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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, CO2 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.

The three‐planet extrasolar system of HD 181433 has been detected with HARPS. The best‐fitting solution, announced by the discovery team, describes a highly unstable, self‐disrupting configuration. In fact, a narrow observational window, only partially covering the longest orbital period, can lead to solutions representing unrealistic scenarios. Taking into account the dynamical stability as an additional observable while interpreting the radial velocity (RV) data, we can analyse the phase space in a neighbourhood of the statistically best fit and derive dynamically stable configurations that reproduce the observed RV signal. Our Newtonian stable best‐fitting model is capable of surviving for at least 250 Myr. The two giant companions are found to be locked in the 5:2 mean motion resonance (MMR) as Jupiter and Saturn in the Solar system. This mechanism does not allow close encounters even in case of highly eccentric orbits. Moreover, planets c and d are located in regions spanned by many other strong low‐order MMRs. We study the dynamics of some plausible scenarios, and we illustrate the behaviours caused by secular apsidal resonances and MMRs. Furthermore, we find a terrestrial planet in the habitable zone of HD 181433 can retain stability. Apart from filling an empty gap in the system, this body could offer a harbour for life indeed. Additional measurements are necessary in order to investigate this hypothesis and can confirm the predictions outlined in the paper.

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