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.

Characteristics

Based on its mass of 2.19 Jupiter masses (MJ), Nu Octantis Ab is classified as a gas giant, a category of planets like Jupiter or Saturn which are made of gases such as hydrogen and helium around a solid core, and have no surface like the terrestrial planets. Since this planet was detected only by the indirect Doppler spectroscopy method, which is based on mass, other properties like size and density are unknown. Nu Octantis Ab takes 404 days (1.11 years) to complete an orbit and has a semi-major axis of 1.24±0.02 astronomical units or 1.24 times the Earth-Sun distance, and equivalent to 47.5% of the binary's semi-major axis. This is the highest ratio ever reported. All other S-type planets, orbiting one component of a star system, have much smaller orbits relative to that of the outer star. Nu Octantis Ab therefore is exceptional due to its tight architecture. Nu Octantis Ab is orbiting retrograde relative to the orbit of Nu Octantis A and B. If it were prograde (i.e. same direction), the orbit would be highly unstable since strong gravitational perturbations of the secondary star would eject it from its current position. The orbital inclination relative to Earth is 108.2°, somewhat coplanar with the binary star's inclnation of 71.8°. The eccentricity is low, at 0.2.

Host star

Nu Octantis A is a subgiant star, which is exhausting the hydrogen at its core and expanding in size; currently it is 5.04 times larger than the Sun, and 13 times more luminous. It lies in the constellation of Octans, which contains the southern celestial pole, and due to its location close to this pole, is invisible to most of the Northern Hemisphere. At a moderate apparent magnitude of 3.73, it is the brightest star of this rather faint constellation, despite its Bayer designation. It is part of a binary star system, and dominates the energy output of the system: The secondary, a white dwarf, is much less luminous, and could only be detected from indirect methods such as spectroscopy or astrometry. Initially both stellar components were separated by 1.31±0.07 AU and had masses of 1.38+0.04−0.03 solar masses (M☉) and 2.36+0.13−0.15 M☉. Around 900 million years after formation, the secondary started to exhaust hydrogen at its core, expanded to a red giant, and then became the current white dwarf. During this process, the orbit become wider, to 2.61±0.03 AU, and the secondary lost most of its original mass (around 1.8 M☉, of which 0.2 M☉ were accreted to the primary). The names Nu Octantis A, Nu Octantis B, and Nu Octantis Ab, derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).

Proposed formation scenarios Based on the current knowledge of planetary formation, it is impossible that Nu Oct Ab formed on its current orbit and at the same time the stellar components formed; the nature of the secondary star implies a closer primordial orbit, and the planet's retrograde orbit does not fit the model. The primordial ν Oct A and ν Oct B are estimated to be separated by 1.31 AU, almost the same value of the planet's separation of 1.24±0.02 AU, implying it could not form in this situation. Planets form by the accretion of planetesimals in a primordial protoplanetary disk, but in this system the binary precluded the formation and evolution of such a disk. The size of a protoplanetary disk around either star should be less than 0.41 and 0.52 AU, respectively, regions so close and so hot that the gas temperature is too high to form a gas giant planet. Two theories that explain the formation of ν Oct Ab were raised in a 2025 study by Ho Wan Cheng et al. These theories are based on the evolution of the system, whose secondary component was initially more massive, but during its transition from main sequence star to a red giant, and then its death as a white dwarf, it lost significant part of its mass. Less mass in the system result in less gravitational force.

Planet-planet scattering This theory proposes that two circumbinary planets (which orbit both stars on a wider orbit) existed. These planets had a stable orbit until ν Oct B evolved to a white dwarf. During the mass loss, the orbits of both planets expanded by about 75%, while the planet-planet separation decreased by 17%. This smaller separation made the planetary system unstable, leading to the one of them being ejected to its current, retrograde orbit.

Second generation formation This theory proposes that Nu Oct B's transition from a red giant to a white dwarf led to the formation of an accretion disk around Nu Oct A with properties resembling a protoplanetary disk. Depending on the disk's inclination, it could have been retrograde and coplanar relative to the binary's motion, or was forced to this configuration via the Kozai mechanism.

Discovery and follow-up studies

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