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The James Webb Space Telescope has detected a hidden moon orbiting Uranus, the seventh planet from the Sun. Researchers suspect additional moons remain unseen around the ice giant.

Webb, the most powerful space telescope ever built, uncovered the faint body in a series of 40-minute-long exposure images on Feb. 2. The tiny new moon is just 6 miles (10 kilometers) across. Measuring just 10 kilometers across, the object is one of the smallest moons ever detected around Uranus. “This new object is tiny and dark, so it was invisible to prior telescopes and spacecraft because of the bright glare of Uranus and its rings,” said astronomer Maryame El Moutamid, lead scientist at the Southwest Research Institute’s Solar System Science and Exploration Division in Boulder, Colorado. “Webb’s exquisite sensitivity has made all the difference.”

The newfound satellite was one of the earliest discoveries made with Webb, just days after the telescope was declared fully operational. El Moutamid is also the principal investigator of a Webb program to observe Uranus’ rings and inner moons, and Webb has already given the first glimpses of Uranus’ rings, weather, and atmosphere. Webb’s new image is helping astronomers fill in the rest of the picture. It shows a previously unknown moon that El Moutamid and colleagues are calling S/2025 U1, a provisional designation used before a new moon’s orbit is precisely measured. It is located some 35,000 miles (56,000 kilometers) from the planet’s center. The new moon orbits nearly circularly in Uranus’ equatorial plane, between two previously known moons: Ophelia just outside the planet’s main ring system, and Bianca just inside the ring. It is thought to have formed at its current location.

“This is a small moon but a significant discovery,” El Moutamid said. “The presence of such a small moon would have been completely missed by previous observations and earlier missions, but it wasn’t dark enough for Voyager 2, nearly 40 years ago. This discovery exemplifies the way in which Webb is continuing the frontier of what we can learn about these worlds, reaching beyond the knowledge gained from previous missions.”

Uranus is already known to have five major moons, plus an entourage of smaller ones. The newly discovered object will become the 14th in the inner group of small moons. No other planet in the solar system has as many small inner moons packed in so closely to one another. Astronomers can’t figure out how they fit together without crossing orbits. The satellite group is so close to Uranus that the tidal pull of its enormous host star should tear them apart. Researchers think they may shepherd the narrow rings of Uranus and keep them in line.

Scott Sheppard, an astronomer at the Carnegie Institution for Science who is leading the next study but was not involved in the new research, said the discovery is “very exciting.” He had co-discovered a Uranus moon in 2024. “It’s just one of the first interesting Uranian satellites to be imaged by Webb. The fact that it is orbiting so close to the planet’s inner ring system makes this discovery even more significant,” he added. “Credit to Webb for its exquisite sensitivity.”

Matthew Tiscareno, an astronomer at the SETI Institute who is also a co-principal investigator in the Webb Uranus program, said in a statement that the discovery “illustrates how hard the line is between moons and rings around Uranus.” “Their complex inter-relationships hint at a chaotic past, and this tiny new moon is smaller and fainter than the faintest known Uranian inner moons, indicating more of them remain hidden,” he added.

Uranus’ moons have been revealed piecemeal through time. Before Voyager 2’s historic 1986 flyby of the planet, only five had been seen — the largest five moons in the system with discovery dates as far back as 1787. Voyager 2 discovered 10 more moons on its flyby that are between 16 and 96 miles (26 and 154 kilometers) in diameter. Ground-based telescopes and the Hubble Space Telescope later detected 13 more small moons, each measuring between 8 and 10 miles (12 and 16 kilometers) in diameter and darker than asphalt. They are Uranus’ inner moons, composed of ice and rock. The satellites beyond Oberon are thought to be captured asteroids.

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 mission. A flight opportunity could come in the early 2030s, although funding has not yet been secured as Congress and the White House negotiate federal budgets. It would explore Uranus’ off-kilter rotation, its complex magnetic field, atmospheric dynamics, and possibly even icy ocean worlds among the planet’s moons.

Sheppard thinks more moons of only a few kilometers across likely await discovery either by additional long-exposure Webb imaging or from future spacecraft missions. El Moutamid and her team plan to continue refining their orbit and search for more hidden Uranian moons.

“Discovering a new moon around Uranus helps scientists better understand how its strange system formed, sheds light on its rings, and helps us prepare for future missions like NASA’s Uranus Orbiter and Probe,” El Moutamid said.