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An international team of astronomers using the James Webb Space Telescope (JWST) has discovered a mysterious new moon around Uranus. The ice giant already had 28 known moons when Webb snapped a set of 40-minute long-exposure images on Feb. 2 that revealed the tiny world, one of the smallest satellites known to orbit the seventh planet from the Sun.

The moon is only 6 miles (10 km) across, and it is faint from great distance—facts that likely explain why the satellite escaped detection during the flyby of NASA’s Voyager 2 spacecraft four decades ago, as well as by all the telescopes on and above Earth. The glare from Uranus’ bright rings probably also hid the satellite from view.

“This is a small moon but an important discovery because it is just one of many new insights that the James Webb Space Telescope is already contributing to planets beyond Jupiter,” said lead scientist Maryame El Moutamid, a solar system 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 to study the rings of Uranus and its inner moons. “The Webb data gives us a sense of how much our knowledge has expanded far beyond previous missions and space telescopes.”

The new moon, which has been temporarily designated S/2025 U1, is about 35,000 miles (56,000 km) from Uranus’ center, and it orbits the planet in the equatorial plane, more or less where Uranus’ 27 faint rings are found. S/2025 U1 is in a nearly circular orbit between the moons Ophelia, just outside the main ring system, and Bianca. “We think the moon may have formed in this location,” El Moutamid said.

Because S/2025 U1 is dark, small, and moves quickly against the background stars, the astronomers had a hard time isolating its image from the glare of Uranus and its rings. The JWST’s Near-Infrared Camera, however, easily picked up its faint infrared light. The telescope has already peered into Uranus’ rings, cloud tops, and weather in short observations, and the discovery adds to that growing record.

“A New Moon for Uranus”

Discovery of the new moon is not only remarkable for adding another member to Uranus’ family of moons but also because the origin of its ring system remains a mystery. Astronomers think the satellite and part of the planet’s rings may share a common origin, perhaps as remnants of a single ancient impact. “This discovery raises questions of how many small moons remain hidden around Uranus and how they interact with its rings,” El Moutamid said.

The planet is currently known to have five big outer moons—Miranda, Ariel, Umbriel, Titania, and Oberon—and a clump of small satellites in the inner system, including the new one, which becomes the 14th small moon known in that system. Uranus is unique in that no other planet in our solar system has as many small inner moons that are so closely packed next to each other. Astronomers are not sure why. The orbits of the satellites are close enough together that their orbits should cross and overlap, which would be unstable and a recipe for collisions. Yet they are not. The researchers believe they may be acting as shepherds to Uranus’ narrow rings.

“I think this is a very exciting discovery,” said Scott Sheppard, an astronomer at the Carnegie Institution for Science in Washington, D.C., who was not involved in the new study but co-discovered a Uranus moon in 2024. “It is very exciting not only because it is a new moon but also because it is so closely associated with the planet’s inner ring system,” Sheppard said. “I am very impressed with how sensitive Webb is.”

“The discovery of this new moon further blurs the line between Uranus’ moons and rings,” said co-principal investigator Matthew Tiscareno, an astronomer at the SETI Institute in Mountain View, California, who also worked on the Webb Uranus program. “Their complex inter-relationships hint at a chaotic history, and yet here is another moon even smaller and fainter than the tiniest known Uranian inner moons that we already know are out there. This is strong circumstantial evidence that there are more hidden satellites yet to be found.”

Moons orbiting Uranus have come into astronomers’ view over the years in a slow trickle. Voyager 2 discovered 10 of the small satellites when it flew by in 1986, including another found with El Moutamid in late 2024. Voyager 2’s moons measured between 16 and 96 miles (26 to 154 km) in diameter. Before Voyager 2, only the five large moons that had been discovered as far back as 1787 were known. In the following years, other small moons were found with ground-based telescopes and the Hubble Space Telescope. Thirteen of them orbit close to the planet. They are no bigger than 8 to 10 miles (12 to 16 km) across and are as dark as asphalt. Inner moons are likely made of ice and rock, and outer moons beyond Oberon are thought to be captured asteroids.

For future exploration of the Uranian system, a planetary decadal survey published in 2022 by the National Academies of Sciences, Engineering, and Medicine recommended that NASA’s next large planetary mission should be a Uranus Orbiter and Probe, perhaps flying in the early 2030s. The mission concept would aim to solve the mystery of Uranus’ strange sideways rotation, its tangled magnetic field, its atmospheric dynamics, and the possibility of ocean worlds beneath the icy shells of some of its moons. The mission is on the House and Senate budgets, though funding is not yet assured as Congress debates its budget.

Sheppard suspects more moons could be lurking as small as a few kilometers across and should be discovered with more long-exposure imaging using Webb or on future spacecraft missions. El Moutamid and her team will continue to work to refine the orbit of S/2025 U1 and continue to look for other hidden moons around Uranus.

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