# A deeper view of the CoRoT-9 planetary system. A small non-zero eccentricity for CoRoT-9b likely generated by planet-planet scattering

## TL;DR

This study refines the parameters of the CoRoT-9 planetary system, confirms a small non-zero eccentricity of CoRoT-9b, and suggests planet-planet scattering as the likely origin of its orbit without causing spin-orbit misalignment.

## Contribution

The paper provides new radial velocity and transit data, confirming the eccentricity and proposing a planet-planet scattering scenario as the origin of CoRoT-9b's orbit.

## Key findings

- Confirmed non-zero eccentricity of CoRoT-9b ($e=0.133$)
- No evidence for additional planets in the system
- Eccentricity likely caused by planet-planet scattering

## Abstract

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.