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Webb astronomers have uncovered a new, previously unknown moon orbiting Uranus, adding to the ringed ice giant’s already mysterious system. The discovery brings Uranus’ total known moon count to 29, and researchers think there could be more.

The new small, distant moon was uncovered on February 2 in a series of 40-minute long-exposure images taken by Webb’s Near-Infrared Camera. At just 6 miles (10 km) across, the tiny world is one of the smallest natural satellites ever found to orbit Uranus. It likely was too dim and too close to Uranus’ bright rings to have been spotted by past telescopes and spacecraft. Not even NASA’s Voyager 2 probe, which flew by Uranus almost 40 years ago, saw it.

“This is a small moon but a significant discovery. It’s another important example of how Webb is building on the knowledge of previous missions and taking us much farther,” said lead author Maryame El Moutamid, a scientist at Southwest Research Institute’s Solar System Science and Exploration Division in Boulder, Colorado. El Moutamid is the principal investigator on a Webb program to study Uranus’ rings and inner moons. She pointed out that even with all the detail Voyager 2 revealed about the Uranus system, something as small as S/2025 U1 would have been invisible to that spacecraft.

In addition to being faint, the moon is dark, small, and moves quickly. Astronomers struggled to separate it from the glare of Uranus and its rings. Detecting faint infrared light beyond the visible spectrum allowed Webb to pick it out. Webb data had already teased out some details of Uranus’ rings, weather, and atmosphere, and this discovery builds on that record.

Adding to Uranus’ Moon Collection, Puzzling Ring Origin

The discovery of S/2025 U1 is more than a single new moon for the Uranus system. Scientists think the new object and a part of Uranus’ ring system may have a common origin, pieces from an event perhaps billions of years ago. “The discovery raises questions about how many more small moons remain hidden around Uranus and how they might interact with its rings,” El Moutamid said.

Presently, Uranus is known to have five large, major moons, Miranda, Ariel, Umbriel, Titania, and Oberon, along with a group of small satellites. The new object is the 14th smallest moon in the inner system. No planet has more small inner moons so close to one another in space, a fact that has long puzzled astronomers. The satellites are so close that their orbits might actually cross, but for some reason, they remain in stable orbits. Astronomers think they may act as shepherds to Uranus’ narrow rings.

Scott Sheppard, an astronomer at the Carnegie Institution for Science who was not involved in the new study but who co-discovered a new Uranus moon in 2024, said the find was “very exciting.” He noted that the object is so close to Uranus’ inner ring system that the two are essentially one system, which is unusual in itself and not what is expected. The brightness of Uranus also made the find special. “The fact that Webb can see such a faint object, given the sensitivity of the telescope, is amazing,” he added.

SETI Institute co-PI Matthew Tiscareno, who is also part of the Webb Uranus project, said that the find “illustrates how hard it is to say where the line is between the moons and the rings.” He added that these complex inter-relationships hint at a chaotic past for the Uranus system. “The newly detected moon is even fainter and smaller than the known inner moons that were also previously too faint to detect with Voyager 2. This small moon is yet another indicator that there are likely many additional small moons yet to be found,” Tiscareno said.

So far, the moons of Uranus have been gradually revealed one by one. Before Voyager 2’s historic 1986 flyby, only five moons, the largest ones, had been spotted, with the first discoveries dating to 1787. Voyager 2 discovered 10 more moons on its flyby that were between 16 and 96 miles (26 and 154 km) in diameter. Additional ground-based telescopes and the Hubble Space Telescope discovered another 13 small moons each between 8 and 10 miles (12 and 16 km) across and so dark that their reflectivity was less than that of asphalt. Inner moons tend to be icy and rocky, while outer moons beyond Oberon are thought to be captured asteroids.

More Uranus Exploration Coming Soon?

Uranus exploration could expand in the coming years as the 2022 planetary decadal survey released by the National Academies of Sciences, Engineering, and Medicine recommended a Uranus Orbiter and Probe mission as NASA’s next large planetary endeavor. The mission could launch in the early 2030s, but is currently up in the air pending budget negotiations. The mission would study the tilted rotation of the planet, the structure of its complex magnetic field, atmospheric dynamics, and possibly icy ocean worlds among the moons.

Sheppard thinks there will be more moons waiting to be discovered, as small as a few kilometers across. He thinks long-exposure imaging, like the new Webb data, could reveal them, or a future spacecraft mission might. El Moutamid and her team will continue to monitor S/2025 U1, improve its orbit, and search for more such hidden satellites.

“Discovering a new moon around Uranus is one more puzzle piece that helps us scientists better understand how its strange system formed and is evolving, sheds light on its ring system, and even prepares us for future missions like NASA’s Uranus Orbiter and Probe,” El Moutamid said.