  We analyse the dynamics of the multiple planet system HD 181433. This
consists of two gas giant planets (c and d) with msin i = 0.65 MJup and 0.53
MJup orbiting with periods 975 and 2468 days, respectively. The two planets
appear to be in a 5:2 mean motion resonance, as this is required for the system
to be dynamically stable. A third planet with mass m_b sin i = 0.023 MJup
orbits close to the star with orbital period 9.37 days. Each planet orbit is
significantly eccentric, with current values estimated to be e_b = 0.39, e_c =
0.27 and e_d = 0.47. In this paper we assess different scenarios that may
explain the origin of these eccentric orbits, with particular focus on the
innermost body, noting that the large eccentricity of planet b cannot be
explained through secular interaction with the outer pair. We consider a
scenario in which the system previously contained an additional giant planet
that was ejected during a period of dynamical instability among the planets.
N-body simulations are presented that demonstrate that during scattering and
ejection among the outer planets a close encounter between a giant and the
inner body can raise e_b to its observed value. We consider the possibility
that an undetected planet in the system increases the secular forcing of planet
b by the exterior giant planets, but we find that the resulting eccentricity is
not large enough to agree with the observed one. We also consider a scenario in
which the spin-down of the central star causes the system to pass through
secular resonance. Spin-down rates below the critical value lead to longterm
capture of planet b in secular resonance, driving the eccentricity toward
unity. If additional short-period low mass planets are present in the system,
however, we find that mutual scattering can release planet b from the secular
resonance, leading to a system with orbital parameters similar to those
observed today.

Earth and Planetary Astrophysics