Astronomers detect first exomoon orbiting brown dwarf CD-35 2722 B

Astronomers have detected a large moon orbiting the brown dwarf CD-35 2722 B, marking the first definitive evidence of an exomoon. The discovery was published in Nature last month, based on observations made between October 2023 and February 2026.

The exomoon candidate orbits CD-35 2722 B, a brown dwarf with a mass about 37 times that of Jupiter, located 73 light-years away in a system with the star CD-35 2722. The brown dwarf and its host star are separated by an apparent distance of about 2.8 arc seconds, and the star has about 40 percent of the sun's mass.

This finding matters because it provides the first clear signal of a satellite outside our solar system, challenging existing frameworks for defining moons. The object's mass ratio to its host—about 2.5 percent—is significantly higher than any moon-planet ratio in our solar system, where Earth and the moon have the highest at approximately 1.2 percent.

Using the CRIRES+ spectrograph on the European Southern Observatory's Very Large Telescope, researchers employed the Doppler method over 23 nights to detect the brown dwarf's wobble caused by the moon's gravity. The measurements were up to 100 times more precise than previous attempts, revealing a signal with a period of approximately 170 days and a moon mass of at least 0.9 times that of Jupiter.

The team ruled out other causes like Earth's orbital motion or the brown dwarf's rotation, and confirmed the orbit is stable within the Roche limit and Hill radius. This follows earlier clues from the HD 206893 system, but no definitive detection until now. Alice Zurlo, an astrophysicist at Diego Portales University, noted the blurring lines between stars, planets, and moons in this system complicate classification.

The researchers use the term "exosatellite" as there is no official definition for exomoons, and the paper acknowledges uncertainty about future criteria. The discovery is expected to catalyze new theories of planet formation and celestial mechanics, and future instruments like the Extremely Large Telescope with a 39-meter primary mirror may detect smaller exomoons.