Have Astronomers Found the First Exomoon at Last?

A giant world circling a brown dwarf may be the first exomoon we have ever caught in the act.

Can a moon be heavier than most planets, and orbit something that is not even a star? In a young system called CD-35 2722, astronomers think they have caught exactly that. Welcome, fellow sky-watcher. Settle in, because the strangest thing about this story is not the object itself but the question it forces on us: what is a moon, really? Read to the end and that question will feel far less obvious than it does right now.

What did astronomers find around CD-35 2722?

They found strong evidence for a giant world, at least as massive as Jupiter, circling the brown dwarf CD-35 2722 B. Because that world orbits a brown dwarf instead of a planet or a star, it would be the first plausible exomoon, or exosatellite, ever detected beyond our Solar System. Kevin Hoy of the Universidad Diego Portales in Chile led the analysis, and the team reported it in Nature after fifteen months of watching the system with the Very Large Telescope.

Hoy calls the system super weird, and he has a point.

Picture a nested set of orbits. A small red star, about half the mass of the Sun, sits at the center. Far out from it drifts a brown dwarf, an object too heavy to count as a planet and too light to ignite as a star. And around that brown dwarf, something roughly Jupiter-sized appears to be looping in a steady rhythm. The star holds the brown dwarf; the brown dwarf holds the new object. Three tiers, one of them entirely new to us.

Why would this exomoon rewrite the rulebook?

Every moon we have ever named orbits a planet. This one does not.

That single fact is what makes astronomers reach for new words. The object at CD-35 2722 loops a brown dwarf, a body that sits in the gray zone between planets and stars. Alice Zurlo, who directs the Millennium Nucleus on Young Exoplanets and their Moons and co-led the work, describes it as a giant gaseous body orbiting a companion several times the mass of Jupiter. Nothing in our own system looks like this arrangement.

Hoy puts the awkwardness plainly. Being the third wheel in the system, he says, makes the team want to call it a moon, even though it resembles nothing like the small rocky moons next door. The label fits the geometry. It does not fit the object.

How do you spot a moon you cannot see?

You do not look at the moon at all. You watch the brown dwarf for a wobble.

The method is called radial velocity, the same technique Michel Mayor and Didier Queloz used in 1995 to find the first planet around a Sun-like star. As a companion tugs on the brown dwarf, the brown dwarf drifts a little toward us and then away from us, over and over. Its light shifts slightly bluer and then slightly redder in step with that pull, and a sensitive spectrograph can read the beat.

One thing usually kills this idea before it starts. A companion sits drowned in the glare of its star, and prying the two apart is punishing work, a problem we walked through in our look at how astronomers overcome stellar glare to find new worlds. CD-35 2722 B gave the team a rare break. It orbits so far out, on a wide and lopsided path, that on the sky it stands about 2.8 arcseconds from its star. That gap is generous enough for the CRIRES+ infrared spectrograph on the Very Large Telescope to capture the brown dwarf’s own spectrum with almost no starlight leaking in.

Between October 2023 and January 2025 the team collected twenty usable nights of these spectra. When they hunted for a repeating signal, one period near 169 days stood out well above the noise, clear enough that a random fluke is very unlikely.

What do we know about the exosatellite’s size and orbit?

The dominant signal points to a gas giant with a minimum mass near three quarters of Jupiter, circling the brown dwarf roughly every 169 days on a nearly circular path. Radial velocity only ever gives a lower bound on mass, so the true figure is likely higher, which is why the public description settles on at least as massive as Jupiter.

Beyond that main signal, the best-fitting model in the preprint added a second, closer, smaller body near an 88-day orbit with a minimum mass around a quarter of Jupiter. If that inner companion is real, the two would sit close to a 2:1 rhythm, one lap for every two of the other. That pattern will feel familiar to anyone who has met the volcanic moons that power Jupiter’s aurorae, since Io, Europa and Ganymede lock into a related 4:2:1 beat.

What stands out either way is how heavy these candidates are compared with the moons we know.

Every moon orbiting a giant planet in our system falls well below that 1.2% line. A satellite carrying 2% of its host’s mass is off the chart by Solar System standards, which is one reason the team suspects it formed the way massive companions do, from the collapse of a disk rather than by slowly gluing together rock and ice.

Could an exomoon like this host life?

Almost certainly not this one. It is a giant gaseous world, far too big to resemble the small rocky moons that make good homes. The interest runs the other way.

Moons can be warmed from within by tides raised by their host, a heat source that does not depend on sunlight. In principle that lets a moon stay warm even outside the comfortable zone around a star, which is why researchers such as René Heller have long argued that exomoons are worth chasing in the search for life. The bodies at CD-35 2722 are the wrong size for that story. What they do is prove the detection works, and the smaller worlds worth getting excited about should follow.

Is it a moon, a planet, or something new?

By the current international rules, an object this heavy orbiting a brown dwarf can be called a planet. That is the knot at the heart of this discovery.

The IAU working definition draws its main line at the mass needed to fuse deuterium, around 13 Jupiter masses, and it says outright that planetary-mass objects orbiting brown dwarfs are planets, whatever their history. By that letter of the law, this Jupiter-mass body qualifies as a planet. By its place in the system, sitting a rung below the brown dwarf, it behaves like a moon.

The team points to a deeper wrinkle. Give this system the age of the present-day Solar System, and the brown dwarf will have faded dramatically while the central star keeps shining. A satellite of a fading brown dwarf faces a very different future from a planet basking beside a steady star. The authors suggest our vocabulary, invented to describe the Solar System, may simply run out of room here, and that objects like this one might deserve a category of their own.

What comes next in the hunt for exomoons?

More data, and bigger mirrors. The team is candid that this is a strong detection rather than a closed case, and follow-up observations are needed to pin down whether there are one or two moons and to sharpen their orbits.

Meanwhile, the wider effort is heating up. Only a few months earlier, a group led by Quentin Kral reported hints of a satellite in the HD 206893 system using the Very Large Telescope Interferometer, though that one fell short of a firm detection. Each oddball system also reshapes how we read the growth and history of worlds, a thread we picked up in our piece on planetary evolution beyond Earth 2.0.

A far bigger leap is coming from raw light-collecting power. ESO’s Extremely Large Telescope, with its 39-metre mirror, should reach exomoons far smaller than anything we can detect today, turning systems like CD-35 2722 from a finish line into a starting one.

So, have we really found the first exomoon?

Our honest answer is a strong maybe, and that is worth celebrating rather than shrugging at. The signal is convincing, the checks against instrument quirks and the brown dwarf’s own spin hold up, and the object orbits where a moon should. What keeps it a maybe is caution, not doubt. The evidence rests on a single system and a result still settling in peer review, and the object itself is so massive it strains the very word we reach for.

Sit with that tension for a moment. We may have found our first moon beyond the Sun’s family, and in the same breath discovered that we no longer have a clean name for it. Nature tends to do that. It answers one question and hands us three sharper ones.

We wrote this piece for you here at FreeAstroScience.com, where we turn tangled science into plain language you can actually carry around. Keep questioning, and never switch off your mind, because the sleep of reason breeds monsters.

Gerd Dani, President, FreeAstroScience — Science and Cultural Group
Never let your mind sleep.

Frequently asked questions

Sources