.

Astronomers have detected a new, previously unknown moon orbiting Uranus with NASA’s James Webb Space Telescope. It’s the smallest natural satellite ever found in Uranus’ extended and still-mysterious system of rings and moons, and only the latest in a series of discoveries Webb is making in the outer solar system.

The newly detected satellite is just 6 miles (10 km) across, and was revealed on February 2 in a series of long-exposure images taken by Webb’s Near-Infrared Camera. Measuring more than 40 minutes in length each, the images were designed to search for new moons around Uranus by capturing their motion against the stars.

Astronomers have only found the moon in one set of images so far, but it is already clear to them that it is orbiting Uranus, even though its brightness is extremely low and is likely concealed by the brightness of Uranus’ rings.

“This is a small moon but a significant discovery,” said Maryame El Moutamid, lead scientist at the Southwest Research Institute’s Solar System Science and Exploration Division in Boulder, Colorado, and principal investigator of a Webb program dedicated to the planet’s rings and inner moons. “This discovery shows that with Webb, we are moving far beyond what previous missions could do and truly exploring what we don’t know.”

The new moon, provisionally known as S/2025 U1, is currently located some 35,000 miles (56,000 km) from the center of Uranus. It orbits in nearly the same plane as Uranus’ equator, between the previously known moons Ophelia, just outside the main ring system, and Bianca.

An orbit in the inner system that is stable suggests S/2025 U1 probably formed where it is now. El Moutamid estimates it has been there for billions of years.

Astronomers only had the new moon in their sights for a few days before Webb was commanded to move on to other targets. To speed their follow-up observations, El Moutamid and her team used data from the archive of previously published Webb images.

Because the moon is small, dark, and moves quickly against the background of the stars, astronomers needed to filter out the glare of Uranus and its rings to be able to spot it. Webb’s ability to view at infrared wavelengths, where the planet’s brightness is much dimmer, allowed the astronomers to tease out the new moon.

Webb previously revealed its rings and revealed hints of the planet’s weather and atmosphere, and the new moon discovery is the second Webb program to detect small Uranian moons. That program is led by Aaron Gronhamer at the Center for Space and Habitability at the University of Mainz in Germany.

Unlocking Uranus’ Rings and Moons

But El Moutamid is using Webb to probe Uranus’ inner ring system and nine inner moons with a set of six programs of her own. By measuring their brightness and other properties in visible and infrared light, the new study found a tiny spot of light in one image that moved from image to image.

Sheppard called it “very exciting.” He praised Webb’s exquisite sensitivity, which made the detection possible, but also noted that the object’s close association with Uranus’ inner ring system makes it particularly interesting.

Study co-author Matthew Tiscareno of the SETI Institute in Mountain View, California, who is also co-principal investigator in the Webb Uranus project, pointed to a blurry line between Uranus’ moons and its rings.

“The discovery of this new Uranian moon that is even smaller and fainter than the smallest previously known inner moons is a stunning discovery and suggests many more hidden small moons exist,” Tiscareno said.

Uranus has long fascinated astronomers with its collection of small moons. The planet has 29 known moons and is suspected to harbor more. Five of these moons—Miranda, Ariel, Umbriel, Titania, and Oberon—are Uranus’ largest and most massive. Many more smaller satellites complete Uranus’ collection, but the satellites orbit in groups in proximity to one another. The inner moons that orbit close to Uranus make up 14 known bodies, which El Moutamid’s team has identified, including the newly discovered moon. None of the solar system’s other planets has this many small inner moons packed so closely together, and that’s a mystery to astronomers. These small satellites are so close together that their orbits should cross, yet somehow they remain stable. Scientists think they may shepherd Uranus’ narrow rings.

Discovering More Moons

Historically, it’s taken decades and missions to explore Uranus’ moons. Before Voyager 2’s famous 1986 flyby of the planet and its system of rings and moons, only five had been discovered, with the first seen as far back as 1787. Voyager 2 discovered 10 new Uranus moons, all between 16 and 96 miles (26 and 154 km) in diameter, during the flyby. More moons, this time all only 8 to 10 miles (12 to 16 km) across and as dark as asphalt, were discovered later with ground-based telescopes and the Hubble Space Telescope. Inner moons like S/2025 U1 are believed to be made of ice and rock, but the outer moons past Oberon are thought to be asteroids that Uranus captured.