On the characterisation and origin of the circumprimary retrograde planetary orbit in ν Octantis

Abstract

Formation and orbital stability of S-type circumstellar planets are severely suppressed in tight binary star systems. nu Octantis, whose stellar components have a mean separation of 2.6 AU, has long been suspected of hosting a retrograde circumprimary planet that resides midway between the stars. Despite all spectroscopic, astrometric, photometric and chromospheric evidence over the past two decades pointing towards the planetary explanation, the absence of observational precedents and the unprecedented planetary parameter space that seriously confronts our theoretical understanding of planet formation and orbital dynamics have cast doubt on the reality of the planet. It is hardly conceivable how an S-type planet ends up in a retrograde and exceptionally wide orbit in the tight binary star system.

In this work, motivated by the latest radial-velocity data obtained with HARPS, which consolidate the planet hypothesis, and also recent verification of the stellar companion as a white dwarf through SPHERE direct imaging, we presumed that the planet is real and analysed the RV data using the Simplex algorithm implemented in the EXO-STRIKER for characterising the orbital configuration of nu Octantis. The best dynamical fit represents a mutually inclined planet-binary orbital configuration, and we also acquired a dynamical fit by forcing the planetary orbit to be coplanar retrograde as it had been found dynamically stable by previous studies. Besides, we considered a Gaussian process that attempts to model correlated noise due to stellar rotational modulations. While a stable orbital solution for nu Octantis was absent in the literature, we managed to find a stable orbital configuration for the system that could reconcile with our orbital solution.

By using a modified Wisdom-Holman integrator and SYMBA, our dynamical analysis confirms that the planetary orbit must be retrograde and almost coplanar. From the dynamical standpoint, the orbital inclination of the planetary orbit has been constrained to within 5 degrees relative to the binary orbital plane. Exploration of possible primordial orbital settings of the binary star suggests that an early in-situ planet formation scenario is not favourable because of the well-constrained binary separation at the periastron of about 1.3 AU. Binary stellar evolution potentially has a significant bearing on the formation and orbital evolution of the planetary system. The planet might have originated from either a second-generation protoplanetary disc or a Tatooine-like P-type orbit.