Astronomers now know of more than 6,000 planets beyond the Solar System – and not one confirmed moon. The gap is conspicuous, given that Jupiter and Saturn alone are circled by well over a hundred satellites. A team led by Kevin Hoy of Universidad Diego Portales in Chile is now reporting the first detection it considers solid. The study appeared on 22 July 2026 in the journal "Nature"; the European Southern Observatory (ESO) presented it the same day.
The setting lies about 73 light-years away and goes by the catalogue name CD-35 2722. At its centre sits a dwarf star of roughly half the Sun's mass. Far out, it is orbited by a brown dwarf of some 30 Jupiter masses – too heavy to count as a planet, too light to ignite like a star. And around that brown dwarf, in turn, moves the newly measured object: a gas giant of at least Jupiter's mass, taking about 170 days to complete one circuit.
Detected through a tremor in the light
The companion was not found by a shadow crossing a disc, but by gravity. For two and a half years the group watched the system with the CRIRES+ spectrograph on ESO's Very Large Telescope in Chile, looking in the spectral lines for the tiny back-and-forth that an orbiting body forces on the object it circles. The same radial-velocity method once yielded the first planet around a Sun-like star. The 170-day signal in the data sits well above the threshold at which false alarms become likely, the researchers report. That sets the find apart from earlier exomoon candidates at Kepler-1625b, Kepler-1708b or WASP-49b, which rested on faint irregularities in light curves or spectra and remained disputed.
Then there is the question of what to call it. Under the International Astronomical Union's definition, planetary-mass bodies orbiting a brown dwarf are planets. The team therefore speaks cautiously of an "exosatellite". It is clearly massive enough to be a planet, Hoy says – it simply does not orbit a star, but an object that itself orbits one. Being the third wheel in the system makes the researchers want to call it a moon, even though it has nothing in common with the small rocky moons close to home. Co-author Alice Zurlo calls the result a breakthrough and the first plausible detection of an exosatellite.
Why it matters: moons reveal a great deal about how planetary systems form and evolve, and that knowledge currently rests almost entirely on our own. The 39-metre mirror of the Extremely Large Telescope, which ESO is building in Chile, should be able to pick out far smaller satellites. The lines between star, planet and moon are likely to blur further still.