Astrophysicists make groundbreaking discoveries about the evolution of a binary star and its role in the origin of a retrograde planet orbiting in the opposite direction. Star Signs around the primary star in the opposite direction to the host star and its stellar companion as they go around each other.” Professor Lee said: “We also discovered that the secondary star ν Oct B is a white dwarf, which is the remnant of a star that has exhausted its nuclear fuel and reached the end of its life cycle. We showed that the planet could not have formed around ν Oct A at the same time as the stars. “This is the first compelling case of a secondgeneration planet, with the planet either captured from an originally circumbinary orbit (going around the binary) or formed from material expelled by ν Oct B, when ν Oct B evolved to become a white dwarf.” The existence of the planet signal was confirmed by taking new measurements using the European Southern Observatory (ESO)’s High Accuracy Radial velocity Planet Searcher spectrograph. “The radial velocity method measures the motion of the host star and it is one of the main methods for finding extrasolar planets. In this case, we now have data for over 18 years, and the planet signal is still there. We found, for the first time, stable fits to all radial velocity data, which require the planetary orbit to be retrograde,” said Professor Lee. To study the companion star, ν Oct B, the team used an adaptive optics imaging instrument, the Spectro-Polarimetric High-contrast Exoplanet Research, at ESO’s Very Large Telescope to observe the system. They knew that if ν Oct B was a small main sequence star (a smaller version of our Sun), it would be visible. “But we couldn’t find it in our imaging observation, so we concluded that the secondary star is a very faint white dwarf,” Professor Lee said. “We explored in detail possible primordial configurations of the binary, which showed that the planet could not have formed around ν Oct A at the same time as the stars. “Since there’s no way that planet could form in that orbit at the same time as the binary formed,” he continued, “we argued that this planet could be second generation and may have formed in a disc around the current primary star when the originally more massive companion star evolved into a white dwarf, because at that time the companion star would have ejected a lot of material, some of which likely formed a disc around the other star.” An international team of astrophysicists led by Professor Lee Man-hoi from HKU’s Department of Earth and Planetary Sciences and MPhil student Mr Cheng Ho-wan, have confirmed the existence of a planet in an unprecedented retrograde orbit in the nu Octantis (ν Oct) binary star system and revealed the role of binary star evolution in the origins of this planet. The existence of a planet around the primary star – ν Oct A, whose mass is about 1.6 times that of the Sun – was put forward in 2009, sparking much debate because the only way for it to be possible would be if its orbit were retrograde – that is, travelling in the opposite direction from the orbit of the companion star, which would be highly unusual. An additional periodic signal in the radial velocity observations of this system was first reported by Dr David Ramm, a co-author of this new paper, during his PhD studies at the University of Canterbury, New Zealand, in 2004. “Our research has resolved a decades-long debate over the existence of an unprecedentedly wide circumstellar planetary orbit in a tight binary star system, previously thought impossible based on standard models of planet formation and dynamics,” said Mr Cheng, who is lead author of the paper, and has been awarded the Li Ka Shing Prize for this research. “For our collaborator Dr Ramm, this has been a very long journey. It is truly exciting that we have now placed the final piece of the jigsaw. We confirmed that the planet is retrograde in the sense that it goes Long debate A conceptual illustration of the ν Oct system. From left: ν Oct A, the primary star; ν Oct Ab, the retrograde-orbiting planet; and ν Oct B, the secondary star which is a white dwarf. Another possibility the team put forward is that during the binary evolution, two original circumbinary planets could have become unstable. “Then one of the planets might have been captured into a retrograde orbit around the current primary star,” he said. Both scenarios are exciting to researchers and indicative of some of the major dramas that planetary systems can undergo. Given the rarity of a retrograde planet, Professor Lee is keen to research further if there are other systems that are similar. “Our work is now going in two directions,” he said. “One is to try and understand and better characterise the system we’ve already reported; and two is to try and see if there are other systems supporting this.” Mr Cheng added: “The backward-moving orbit in ν Oct is the first of its kind among the approximately 6,000 exoplanets discovered to date. Our discovery that the companion star is a white dwarf has shed light on the role of late-stage stellar evolution in shaping planetary systems. This finding opens new avenues for exploring planet formation and evolution scenarios, and prompts us to rethink the possible parameter space of exoplanets.” Two planets HKU Bulletin | Nov 2025 Research 28 29 We found, for the first time, stable fits to all radial velocity data, which require the planetary orbit to be retrograde. Professor Lee Man-hoi
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