Astronomers say they have found evidence of a moon-like object, which they are calling an "exosatellite," orbiting a brown dwarf in a young star system called CD-35 2722 — a find that, if confirmed, would be the first moon ever detected outside our solar system.
The suspected exosatellite is at least as massive as Jupiter, orbiting a brown dwarf more than 30 times Jupiter's own mass that in turn orbits a star roughly half the mass of our sun. Researchers made the detection using the CRIRES+ instrument on the European Southern Observatory's Very Large Telescope in Chile, applying the radial velocity method, which detects the tiny gravitational wobble a companion object induces in whatever it orbits. The same basic technique was used decades ago to find the first exoplanet discovered around a sun-like star.
"The exosatellite is clearly massive enough to be a planet" in its own right, said lead author Kevin Hoy, an ESO student based in Chile, but it does not orbit a star directly, making its classification unusual. Hoy described the arrangement as "super weird" compared with our own solar system. Alice Zurlo, director of the Millennium Nucleus of Young Exoplanets and their Moons, called it "the first plausible detection of an exosatellite," while cautioning that the evidence, though strong, is not yet definitive. Astronomers have no universally agreed definition of what qualifies as an exomoon, and the object's mass means it could ultimately be classified as a planet under some schemes. More than 6,000 exoplanets have been confirmed to date, but only a handful of candidate exomoons have ever been proposed, and none has been conclusively confirmed.
