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Astronomers with the James Webb Space Telescope (JWST) have identified a previously unknown moon orbiting Uranus. The discovery adds to the ice giant’s mysterious moonscape. Uranus now has 29 known moons, and astronomers suspect more hidden ones remain.

Webb detected the ultradistant moon on Feb. 2 in a series of 40-minute long-exposure images. It spans only about 6 miles (10 kilometers) in diameter and is one of the smallest natural satellites ever discovered around Uranus. Its tiny size, along with Uranus’ brighter rings, likely hid the moon from view in earlier spacecraft data and telescopes, including NASA’s Voyager 2 mission that performed a historic flyby of Uranus more than 40 years ago.

“This is a small moon but a significant discovery,” said lead scientist Maryame El Moutamid of 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 that is targeting Uranus’ rings and inner moons. She said the finding demonstrates that Webb is providing new insights “far beyond the threshold of any previous mission.”

S/2025 U1’s small size and distance from Uranus indicate it orbits the planet in a nearly circular path that resides in Uranus’ equatorial plane. Astronomers say it orbits between the known Uranian moons Ophelia, just outside the main ring system, and Bianca. The moon’s orbit indicates it may have formed near its current position.

Webb’s ability to measure faint infrared light was key to unlocking the new moon. The telescope’s view of the small satellite is also a “natural continuation” of the Webb research that has previously detected Uranus’ rings and glimpses of the planet’s weather and atmosphere.

Unlocking Uranus’ Rings and Moons Mysteries

The newly discovered satellite adds to the complex system already orbiting Uranus. The planet is known to have five large inner moons: Miranda, Ariel, Umbriel, Titania, and Oberon. Uranus also has many smaller satellites in its inner system, and S/2025 U1 is the 14th inner satellite. Uranus is unique in the solar system because it has so many small, inner moons in orbits so close to each other that their orbits may cross and yet they remain stable.

“The discovery of S/2025 U1 just 2,900 miles (4,700 kilometers) from Uranus’ massive rings hints that they may have a common origin,” El Moutamid said. “The discovery raises questions about how many small moons remain hidden around Uranus and how they interact with the planet’s rings.”

Scientists suspect the new moon and pieces of Uranus’ ring system may be fragments of a larger satellite that broke apart in a low-energy event. Some 200 years ago. S/2025 U1 may be a piece of that event, she said.

No other planet in the solar system has so many small, inner moons in such proximity to each other, said Scott Sheppard, an astronomer at the Carnegie Institution for Science and a co-discoverer of a Uranus moon in 2024. “It is also very exciting because the object is very close to the planet and so physically associated with the inner ring system,” said Sheppard, who was not part of the new study.

He added, “It’s a testament to the exquisite sensitivity of Webb that it was able to make this detection.” The Carnegie Institution’s Matthew Lane and Eliot Young, who are also co-authors of the Uranus moon co-discovered by Sheppard, helped El Moutamid’s team analyze the new moon data.

Matthew Tiscareno of the SETI Institute, who is also co-principal investigator (Co-PI) in the Webb Uranus project with El Moutamid, noted the discovery demonstrates the “morphological fuzziness” between the line that separates Uranus’ moons from the planet’s rings. “Their complex inter-relationships hint at a chaotic history of formation and interactions with material from the parent planet itself and with other moons,” Tiscareno said.

The newly discovered Uranus moon is even smaller and fainter than the faintest known Uranian inner moons, Tiscareno added. He said that fact suggests there are additional moons that remain hidden.

Five moons were known around Uranus before Voyager 2’s historic flyby in 1986. The moons were large and could be seen with early telescopes, with the earliest discovery dating back to 1787. Voyager 2 discovered 10 more moons during its Uranus encounter, each with diameters between 16 and 96 miles (26 to 154 kilometers).