If you examine the solar system’s giant planets, you’ll immediately notice that they all have moons—one much of moons. While Earth only has one, Jupiter has about 100 that we know of (and probably 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 more powerful planets (including Uranus and Neptune) also rings. Saturn’s, of course, are the most obvious and iconic, but the others have rings too, albeit fainter and harder to see.
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So both moons and rings appear to be easy for giant planets to make – at least around the Sun. Presumably this applies to the countless large worlds we have discovered orbiting other stars; many of these exoplanets should also have exomoons and exorings.
But could we detect them?
The answer, which is so common with astronomy, is maybe.
Astronomers have already found several exomoon candidates. We cannot see them directly – they are too faint and too close to their parent planets to be resolved – but their presence can be inferred.
One of the most notable 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 be watching the edge of the planet’s orbit, so once per orbit we can see the planet pass—in transit—right in front of its star clip. These transitions usually manifest as a U- or V-shaped drop in the star’s brightness when plotted over time. Such a plot is called a light curve.
However, with the exoplanet Kepler-1625b there were some asymmetries – strange irregularities in the associated light curve that were difficult to explain. Astronomers argued that this could be caused by an orbiting exomoon that sometimes trails and sometimes leads the planet itself during their mutual transit, changing the shape of the light curve. If real, this exomoon must be quite large; its telltale bump in the light curve would correspond to something roughly the size of Neptune. (The exoplanet itself is a so-called super-Jupiter, a giant world that may have the equivalent mass of a dozen Jupiters.) However, this alleged exomoon has proven controversial, with papers going back and forth arguing for or against its existence. At the moment it is still a candidate, unconfirmed.
Another exomoon hunting method relies on transit time variations. As the exomoon orbits its host, 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 supposed onset or duration by small amounts. Certain configurations—for example, a very large moon orbiting a relatively low-mass planet—should produce temporal variations detectable in existing data, although non-transiting planets can induce similar signals, complicating the exomoon search.
Astrometry is another promising technique; this is the highly accurate measurement of an astronomical object’s position and movement in the sky. It could potentially reveal an unseen exomoon by shifting 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 it may be possible to detect the wobbles of hidden exomoons 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. Although not technically an exoplanet, this brown dwarf may still harbor a detectable exomoon. And indeed, the team found some borderline evidence for a companion. If their observed astrometric wobble is real, it implies that HD 206893 B is accompanied by something in a nine-month orbit of estimated mass almost half of Jupiter.
This “moon” would be more than 100 times the mass of Earth – hence the quotation marks – and, like all other exomoon candidates, is as yet unconfirmed. However, astronomers are currently testing a sharper upgrade to GRAVITY (aptly called GRAVITY+) that should ultimately be able to validate or rule out this particular candidate.
Yet another exomoon search method involves looking for them via—of all things—volcanic activity. This is not as far-fetched as it sounds; Jupiter’s moon Io erupts constantly, spewing sulfur into space as its interior is heated by gravitational pull lifted 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 have discovered a nearby cloud containing varying amounts of sulfur dioxide and other compounds. The fluctuations suggest an episodic, external source—potential outbursts from some sort of super-Io satellite being tidally squeezed by its fierce planetary host. This detection—and another much like it, around another exoplanet, WASP-49Ab—is not conclusive, but it does show promise as a new avenue for finding these elusive exomoons.
And what about exorings? In some ways, they can be more difficult to detect than exomoons. Rings, while wide and bright, can actually be quite ethereal; all the material in Saturn’s rings forms just a sphere about 400 kilometers in diameter, about the size of its medium-sized moons. The gravitational effects of such puny equipment would be too small for astronomers to see.
But exoplanets around a transiting exoplanet can 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) showed a series of extreme dimming events in 2007. One possible explanation is that the dimming was the shadow transition of a planet, J1407b, surrounded by a huge disc of material. If so, the ring system is huge, possibly 180 million km across, greater than the distance of the Earth from the Sun. Neither the planet nor the rings have been confirmed in follow-up observations, but this led astronomers to pursue other possible explanations.
There may be another way to detect exorings. In the November 2025 issue of Astronomical Journal, a team of astronomers assumed to use JWST to look for them. While some rings would be far too small to see directly, the researchers note that icy rings will reflect infrared light strongly at certain shorter wavelengths, but not nearly as well at longer ones. If an exoplanet seen by JWST shows this pattern, it may be due to the presence of exorings.
The team found that an exoring system would have to be quite large for this to work because JWST could not detect this effect for any ring system smaller than about three times the extent of Saturn’s. However, one that was 10 times the size of Saturn could be within JWST’s range, provided the exoplanet host was not too close to the much brighter star. The researchers also note in their paper that NASA’s proposed Habitable Worlds Observatory and the space agency’s soon-to-be-launched Rome Nancy Grace Space Telescope may also be able to detect exoplanets in this way.
We may have a long way to go before we find either exomoons or exorings with certainty. But looking at our own giant planets, I suspect these discoveries are a matter of when, not if.
