Astronomers Have Detected an Exomoon for the First Time

To confirm that the observed signal was indeed caused by a moon, the research team also examined other possible factors. These included apparent periods resulting from errors in corrections for Earth’s orbital motion, seasonal variations in atmospheric conditions, and the effects of the brown dwarf’s own rotation.

Furthermore, calculations of the Roche limit—the boundary beyond which a satellite would be torn apart by the brown dwarf’s tidal forces—and the Hill radius—the radius of the brown dwarf’s gravitational influence—confirmed that the satellite’s orbit falls within a range where it can exist in a physically stable manner.

According to the researchers, this is the first evidence of a satellite orbiting a brown dwarf companion obtained using this method. A few months earlier, another team had gathered clues suggesting the presence of a satellite during observations of the HD 206893 star system using the VLT Interferometer but had not yet achieved a definitive detection.

The object discovered in this study occupies a unique position that cannot be fully understood using the conventional framework for moons in our solar system. “We have a clear delineation between the planets and the Sun in the Solar System, so defining things like moons is simple,” says Alice Zurlo, an astrophysicist at Diego Portales University, in a news release. “In the CD-35 2722 system, where we are blurring the lines between stars, planets, and moons, the whole thing becomes more complicated to describe.”

It remains unclear whether this object should be called a moon. ESO also notes that there is no officially recognized definition for exomoons and the researchers use the term “exosatellite.” The paper itself acknowledges that it is uncertain whether this object will meet future criteria to be considered a moon, but that the discovery is a step toward creating a definitive detection.

Researchers believe this discovery will serve as a catalyst for identifying new directions in future theories of planet formation and celestial mechanics. Furthermore, if there are smaller, rocky moons like the one in the new paper, they could be subjected to tidal heating from brown dwarfs, potentially creating environments suitable for life even at greater distances from their stars—a development that might also have implications for the search for extraterrestrial life.

It’s also possible that once the next-generation Extremely Large Telescope equipped with a 39-meter primary mirror is completed, it will be possible to detect even smaller exomoons.

This story originally appeared on WIRED Japan and has been translated from Japanese.