Letter to the Editor

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

¹ Observatoire de l'Université de Genève, 1290 Sauverny, Switzerland e-mail: michael.gillon@obs.unige.ch
² Institut d'Astrophysique et de Géophysique, Université de Liège, 4000 Liège, Belgium
³ Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001, USA
⁴ Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal
⁵ School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel

Received: 16 July 2007
Accepted: 21 July 2007

Abstract

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.