Astronomers have discovered an object orbiting a brown dwarf or “failed star”. Is it the first exomoon, or moon outside our solar system? Astronomers are debating what to call it. Why? Because moons orbit planets … And this exomoon is orbiting a brown dwarf, which is an object that’s too massive to be a planet but not massive enough to shine like a star. Video via ESO.
Did we find an exomoon around a brown dwarf?
Exomoons are moons that exist outside our own solar system. We’ve yet to definitively find a single one! That’s because they are small and dim, circling exoplanets that are also small and dim, hidden in the glare of the stars they orbit. But on July 22, 2026, the European Southern Observatory said astronomers using the Very Large Telescope in Chile believe they have detected an exomoon.
Only, there’s one problem: it doesn’t orbit a planet. Instead, it orbits a brown dwarf. A brown dwarf is an object that’s too massive to be considered a planet, but not massive enough to ignite and shine like a star. And this particular brown dwarf orbits a star, named CD-35 2722. So astronomers are debating what to call the newly discovered object. Is it the first discovered exomoon, or something else?
The researchers published their peer-reviewed paper on July 22, 2026, in the journal Nature.
A ‘super weird’ system
Not all brown dwarfs orbit stars, but this one does. The star in this system is named CD-35 2722, and it’s about half the mass of our sun. But even though the brown dwarf orbits this star, it’s not a planet, because it’s too massive. Brown dwarfs are so massive compared to planets that they’re sometimes called “failed stars”. Though far heavier than the objects we call planets, they aren’t quite massive enough to begin nuclear fusion and shine like a star does.
And then we have the newly discovered object, which orbits the brown dwarf. Normally we think of a moon as something that orbits a planet, which orbits a star. So what orbits a brown dwarf? Is it an exomoon? That’s what astronomers are having to decide: the semantics of the system. In any case, the lead author of the new study, Kevin Hoy, an ESO student in Chile, calls it:
super weird.
Right now, the team is calling the object an exosatellite. The object is at least as massive as Jupiter. Meanwhile, the brown dwarf it orbits is about 30 times as massive as Jupiter. Hoy said:
This system is somewhat hard to define using solar-system-based words like ‘planet’ and ‘moon’. The exosatellite is clearly massive enough to be a planet, but it does not orbit a star, though it orbits an object that orbits a 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.
The name game
Humans have a need to neatly categorize the things they encounter in the world. But nature is not so rigid. Pluto itself did not change when its classification changed from planet to dwarf planet. And asteroids, comets and meteors are all leftover material from the formation of our solar system under different names.
But naming things – or nomenclature – is part of the scientific process. It’s just not always easy to do. Co-author Alice Zurlo of Universidad Diego Portales said:
We have a clear delineation between the planets and the sun in the solar system, so defining things like moons is simple. In the CD-35 2722 system, where we are blurring the lines between stars, planets and moons, the whole thing becomes more complicated to describe.
Exomoons around exoplanets
Astronomers didn’t confirm the first exoplanet – or planet around a star other than the sun – until 1992. Because they are so small and dim compared to their parent star, they are particularly hard to find. But astronomers long thought they were there. And now we’ve confirmed some 6,000 exoplanets.
So far, it’s been a similar story with exomoons. Astronomers assume there are many exomoons in other stellar systems, just as there are many moons in our solar system. It’s simply a challenge to find them. Astronomers have found exomoon candidates around distant exoplanets, but no confirmation yet.
This candidate exomoon – if it ends up with that label – has strong evidence. It’s causing the brown dwarf that it orbits to wobble. And those wobbles show up in the brown dwarf’s spectra. Zurlo said:
As exotic as it is, this system is truly unique and represents a breakthrough: the first plausible detection of an exosatellite.
Bottom line: Have astronomers found the first exomoon? We think of moons as objects that orbit planets. So what do we call this object, which orbits a brown dwarf?
Source: Planetary-Mass Exosatellite Detected Around a Star’s Substellar Companion
Via ESO