New moon discovered orbiting Uranus is its smallest one

A small and dim moon has been discovered in orbit around Uranus, bringing the planet’s total to 29. Many of the gas giant’s other moons are named after characters from works by William Shakespeare, and scientists are now debating which of his characters may lend the body its name.

The new moon was discovered by a team led by Maryame El Moutamid at Southwest Research Institute in Colorado, using 10 long-exposure infrared images taken by NASA’s James Webb Space Telescope (JWST) on 2 February this year.

The moon currently has the provisional name S/2025 U 1. But in time it will probably be named along the same lines as 27 of Uranus’ moons: taking a character’s name from one of Shakespeare’s plays. This convention dates back to the discovery of the planet’s first two moons, Titania and Oberon, in 1787.

The chosen name for the newly-found moon will need to be approved by the International Astronomical Union (IAU), the leading authority on assigning official names and designations to astronomical objects. Mark Showalter at the SETI Institute, who was part of the research team and is a self-confessed theatre fan, says there have been discussions but no shortlist yet.

Showalter says spotting a moon so small and dim was a difficult task. “It’s a tiny object right next to a very, very bright object. It’s like staring into the headlight of a car and trying to look at a fly,” he says. “The James Webb telescope is an extraordinary instrument that is vastly more sensitive than any other telescope that has ever existed, frankly.”

There is hope more moons may be found around Uranus, says Showalter. “We certainly haven’t finished the job,” he says. “I think it’s reasonable to assume that there are more satellites out there. We always suspected that there might be some satellites within the ring system that are kind of what we call shepherding moons, that push the ring material around. We haven’t found any of those yet, but that’s a major focus for our ongoing work.”

El Moutamid says the sharpness of Uranus’ rings indicate there are likely to be more moons, yet undiscovered, that helped with the rings’ formation. “There are probably a lot of more out there that are just waiting for us to detect them,” she says. Some of these may be found by JWST, but there could be a rash discovered by the proposed Uranus Orbiter and Probe mission in 2044 if it goes ahead as planned. “Probably there are plenty of moons out there that are very tiny, and we just cannot detect them yet because of the resolution that we have so far,” she says.

S/2025 U 1 is estimated to be around 10 kilometres in diameter, meaning it is too small to have been seen by the cameras aboard the Voyager 2 probe. Voyager 2 launched in 1977 and passed by Uranus in 1986, coming as close as 81,500 kilometres. It remains the closest pass to Uranus by a spacecraft from Earth.

The new moon is at the edge of Uranus’ inner rings, located around 56,250 kilometres from its centre in the planet’s equatorial plane. This places it between the orbits of the moons Ophelia and Bianca.

NASA operates a “General Observer” program for JWST, which allows any scientist around the world to propose targets that require one of the telescope’s advanced sensors. El Moutamid applied for time on JWST’s NIRCam instrument – a high resolution infrared sensor – to study the rings of Uranus. That application led to the discovery of this new little moon.