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Astronomers using NASA’s James Webb Space Telescope have discovered a hidden moon orbiting the planet Uranus, and it’s just one of likely many small moons yet to be found in that system. The object is the 29th known moon around the ice giant planet.

Because this moon is dark, small, and moves quickly across the sky, it was difficult to tease it out from Uranus and the planet’s bright rings. But Webb’s ability to pick up on faint infrared light enabled the discovery. The telescope has recently given astronomers their first views of Uranus’ rings, weather, and atmospheric composition. The discovery builds on that existing record.

Deciphering Uranus’ Many Mysteries

Finding the moon not only adds one more member to Uranus’ family of natural satellites, it also provides new clues into the origin of the planet’s ring system. The moon S/2025 U1 and portions of Uranus’ rings may have a shared past, including a common origin in some ancient, as-yet-unknown event. “The discovery raises questions about how many more small moons remain hidden around Uranus and how they interact with its rings,” said El Moutamid.

The planet is known to have five major moons, Miranda, Ariel, Umbriel, Titania, and Oberon, as well as a collection of smaller satellites. The newly discovered object becomes the 14th small moon in Uranus’ inner system. No other planet has so many small inner moons so close together, which has astronomers scratching their heads. The satellites are so close that their orbits should cross and cause them to collide, and yet somehow they stay in place. Astronomers think that some of them may be shepherding Uranus’ narrow rings.

“This is a very exciting discovery,” said Scott Sheppard, an astronomer at the Carnegie Institution for Science who was not part of the new study but co-discovered one of Uranus’ moons in 2024. He called the object especially interesting because it’s closely associated with Uranus’ inner ring system. “We can only thank Webb for being sensitive enough to be able to find this new moon,” he said.

Observations with Webb, as well as previous discoveries by the Hubble Space Telescope and ground-based telescopes, have also found small moons clustered in the outer Uranian system, in orbits far beyond that of Oberon, Uranus’ next outermost major moon. These are thought to be captured asteroids.

The SETI Institute’s Matthew Tiscareno, co-principal investigator in the Webb Uranus program, called the discovery one that “confirms a blurry line between Uranus’ moons and rings. Their complex inter-relationships hint at a chaotic past,” he said. Tiscareno also noted that the new object is even smaller and fainter than Uranus’ tiniest known inner moons. There are likely more yet to be found.

Exploration of Uranus is about to get a lot more interesting. The planetary decadal survey released by the National Academies of Sciences, Engineering, and Medicine in 2022 chose Uranus Orbiter and Probe as NASA’s next big planetary science project. The mission is a top choice to launch in the early 2030s, but that’s far from certain as budget battles rage and other missions need money as well. If it goes forward, it will investigate how Uranus’ weird rotation is tilted on its side, its complex magnetic field, atmospheric dynamics, and possibly worlds with subsurface oceans in ice among the planet’s moons.

Sheppard thinks that more moons are out there waiting to be discovered, possibly as small as just a few kilometers across. Webb long-exposure imaging will be one way to find them, as will any future spacecraft missions. El Moutamid and her team plan to continue refining the orbit of S/2025 U1 and to continue the search for additional hidden satellites.