Huge cosmic object could force astronomers to rethink what a moon is

When is a moon not a moon? That’s the conundrum raised by a new study, and the solution appears to be “when it’s a planet.” Or maybe the answer is vice versa.

These are the strange semantic questions raised by studies of CD-35 2722 B, a brown dwarf with about 30 times the mass of Jupiter orbiting a small star located about 73 light years from Earth in the system CD-35 2722. The brown dwarf appears to have its own satellite, much like Earth has the Moon, although it is almost as large as the largest planet in our solar system. While astronomers have discovered thousands of exoplanetsCD-35 2722 could be the first star system discovered to exhibit this three-level hierarchy “star, brown dwarf, satellite”.

Previous observations of CD-35 2722 B, taken at the WM Keck Observatory in Hawaii, had shown evidence of an exomoon, a type of object that has still to be found definitively– but the data was inconclusive. To settle the matter, Kevin Hoy, a Ph.D. student at the Institute for Astrophysical Studies in Santiago, Chile, turned to the European Southern Observatory’s Very Powerful Telescope (VLT) and its CRIRES+ instrument, located atop a mountain in the Chilean high desert.

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CRIRES+ is a spectrograph, an instrument that analyzes the light of a star into a spectrum of colors similar to a rainbow; Fluctuations in this spectrum follow the movement of a star across the sky, with the spectrum becoming bluer as the star approaches Earth and redder as it moves further away. Astronomers search for hidden orbital companions thanks to the way their gravity can pull a star back and forth, creating subtle stellar wobbles that register as a telltale series of “blueshifts” and “redshifts” in the star’s spectrum.

For nearly two and a half years, until February 2026, Hoy and his colleagues used the VLT and CRIRES+ to periodically measure the brown dwarf’s spectrum for such oscillations, a technique known as radial velocity analysis. As they described in a new paper published in Nature On Thursday, CD-35 2722 B indeed wobbles because of an otherwise invisible satellite that orbits it once every 170 days.

What exactly to call this companion is a matter of debate. Although it acts like a moon, it is large – really, really large – with a minimum mass that is 90% of that of Jupiter. “The mass could be even higher,” Hoy notes, adding that the object’s size likely means it is mostly gas rather than rock, ice or metal. If it were orbiting a full-fledged star rather than a distinctly substellar brown dwarf, astronomers would simply call the object an exoplanet – and a relatively mundane one at that.

Brown dwarfs occupy a special place in the cosmic hierarchy. They are gaseous orbs several times larger than Jupiter, but too small to have reached the critical mass necessary for the thermonuclear fusion reactions that make stars shine. (This is why brown dwarfs are often referred to as failed stars.) Given their strangeness, it makes sense that the bodies orbiting them are also oddities.

The closest analogue to Hoy’s discovery might be HD 206893 b, a visibly wobbly 28-Jupiter-mass exoplanet located 129 light-years from Earth, says David Kipping, an exomoon-hunting astronomer at Columbia University who did not work on the new study. But even if HD 206893 b’s wobble is consistent with the persistent tugging of an accompanying satellite about half Jupiter’s mass, he says, a paper published in January showed that this signal could just as easily I have other explanationsproviding at best shaky evidence of a “candidate” exomoon. The putative exomoon of CD-35 2722 B is technically only a candidate, but its associated wobble is much clearer and more difficult to explain.

“It’s a candidate object, but it’s probably real,” Kipping says. “In this case, they feel like it’s safer, and I would probably buy it.” [like] truly clean detection.

As astronomical instruments continue to improve, it’s likely that more and more small and strange candidate companions will be found for brown dwarfs and exoplanets, Hoy says. This means that astronomers will have to start by finding better definitions of what constitutes an exomoon versus an exoplanet. For now, he suggests, we could stick to a more neutral term for all these objects: exosatellites.

An already borderline object such as the brown dwarf CD-35 2722 B is “the perfect place to start this kind of conversation, because the system is like the perfect testing ground for discussing things,” says Hoy. “The star is clearly a star. The brown dwarf is also clearly a brown dwarf. We now have this new object that is too big to obviously be a moon. The way we draw the line between what is a moon and what is a binary planet is very fuzzy around the edges.”