Astronomers using the NASA/ESA/CSA James Webb Space Telescope have detected a previously unknown tiny moon — provisionally designated S/2025 U 1 — orbiting Uranus. Made in a series of Webb images taken on February 2, 2025, the detection brings Uranus’ total moon count to 29.
This Webb/ NIRCam image shows S/2025 U1 as well as 13 of the 28 other known moons orbiting Uranus. Image credit: NASA / ESA / CSA / STScI / M. El Moutamid, SwRI / M. Hedman, University of Idaho.
Located in the outer Solar System, Uranus is the seventh planet from the Sun.
Known as ‘the sideways planet’ for its extreme axial tilt, the cyan-colored ice giant has a deep atmosphere composed of hydrogen, helium and methane.
Uranus’ 28 moons include five major moons — Titania, Oberon, Umbriel, Ariel and Miranda — discovered between 1787 and 1948.
Known as ‘the literary moons,’ Uranus moons are named for characters in Shakespeare and the works of Alexander Pope.
Astronomers think Uranus’ larger moons are roughly equal parts water ice and silicate rock.
“As part of Webb’s guest observer program, we found a previously unknown satellite of the ice giant,” said Dr. Maryame El Moutamid, a researcher at Southwest Research Institute.
“This object, by far the smallest object discovered to date, was detected in a series of 10 long exposures obtained by Webb’s Near-Infrared Camera (NIRCam).”
The new moon, provisionally designated S/2025 U 1, is at the edge of Uranus’ inner rings.
The object is estimated to be just 10 km (6 miles) in diameter, assuming it has a similar reflectivity (albedo) to Uranus’ other small satellites.
It is located about 56,250 km (35,000 miles) from its center in the planet’s equatorial plane, between the orbits of Ophelia and Bianca.
Ophelia is about 43 km (13 miles) in diameter, while Bianca is an elongated object around 64 by 46 km (40 by 29 miles) in dimension.
“It’s a small moon but a significant discovery, which is something that even NASA’s Voyager 2 spacecraft didn’t see during its flyby nearly 40 years ago,” Dr. El Moutamid said.
S/2025 U 1 is the 14th member of the intricate system of small moons orbiting inward of the largest moons Miranda, Ariel, Umbriel, Titania, and Oberon.
“No other planet has as many small inner moons as Uranus, and their complex inter-relationships with the rings hint at a chaotic history that blurs the boundary between a ring system and a system of moons,” said Dr. Matthew Tiscareno, a researcher at the SETI Institute.
“Moreover, the new moon is smaller and much fainter than the smallest of the previously known inner moons, making it likely that even more complexity remains to be discovered.”
