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Nu Octantis Ab

Discovery
Discovery date	April 2009 (suspected) May 2025 (confirmed)
Designations
Alternative names	ν Octantis Ab, ν Oct Ab
Orbital characteristics
Semi-major axis	1.24±0.02 AU
Eccentricity	0.195+0.050 −0.037
Orbital period (sidereal)	402.4+7.7 −6.0 d (1.102+0.021 −0.016 years)
Inclination	108.2°
Longitude of ascending node	266.5
Argument of perihelion	100+32 −15
Semi-amplitude	42.7+2.6 −1.6 m/s
Star	Nu Octantis A
Physical characteristics
Mass	2.19±0.11 MJ

Nu Octantis Ab (ν Octantis Ab) is a gas giant exoplanet orbiting around the subgiant star Nu Octantis A. It was discovered by the analysis period 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 configuration is challenging, since gravitational interactions with one of the stars would make its orbit unstable, and current models for planetary formation preclude the formation of a planet on such orbit.

Due to difficulties on the stability and formation, the planet has been disputed until a 2025 study provided unambigous evidence of its existence. Alternative explanations for the radial velocity variations were discredited time after time, and stable configurations were found, but these imply a retrograde orbit relative to the binary's motion, further difficulting an explanation 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 circumbinary or the formation of a planet with the lost mass.

Characteristics

Based on its mass of

Proposed formation scenarios

Based on the current knowledge of planetary formation, the nature of the secondary star which implies a closer primordial orbit, and the planet's retrograde orbit, it is impossible Nu Oct Ab formed on its current orbit at the same time the stellar components formed. The primordial ν Oct A and ν Oct B are estimated to be separated by 1.31±0.07 AU, almost the same value of the planet's separation of 1.24±0.02 AU, implying it could not form in situ. The sizes of protoplanetary disks 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 Nu Octantis, whose secondary component, initially of a mass 2.36+0.13
−0.15 M_☉, started to exhaust all the hydrogen at its core, expand into a red giant, and then evaporated to a white dwarf of mass 0.57±0.01 M_☉, losing most of its mass during the proccess.

Planet-planet scattering

This theory propose 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 propose that the transition for a red giant to a white dwarf led to the formation of an accretion disk around the primary star, with properties that resemble protoplanetary disks that form regular planets. 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

Diagram showing how an extrasolar planet orbiting a star could produce changes in position and velocity of the latter as they orbit their common center of mass (red cross).

Doppler spectroscopy detects periodic shifts in radial velocity by recording variations in the color of light from the host star. When a star moves towards the Earth, its spectrum is blueshifted, while it is redshifted when it moves away from us. By analyzing these spectral shifts, astronomers can deduce the gravitational influence of extrasolar planets.

The first evidence for the planet's existence was gathered in a April 2009 study made by David J. Ramm and other three astronomers. The researchers found a periodic variation on ν Oct A's radial velocity, which could not be explained by intrinsic variability, stellar activity or starspots, leaving the orbital motion of an exoplanet as a plausible scenario. The data suggested a minimum mass of 2.4 M_J and a low-eccentricity orbit of separation 1.3 AU that has a 5:2 resonance with ν Oct B. However, the existence of a planet on the proposed orbit would be unlikely: It would lack orbital stability and models of planetary formation do not allow the formation of a planet on such orbit. However, the astronomers suggested the planet could still exist, since subsequent research could revise the system's dynamical stability.

In October 2010, a study by J. Eberle and M. Cuntz found that if the planet's orbit were retrograde relative to the movement of the companion star, it would be stable for at least 10 million years. This discovery mean that the existence of Nu Octantis Ab could still be a possibility.

The planet was again challenged in a January 2012 study by M. H. M. Morais and A. C. M. Correia. They found that the radial velocity data showed a retrograde precession of −0.86 °/yr. For the precession to be explained by the planet, its inclination relative to the binary's orbit should be over 45°, but the researchers did not find a stable orbit on such inclination. This raised the possibility that ν Oct B is actually a close binary, and the precession due to this binary system could provide an alternative explanation to the radial velocity variations on ν Oct A, as opposed to a planet. More radial velocity observations were needed to decide which explanation is correct.

A March 2013 study by Krzysztof Gozdziewski and other three astronomers also challenged the planet's existence based on stability concerns.

In 2016, D. J. Ramm and more eight astronomers published an updated study with more radial velocity observations. The alternative hypothesis gathered by Morais & Correia (2012) was ruled out, with a much smaller and prograde precession been found, as well as no evidence for a third star. The study found an orbital configuration that is stable for at least 100 million years. The presence of a retrograde planet is the best scenario as per the updated data, but its formation on the current orbit would be highly unlikely. As of such, the researchers raised exotic alternative possibilities, such as ν Oct Ab being captured from another star system.

A 2021 study by D. J. Ramm and six other astronomers, analysing photometry from the Hipparcos, TESS and Gaia satellite snd spectroscopic data, again disfavour non-planetary explanations for the RV variations, further supporting the existence of the planet. In particular, the star does not show any photometric or spectroscopic variability. The study also posited that ν Oct B could be a white dwarf rather than a main sequence star, a theory which has some evidence from observations taken in the 70s. If the companion is a white dwarf, the planet could have formed during the progenitor's transition from a red giant to a white dwarf, creating an accretion disk that formed the planet.

The planet would finally be confirmed in May 2025, by a study by Ho Wan Cheng and other six astronomers, including David Ramm. This study definitively ruled out any non-planetary hypothesis to the radial velocity variations, and 70% of their orbital configurations were stable for long timescales. A remarkable discovery reported in this study is that the compaion is a faint white dwarf, as adaptive optics failed to find a star at its distance, further supporting the theory of second-generation formation that was raised by Ramm et al. (2021).