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Astronomers using NASA’s James Webb Space Telescope (JWST) have discovered a previously unseen moon around Uranus, the ice giant with a mysterious and lopsided system. The new satellite adds to the 28 other known moons circling the planet, with more likely still hidden.

Webb’s Near-Infrared Camera spied the tiny moon on Feb. 2 through long-exposure, 40-minute images of Uranus and its faint rings. The newly found satellite measures just 6 miles (10 km) across, and is one of the smallest natural satellites ever discovered orbiting Uranus. It had been too faint to see in previous telescopic observations or spacecraft flybys, due to its small size and the glare of Uranus’ rings. “Even NASA’s Voyager 2 spacecraft, which flew past Uranus almost 40 years ago, was not able to see it,” said study lead author Maryame El Moutamid.

“This is a small moon, but a significant discovery,” said El Moutamid, who is based at the Southwest Research Institute’s Solar System Science and Exploration Division in Boulder, Colo., and is the principal investigator of a Webb program to study Uranus’ rings and inner moons. She added that the find is evidence of how much Webb will learn about the solar system beyond what has been achieved by earlier missions.

The moon, currently named S/2025 U1, is about 35,000 miles (56,000 km) from Uranus’ center, and orbits in the planet’s equatorial plane, moving in nearly circular motion in the space between Uranus’ moons Ophelia, just beyond the planet’s main ring system, and Bianca. The orbit suggests it formed in its current location.

Astronomers needed Webb to see the moon due to its dark color, small size, and rapid motion. Uranus’s rings, weather, and atmosphere are also difficult to discern, but Webb’s ability to image faint infrared light has allowed researchers to peer through. The team published its discovery in The Astronomical Journal.

“The world is dark, it’s tiny, and it’s moving pretty quickly,” El Moutamid said of S/2025 U1. “So, it was quite a challenge to separate it from the background of the planet and the glare of the rings.”

More Hidden Worlds to Come

A major mystery is how Uranus’s ring system formed. Scientists think the tiny moon and portions of its rings are related, fragments of some ancient cataclysmic collision that pulverized one or more moons into dust and detritus. “The discovery raises questions about how many more small moons remain hidden around Uranus and how they interact with the rings,” El Moutamid said.

Today, astronomers know that Uranus has five major moons — Miranda, Ariel, Umbriel, Titania, and Oberon — and a handful of much smaller satellites. S/2025 U1 becomes the 14th small moon orbiting in the inner system. This is the most any planet has, packed into a region of space that is roughly the size of our solar system’s asteroid belt. The satellites are orbiting so close together that their orbits could cross, yet they somehow don’t. Astronomers think that some may act as shepherd moons, keeping Uranus’ narrow rings in line.

“I think it is very exciting,” said Scott Sheppard, an astronomer at the Carnegie Institution for Science in Washington, D.C., who was not involved in the study but was the co-discoverer of a Uranus moon earlier this year in 2024. “It is tiny, but it’s in the immediate vicinity of Uranus’ inner ring system, and that’s what makes it so interesting,” he added. “We need the sensitivity of Webb to be able to detect it.”

NASA’s Voyager 2 spacecraft during its historic flyby of Uranus almost 40 years ago. Voyager 2 did not spot the newly discovered moon due to its small size and dimness.

Matthew Tiscareno, a planetary scientist at the SETI Institute and co-principal investigator in the Webb Uranus program, said the find is especially important because it blurs the line between Uranus’ moons and its rings. “Their complex inter-relationships hint at a chaotic history, like fragments and rubble with an uncertain future,” he added.

The moon detected by Webb is even smaller and fainter than the known smallest inner moons of Uranus, according to El Moutamid. As such, the study suggests that there are likely more small moons waiting to be discovered.