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By using the James Webb Space Telescope, astronomers have discovered a previously unknown moon around Uranus. The discovery brings the known number of moons around the ice giant planet to 29, with more expected to be lurking.

Observed on February 2, the tiny moon was detected in a set of long-exposure 40-minute images captured by Webb’s Near-Infrared Camera. Just 6 miles (10 kilometers) across, the newfound moon is among the smallest natural satellites ever discovered around Uranus. It is dark, fast-moving, and small enough that its faint light was hidden by Uranus’ bright rings from past missions and telescopes. This was the case even for NASA’s Voyager 2, which flew by Uranus more than 38 years ago.

“This is a small moon, but a big discovery,” said Maryame El Moutamid, a lead scientist at the Southwest Research Institute’s Solar System Science and Exploration Division in Boulder, Colorado. El Moutamid is also the principal investigator for a Webb program observing Uranus’ rings and inner moons. “This discovery is also showing that Webb can do for astronomy what other missions only dreamed of. Webb is breaking through the next frontier of space.”

Unlocking the Mystery of Uranus’ Complex Ring System

S/2025 U1 is not the only one to bring a mystery. This discovery could hold the clue to how Uranus’ complex ring system formed. S/2025 U1 and portions of the planet’s rings could be part of the same family. The moon and the rings may be relics of an ancient breakup event.

“The discovery raises the question of how many more of these small, hidden moons may be lurking around Uranus and how they might affect its rings,” El Moutamid said.

Five major moons are known around Uranus – Miranda, Ariel, Umbriel, Titania, and Oberon. In addition, there is a menagerie of small satellites. S/2025 U1 now makes 14 small moons in the inner system. Uranus is the only planet known to have this many small inner moons packed so closely together. Astronomers are puzzled as to how so many satellites are packed so closely together with orbits that would have the potential to cross, but somehow don’t. The theory is that they may serve to shepherd Uranus’ narrow rings, keeping them in line.

“The discovery is very exciting because this object is a new member of Uranus’ small moon population and its close association with Uranus’ inner ring system makes it particularly interesting,” said Scott Sheppard, an astronomer at the Carnegie Institution for Science in Washington, D.C. Sheppard was not involved in the new study but did co-discover one of Uranus’ moons in 2024. “I am very impressed with the sensitivity of Webb to detect it.”

The SETI Institute’s Matthew Tiscareno, also the co-principal investigator in the Webb Uranus project, said this discovery is also helping to erase the lines between where Uranus’ moons end and its rings begin. “The complex inter-relationships of these bodies hint at a chaotic past,” Tiscareno said. “The new moon, S/2025 U1, is even smaller and fainter than Uranus’ tiniest known inner moons, and is likely not the only one still to be discovered.”

History of the Discovery of Uranus’ Moons

The discovery of Uranus’ Moons had not come on the rapid-fire pace in recent years, with all satellites being spotted over a longer timeframe. Five moons of Uranus, the largest ones in the system, had been observed when Voyager 2 made a flyby of the planet in 1986, with the first moons having been discovered as early as 1787. Voyager 2 added 10 moons in the past, with diameters measuring 16 to 96 miles (26 to 154 kilometers). A handful of small moons, each just 8 to 10 miles (12 to 16 kilometers) across and darker than asphalt, were found by ground-based telescopes and the Hubble Space Telescope after Voyager 2. These inner moons are thought to be made of ice and rock, while those beyond Oberon are thought to be captured asteroids.

The Future of Uranus Exploration

Uranus could be the target of a future planetary mission. The planetary decadal survey published by the National Academies of Sciences, Engineering, and Medicine in 2022 recommended that NASA’s next large planetary endeavor be a Uranus Orbiter and Probe mission. The mission could launch in the early 2030s, but a budget standoff makes that funding in doubt. It would address Uranus’ mysterious tipped-over rotation, irregular magnetic field, atmospheric dynamics, and icy ocean worlds that could exist among its moons.

“I would be surprised if there weren’t more moons in the system as small as a few kilometers,” Sheppard said. More may be found by Webb’s long-exposure imaging or during future spacecraft missions.

“Now that we’ve found a new moon, El Moutamid and her team plan to follow up and refine the orbit of S/2025 U1 and look for more,” said a statement.

“Discovering a new moon around Uranus gives scientists a better sense of how the planet’s strange system formed, illuminates its rings, and will help us prepare for future missions like NASA’s Uranus Orbiter and Probe,” El Moutamid said.