The occurrence rate of S-type planets in binary star systems is notably suppressed due to gravitational perturbation of close stellar companions. Nevertheless, a planet around the primary star has been found midway between the two stellar components of nu Octantis characterised by an exceptionally small orbital semi-major axis of 2.6 au. Despite the theoretical challenges, all standard observational tests have hitherto ruled out any non-planetary alternatives. We have performed nested sampling fitting to the radial velocity data and analysed the dynamical stability of the resulting samples. The mutually inclined fits were found to have mutual inclination of about 154° between the planetary orbit and the binary orbit. While almost all of the mutually inclined fits are dynamically unstable beyond 10? yr, a significant fraction of the coplanar retrograde fits remains stable for at least 10? yr. This leaves a nearly coplanar retrograde planetary orbit the only option for dynamical stability, which must be explained by any scenario for the planet’s origin. The possibility that the secondary star of 0.57 M? is a white dwarf has been investigated. Besides, we have developed a novel approach combining a binary stellar evolution code with a nested sampling scheme to explore plausible primordial orbital configurations of the binary that might shed light on the origin of the nu Octantis planet.
[Poster PDF File]