Probing structures of distant extrasolar planets with microlensing

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 starlight reflecting off the atmosphere or surface of the planets. The magnitude of the deviation is δ_p ∼ ∈_pρ_p^-1/2 where ∈_p is the fraction of starlight reflected by the planet and ρ_p is the angular radius of the planet in units of the angular Einstein ring radius. Because of 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 the order of 10%δ_p, whereas satellites, spots, and zonal bands produce deviations of the order of 1%δ_p. We consider the detectability of these features using current and future telescopes and find that, with very large apertures (>30 m), 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 the order of 105 yr, unless they are composed of large (≫ 1 cm) particles or are stabilized by satellites.