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Astronomers working with NASA’s James Webb Space Telescope have detected an unknown moon orbiting Uranus. The newly discovered moon increases the number of moons known to orbit Uranus to 29, and scientists suspect that several others remain to be found.

The faint moon first showed up in a series of 40-minute long-exposure images captured by Webb’s Near-Infrared Camera (NIRCam) on February 2. It is only 6 miles (10 km) in diameter and is one of the smallest natural satellites ever discovered around Uranus. Its small size and the bright glare of Uranus’ rings had likely hidden the object from previous missions and telescopes, including NASA’s Voyager 2 spacecraft, which flew past the ice giant almost 40 years ago.

“This is a small moon but a big deal,” said lead scientist Maryame El Moutamid, a solar system researcher in the Southwest Research Institute’s Solar System Science and Exploration Division in Boulder, Colorado. El Moutamid is also the principal investigator (PI) of a Webb program dedicated to studying Uranus’ rings and its inner moons. “The new moon represents what Webb can do that was beyond previous missions,” she added.

Located about 35,000 miles (56,000 km) from the planet’s center, the new moon orbits the planet between two known moons: Ophelia, just beyond Uranus’ main ring system, and Bianca. In between, it follows a nearly circular orbit in the equatorial plane of Uranus. The fact that its orbit is near Uranus’ equator suggests that it formed in that neighborhood.

Astronomers found the moon difficult to separate from the glare of Uranus and its rings since it is dark, small, and fast-moving. Webb’s detection capability of faint infrared light was the breakthrough they needed to detect the object, which they named S/2025 U1, where “U” stands for Uranus and “1” indicates that it is the first one discovered by Webb. The telescope has provided insights into Uranus’ rings, weather, and atmosphere so far. The new finding is another first for the observatory.

Unlocking the secrets of Uranus’ moons

Scientists speculate that S/2025 U1 and a part of Uranus’ rings might be related. Possibly, both could be fragments of the same object that broke up in ancient times. “The discovery raises questions about how many more small moons remain hidden around Uranus and how they interact with its rings,” El Moutamid said.

In total, Uranus has five major moons: Miranda, Ariel, Umbriel, Titania, and Oberon. In addition, there is a collection of smaller satellites. The newly discovered satellite joins the 13 other small moons of the inner system. No other planet has as many small inner moons that are packed so close to one another, a fact that astronomers find hard to explain. The satellites are so close to one another that their orbits should cross. Yet, they appear to be stable. Astronomers think that some of these moons act as shepherds that control Uranus’ narrow rings.

“The new satellite is very exciting since it’s one of the inner moons and very closely associated with Uranus’ inner ring system,” said Scott Sheppard, an astronomer at the Carnegie Institution for Science, who was not involved in the new study but co-discovered another Uranus moon in 2024. Sheppard noted that Webb’s sensitivity is amazing, and no other telescope would be able to spot the object. “It just speaks to the sensitivity of Webb that we can start to find these satellites,” he said.

The SETI Institute’s Matthew Tiscareno, who is also co-principal investigator in the Webb Uranus project, said the discovery continues to blur the line between Uranus’ moons and its rings. “Their complex inter-relationships hint at a chaotic history,” he said. Tiscareno added that S/2025 U1 is even smaller and fainter than the smallest known Uranian inner moons. That means, there is more to be found.

Moons of Uranus have been slowly emerging over the years. Before Voyager 2’s historic flyby in 1986, only the five largest moons had been discovered, with the oldest observations dating back to 1787. Voyager 2 discovered another 10 moons as it flew past the ice giant. These range from 16 to 96 miles (26 to 154 km) in diameter. Later, telescopes on the ground and the Hubble Space Telescope also found 13 other small moons that orbit in the inner system. These range from 8 to 10 miles (12 to 16 km) across and are so dark that they are even blacker than asphalt. While inner moons are thought to be made of ice and rock, the moons outside of Oberon are believed to be captured asteroids.

Uranus exploration will become more extensive in the coming years. A planetary decadal survey released by the National Academies of Sciences, Engineering, and Medicine in 2022 selected a Uranus Orbiter and Probe mission as NASA’s next large planetary effort. A mission could be launched in the early 2030s. Still, money is uncertain amid debates over NASA’s overall budget. The mission would address questions about Uranus’ sideways rotation, peculiar magnetic field, and atmospheric dynamics, and would also explore potentially icy ocean worlds among its moons.

Sheppard speculates that more moons with diameters as small as a few kilometers could still be hiding around Uranus and are waiting to be detected by long-exposure Webb images or future spacecraft missions. El Moutamid and her team also plan to continue refining the orbit of S/2025 U1 and searching for more.

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