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Astronomers using NASA’s James Webb Space Telescope have detected a mysterious new moon orbiting Uranus. The small satellite, which had previously been hidden from view, is one of at least 29 moons that orbit the ice giant. Many more may be lurking undiscovered, researchers say.

The tiny satellite, which measures just 6 miles (10 km) across, appeared on Feb. 2 in a series of 40-minute long-exposure images taken by Webb’s Near-Infrared Camera. At that size, it is one of the smallest natural satellites ever detected around Uranus. Its small size, as well as the bright light reflected by Uranus’ rings, probably hid it from the view of earlier telescopes and spacecraft. Even NASA’s Voyager 2, which made a historic flyby of Uranus nearly 40 years ago, didn’t spot it.

“This is a small moon but a big discovery,” said lead author Maryame El Moutamid, a 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 the study of Uranus’ rings and inner moons. “This finding just shows what Webb is allowing us to do, way beyond the reach of previous missions and telescopes.”

Webb Detected the Elusive Moon Using Infrared Light

The moon was designated as S/2025 U1, with the large number representing the year its orbit was first determined. It orbits at a distance of about 35,000 miles (56,000 km) from Uranus’ center, in a nearly circular orbit within the planet’s equatorial plane. The small moon orbits between the previously known moons of Ophelia (just outside Uranus’ bright main ring system) and Bianca. In fact, its orbital location suggests it likely formed near its present location.

Dark, tiny, and rapidly moving, the new satellite was difficult for astronomers to distinguish from the bright light coming from Uranus and its rings. Webb’s powerful instruments were able to see the faint infrared light that the new moon was giving off.

Webb has already provided astronomers with new glimpses of Uranus’ rings and weather, as well as its atmosphere. The recent discovery builds on that record.

Uncovering the Mystery of Uranus’ Moons and Rings

The discovery of the new moon has astronomers curious about the origin of Uranus’ complicated ring system as well. “The discovery of S/2025 U1 poses questions about how many other small moons are hidden around Uranus and how they interact with its rings,” El Moutamid added.

Scientists think the new moon and part of Uranus’s ring system may have been involved in the same ancient event. One possibility is that a chunk of a bigger moon broke away in the past. All the inner moons and most of the small rings orbit in the same direction and in nearly the same plane.

Uranus is known to have five big moons: Miranda, Ariel, Umbriel, Titania, and Oberon, as well as a swarm of small satellites. The newly found body is the 14th inner small moon. No other planet is known to have as many small inner moons that are all clustered together so closely. Astronomers are not quite sure why this is. The satellites are so close to one another that their orbits cross and could cause them to crash into each other or knock each other out of their orbits. Somehow, they are not doing so. Astronomers think they may act as shepherds, which helps keep Uranus’ narrow rings in place.

“This is very exciting,” said Scott Sheppard, an astronomer at the Carnegie Institution for Science who was not involved in the new study but is a co-discoverer of a new Uranus moon, also in 2024. He added that “the newly discovered object’s very close association with Uranus’ inner ring system makes this finding even more important. We have to give credit to the exquisite sensitivity of Webb.”

SETI Institute’s Matthew Tiscareno, co-principal investigator in the Webb Uranus project, said this discovery illustrates how the line between Uranus’ moons and rings becomes blurred. “The inner moons and rings of Uranus have a complex set of inter-relationships that hint at a chaotic history,” Tiscareno added. “In this discovery, Webb is again stretching the boundaries of what we know. This new inner moon is even smaller and fainter than the smallest known Uranian inner moons. So there are sure to be more satellites waiting to be discovered.”

The search for Uranus’ moons began more than two centuries ago. Before Voyager 2’s historic flyby in 1986, only five moons had been detected. These were the largest five that have all been known since at least 1787. Voyager 2 discovered 10 more during its flyby, measuring 16 to 96 miles (26 to 154 km) in diameter. Later, astronomers using ground-based telescopes and the Hubble Space Telescope found another 13 small moons, each 8 to 10 miles (12 to 16 km) across and darker than asphalt. Inner moons are believed to be made of ice and rock, while the outer moons beyond Oberon are thought to be captured asteroids.

In the future, NASA and other space agencies may be able to study Uranus’ moons even more closely. The planetary decadal survey, published in 2022 by the National Academies of Sciences, Engineering, and Medicine, recommended a Uranus Orbiter and Probe mission as NASA’s next large planetary mission. The mission could launch in the early 2030s if NASA receives the funding, which is currently tied up in agency budget debates. The mission would explore Uranus’ tilted rotation and mysterious magnetic field, its atmospheric dynamics, and perhaps icy ocean worlds among the moons.

Sheppard said there are likely additional moons out there that are only a few kilometers across waiting to be detected. Webb might spot them in long-exposure images. In the future, spacecraft missions might be able to find them as well. El Moutamid and her colleagues plan to further refine the orbit of S/2025 U1, as well as look for more hidden satellites.

“A new moon around Uranus is helping us better understand how this peculiar system formed and how its strange rings work, and preparing the way for future missions like NASA’s proposed Uranus Orbiter and Probe,” El Moutamid said.