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A team of scientists from the European Southern Observatory in Santiago, Chile have recently published a paper in Nature magazine titled Planetary mass exosatellite detected around the substellar companion of a star. It describes a 0.9 Jupiter-mass companion, orbiting a 37 Jupiter-mass brown dwarf designated CD352722B, which in turn orbits an M-dwarf star. Now, exoplanet nomenclature is not designed to handle such hierarchical systems, and brown dwarfs themselves are oddballs, straddling the line between stars and planets. Although planetary scientists, especially those studying brown dwarfs specifically do not like the term, ‘failed stars’ is an accurate description for such objects because they are formed by the same process by which stars are born, but do not grow massive enough to sustain nuclear fusion.

Now, let us take a look at how the media covered this paper. Wired magazine ran with the headline ‘Astronomers Have Detected an Exomoon for the First Time‘ while Time described the discovery as ‘Astronomers Discover the First Known Moon Outside our Solar System—and What a Moon it Is‘. Both headlines are misleading, and conflict with the official, albeit broken approach by the IAU, that does not consider planets in orbits around stars other than the Sun as planets at all. According to the International Astronomical Union’s (IAU) 2018 Working Definition of an Exoplanet, any object below 13 Jupiter masses that orbits a star or brown dwarf with a mass ratio below 1/25 is classified as an exoplanet, regardless of how it formed. Under standard astronomical taxonomy, CD352722B’s companion is unambiguously an exoplanet, not an exomoon. The authors state that their discovery is “the most compelling detection of a satellite around a substellar companion to date,” yet they fail to cite earlier published papers that discovered nearly identical systems years prior, with the earliest of these being ‘Discovery of a planetary-mass companion to brown dwarf in Taurus‘, published in The Astrophysical Journal Letters by Todorov, Kevin Luhman, and Kim McLeod.

Why brown dwarfs cannot fuse hydrogen into helium, they can fuse deuterium, a heavier isotope of hydrogen into helium-3. Brown dwarfs can host cloudy, planet-like atmospheres, but do not have a corona like a star. Companions to them have therefore been called planets, binary brown dwarfs, or simply companions, depending on mass ratio and separation. Physics World emphasized the contrast with 2M1207 b: that object is ~25 percent of its host’s mass on a ~40 AU orbit, whereas the new candidate is ~2.5 percent of CD−35 2722 B on a 170-day path. Closer and more hierarchical, yes. Still not a moon.

Nature’s own press release framed the discovery as ‘the first exomoon detection.‘ Commentators, including brown-dwarf specialists discussing earlier imaged pairs that were never marketed this way, have argued that inflating a brown-dwarf companion into a ‘first moon’ spends public attention that should be reserved for a satellite of a true planet. When that object is eventually confirmed, that is, when the first exomoon is discovered for real, the milestone will already have been ‘used’. Sky & Telescope captured the field’s reaction more cleanly than the headlines: the hierarchy is ‘hard to classify,’ and calling the body a moon is optional rather than required.

None of this subtracts from the measurement. Isolating the brown dwarf’s spectrum from the star and extracting a stable Keplerian residual is a genuine instrumental advance. David Kipping, who has spent a decade on exomoon searches, called the wobble ‘really clean‘ even while treating the object as a candidate. Follow-up astrometry, imaging, and ELT-era spectroscopy can shed more light on the object.

The semantic problem is not pedantry. Words allocate discovery credit and set the public’s expectation of what ‘first’ means. A Jupiter-mass body on a 170-day orbit around a an M dwarf is an important hierarchical companion. Keeping those categories distinct does not diminish Hoy et al.; it keeps the first uncontroversial exomoon, when it arrives, from landing in a landscape already exhausted by a mislabeled brown-dwarf planet. The semantics of the issue and its complexities are explored by Kipping in a solo episode of his Cool Worlds podcast.

Kipping, one of the foremost exomoon hunters on the planet, has even proposed an elegant solution! He is the author of the paper titled ‘What Even Is an “Exomoon”?‘ published in the Research Notes of the American Astronomical Society. This does not propose a definition, but accepts that nature often defies the tendency of scientists to put everything into buckets. In the paper, Kipping argues that the recent rush to label Jupiter-mass companions of brown dwarfs as exomoons or satellites is not merely sloppy language, or a semantic problem. It is a symptom of a deeper problem: our vocabulary for worlds was built around the Solar System, and nature has now begun to ignore that vocabulary.

The trigger is a pair of 2026 detections. Kral and colleagues reported a roughly 0.4 Jupiter mass​ companion to HD 206893 B, a ∼20 Jupiter mass​ brown dwarf itself orbiting an F-type star. Hoy and colleagues reported a ∼0.9 Jupiter mass​ companion to CD-35 2722 B, a ∼37 Jupiter mass​ brown dwarf orbiting an M dwarf. As previously mentioned, hierarchical systems known for decades already contained such arrangements. In press and even in technical writing these objects have been called exomoons or satellites. The International Astronomical Union, however, has no formal definition of an ‘exomoon’ at all, or even an exoplanet!

What the IAU does have is a Working Definition of an Exoplanet, first drafted in 2003 and revised in 2018. Substellar objects below 0.08 solar masses​ whose true masses exceed the deuterium-burning limit of ∼13 Jupiter masses​ are brown dwarfs, regardless of how they formed or where they sit. That clause settles the hosts: HD 206893 B and CD-35 2722 B are brown dwarfs. The same document then states that objects below the deuterium limit that orbit stars, brown dwarfs, or stellar remnants, and that have a mass ratio with the central body below the L4/L5 instability threshold, are planets—again, regardless of formation history. Under that wording the new companions are planets. If ‘planet’ and ‘moon’ are treated as mutually exclusive, they cannot also be moons.

Some authors reply that an object can be both. Hoy et al. note that Earth is, in a dynamical sense, a satellite of the Sun. Kipping finds this unhelpful. Stretch the word ‘satellite’ that far and every one of the six thousand known exoplanets becomes an ‘exosatellite.’ The term loses the contrast that made it useful in the first place. The argument does not settle the taxonomy; it merely shows why an updated IAU definition is overdue.

The physical mismatch is sharper still. Solar System moons formed either in a giant impact, as Earth’s Moon almost certainly did, or in a circumplanetary disk, as the Galilean satellites did. The disk channel appears to produce satellites thousands of times less massive than their hosts. The new companions, by contrast, comprise ≳2% of the combined mass of their systems. They are not scaled-up Ios. They are objects that assembled by a different channel, in a different mass regime, around objects that themselves sit in the awkward interval between planet and star.

Kipping does not offer a replacement lexicon. He sketches the limits of any formal scheme. Companions of free-floating planetary-mass objects, true binary planets, and the still-contested boundary of the brown-dwarf class will keep producing edge cases. Rigid lines may simply fail. Nature often supplies a continuum rather than a partition: Jupiter’s Galilean system already looks uncannily like a miniature TRAPPIST-1. Architecture and mass ratio alone will not always decide the name.

Yet the names are not the point. The guiding question of exoplanetary science is how ordinary or peculiar our own arrangement is. Analogues to the Earth–Moon system, if they exist, will tell us something about that question. Companions of brown dwarfs, whatever we call them, almost certainly will not. Their formation is alien to the processes that built the worlds Galileo first resolved. That difference is what makes them scientifically interesting. Astrophysics has always been the most exciting at the places where the old categories break.