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 mu m band of the InfraRed Array Camera (IRAC). The high accuracy of the transit data and the weak limb-darkening in the 8 mu m IRAC band allow us to derive ( assuming M = 0.44 +/- 0.04 M-circle dot for the primary) a precise value for the planetary radius (4.19(+ 0.21) (-0.16) R-circle plus), the stellar radius (0.463(+0.022) (-0.017) R-circle dot), the orbital inclination (85.90 degrees(+0.19 degrees)(-0.18 degrees)) and transit timing (2454280.78186(-0.00008)(+ 0.00015) 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.