The conjectured S-type retrograde planet in ν Octantis: more evidence including four years of iodine-cell radial velocities

We report 1212 radial-velocity (RV) measurements obtained in the years 2009–2013 using an iodine cell for the spectroscopic binary ν Octantis (K1 III/IV). This system ( $a_{\rm \scriptscriptstyle bin} \sim$ 2.6 au, P ∼ 1050 d) is conjectured to have a Jovian planet with a semimajor axis half that of the binary host. The extreme geometry only permits long-term stability if the planet is in a retrograde orbit. Whilst the reality of the planet (P ∼ 415 d) remains uncertain, other scenarios (stellar variability or apsidal motion caused by a yet unobserved third star) continue to appear substantially less credible based on cross-correlation function bisectors, line-depth ratios and many other independent details. If this evidence is validated but the planet is disproved, the claims of other planets using RVs will be seriously challenged. We also describe a significant revision to the previously published RVs and the full set of 1437 RVs now encompasses nearly 13 yr. The sensitive orbital dynamics allow us to constrain the 3D architecture with a broad prior probability distribution on the mutual inclination, which with posterior samples obtained from an N-body Markov chain Monte Carlo is found to be $152{^{\circ}_{.}}5\pm^{0.7}_{0.6}$ . None of these samples are dynamically stable beyond 106 yr. However, a grid search around the best-fitting solution finds a region that has many models stable for 107 yr, and includes one model within 1σ that is stable for at least 108 yr. The planet's exceptional nature demands robust independent verification and makes the theoretical understanding of its formation a worthy challenge.