A small international team of astronomers has confirmed that a binary star system with an odd signal has a companion—a planet roughly twice the size of Jupiter, which may have emerged from a circumbinary orbit or from a second-generation protoplanetary disk.
In their study, published in the journal Nature, the group used improved measuring devices to confirm the existence of the planet and to explain how the system can remain stable. Kendall Sullivan, with the University of California, Santa Cruz, has published a News and Views piece in the same journal issue outlining the work done by the team.
In 2004, an astronomer at the University of Canterbury, in New Zealand, observed something odd in the v Octantis binary star system—a repeating signal between the two stars that made up the system. That finding led to debate within the astronomy community regarding its nature. Many felt that it was evidence of a large planet; others disagreed, suggesting that the characteristics of the stars would make the existence of such a planet impossible.
In this new study, the researchers found evidence via HARPS spectrograph data from the European Southern Observatory’s telescope in Chile, showing that not only is the signal from ν Octantis evidence of the existence of a planet, it also offers evidence of the planet as retrograde.
They also found other planetary evidence using other new technology and point out that the signal has remained stable for over two decades, proving it was not due to stellar activity.
The main star in the system is a sub-giant. The smaller star, a white dwarf, and the planet both orbit the larger star. But, oddly enough, they go around the star in opposite directions. The researchers suggest that is why the system remains stable.
They also note that for such a system to have developed, the planet may have once have circled both stars or developed from the material ejected from the white star as it was transitioning from a red giant after it ran out of fuel and its core collapsed.
More information:
Ho Wan Cheng et al, A retrograde planet in a tight binary star system with a white dwarf, Nature (2025). DOI: 10.1038/s41586-025-09006-x
Kendall Sullivan, Dying stars give a second wind to exoplanet formation, Nature (2025). DOI: 10.1038/d41586-025-01445-w
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Odd binary star system has a huge planetary companion (2025, May 22)
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