopes, and find that, with very
large apertures (>30m), ring systems may be detectable, whereas spots,
satellites, and zonal bands will generally be difficult to detect. We also
present a short discussion of the stability of rings around close-in planets,
noting that rings are likely to be lost to Poynting-Robertson drag on a
timescale of order 10^5 years, unless they are composed of large (>>1 cm)
particles, or are stabilized by satellites.
