A moon has been found 72 light-years (1 light-year is the distance light travels in one year, about 9.46 trillion km) from Earth. As Earth orbits the sun and the moon orbits Earth, it is a satellite that orbits a body that circles a star. More than 6,000 exoplanets have been found outside the solar system so far, but this is the first time an exomoon has been discovered. However, because the body the satellite orbits is not a planet like Earth, debate is expected to continue over whether the exomoon is truly a moon.
The European Southern Observatory (ESO) said on the 23rd that it "captured an exomoon in the CD-35 2722 system with the Very Large Telescope (VLT) in the Atacama Desert of Chile," in the journal Nature. This star has about half the sun's mass. The researchers said that, just as Earth orbits the sun and the moon orbits Earth, the CD-35 2722 system also has a three-tier hierarchical structure.
◇Exomoon as massive as Jupiter spotted
The team detected a brown dwarf (CD-35 2722 B) that orbits the luminous star CD-35 2722 on a 5,000-year period. They also found a body orbiting that brown dwarf with a period of 171 days. As in our solar system, they confirmed a three-tier system composed of the star CD-35 2722 at the center, a brown dwarf, and an exomoon.
A brown dwarf refers to an object 13 to 80 times more massive than Jupiter, the heaviest planet in the solar system. CD-35 2722 B has a mass about 37 times that of Jupiter. The team analyzed so-called radial velocity signals from Oct. 2023 to Feb. 2026 and detected a brown dwarf hosting a moon.
Exoplanets are usually found using the transit method. When a planet passes in front of a star, it blocks some of the light. Like a solar eclipse when the sun disappears, observers on Earth detect the star dimming briefly to confirm the presence of an exoplanet. Radial velocity analysis is used when a transit is hard to confirm because the system is not directly visible from Earth.
When a planet orbits its host star, not only the planet but also the star is tugged by the planet's gravity and wobbles ever so slightly. As a result, when the star moves toward Earth, the light's wavelength shortens and shifts blueward, and when it recedes, it shifts to longer, red wavelengths. Using radial velocity analysis, ESO researchers previously discovered the first planet in the CD-35 2722 system.
This time, they caught the wobble caused by at least one moon orbiting the brown dwarf. They confirmed that a body with a mass about 0.9 times that of Jupiter orbits the brown dwarf with a period of 171 days. It is the first time a moon has been found outside the solar system.
◇A brown dwarf that is neither a star nor a planet
The problem is that the brown dwarf orbited by the exomoon is ambiguous to classify as a planet like Earth. The brown dwarf CD-35 2722 B falls somewhere between a star and a planet. With a mass 37 times that of Jupiter, it is too heavy to be called a planet. Still, it lacks the mass to shine by nuclear fusion like a star.
Stars have different colors depending on their surface temperatures. The hottest stars have surface temperatures close to 30,000 degrees and emit bluish light. Next are white stars at 10,000 degrees, and yellow stars like the sun have surface temperatures of 6,000 degrees. Red dwarfs, which are red, are small stars cooler than the sun, and the next step down is brown dwarfs.
Brown dwarfs do not actually appear brown; they emit invisible infrared light. They are called brown because they fall between the smallest stars, red dwarfs, and black dwarfs that cannot emit light at all. The researchers said that because the newly found exomoon orbits a brown dwarf rather than a planet, it may be inappropriate to call it an exolunar "moon."
The ESO team said, "This study shows that the radial velocity method used to find exoplanets can be used to find exomoons," and "at the same time raises the need for a clear system to classify exomoons."
Kevin Hoy, the paper's first author, said, "The exomoon we found is clearly large enough to be a planet, but it does not orbit a star directly," and "because it plays the role of a 'third body' orbiting a body that orbits a star, we want to call it a moon even if it is completely different from the small rocky moons in our solar system."
Some noted, however, that it is premature to interpret the signal from the radial velocity analysis as an exomoon right away. David Kipping of Columbia University said that in Jan., he detected a similar wobble in the exoplanet HD 206893 B, 129 light-years from Earth, to what ESO observed around a brown dwarf.
HD 206893 B has a mass 28 times that of Jupiter. Kipping said the wobble in HD 206893 B matches the pull of a moon about half Jupiter's mass, but that the signal can be explained in other ways as well, so it is only evidence of a candidate exomoon.
References
Nature (2026), DOI: https://doi.org/10.1038/s41586-026-10751-w
Astronomy & Astrophysics (2026), DOI: https://doi.org/10.1051/0004-6361/202557127