The JWST has another feather in its cap. The perceptive space telescope has taken a break from peering into the ancient, distant Universe and probing the formation and evolution of galaxies. It’s turned its gaze closer to home, examining Uranus for the presence of undiscovered moons, and it found one.
The discovery of one more tiny moon might not seem like a big deal. But if the Solar System is a puzzle, it can’t be completed without the small pieces. As scientists build a more complete understanding of Uranus, the tiny moon S/2025 U1 will be a part of it.
Astronomers found the small moon in JWST NIRCam images from February. It orbits about 56,000 km from Uranus’ center, and has an orbital period of 9.6 hours. It follows a nearly circular orbit. That suggests that it formed there, rather than being captured, since captured moons tend to follow eccentric orbits.
“It’s located about 35,000 miles (56,000 kilometers) from Uranus’ center, orbiting the planet’s equatorial plane between the orbits of Ophelia (which is just outside of Uranus’ main ring system) and Bianca,” said Maryame El Moutamid, a lead scientist in SwRI’s Solar System Science and Exploration Division based in Boulder, Colorado. “Its nearly circular orbit suggests it may have formed near its current location.”
“This object was spotted in a series of 10 40-minute long-exposure images captured by the Near-Infrared Camera (NIRCam),” said El Moutamid. “It’s a small moon but a significant discovery, which is something that even NASA’s Voyager 2 spacecraft didn’t see during its flyby nearly 40 years ago.”
Uranus is unique among the Solar System’s planets because it’s tipped on its side. So instead of a side view, the JWST gets a ‘top down’ view of the planet, its rings, and its moons.
In 1986, Voyager 2 came to within 81,500 km of Uranus and didn’t spot the moon. At that time, only five of the planet’s moons had been discovered, and Voyager discovered a sixth, Puck, named after a character in Shakespeare’s A Midsummer Night’s Dream. Eventually, Voyager 2’s images revealed the presence of 11 new moons.
S/2025 U1 is only about 10 km in diameter. That measurement is based on its albedo. If it has the same albedo as Uranus’ other moons, then that measurement should stand. At only 10 km diameter, it’s easy to see how it’s gone undetected for so long.
The tiny moon adds more complexity to one of the Solar System’s most complex environments. In fact, its discovery hints at even greater complexity yet to be discovered.
Moons around Saturn and Uranus can act like shepherds that maintain and shape the structure of the rings. Rings can also form from moons that get too close to their planets. When a moon exceeds the Roche Limit, the planet’s gravity pulls the moon apart and the debris creates a ring. Around Saturn, there’s growing evidence that the same material can then coalesce into another moon, and that this cycle has been repeated. In fact, Saturn’s rings contain about 150 moonlets embedded in its rings, which could be evidence of new moons forming.
A similar cycle may happen at Uranus, though on a much shorter timescale. Uranus’ moons are more densely packed than Saturn’s, and collisions between moons may create the debris that forms rings.
“No other planet has as many small inner moons as Uranus, and their complex inter-relationships with the rings hint at a chaotic history that blurs the boundary between a ring system and a system of moons,” said Matthew Tiscareno of the SETI Institute in Mountain View, California, a member of the research team. “Moreover, the new moon is smaller and much fainter than the smallest of the previously known inner moons, making it likely that even more complexity remains to be discovered.”
This discovery isn’t completely unexpected. Astronomers have been studying Uranus’ moons and some have concluded that there must be more smaller moons. Only they can explain the sizes and edges of Uranus’ rings. A 2020 paper said, “Given that 17 of the 20 sharp ring edges remain unexplained, one would expect several more moons to be required.” These moons would be in the 5 to 10 km range, and S/2025 U1 is about 10km in diameter. Though the same paper said that Cassini should’ve found them, it was incorrect; only the JWST has the power to spot them.
Will the JWST find more tiny moons in the Solar System? Much of the space telescope’s observing time is already spoken for. Its observations focus on its four main science themes. But a portion of its time is allocated to General Observer programs, and astronomers compete for this time with observing proposals. It seems likely that more GO programs will focus on the Solar System.
“Through this and other programs, Webb is providing a new eye on the outer solar system. This discovery comes as part of Webb’s General Observer program, which allows scientists worldwide to propose investigations using the telescope’s cutting-edge instruments. The NIRCam instrument’s high resolution and infrared sensitivity make it especially adept at detecting faint, distant objects that were beyond the reach of previous observatories,” said El Moutamid.
“Looking forward, the discovery of this moon underscores how modern astronomy continues to build upon the legacy of missions like Voyager 2, which flew past Uranus on Jan. 24, 1986, and gave humanity its first close-up look at this mysterious world. Now, nearly four decades later, the James Webb Space Telescope is pushing that frontier even farther,” El Moutamid said.