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An unidentified moon has been added to the tally of Uranus’ moons, discovered by astronomers using NASA’s James Webb Space Telescope (JWST). With 29 now known, there are thought to be more yet to be found orbiting Uranus.

Webb uncovered the new moon after a series of 40-minute-long exposure images revealed a new faint object on Feb. 2 with its Near-Infrared Camera. The new moon is only about 6 miles (10 kilometers) wide and is one of the smallest known natural satellites orbiting Uranus. Its small size and the bright reflected light from the planet’s rings likely obscured it in earlier missions and ground and space telescopes. It’s even smaller than what NASA’s Voyager 2 spacecraft was able to detect in its historic flyby of Uranus nearly 40 years ago.

“This is a small moon, but a significant discovery,” said lead author Maryame El Moutamid, a solar system scientist at the Southwest Research Institute’s Solar System Science and Exploration Division in Boulder, Colorado. El Moutamid is also the principal investigator of a Webb program dedicated to observing Uranus’ rings and inner moons. “Discovering this moon with Webb highlights how it is extending our knowledge much further than what previous missions have done,” she added.

The new moon, named S/2025 U1, is orbiting about 35,000 miles (56,000 kilometers) from Uranus’ center. It orbits close to the planet in its equatorial plane on a nearly circular path, between two known moons, Ophelia just outside Uranus’ main ring system, and Bianca. Based on its orbit, astronomers believe the moon may have formed near its present location.

Because the new moon is dark, small, and moves rapidly, astronomers had difficulty observing it against the bright background of Uranus and the planet’s ring system. The faint object was finally picked out by Webb because it is sensitive to infrared light. The telescope has already provided new insights into Uranus’ rings, weather, and atmosphere, and this is the first of a series of discoveries it is expected to make.

Study in Orbit

The discovery has piqued astronomers’ interest in how Uranus’ complex ring system may have formed. They think that S/2025 U1 and part of Uranus’ rings may have formed at the same time and be fragments from the same original event. “The discovery raises questions about how many more small moons remain hidden around Uranus and how these moons interact with its rings,” said El Moutamid.

At present, Uranus is known to have five major moons, Miranda, Ariel, Umbriel, Titania, and Oberon, as well as a collection of small satellites, including the newly discovered body, which now becomes the 14th small moon of Uranus’ inner system. No other planet has as many inner moons in such a small volume, all clustered so close together, which has long puzzled astronomers. These satellites are so close together that their orbits would tend to cross one another, but for some reason, they have not. Astronomers think that some of the satellites may be shepherds, helping keep Uranus’ narrow rings in check.

“This is a very exciting discovery,” said Scott Sheppard, an astronomer at the Carnegie Institution for Science who was not involved in the new study but co-discovered one of Uranus’ moons in 2024. The object’s association with Uranus’ inner ring system makes the discovery especially intriguing, he said. “I’m impressed by the sensitivity of Webb that allowed the discovery,” he added.

A co-principal investigator in the Webb Uranus project, Matthew Tiscareno, an astronomer at the SETI Institute, said the find points out that there is no clear boundary between Uranus’ moons and its rings. “Their complex inter-relationships hint at a chaotic history,” he said, with “additional icy fragments and potentially small satellites orbiting Uranus that are still hiding in the shadows.”

The faintest inner moon known is about 6 miles (10 kilometers) across, so the new moon is even smaller and fainter, Tiscareno said. “This discovery is not only important for continuing to map out the population of Uranus’ small inner moons, but suggests more remain to be found,” he added.

Slow discoveries

Slowly, many of Uranus’ moons have been coming to light. Voyager 2 was the first to see moons beyond the five largest known, with the first discovery in the 20th century in 1787. Voyager 2 revealed 10 during its flyby in 1986. These are 16 to 96 miles (26 to 154 kilometers) across. More recently, ground-based telescopes and the Hubble Space Telescope have identified another 13 small moons, each 8 to 10 miles (12 to 16 kilometers) across and blacker than asphalt. The inner moons are likely composed of ice and rock, whereas the moons outside Oberon, Uranus’ outermost large moon, are thought to be captured asteroids.

In the future, NASA and other agencies have discussed more Uranus exploration. A planetary decadal survey, published by the National Academies of Sciences, Engineering, and Medicine in 2022, recommended a Uranus Orbiter and Probe as NASA’s next large planetary science mission. The mission could launch in the early 2030s, depending on funding discussions amid NASA’s budget debate. If realized, it would study Uranus’ tilted rotation, complex magnetic field, atmospheric dynamics, and potential icy ocean worlds in its moons.

Sheppard thinks many more moons as small as a few kilometers in diameter are likely to be found either in the long-exposure Webb images or in future spacecraft missions. El Moutamid and her colleagues plan to further refine the new object’s orbit and continue to search for additional hidden satellites.

“Discovering a new moon around Uranus will help scientists better understand how its strange system formed, give insights into its rings, and prepare for future missions such as NASA’s Uranus Orbiter and Probe,” El Moutamid said.