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Astronomers have spotted an unknown moon orbiting Uranus, the second ice giant of the solar system. The discovery, using Webb’s Near-Infrared Camera, increases the number of moons known to orbit Uranus to 29, and many more may be hidden.

The new, fuzzy object appeared in Webb images taken on Feb. 2 using long 40-minute exposures that enhance the visibility of faint objects. At just 6 miles (10 km) across, it’s one of the smallest natural satellites found around Uranus. It was likely hidden in plain sight, too small and faint to be seen by earlier space missions, telescopes, and even NASA’s Voyager 2 spacecraft, which flew by Uranus nearly 40 years ago.

“This is a small moon but a very significant discovery,” said lead scientist Maryame El Moutamid, of the Southwest Research Institute’s Solar System Science and Exploration Division in Boulder, Colorado. She is also the principal investigator (PI) of a Webb program dedicated to Uranus’ rings and inner moons. “This discovery highlights what Webb is allowing us to learn well beyond the capabilities of other missions.”

The newfound moon, currently known as S/2025 U1, orbits Uranus at a distance of about 35,000 miles (56,000 km) from the planet’s center. It orbits within the planet’s equatorial plane, where the other inner satellites are, between the known moons Ophelia, just outside Uranus’ main ring system, and Bianca. Its orbit is nearly circular, suggesting the moon formed in its current location.

The tiny moon is also dark, fast-moving moving and lost in the glare of Uranus and its brighter rings. Webb’s sensitivity to faint infrared light made all the difference. The telescope’s images have given scientists their first views of Uranus’ rings, weather, and atmosphere. This discovery is just another part of that ongoing record.

New Uranus Moon Reveals the Complex System’s Mysteries

The latest discovery is important for another reason: studying moons can provide clues to the planet’s more mysterious ring system. Researchers suspect S/2025 U1 and a portion of Uranus’ ring system might have formed together at some point in the past. They could be fragments of the same ancient collision, El Moutamid said. “The discovery raises questions about how many other small moons remain hidden around Uranus and how the small moons and the rings interact,” she said.

Uranus is known to have five large moons called Miranda, Ariel, Umbriel, Titania, and Oberon. But Uranus also has a host of smaller moons. The newly discovered object is now the 14th moon in that group, which scientists call the inner system. No planet has as many small inner moons as Uranus, and they are packed so closely together, their orbits should intersect and become unstable. Researchers have speculated that they may shepherd Uranus’ narrow rings, like a moonlet between Saturn’s F ring and the planet. It’s not yet clear why Uranus has so many of them.

“It’s very exciting,” said Scott Sheppard, an astronomer at the Carnegie Institution for Science who co-discovered one of Uranus’ moons in 2024. “This object is in such close association with Uranus’ inner ring system that it’s especially interesting.” Sheppard was not involved in the study. “I’m sure we’ll find more of these moons. Webb’s sensitivity will really allow us to open this region and see what’s there.”

Webb team member Matthew Tiscareno, an astronomer with the SETI Institute in Mountain View, California, is co-PI in the Webb Uranus project. He said the discovery confirms the “continuum between Uranus’ moons and the rings” and blurs any clear distinction between the two. “The existence of a small moon even fainter than the known faintest inner moons suggests that more small satellites are still hidden from view,” he said.

Scientists have been gradually adding Uranus’ moons since Voyager 2 first arrived in 1986. Before Voyager 2, only five moons were known, the largest ones discovered between 1787 and 1949. Voyager 2 added 10 more during its flyby. Voyager measured the new moons as between 16 and 96 miles (26 to 154 km) across. Beginning in the 2000s, astronomers found 13 more with ground-based telescopes and the Hubble Space Telescope. They range in size from 8 to 10 miles (12 to 16 km) across and are so dark that they would look blacker than asphalt if viewed from Earth. Astronomers think the inner moons are made of ice and rock. Moons outside Oberon are likely captured asteroids.

NASA’s future Uranus exploration is also on the horizon. The National Academies of Sciences, Engineering, and Medicine published a planetary decadal survey in 2022, a research roadmap developed by scientists and planetary researchers, that recommended a Uranus Orbiter and Probe mission as NASA’s next large planetary mission. The launch could come as soon as the early 2030s, but funding is uncertain while budgets are negotiated. Such a mission would explore Uranus’ tilted rotation, complex magnetic field, atmospheric dynamics, and intriguing potential icy ocean worlds that might exist among its moons.

Sheppard suspects there are many more moons out there, even smaller than a few kilometers in size. He and others plan to search for them with Webb’s long exposure imaging or with a spacecraft. El Moutamid and her team also plan to refine the orbit of S/2025 U1 and look for other hidden moons in future research.

“Discovering a new moon around Uranus helps us understand how Uranus’ strange system formed, helps us learn more about its rings, and helps us prepare for future missions to Uranus, such as NASA’s Uranus Orbiter and Probe mission,” El Moutamid said.