Uncovering the Role of Binary Star Evolution in the Origin of a Retrograde Second-generation Planet

Most stars in the Universe exist in binary or multiple star systems, where the presence of close-in companion stars in such systems can adversely influence the formation and orbital stability of planets around one of the stars. An international team of astrophysicists, involved scientist from institut de planétologie et d’astrophysique de Grenoble (IPAG-OSUG, CNRS/UGA), led by Professor Man Hoi LEE from the Department of Earth Sciences and the Department of Physics at The University of Hong Kong (HKU) and Mr Ho Wan CHENG, an MPhil student in his team, has confirmed the existence of a planet in an unprecedented retrograde orbit (moving in the opposite direction to the binary’s orbit) in the nu Octantis binary star system and revealed the role of binary star evolution in the origin of this planet. The finding has been published in the journal Nature.

nu Octantis is a tight binary star system comprising a primary subgiant star, nu Oct A, with about 1.6 times the mass of the Sun, and a secondary star, nu Oct B, with about half the mass of the Sun. The two stars orbit each other with a period of 1,050 days. In 2004, an additional periodic signal in the radial velocity observations (measurements of how a star moves toward or away from us) of this system was reported, consistent with the presence of a Jovian planet of about twice the mass of Jupiter orbiting around the primary star, nu Oct A with a period of about 400 days.

However, the existence of this planet has been controversial because its orbit would be so wide that it could only remain stable if it were retrograde and moved in the opposite direction to the orbit of the binary. There were no observational precedents for such a planet and strong theoretical grounds against its formation. To settle the debate, the research team obtained new high-precision radial velocity observations with the European Southern Observatory (ESO)’s HARPS spectrograph, which confirmed the existence of the planet signal.

Another key focus of the new study was the determination of the nature of the secondary star nu Oct B. The mass of nu Oct B suggests that it could be either a low-mass main-sequence star or a white dwarf. To identify which type of star nu Oct B is, the research team used the adaptive optics imaging instrument SPHERE at ESO’s Very Large Telescope to observe the system. The fact that nu Oct B was not detected in these observations indicated that it must be a very faint white dwarf. This suggests that the binary system has evolved significantly since its formation, as nu Oct B has already ejected most of its mass and entered the final stage of its stellar evolution.

From the analysis, the authors have found out that the system is about 2.9 billion years old and that nu Oct B was initially about 2.4 times the mass of the Sun and evolved to a white dwarf about 2 billion years ago: the planet could not have formed around nu Oct A at the same time as the stars. When nu Oct B evolved into a white dwarf about 2 billion years ago, the planet could have either formed in a retrograde disc of material around nu Oct A accreted from the mass ejected by nu Oct B, or it could be captured from a prograde orbit around the binary into a retrograde orbit around nu Oct A.

With this system, we might be witnessing the first compelling case of a second-generation planet; either captured, or formed from material expelled by nu Oct B, which lost more than 75% of its primordial mass to become a white dwarf.

Reference

Ho Wan Cheng, Trifon Trifonov, Man Hoi Lee, Faustine Cantalloube, Sabine Reffert, David Ramm, and Andreas Quirrenbach. A retrograde planet in a tight binary star system with a white dwarf. Nature 2025.
DOI : https://doi.org/10.1038/s41586-025-09006-x

This news was initially published by The University of Hong Kong (HKU).

Updated on 4 June 2025