Pushing a Definition to its Limits - What Defines a Moon?
In July 2026, astronomers at the European Southern Observatory announced evidence of what could be the first discovery of an exomoon—a moon that exists in a solar system beyond our own. If confirmed, this so-called moon would be a gaseous body at least the mass of Jupiter, which raises the question, what makes it a moon instead of a gas giant planet?
What is a Moon?
It might seem easy to define what a moon is. After all, Earth’s Moon makes an intuitive definition pretty clear. A moon is a large body that stably orbits a planet. We see bodies similar to our own Moon around other planets. Mars has two of them, and, as of October 2026, Jupiter has 115, Saturn has 293, Uranus has 29, and Neptune has 16. For the most part, these objects we call moons are similar enough to Earth’s Moon that sharing a category makes sense.
Despite that, we don’t actually have an agreed-upon definition of a moon like we do for planets. One point of contention is the size requirement. While we say Saturn has 293 moons, most of these are very small. Rather than forming as large objects around Saturn, they are likely smaller asteroids that got captured by Saturn’s gravity and now permanently orbit it. They can be as small as a kilometer in diameter. We consider these to be moons, despite their size. But there has to be a cut-off somewhere. Saturn’s rings are composed of countless chunks of ice that range in size from micrometers to meters in diameter. Surely these shouldn’t be considered moons, despite having a stable orbit around the planet.
Then there’s the other part of the intuitive definition, where it says that a moon orbits a planet. We can’t just say that a moon is any object that orbits another, or the planets themselves would be considered moons of the Sun. And yet, there are moon-like objects that orbit dwarf planets, Kuiper Belt objects, and large asteroids. We already call Charon, an object orbiting Pluto, a moon. Should they still be considered moons, despite not orbiting a planet? It’s this problem that brings the potential exomoon and its category back into question.
Is This Exomoon Even a Moon?
The problem with the exomoon, and why it may not even be a moon at all, is that it doesn’t orbit a planet. Instead, it orbits a brown dwarf. A brown dwarf is an object that ranges from roughly 13 to 80 times the mass of Jupiter, and is often referred to as a failed star. There’s a clear cut-off line on the what we would consider to be the largest possible planet and a brown dwarf, as brown dwarfs form and behave much more like stars. But, unlike stars, a brown dwarf does not have enough mass to sustain hydrogen fusion, which is the minimum requirement for a star. This leaves the brown dwarf as an awkward middle ground that is neither a planet nor a star.
The system we might have found an exomoon in, CD-35 2722, features a brown dwarf orbiting a star, and, possibly, a gas giant-like object orbiting the brown dwarf as the exomoon. But therein lies the problem. Traditionally, we would never consider a gas giant-like object to be a moon rather than a planet. Gas giants form independently from one another during the formation of a solar system. And even if two of them ended up close enough to force one into orbit around the other, they would be similar enough in mass to instead orbit each other as a binary planet system. But this object in CD-35 2722 isn’t orbiting a planet—it’s orbiting a far more massive brown dwarf.
From afar, the system certainly looks like a more traditional moon system, with the brown dwarf orbiting a star and the gaseous moon orbiting the brown dwarf. But when ignoring the star and looking closer, the brown dwarf and its satellite could be argued to appear more like a star-like object with a gas giant planet orbiting it. There’s also the fact that not all brown dwarfs orbit stars. In fact, brown dwarfs are more often found on their own, not orbiting a star, making them usually the center of their planetary system. Would a Jupiter-size object orbiting a brown dwarf that doesn’t orbit a star be considered a planet, a moon, or something else entirely? A planet seems like the intuitive answer in this case, but why would the brown dwarf orbiting a star instead change that classification to a moon? Even the researchers at the ESO refer to this object as an exosatellite, rather than an exomoon.
So, the problem with the potential exomoon in CD-35 2722 is that it comes at the intersection of several different limits and definitions. In our Solar System it seems fairly obvious without a definition what’s a star, what’s a planet, and what’s a moon, but those clear distinctions don’t hold, especially when looking at a potential exomoon that may be larger than any planet in our Solar System. So, even if the object is confirmed to exist, the jury will still be out on whether or not we’ve actually discovered an exomoon.
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