- calendar_today August 16, 2025
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Astronomers using NASA’s James Webb Space Telescope (JWST) have found an as-yet unnamed moon orbiting Uranus. Webb’s discovery makes it the 29th known moon of Uranus, the ice giant with the sideways spin, and the latest member of an unusually populous and diverse family of satellites. The discovery also leaves astronomers wondering if additional Uranian moons await discovery.
Webb imaged the ultra-faint object on Feb. 2 with its Near-Infrared Camera as part of a sequence of 40-minute long-exposure frames. Tiny and only 6 miles (10 kilometers) across, the new Uranian moon is the smallest natural satellite of Uranus ever found. So dim was the icy object, and so bright the planet’s rings, that the new moon had been hidden in plain sight from telescopes and previous spacecraft missions. It even escaped detection during NASA’s Voyager 2 spacecraft’s Uranus flyby more than 38 years ago.
“This is a small moon, but a very important discovery,” said Maryame El Moutamid, a planetary astronomer at the Southwest Research Institute’s Solar System Science and Exploration Division in Boulder, Colorado, and a principal investigator of a Webb program that is characterizing Uranus’ rings and inner moons. “This discovery is showing us what Webb is capable of and how it’s extending our knowledge well beyond any previous mission.”
The moon, which has been temporarily named S/2025 U1, lies just 35,000 miles (56,000 kilometers) from Uranus’ center and orbits within the planet’s equatorial plane. S/2025 U1 orbits Uranus nearly circularly, just outside the planet’s main ring system between the known moons Ophelia and Bianca. The location and orbit of the new moon suggest that it may have formed where it was found.
This past discovery may not be the last time Webb will see Uranus’ missing satellites, since S/2025 U1 is only about 6 miles (10 kilometers) wide. “There are likely to be other small moons yet to be found that Webb can image as it continues to explore the solar system,” El Moutamid said. “This is a very bright field for Webb to image, which is why we were able to find this.”
Astronomers know Uranus has five major moons (Miranda, Ariel, Umbriel, Titania, and Oberon), but they have also counted a group of at least 24 smaller satellites. They call these additional satellites “small moons.” The newly discovered object is the 14th inner small moon that astronomers have discovered so far. No planet has as many small moons found in its innermost reaches as Uranus does. The inner moons of Uranus are so close together that the orbits of some of them could cross, yet amazingly, they are stable. Scientists think the moons could act as shepherds of Uranus’ narrow rings, keeping them in line.
Scott Sheppard, an astronomer at the Carnegie Institution for Science in Washington, D.C., who is not involved in the new study but co-discovered a Uranus moon in 2024, said the new finding was “very exciting.” The object is located very close to Uranus’ inner ring system, which makes it particularly important, he said. “I’m amazed it was missed for so long,” Sheppard said, “and really thrilled that Webb’s sensitivity allowed us to find it.”
The SETI Institute’s Matthew Tiscareno, who is co-principal investigator in the Webb Uranus project, added that the find illustrates the fuzzy line that exists between the planet’s moons and the ring system. “The connections between its satellites and ring system are far more complex than we imagined, and hint at a chaotic past,” he said. S/2025 U1’s size and faintness also hint that there are more moons of Uranus out there waiting to be found. The discovery is even smaller and fainter than the known smallest Uranian inner moons, with a diameter of 8 to 10 miles (12 to 16 kilometers) and darker than asphalt. If discovered, it would be smaller than any other known Uranian moon.
Some moons of Uranus have taken a long time to be found. In fact, astronomers had only seen Uranus’ five largest moons before Voyager 2’s 1986 visit to the outer planet. (Some had been seen as far back as 1787.) Voyager 2 added another 10 moons that Voyager 2’s flyby measured between 16 and 96 miles (26 and 154 kilometers) across. Follow-up searches with Earth-based telescopes and the Hubble Space Telescope later found 13 additional small moons. Each of these orbits Uranus at a distance that places them in the inner system and measures between 8 and 10 miles (12 and 16 kilometers) across. Each of these inner moons is darker than asphalt. The satellites found farther out than Oberon are thought to be asteroids that Uranus captured.
An expanded Uranus exploration program may be on the way. A planetary decadal survey that the National Academies of Sciences, Engineering, and Medicine released in 2022 recommended NASA’s next big planetary exploration project as a Uranus Orbiter and Probe mission. It could fly in the early 2030s but has not been funded in recent NASA budget battles. Such a mission could look for evidence of Uranus’ tipped rotation and off-center rotation axis, study Uranus’ strange magnetic field, learn how the planet’s atmospheric dynamics work, and look for icy ocean worlds on the moons.
Sheppard believes astronomers will likely find additional moons around Uranus as small as a few kilometers across in the future, whether by Webb’s long-exposure images or by using a spacecraft to visit the planet in the future. El Moutamid and her colleagues hope to measure S/2025 U1’s orbit to a higher precision and then conduct deeper, long-exposure searches in Webb images for still more hidden satellites.
“Discovering a new moon around Uranus helps scientists better understand how its strange system formed, how its rings behave, and prepares us for future missions like NASA’s Uranus Orbiter and Probe,” El Moutamid said.
