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NASA’s James Webb Space Telescope has identified a new moon orbiting Uranus. The discovery brings the known total number of Uranian moons to 29, and there could be more.

The small, previously hidden world was discovered in a stack of images taken on Feb. 2. The exposures, each 40 minutes long, are part of a series of long exposures the telescope is taking with its Near-Infrared Camera, or NIRCam. The moon itself is only 6 miles (10 kilometers) across, a record-setter for one of the smallest natural satellites discovered around Uranus.

Given its size, its discovery required Webb. Smaller than a deck of cards, it was hidden in Uranus’ bright glare, as well as by the bright glare of its rings, which, for the record, were imaged in separate exposures. For that reason, the small satellite was likely missed by earlier missions and telescopes, even NASA’s Voyager 2 spacecraft, which flew by Uranus almost 40 years ago.

“This is a small moon, but a significant discovery,” said Maryame El Moutamid, a planetary astronomer and one of the lead scientists for the Southwest Research Institute’s Solar System Science and Exploration Division, in Boulder, Colorado. El Moutamid is also the principal investigator of the Webb program that is studying Uranus’ rings and inner moons. “We are continuing to unlock the secrets of Uranus’ complex ring and inner moon system, with Webb showing us that we can push far beyond the frontiers of what was previously discovered,” she added.

Preliminary observations reveal the moon, which astronomers have temporarily named S/2025 U1, orbits about 35,000 miles (56,000 kilometers) from Uranus’ center. It has a nearly circular orbit in the planet’s equatorial plane. Astronomers think the moon was formed near its present location and orbits between two known moons, Ophelia (immediately outside Uranus’ main ring system) and Bianca.

Unlocking Uranus’ Rings

There are at least five major moons around Uranus: Miranda, Ariel, Umbriel, Titania, and Oberon. In addition to these, astronomers know of 23 smaller satellites. S/2025 U1 becomes the 14th small moon in Uranus’ inner system.

Astronomers do not understand why the planet has so many small inner moons, clustered close to each other. This makes Uranus unique in the solar system. Satellites are so close together that their orbits should cross, but somehow they remain in stable orbits. Scientists think the moons may be acting as shepherds, keeping Uranus’ narrow rings in place.

The discovery, made by Webb astronomers not connected to the telescope’s Uranus program, has piqued the interest of other scientists, such as Scott Sheppard, an astronomer at the Carnegie Institution for Science in Washington. He was not involved in the new study, but he is a co-discoverer of the 2024 Uranus moon. “It’s very exciting to find a new object associated with Uranus that’s that small,” he told Space.com. “Very few things of that size have been observed around a planet so far,” Sheppard added. He was impressed with Webb’s sensitivity, he said.

“The new moon is dark, tiny, and moving very quickly around Uranus, but it’s the very close association with the inner ring system that makes it even more interesting,” said Matthew Tiscareno, a planetary scientist at the SETI Institute in Mountain View, California, and co-principal investigator in the Webb Uranus program.

The line between Uranus’ moons and rings appears blurry. “Their complex inter-relationships hint at a chaotic history,” he said in a statement. “This new moon is even smaller and fainter than the smallest known Uranian inner moons, suggesting that more satellites are waiting to be discovered.”

Uranus Moon Discovery

Uranus’ moons are historically a long game. Before Voyager 2’s flyby in 1986, only five moons were known (the biggest ones in the system), and discoveries go back to 1787. Voyager 2 discovered 10 moons during the flyby, all between 16 and 96 miles (26 to 154 kilometers) across. Later ground-based telescopes and the Hubble Space Telescope added 13 more small moons, each only 8 to 10 miles (12 to 16 kilometers) across and so dark that they’re even blacker than asphalt. Satellites orbiting within the orbit of Oberon are generally thought to be made of ice and rock, while moons outside Oberon are thought to be captured asteroids.

Exploration of Uranus may increase in the coming decades. A planetary decadal survey released by the National Academies of Sciences, Engineering, and Medicine in 2022 put a Uranus Orbiter and Probe mission at the top of NASA’s list of recommended next large planetary missions. The launch could come in the early 2030s if the mission can find the required funding and make it through the budget battles, but no one expects this to happen easily or quickly. If it goes, the mission would send a spacecraft to study Uranus’ tipped rotation axis, complex magnetic field, atmospheric dynamics, and perhaps icy ocean worlds on its moons.

“We suspect there are many more small moons like this one to be discovered, with sizes as small as a few kilometers,” said Sheppard, who thinks more moons will be discovered in long-exposure Webb images or by a spacecraft. El Moutamid and her team will continue to refine the moon’s orbit and look for more.

“The discovery of a new moon around Uranus helps us understand how this strange system formed,” El Moutamid said. “We will be able to learn more about Uranus’ rings and their connections with moons, shedding light on one of the least explored planets in our solar system,” she added. “And these results pave the way for upcoming missions like NASA’s Uranus Orbiter and Probe,” El Moutamid said.