“There’s probably a lot more of them and we just need to keep looking,” said Matthew Tiscareno, a senior research scientist at the SETI Institute. He made the comment after a find that is a testament to the technical prowess of astronomy now insofar as what occurs internally within the outer solar system: the James Webb Space Telescope has spotted a hitherto unsuspected moon orbiting around Uranus, only six miles in diameter.
The moon, now formally referred to as S/2025 U1, was first seen in a series of ten 40-minute exposures across February by the Near-Infrared Camera (NIRCam) on Webb. The camera’s sensitivity to far-infrared radiation allowed it to register feeble glints of sunlight off the object since previous missions could not. Even Voyager 2, whose Uranus flyby had placed it within 50,600 miles in 1986, flew by. While the spacecraft cameras and instruments, meanwhile, were slicing through a compressed magnetosphere a fairly uncommon solar wind occurrence that occurred only roughly 4% of the time and maybe making it more difficult to detect faint inner moons.
S/2025 U1 orbits about 35,000 miles above the center of Uranus in the planet’s faint inner rings. Its nearly circular path indicates that it has remained near its origin. This position, distant in a ring system of 13 and 14 other inner minor moons, explains why it was so hard to find until now. The rings of Uranus are made up of low-albedo material, and therefore are difficult to observe even with enormous ground telescopes, and any moon within them will not reflect much light.
The find increases the count of named moons of Uranus to 29, sustaining a 19th-century tradition where the initial four notable moons were named by Sir John Herschel from plays and poetry of Shakespeare and Alexander Pope. Although the International Astronomical Union has not yet formally approved a name for S/2025 U1, it will most likely continue the tradition, joining its name to the list headed by Titania, Oberon, Miranda, and Puck.
Technically, Webb’s discovery is a testament to advances in detection capability achieved through near-infrared imagery. NIRCam operates from 0.6 to 5 microns, ideally positioned to detect cold, distant objects which are poor reflectors of sunlight. It requires long exposure times and precise pointing stability to detect signals from objects which are billions of times fainter than the Sun. In S/2025 U1, not only the size but even the sheen of Uranus itself was an issue, and calibration and image processing with caution was necessary to settle down. The discovery also sheds light on larger issues of the creation and construction of the system of Uranus.
Uranus is different from the giant planets in having a tilted 98-degree axis, and with its displaced magnetic field, it has one of the special-shape magnetospheres. Voyager 2 observations now found to have been obtained when the solar wind was experiencing extreme compression indicated radiation belts equivalent to those of Jupiter but apparently no plasma from moon-born sources. New re-analyses suggest that the compression event might have had the result of puffing plasma temporarily out of the way, obscuring any geological activity on the giant moons. If this is the case, tiny moons like S/2025 U1 might be stealthily accumulating matter into the planet’s inner environments. Such findings are a bit more than a taxonomy drill. There are small moons that may act as shepherds, holding back ring material gravitationally and controlling ring shape. Their dynamics also monitor collision and accretion history in the system. As Tiscareno suggests, the ring system and inner moons of Uranus are an entwined, complicated entity which blurs the boundary between a ring system and a system of moons, with a suggestion of unpacified history.
It was found under Webb’s General Observer programme, which provides observing time for a range of scientific studies.
While Webb has delivered stunning pictures of exoplanets and distant galaxies, this find is a testament to what it can do to probe the fringes of our solar system. For planetary scientists, each new finding close to Uranus is a reminder that, nearly four decades after Voyager 2’s flyby, the ice giant remains full of secrets and just the appropriate mix of technology, timing, and tenacity to uncover them.
