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Do alien exoplanets have exomoons and exorings?
Although astronomers have found thousands of exoplanets, the number of confirmed exomoons—and exorings—is still zero. But that may soon change
The UniverseFridays
February 27, 2026
5 min read
How far are we from finding exomoons and exorings?
Although astronomers have found thousands of exoplanets, the number of confirmed exomoons—and exorings—is still zero. But that may soon change
By Phil Plait edited by Lee Billings
[A large, fiery moon whirls around a storm-wracked gas giant planet; the duo’s host star appears in the background.]
This artist’s concept depicts a potential volcanic exomoon between the exoplanet WASP-49Ab, left, and its parent star.
NASA/JPL-Caltech
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.
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