Problems of definition will long be with us as we take ever closer looks at exoplanets. But they’re suddenly on everyone’s mind because of the detection of what some are calling an ‘exomoon’ in the system CD-35 2722, found in the constellation Columba. The primary in this system is an M-dwarf thought to be 50–200 million years old. I imagine there is no shortage of flare activity on this star, although the paper doesn’t get into that. The interesting finding in this work just published in Nature is expressed in its title: “Planetary-Mass Exosatellite Detected Around a Star’s Substellar Companion.”

Image: This illustration shows the system around the star CD-35 2722, with the newly found moon-like object at the centre. The star –– the point source to the left –– has about half the mass of our Sun, and it is orbited by a brown dwarf, the reddish-brown object seen here in the foreground (right). The brown dwarf has about 37 times the mass of Jupiter: too massive to be a planet, but not massive enough to have sustained nuclear fusion like stars. This brown dwarf is, in turn, orbited by a newly discovered object at least as massive as Jupiter, seen at the centre of this image. This new object, found with ESO’s Very Large Telescope (VLT), is difficult to label. It behaves like a moon in the sense that it orbits an object that orbits a star. But this ‘moon’ is massive enough to be a planet, and the object it orbits, a brown dwarf, is neither a planet nor a star. Credit: ESO.

So is this the first solid detection of a ‘exomoon’? I can’t describe it as that, and ‘exosatellite’ is an awkward coinage. What we have here is an M-dwarf orbited by a brown dwarf about 30 times the mass of Jupiter. It is the brown dwarf, not the star, that is being orbited by a third object, evidently a gas giant with a minimum mass of 0.743 Jupiter masses. How this arrangement drives orbital mechanics of any other objects in this system (none have yet been found) is well worth pondering. For now, we see again the problem of definition.

Kevin Hoy (Universidad Diego Portales, Chile) is lead author of the paper on this work:

“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.”

I think this discovery is straightforward. The object around brown dwarf CD−35 2722B fully qualifies as a planet. Brown dwarfs cannot sustain stable hydrogen fusion in their core for any length of time, so that they’re basically ‘failed stars’ that are cooling down throughout their lifetimes. Even if we demand that the term ‘star’ be defined by hydrogen burning, then whatever category we create to include brown dwarfs is clearly one that can sustain planets around it.

I’m seeing a lot of chatter about this detection, but let’s leave ‘exomoon’ out of the discussion. Anyway, it’s also interesting to see that brown dwarf CD−35 2722B has been directly imaged, as reported in 2011 in The Astrophysical Journal (citation below). Directly imaged planets are still a rarity.

Image: The discovery image of CD-35 2722B, the brown dwarf in this intriguing system. Credit: Wahhaj et al. 2011, ApJ 729, 139.

So this is an intriguing find, and it also points to our continuing inability to locate what could indisputably be called an ‘exomoon,’ though the HD 206893 system continues to be interesting as a possibility for further research. Until we have a confirmed exomoon, though, this odd configuration will have to do. It reminds me as well that as our explorations continue, we still find how unusual our own Solar System is. Looking for commonality between different star systems, we find over and over again that any facile Copernican notion that we live in an ordinary stellar environment continues to be proven wrong.

Indeed, just what constitutes an ‘ordinary’ stellar system? The beauty of this work is that we seem to find surprises almost everywhere we turn.

The paper is Hoy et al., “Planetary-mass exosatellite detected around the substellar companion of a star,” Nature 16 July 2026. Full text. The discovery paper for CD-35 2722B is Wahhaj et al., “The Gemini NICI Planet-Finding Campaign: Discovery of a substellar L dwarf companion to the nearby young M dwarf CD−35 2722. The Astrophysical Journal 729(2), (2011), 139. Full text.