Can We Probe the Atmospheric Composition of an Extrasolar Planet from Its Reflection Spectrum in a High–magnification Microlensing Event?

We revisit the possibility of detecting an extrasolar planet around a background star as it crosses the fold caustic of a foreground binary lens. During such an event, the planet’s flux can be magnified by a factor of ∼ 100 or more. We find that the detectability of the planet depends strongly on the orientation of its orbit relative to the caustic. If the source star is inside the inter–caustic region, detecting the caustic–crossing planet is difficult against the magnified flux of its parent star. In the more favorable configuration, when the star is outside the inter–caustic region when the planet crosses the caustic, we find that a close–in Jupiter–like planet around a Sun–like star at a distance of 8 kpc is detectable in 8-minute integrations with a 10m telescope at S/N ∼ 15. In this example, we find further that the presence of methane, at its measured abundance in Jupiter, and/or water, sodium and potassium, at the abundances expected in theoretical atmosphere models of close–in Jupiters, could be inferred from a non– detection of the planet in strong broad absorption bands at 0.6 − 1.4μm caused by these compounds, accompanied by a S/N ∼ 10 detection in adjacent bands. We conclude that future generations of large telescopes might be able to probe the composition of the atmospheres of distant extrasolar planets. Subject headings: gravitational lensing – planetary systems – stars: atmospheres – stars:individual (HD209458) – astrobiology – astrochemistry