Treating dynamical stability as an observable: a 5:2 mean motion resonance configuration for the extrasolar system HD 181433

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.