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The hot Jupiter HD 209458b was observed during primary transit at 3.6, 4.5, 5.8 and 8.0 μm using the Infrared Array Camera (IRAC) on the  Spitzer Space Telescope . We describe the procedures we adopted to correct for the systematic effects present in the IRAC data and the subsequent analysis. The light curves were fitted including limb‐darkening effects and fitted using Markov Chain Monte Carlo and prayer‐bead Monte Carlo techniques, obtaining almost identical results. The final depth measurements obtained by a combined Markov Chain Monte Carlo fit are at 3.6 μm,  1.469 ± 0.013  and  1.448 ± 0.013 per cent ; at 4.5 μm,  1.478 ± 0.017 per cent ; at 5.8 μm,  1.549 ± 0.015 per cent ; and at 8.0 μm,  1.535 ± 0.011 per cent . Our results clearly indicate the presence of water in the planetary atmosphere. Our broad‐band photometric measurements with IRAC prevent us from determining the additional presence of other molecules such as CO,  CO 2   and methane for which spectroscopy is needed. While water vapour with a mixing ratio of  combined with thermal profiles retrieved from the day side may provide a very good fit to our observations, this data set alone is unable to resolve completely the degeneracy between water abundance and atmospheric thermal profile.

Water in the atmosphere of HD 209458b from 3.6–8 μm IRAC photometric observations in primary transit