A gas giant outside our solar system is showing signs of a massive moon. This planet, known as HD 206893 B, is about 28 times heavier than Jupiter and orbits a star 133 light-years away. The exciting part? If this moon exists, it could weigh roughly half as much as Jupiter, which would challenge our understanding of what a moon really is.
The research team used the GRAVITY instrument at the Very Large Telescope in Chile to observe HD 206893 B. They noticed a “wobble” in the planet’s orbit, a signal that suggests it might be influenced by an unseen moon. “We detected a small but measurable back-and-forth wobble,” said Quentin Kral, the study’s leader. This wobble happens over about nine months and is similar to the distance between the Earth and its moon.
This finding comes from astrometry, a technique that measures the positions of stars and other objects over time. Kral pointed out that while this method has typically been used for bigger exoplanets, they applied it to slightly shorter timescales to uncover new details about HD 206893 B.
The potential moon is thought to orbit its giant parent planet every nine months, at a distance of about one-fifth of the distance between Earth and the Sun. Interestingly, its orbit is tilted by about 60 degrees, suggesting some past disruptions in the system.
Currently, the most massive moon we know in our solar system is Ganymede, which is significantly smaller than this potential exomoon. Kral emphasized that the weight difference raises questions about terminology. He stated, “There’s a huge gap between the largest known moons and what we’re observing here.” If confirmed, this moon may redefine what we think of as a moon.
Despite several suspected exomoon sightings in the past, astronomers have not officially confirmed any. Kral hopes this research will pave the way for that. “Exomoons are tricky to find,” he noted. The common transit method, which detects planets based on light dips as they cross in front of their stars, doesn’t work as well for moons. Astrometry provides a more reliable way to spot them, especially those farther from their stars, which might help in future discoveries.
Kral believes we’re just beginning to scratch the surface of exomoon research. He remarked, “As our observational tools improve, so will our definitions of what constitutes a moon.” This expanding knowledge could have exciting implications for our understanding of other planetary systems.
For more details, you can read the full research on arXiv, which has been accepted for publication in Astronomy & Astrophysics.
