An international team of researchers has identified evidence of a moonlike body circling a brown dwarf in a system located 73 light-years from Earth, marking what could be the first detection of such an object beyond our solar system. The findings, published in Nature last month, focus on an object orbiting the brown dwarf CD-35 2722 B. While more than 6,000 exoplanets have been found to date, definitive proof of moons in other planetary systems has remained elusive until now.
The research team tracked CD-35 2722 B across 23 nights spanning October 2023 to February 2026 using the CRIRES+ infrared spectrograph on the European Southern Observatory's Very Large Telescope. Their analysis revealed a strong signal pointing to a large moon with an orbital period of roughly 170 days and a mass at least 0.9 times that of Jupiter. The mass ratio between this body and the brown dwarf it circles is about 2.5 percent—substantially higher than any moon-to-planet ratio in our solar system. For comparison, Earth and its moon have the highest mass ratio in our system at approximately 1.2 percent, while the moons around Jupiter and Saturn have much smaller ratios. The brown dwarf CD-35 2722 B itself weighs about 37 times Jupiter's mass and orbits the star CD-35 2722, which contains roughly 40 percent of the sun's mass.
The researchers used the Doppler method to capture how CD-35 2722 B wobbles slightly under the gravitational tug of another celestial body orbiting nearby—the same technique that led to the first discovery of an exoplanet around a sunlike star in 1995. Because CD-35 2722 B sits far enough from its host star, the team could obtain its spectrum directly with almost no light interference from the star, enabling measurements up to 100 times more precise than previous attempts. The team also ruled out alternative explanations for the signal, including errors from corrections for Earth's orbital motion, seasonal atmospheric variations, and effects from the brown dwarf's own rotation. Calculations of the Roche limit and Hill radius confirmed the satellite's orbit falls within a physically stable range.
According to Alice Zurlo, an astrophysicist at Diego Portales University, the object occupies "a unique position that cannot be fully understood using the conventional framework for moons in our solar system." She notes that while there's "a clear delineation between the planets and the Sun in the Solar System," making moon definitions simple, "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." The European Southern Observatory notes there's no officially recognized definition for exomoons, and the researchers use the term "exosatellite" instead. The paper acknowledges uncertainty about whether this object will meet future criteria to be considered a moon, though it represents a step toward creating a definitive detection standard.
Researchers believe this finding will spark new directions in theories of planet formation and celestial mechanics. If smaller, rocky moons exist around brown dwarfs, they could experience tidal heating that creates environments potentially suitable for life even at greater distances from their stars—a possibility with implications for the search for extraterrestrial life. Once the next-generation Extremely Large Telescope with its 39-meter primary mirror is completed, detecting even smaller exomoons may become possible.