Editorial NexoScience
In 1610, Galileo Galilei pointed a new telescope at Jupiter and spotted four points of light that clearly weren’t stars. They turned out to be the first moons ever confirmed beyond our own. More than four centuries and over 6,000 confirmed exoplanets later, astronomers still couldn’t say the same about a single world orbiting another star. Not one moon beyond the solar system had ever been confidently detected, until now.
A team led by researchers in Chile has found the strongest evidence yet for a moon outside our solar system. The signal comes from CD-35 2722 B, a brown dwarf roughly 70 light-years away in the constellation Columba, first spotted through direct imaging back in 2011. At 37 times the mass of Jupiter, it is too heavy to count as a planet but not massive enough to ignite the nuclear fusion that powers a star, a kind of cosmic in-between object orbiting a small, young red dwarf star about half the mass of the Sun.
The evidence comes from tracking tiny wobbles in the brown dwarf’s light. Using the CRIRES+ instrument on the European Southern Observatory’s Very Large Telescope in Chile, the team applied radial velocity analysis, the same technique that found the first exoplanet ever confirmed around a Sun-like star, to 21 observations of CD-35 2722 B collected between October 2023 and January 2025. A clear periodic signal emerged: something orbiting the brown dwarf every 169 days.
The best explanation for that signal is not one satellite, but two. The larger candidate has a minimum mass of 0.74 times Jupiter, on a nearly circular 169-day orbit. A second, smaller and less certain candidate weighs in at a minimum of 0.28 Jupiter masses on a shorter, roughly 87-day orbit. Their periods sit close to a 2:1 resonance, the same rhythmic relationship seen between Io and Europa, two of Jupiter’s own Galilean moons.
Relative to their host, these would be unusually massive moons. The larger candidate weighs in at about 2% of the brown dwarf’s mass and the smaller one at 0.7%, both higher than any moon-to-planet mass ratio found in our own solar system, where the Earth-Moon system holds the record at 1.2%. That size hints at a different formation story than the one that built the Moon or Jupiter’s moons, more in line with how giant planets themselves are thought to form at large distances from their star.
The researchers were careful to rule out imitators before calling this a detection. They checked whether the signal could be an artifact of Earth’s own orbit, instrument drift, or the brown dwarf’s rotation, and found none of those explanations fit. CD-35 2722 B is expected to spin roughly once every 16 hours, far too fast to produce a wobble that repeats every 169 days. The authors describe this as the most compelling evidence yet for a satellite around a body outside our solar system, while stressing that it remains a candidate, not a confirmed discovery, until further observations pin down the orbits with more precision.
The find is already testing the definition of the word “moon.” Because both the brown dwarf and its satellites are so unusually massive, some no heavier than a large planet, the discovery sits in a gray zone between moon, planet and star that current astronomical definitions were never built to handle. The authors argue the case may need its own category altogether, rather than forcing it into vocabulary invented for our own solar system.
Source: Hoy, K., Zurlo, A., Peña R., P.A. et al. “Planetary-Mass Exosatellite Detected Around the Substellar Companion of a Star.” Nature, 2026. DOI: 10.1038/s41586-026-10751-w.
