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The Webb team and outside researchers have discovered a new, previously unknown moon around the ice giant Uranus, increasing the known number of Uranian moons to 29 and likely indicating that others remain to be found.

The new moon was first seen on 2 February 2025 in a set of 40-minute-long exposure images taken with Webb’s Near-Infrared Camera. It has an estimated diameter of only 6 miles (10 kilometers), making it one of the smallest natural satellites discovered around Uranus to date. The moon’s size and the bright emissions of Uranus’ rings likely hid it from earlier spacecraft and telescopes, including the Voyagers and Hubble. Even NASA’s Voyager 2 spacecraft, which flew by Uranus 37 years ago this September, did not see it.

“This is a small moon, but an important discovery. It is another example of how Webb is building on the discoveries of previous missions and taking our knowledge much farther,” said Maryame El Moutamid, principal investigator of a Webb program for imaging the rings and inner moons of Uranus and a lead scientist in the Solar System Science and Exploration Division at Southwest Research Institute’s (SwRI) Boulder, Colorado, office. She and colleagues published their findings in a report that will appear in the Astrophysical Journal Letters.

Located approximately 35,000 miles (56,000 kilometers) from the center of Uranus, the new moon has a nearly circular orbit in Uranus’ equatorial plane, between the moons Ophelia (outside the main ring system) and Bianca. It orbits in a region where it may have formed.

To see the moon and distinguish it from the glare of Uranus and its rings, astronomers have had to make a trade-off. Being so small, the moon moves quickly and needs to be separated in time from the much brighter disk of Uranus and its rings. To get an image with enough detail to do this, astronomers had to use longer exposure times, and that washed out the bright emissions of Uranus and its rings. Webb’s sensitivity to faint infrared light did the trick. It also has glimpsed Uranus’ rings, weather, and atmosphere, and this discovery builds on the previous record.

Unlocking Uranus’ Complex Moons and Rings

The moon discovery is “yet another piece of the puzzle of the Uranus system,” El Moutamid said. Scientists have been surprised to find the largest of Uranus’ moons spread out among smaller satellites, many in the inner system just beyond the bright rings. Astronomers think that these satellites may act like shepherds, keeping Uranus’ narrow rings in line. “Their complex inter-relationships hint at a chaotic history,” she added.

Scientists suspect that S/2025 U1, as the newly discovered object has been temporarily named, may have formed from the same material and event that created one of Uranus’ rings. It could be a fragment of a past break-up or an object that has broken up after being captured by Uranus.

Uranus has five big moons—Miranda, Ariel, Umbriel, Titania, and Oberon—and an assortment of 24 smaller satellites. The newly discovered moon, being only 6 miles (10 kilometers) across, becomes the 14th small moon in the inner system. It is not known if other planets have so many small, inner moons packed so close together in a region so narrow. Astronomers are interested in learning how these satellites can remain so close together without their orbits crossing. That is considered a violation of one of Isaac Newton’s laws of gravity.

Astronomer Scott Sheppard of the Carnegie Institution for Science, not involved with the new study but co-discoverer of a Uranus moon with colleagues in 2024, said, “This is a very exciting detection,” of a small natural satellite “very close to Uranus.” He added, “It is very close to Uranus’ inner ring system, and this association makes this moon discovery especially noteworthy.” Sheppard praised Webb’s sensitivity, which made the detection possible.

Scientists have been learning about the moons of Uranus over the centuries, with Voyager 2 showing scientists in particular a rich trove of previously unknown natural satellites, El Moutamid said. Yet its moons are the least studied of the four ice giants (the other three being Neptune, Saturn, and Jupiter). Before Voyager 2’s visit in 1986, only five moons had been seen, the largest of Uranus’ satellites that had been discovered to date, with observations dating back to as early as 1787. Voyager 2 found 10 additional moons in its flyby, measuring from 16 to 96 miles (26 to 154 kilometers) in diameter. Ground-based telescopes and the Hubble Space Telescope found another 13 small moons that measure just 8 to 10 miles (12 to 16 kilometers) across and are darker than asphalt. The inner moons are made of ice and rock, while the outer moons beyond Oberon are thought to be captured asteroids.

Future studies could shed light on Uranus. A planetary decadal survey, which the National Academies of Sciences, Engineering, and Medicine published in 2022, recommended as the next large planetary mission to be funded by NASA, a Uranus Orbiter and Probe mission. The spacecraft would launch in the early 2030s to study the ice giant’s tilted rotation and crazy magnetic field as well as atmospheric processes, and potentially the habitability of the moons’ presumed icy ocean worlds. While NASA had been considering such a project, the budget has come under a lot of discussion, so it is not known whether it will be funded. “We hope that a mission will be put forward because there are so many important mysteries at Uranus that need to be studied,” El Moutamid said.

Finding new satellites is important for understanding how Uranus’ strange system formed, said Sheppard, who studies Solar System objects with colleagues using ground-based telescopes. There are likely moons as small as a few kilometers still to be found, he said, that may be revealed by long-exposure Webb images or a future spacecraft flyby.

As for this new moon, El Moutamid and her collaborators hope to continue to characterize its orbit and search for other hidden satellites.

“Discovering a new moon around Uranus helps us better understand the complex and strange system of moons and rings at Uranus, helps us better understand the rings, and prepares us for future missions like NASA’s Uranus Orbiter and Probe mission,” El Moutamid said.