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Astronomers using the James Webb Space Telescope (JWST) have discovered an as-yet-unnamed moon orbiting Uranus. The icy giant already has a fascinating system of moons and rings, and this brings the count of known moons to 29, but the team thinks more should be hiding in its skies.

The discovery of the ultradark, 6-mile-wide (10-kilometer-wide) object was reported on February 2, in a series of images that took about 40 minutes each to expose. Webb’s Near-Infrared Camera revealed the newfound moon. It is one of the smallest natural satellites known to orbit Uranus. It is so small, and Uranus’ rings are so bright, that the tiny moon had been hidden from previous spacecraft and telescopes. NASA’s Voyager 2 spacecraft missed it when it flew past Uranus about 39 years ago.

“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, Boulder, Colorado. El Moutamid is also a Webb program principal investigator and co-principal investigator of a program observing Uranus’ rings and inner moons. “It shows what Webb is doing in the exploration of Uranus to go far beyond what Voyager 2 could do 40 years ago,” she said.

The Latest Moon for Uranus’ Odd System

The moon is a new entrant to Uranus’ growing family, but its discovery also helps astronomers understand the origin of the planet’s ring system. The astronomers suspect that the moon and parts of Uranus’ rings share a common origin that is still waiting to be uncovered. In particular, the small moon could be a remnant of an ancient event that created one or more of the rings, or the ring or rings may have created the moon.

“The discovery raises questions about how many more small moons remain hidden around Uranus and how these new and small moons interact with Uranus’ rings,” El Moutamid said.

At present, Uranus is known to have five big moons—Miranda, Ariel, Umbriel, Titania, and Oberon—and a group of small satellites with radii generally between 4 and 15 miles (7 and 25 kilometers). The new object is the 14th smallest moon in the inner system. This is the smallest of inner moons for any planet, and no other planet has small inner moons that are clustered so close together. Astronomers do not know how many such satellites can be packed so close to each other, without their orbits crossing and becoming unstable. It is thought that some of these satellites may shepherd the planet’s narrow rings, but much remains to be learned about this system.

The SETI Institute’s Matthew Tiscareno, the other co-principal investigator in the Webb Uranus project, said the discovery “blurs the distinction between the moons and rings of Uranus, as is the case at Saturn and Jupiter. Their complex inter-relationships hint at a chaotic history. This new moon is even smaller and fainter than the smallest Uranian inner moons previously known, suggesting more satellites must be hidden.”

Previously, the moons of Uranus have been revealed in dribs and drabs. Before Voyager 2’s historic encounter with the planet in 1986, only five moons, the largest ones, had been discovered. The earliest were found as far back as 1787. Voyager 2 observed 10 more moons during its flyby, all measuring between 16 and 96 miles (26 and 154 kilometers) in diameter. Additional ground-based telescopes and the Hubble Space Telescope have uncovered another 13 small moons since, all between 8 and 10 miles (12 and 16 kilometers) across, and darker than asphalt. While the inner moons are likely made of ice and rock, the outer moons beyond Oberon are likely captured asteroids.