Astronomers have found something 73 light-years away that they cannot quite name. It is at least as massive as Jupiter. It orbits a brown dwarf – an object too heavy to be a planet and too light to ignite as a star – and that brown dwarf in turn circles a star about half the mass of the Sun. Three tiers, and none of the words built for our own solar system fits the bottom one.
The team behind the detection, reported in Nature on 22 July, has settled on "exosatellite". It is not an exomoon, since it orbits no planet; it is not an exoplanet, since it orbits no star. "Being the third wheel in this system makes us want to call it a moon, even if it is nothing like the small, rocky moons we have in our system," says lead author Kevin Hoy, a student at the European Southern Observatory in Chile who is also affiliated with Universidad Diego Portales and the Millennium Nucleus YEMS.
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The find matters as much for its method as for the object. Astronomers have catalogued more than 6,000 exoplanets since the first two were confirmed in 1992, yet moons around them have stayed out of reach, and the handful of candidates rest on thin evidence. Hoy's team took the radial-velocity technique used to hunt planets and pointed it somewhere nobody had: at the brown dwarf itself. Because that companion can be imaged separately from its host star, the Very Large Telescope's CRIRES+ spectrograph could track wobbles in its own spectral lines. Between October 2023 and February 2026 the team gathered 23 high-quality observations, and the simulations that followed pointed to a single companion on an oval, 170-day orbit.
Not everyone reads the system the same way, and the researchers seem pleased about that. Mary Anne Limbach of the University of Michigan, who was not involved, thinks "satellite" is the broader term the field is drifting towards for companions that fit neither traditional category. Others reject the framing outright: Nader Haghighipour of the Planetary Science Institute sees three substellar bodies and no satellite at all, while Roman Rafikov of the University of Cambridge treats the star and brown dwarf as a binary, which would make the new object simply a planet orbiting one member of that pair.
Hoy welcomes the argument and calls the system a useful test bed for it. The star is clearly a star, he notes, and the brown dwarf clearly a brown dwarf; it is the third object, too large to be an obvious moon, that exposes how fuzzy the boundary between a moon and a binary planet really is. More data may settle the question – or show the object is heavier than the current floor of roughly 0.9 Jupiter masses, and, as its size suggests, made mostly of gas rather than rock, ice or metal.