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Description
We have measured the infrared transit of the extrasolar planet HD 209458b using the Spitzer Space Telescope. We observed two primary eclipse events (one partial and one complete transit) using the 24um array of the Multiband Imaging Photometer for Spitzer (MIPS). We analyzed a total of 2392 individual images (10s integrations) of the planetary system, recorded before, during, and after transit. We performed optimal photometry on the images and used the local zodiacal light as a short-term flux reference. At this long wavelength, the transit curve has a simple boxlike shape, allowing robust solutions for the stellar and planetary radii independent of stellar limb darkening, which is negligible at 24um. We derive a stellar radius of R*=(1.06+/-0.07)R_{sun}_, a planetary radius of Rp=(1.26+/-0.08)R_J_, and a stellar mass of 1.17M_{sun}_. Within the errors, our results agree with the measurements at visible wavelengths. The 24um radius of the planet therefore does not differ significantly compared to the visible result. We point out the potential for deriving extrasolar transiting planet radii to high accuracy using transit photometry at slightly shorter IR wavelengths where greater photometric precision is possible.
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