Astronomers have discovered a planetary system so alien that scientists are struggling to find the right words to describe it. Using the European Southern Observatory's Very Large Telescope in Chile, researchers detected a Jupiter-sized world orbiting a brown dwarf 71 light-years away — and they deliberately refuse to call it a moon.
Published Wednesday in the journal Nature, the discovery centers on the star system CD-35 2722, located in the constellation Columba. The system contains a red dwarf star about 40% the mass of our Sun, circled by a brown dwarf — a celestial object that straddles the boundary between planet and star. That brown dwarf, in turn, hosts a newly detected gaseous world with at least 90% of Jupiter's mass.
A crisis of terminology
By earthly definitions, a body orbiting another body that orbits a star should be called a moon — or, since it lies beyond our solar system, an exomoon. But this object shares almost nothing with the small, rocky or icy moons of our solar system.
"This will certainly be a matter of debate," said Kevin Hoy, a doctoral student at Universidad Diego Portales in Chile and lead author of the study. "In general, most astronomers are fine with the term exosatellite, at least as a placeholder until we formally adopt definitions for objects like the one we found."
Co-author Alice Zurlo, an astrophysicist at the same university, was blunter: "We deliberately chose not to call it a moon. That's because this object is unlike any moon in our solar system. It's much more massive, and it doesn't orbit a planet — it orbits a brown dwarf, an object that's somewhere between a planet and a star."
A 'Pandora's box'
The exosatellite completes an orbit of the brown dwarf every 170 days at a distance about one-fifth of that separating Earth from the Sun. The brown dwarf itself is roughly 33 times more massive than Jupiter, 50% larger, and still hot from its formation.
"I think this discovery has opened a Pandora's box for the scientific community," Zurlo said. "This type of object is completely new and unusual, truly different from what we are familiar with in our solar system."
Open questions
Researchers are trying to determine the exosatellite's origins. It may have formed from the disk of gas and dust around the brown dwarf, similar to how planets form around stars. Or it may have been captured by the brown dwarf's gravity later in the system's chaotic history.
"Yep, this system is definitely weird," Hoy said. "There was likely a pretty chaotic dynamical history that left it in its current state, but it's really hard to tell exactly what kind of chaos it went through."
While more than 6,300 exoplanets have been discovered, finding exomoons has proven far more challenging, with only a handful of prior candidates. This discovery — the first confirmed exosatellite of its kind — could reshape how scientists classify objects beyond our solar system entirely.
