Nu Octantis Ab

Nu Octantis Ab (ν Octantis Ab) is a gas giant exoplanet. It is orbiting around the subgiant star Nu Octantis A, which is part of a binary star system. Nu Octantis Ab was discovered by analysing periodic oscillations on its host star's radial velocity, the so-called radial velocity method. This planet is notable for its unusual orbit, which lies halfway between the orbit of the two stars in the Nu Octantis system. Such orbital configuration is problematic, since gravitational perturbations with the secondary star would make its orbit unstable, and current models for planetary formation preclude the formation of a planet on such an orbit.

It was initially discovered in 2009, but due to difficulties on the stability and formation, the planet has been disputed by multiple studies. However, alternative explanations for the observed variations in radial velocity have been discredited or weakened, and in 2025 a study reported the confirmation of the planet's existence.

As of 2025, stable orbits have been found, but these imply a retrograde orbit relative to the binary's motion, making more difficult an explanation to the formation of a planet on such a tight orbit. Two hypotheses have been proposed, and are based on the secondary being a white dwarf that lost mass during its evolution, leading to the instability of a former planetary system on a circumbinary orbit, or the formation of a planet via an accretion disk made up of the lost mass.