What do you call an object that has all the characteristics of a moon, but whose mass compares to that of a giant planet? The latest observations of a system located 133 light-years from Earth pose this very question.
An international team examined the planet HD 206893 B, a gas giant far more massive than Jupiter. Using the GRAVITY instrument on the Very Large Telescope in Chile, astronomers detected a regular oscillation in the planet's trajectory around its star. This back-and-forth motion, which repeats every nine months, cannot be explained by the planet's orbit alone.
According to the researchers, this oscillation reveals the presence of a massive companion that is gravitationally perturbing the planet. The candidate object would orbit HD 206893 B at a distance equivalent to one-fifth of the Earth-Sun distance. Furthermore, its inclined orbit could point to a turbulent history for this system.
The most notable element of this potential observation concerns the size of the companion object. Its mass is estimated at about 40% that of Jupiter, which represents nine times the mass of Neptune.
Such a difference calls into question the very definition of a moon. Although there is not yet an official classification for exomoons, astronomers generally use this term for any body orbiting a planet. However, the boundary between a very massive moon and a low-mass companion blurs in such an extreme case.
This detection was made possible by an astrometry technique, which measures the positions of celestial objects with great accuracy. The team spotted tiny variations in the planet's motion, betraying the gravitational influence of another body.
This approach seems promising for identifying other extrasolar moons, particularly those orbiting far from their host star, where they have a greater chance of being stable. As with the first exoplanets discovered a few decades ago, the first exomoons detected will likely be the most massive.
If validated, this observation would offer a new perspective on the diversity of planetary systems. It reveals that our current view, based solely on the example of the Solar System, remains very incomplete.