If you examine our solar system’s giant planets, you’ll notice right away that they’ve all got moons—a lot of moons. While Earth only has the one, Jupiter has about 100 that we know of (and likely hundreds more, depending on what you define as a “moon,” that is). Saturn has almost 275!
Many of these moons are huge; Saturn’s Titan and Jupiter’s Ganymede are both about the size of Mercury, and if they orbited the sun on their own, we’d be sorely tempted to call them planets in their own right.
As if moons weren’t enough, our quartet of beefier planets (including Uranus and Neptune) also sport rings. Saturn’s, of course, are the most obvious and iconic, but the others have rings as well, albeit ones that are fainter and harder to see.
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So moons and rings alike seem to be easy for giant planets to make—at least around the sun. Presumably this holds true for the myriad large worlds we’ve discovered orbiting other stars; many of these exoplanets should have exomoons and exorings, too.
But could we detect them?
The answer, so common with astronomy, is maybe.
Astronomers have found several exomoon candidates already. We can’t see them directly—they’re too faint and too close to their parent planets to resolve—but their presence can be inferred.
One of the most noteworthy exomoon candidates, Kepler-1625b I, was first identified in 2017. The year before, astronomers had discovered its exoplanet via the transit method: we happen to see the planet’s orbit edge on, so once per orbit, we can see the planet passing—transiting—directly in front of its star, creating a mini eclipse. These transits usually manifest as a U- or V-shaped dip in the star’s brightness when plotted over time. Such a plot is called a light curve.
With the exoplanet Kepler-1625b, there were some asymmetries, however—odd bumps in its associated light curve that were difficult to explain. Astronomers posited that this could be caused by an orbiting exomoon that sometimes trails and sometimes leads the planet itself during their mutual transit, changing the light curve’s shape. If it’s real, this exomoon would have to be quite large; its telltale bump in the light curve would correspond to something about the size of Neptune. (The exoplanet itself is a so-called super-Jupiter, a gargantuan world that could have the equivalent mass of a dozen Jupiters.) This claimed exomoon has proved controversial, however, with papers going back and forth arguing for or against its existence. For the moment, it’s still a candidate, unconfirmed.
Another exomoon-hunting method relies on transit timing variations. As the exomoon orbits its host, its gravity swings the planet around their common center of gravity, called the barycenter. This subtly changes the timing of the planet’s transits, altering their predicted onset or duration by small amounts. Certain configurations—such as a very large moon orbiting a relatively low-mass planet—should produce timing variations that could be detected in existing data, although nontransiting planets can induce similar signals, complicating the exomoon search.
Astrometry is another promising technique; this is the very precise measurement of an astronomical object’s position and movement in the sky. It can potentially reveal an unseen exomoon by its offset to its host’s barycenter, which manifests as a wobble in the planet’s motion around the star. Some interferometers, such as the GRAVITY instrument on the Very Large Telescope in Chile, can measure positions with such astonishing accuracy that detecting the wobbles of hidden exomoons might be possible for some giant exoplanets around nearby stars.
In January a team of astronomers reported how they used GRAVITY’s astrometric measurements to study HD 206893, a star with a companion called HD 206893 B, which is likely a brown dwarf with a mass about 20 times that of Jupiter. While it’s not technically an exoplanet, this brown dwarf could still harbor a detectable exomoon. And indeed, the team found some borderline evidence for a companion. If their observed astrometric wobble motion is real, it implies that HD 206893 B is accompanied by something in a nine-month orbit with an estimated mass nearly half that of Jupiter.
This “moon” would be more than 100 times Earth’s mass—hence the quotation marks—and, like all other exomoon candidates, remains as yet unconfirmed. Astronomers are, however, currently testing a sharper-eyed upgrade to GRAVITY (aptly called GRAVITY+) that should be able to eventually validate or rule out this particular candidate.
Yet another exomoon search method involves looking for them via—of all things—volcanic activity. This isn’t as farfetched as it sounds; Jupiter’s moon Io erupts constantly, blasting sulfur into space as its innards are heated by gravitational tides raised by the giant planet and other nearby moons. In recent years astronomers have used the James Webb Space Telescope (JWST) and other observatories to look at the exoplanet WASP-39b, and they’ve detected a cloud in its vicinity that contains fluctuating amounts of sulfur dioxide and other compounds. The fluctuations hint at an episodic, external source—potentially eruptions from a sort of super-Io satellite being tidally squeezed by its hefty planetary host. This detection—and another much like it, around a different exoplanet, WASP-49Ab—isn’t conclusive, but it shows promise as a new pathway for finding these elusive exomoons.
And what of exorings? In some ways, they may be harder to detect than exomoons. Rings, while wide and bright, can actually be fairly ethereal; all the material in Saturn’s rings only add up to a sphere about 400 kilometers across, about the size of its midsize moons. The gravitational effects from such puny accoutrements would be too small for astronomers to see.
But exorings around a transiting exoplanet may sometimes block enough starlight to register as a series of shallow dips in a star’s light curve. Something like this has already been seen; the star 1SWASP J140747.93-394542.6 (or J1407 for short) exhibited a series of extreme dimming events in 2007. One possible explanation is that the dimming was the shadowy transit of a planet, J1407b, surrounded by a huge disk of material. If so, the ring system is immense, possibly 180 million km across, bigger than the distance of Earth from the sun. Neither the planet nor its rings have been confirmed in follow-up observations, however, leading astronomers to pursue other possible explanations.
There may be another way to spot exorings. In the November 2025 edition of the Astronomical Journal, a team of astronomers posited using JWST to look for them. While any rings would be far too small to see directly, the scientists note that icy rings will reflect infrared light strongly at certain shorter wavelengths but not nearly so well at longer ones. If an exoplanet seen by JWST exhibits this pattern, it may be because of the presence of exorings.
The team found that an exoring system would have to be fairly large for this to work because JWST couldn’t detect this effect for any ring system smaller than about three times the extent of Saturn’s. One that was 10 times the size of Saturn’s, however, could be within JWST’s reach, provided its exoplanet host wasn’t too close to its far brighter star. The scientists also note in their paper that NASA’s proposed Habitable Worlds Observatory and the space agency’s soon-to-launch Nancy Grace Roman Space Telescope might also be able to detect exorings this way as well.
We may have far to go before we find either exomoons or exorings with certainty. But looking at our own giant planets, I suspect those discoveries are a matter of when, not if.