A binary star system is a pair of stars gravitationally bound and orbiting a common centre of mass. In 2004, David Ramm at the University of Canterbury in New Zealand spotted a mysterious repeating signal while observing the motion of a pair of stars in a system called Nu Octantis. The signal hinted that a massive planet, twice Jupiter’s size, might exist in that system. In a new study, a small group of astronomers used improved measuring devices to confirm the planet’s existence and explain how the system can remain stable.

Retrograde motion of the planet

According to the study, new data from the HARPS spectrograph at the European Southern Observatory, 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. These reversed trajectories reduce the risk of gravitational disruption and make the system stable.

The planet’s signal has remained consistent for more than 20 years, which strongly suggests it is not caused by stellar activity. According to Man Hoi Lee, co-author of the study, researchers are pretty sure about the planet’s existence. This highlights how long-term stability in the data supports the existence of this strange planet with a tight but stable path through the binary system.

Origin of the planet

There are two possibilities: the planet either used to orbit both stars at once but then radically shifted trajectory when one of the two stars became a white dwarf, or it was formed from the mass that the star ejected as it transformed into a white dwarf. Future observations and a lot more mathematical modelling may be able to pinpoint which of these scenarios is more likely to have occurred, but both are rather novel.