Accurate Spitzer infrared radius measurement for the hot Neptune GJ 436b

We present Spitzer Space Telescope infrared photometry of a primary transit of the hot Neptune GJ 436b. The observations were obtained using the 8 μm band of the InfraRed Array Camera (IRAC). The high accuracy of the transit data and the weak limb-darkening in the 8 μm IRAC band allow us to derive (assuming M = 0.44 ± 0.04 for the primary) a precise value for the planetary radius (4.19 ), the stellar radius (0.463 ), the orbital inclination (85.90°) and transit timing (2454280.78186 HJD). Assuming current planet models, an internal structure similar to that of Neptune with a small H/He envelope is necessary to account for the measured radius of GJ 436b.