  CoRoT-9b is one of the rare long-period (P=95.3 days) transiting giant
planets with a measured mass known to date. We present a new analysis of the
CoRoT-9 system based on five years of radial-velocity (RV) monitoring with
HARPS and three new space-based transits observed with CoRoT and Spitzer.
Combining our new data with already published measurements we redetermine the
CoRoT-9 system parameters and find good agreement with the published values. We
uncover a higher significance for the small but non-zero eccentricity of
CoRoT-9b ($e=0.133^{+0.042}_{-0.037}$) and find no evidence for additional
planets in the system. We use simulations of planet-planet scattering to show
that the eccentricity of CoRoT-9b may have been generated by an instability in
which a $\sim 50~M_\oplus$ planet was ejected from the system. This scattering
would not have produced a spin-orbit misalignment, so we predict that CoRoT-9b
orbit should lie within a few degrees of the initial plane of the
protoplanetary disk. As a consequence, any significant stellar obliquity would
indicate that the disk was primordially tilted.
