The space around Uranus just got a bit more crowded.
On Tuesday, astronomers using the James Webb Space Telescope announced the discovery of a moon orbiting around the pale blue ice giant, bringing its total number of satellites to 29. The latest addition, tentatively known as S/2025 U1, is tinier and fainter than any of the planet’s other known moons. That is the likely explanation for why it was missed in observations by previous telescopes or spacecraft.
“We were very happy to see it,” said Maryame El Moutamid, an astrophysicist at the Southwest Research Institute in Colorado. “It was acting exactly like a moon.”
All four of the largest planets in our solar system have a substantial number of moons of varying sizes. In recent years, scientists have discovered additional objects orbiting Jupiter, Saturn and Neptune that are considered moons.
Several of the now 29 moons of Uranus were discovered during a flyby of NASA’s Voyager 2 spacecraft in 1986. But the icy world also has 13 narrow, sharply defined rings. Some astronomers believe these rings to have been sculpted by the presence of unknown “shepherd” moons on either side, whose gravity prevents the material from spreading outward.
With the high resolution of the Webb telescope’s Near-Infrared Camera, or NIRCam, Dr. El Moutamid and her team “were almost sure that we would find plenty of moons around these rings,” she said.
The astronomers discovered S/2025 U1 in a series of images collected by the NIRCam in February. The moon was found lurking outside of the planet’s brightest ring, Epsilon, between two other satellites, Ophelia and Bianca.
This moon is not a shepherd moon for the Epsilon ring, as the astronomers expected. The team estimates that it is about six miles wide, and in a nearly circular orbit some 35,000 miles away from Uranus’s center. It is the 14th inner moon to be discovered orbiting the planet.
Over millions of years, Uranus’s inner moons may have collided and spread out into rings. As the material in the rings diffused, they moved farther away from the planet. Eventually, that material can start to accumulate, recycling itself back into a moon.
For S/2025 U1, “this is the most likely scenario,” Dr. El Moutamid said, adding that this process likely occurred sometime within the past 50 million years.
The astronomers are working on giving the newly discovered moon a permanent name. According to Dr. El Moutamid, they have an idea, but the name will have to go through an approval process by the International Astronomical Union. Thus far, all of Uranus’s moons have been given the names of literary characters, many from the works of Shakespeare.
More Uranian moons may be found in the future with the Webb telescope. But they will likely be even smaller than S/2025 U1, Dr. El Moutamid said.
Uranus remains one of the least explored planets in our solar system. Learning about its moons — if their orbits are in sync, how they influence each other and what relationship they have with the planet’s rings — can help astronomers better understand the evolution of the ice giant in our solar system. Many astronomers see it as a close analog for many exoplanets around other stars. That means studying Uranus may also offer insight for many worlds beyond our solar system.
Uranus “is just waiting for us to know more about it,” Dr. El Moutamid said. “We need to go there, study it, to better understand our universe.”
Katrina Miller is a science reporter for The Times based in Chicago. She earned a Ph.D. in physics from the University of Chicago.
The post Uranus Was Hiding a Moon Outside Its Rings appeared first on New York Times.
