New Moons Found at Uranus and Neptune

Astronomers have found three new moons orbiting our Solar System’s ice giants. One is orbiting Uranus, and two are orbiting Neptune. It took hard work to find them, including dozens of time exposures by some of our most powerful telescopes over several years. All three are captured objects, and there are likely more moons around both planets waiting to be discovered.

This is the first new moon found around Uranus in 20 years and brings the planet’s total to 28. One of the new moons around Neptune is the smallest ever detected with a ground-based telescope, and the pair of new discoveries bring Neptune’s total to 16.

Uranus’ new moon has the provisional title S/2023 U1 and was first observed on November 4, 2023, by Scott Sheppard from Carnegie Science. Like the planet’s other outer satellites, it will eventually be given a name from a Shakespeare play. Other moon names include Oberon, Titania, and Ariel. S/2023 U1 is only 8 km in diameter, tiny compared to the ice giant’s largest moon, Titania, which is almost 800 km. The tiny moon takes 680 days to orbit Uranus.

Neptune’s pair of new moons are likewise tiny. The brightest one has the provisional name S/2002 N5, is about 23 km in diameter, and takes nearly nine years to orbit Neptune. The fainter one has the provisional name S/2021 N1, is about 14 km in diameter, and takes almost 27 years to orbit the planet. They’ll both be given names from Greek mythology.

All of the easy-to-observe moons were found long ago. These small moons required much more work. While Scott Sheppard played a leading role, he had a lot of help.

“The three newly discovered moons are the faintest ever found around these two ice giant planets using ground-based telescopes,” explained Sheppard. “It took special image processing to reveal such faint objects.”

Sheppard used the Magellan telescopes at Carnegie Science’s Las Campanas Observatory in Chile to first spot Uranus’ new moon in November 2023. One month later, he performed follow-up observations with Magellan. Sheppard worked with those observations and alongside Marina Brozovic and Bob Jacobson of NASA’s Jet Propulsion Laboratory to determine the moon’s orbit. He also found the moon in older images from 2021 from the Subaru Telescope and the Magellan telescopes.

Sheppard also found the brighter of Neptune’s new moons with the Magellan telescope. He collaborated with David Tholen of the University of Hawaii, Chad Trujillo of Northern Arizona University, and Patryk Sofia Lykawa of Kindai University to find Neptune’s other new moon. It was extremely faint but still detectable with the Subaru Telescope. Both of Neptune’s new moons were first observed in September 2021. In October 2021, October 2022 and in November 2023, follow-up observations with the Magellan telescopes confirmed the brighter Neptune moon.

Confirming the fainter moon with follow-up observations was more challenging. To do that, the big guns in the telescope world were brought to bear. The ESO’s Very Large Telescope and the Gemini Observatory’s 8-meter telescope were used under pristine observing conditions to confirm the tiny, faint moon. Even then, the astronomers used the 2021 observations to know where to point the powerful telescopes and locate the moon.

Neptune’s brighter new moon was first spotted a couple of decades ago but was never recognized as a moon.

“Once S/2002 N5’s orbit around Neptune was determined using the 2021, 2022, and 2023 observations, it was traced back to an object that was spotted near Neptune in 2003 but lost before it could be confirmed as orbiting the planet,” Sheppard explained. 

It took dozens of five-minute exposures over three- or four-hour periods on several nights to gather enough data to see the moons. In these long exposures, the position of the planet and the moons shifts. The exposures were combined to create one deep image. This was a demanding use of time on the pair of large telescopes. But the result was the deepest images yet of the Uranus and Neptune systems.

“Because the moons move in just a few minutes relative to the background stars and galaxies, single long exposures are not ideal for capturing deep images of moving objects,” Sheppard said. “By layering these multiple exposures together, stars and galaxies appear with trails behind them, and objects in motion similar to the host planet will be seen as point sources, bringing the moons out from behind the background noise in the images.”

The three new moons are most likely captured objects. Their orbits are eccentric and inclined, and they’re quite distant from their planets. But regardless of their moons’ origins, all four giant planets in the Solar System have similar lunar arrangements and configurations.

“Even Uranus, which is tipped on its side, has a similar moon population to the other giant planets orbiting our Sun,” Sheppard explained. “And Neptune, which likely captured the distant Kuiper Belt object Triton—an ice-rich body larger than Pluto—an event that could have disrupted its moon system, has outer moons that appear similar to its neighbours.”

What can these moons tell us about the Solar System’s history? They’re more evidence of the Solar System’s chaotic, formative days.

Specifically, these moons show us that the groupings of moons around the ice giants are similar to the groupings around the other giants, Jupiter and Saturn. The gas giants have dynamic orbital groupings of outer moons, and now we know that Uranus and Neptune do, too.

The grouping of the moons is evidence of much larger parent moons smashed into pieces by collisions with other objects, probably comets or asteroids. The collisions left the broken fragments in similar orbits as the parent moons. There are almost certainly other smaller fragment moons from these collisions, but our current technology can’t find them.

If there are other moons around the ice giants, they’re likely smaller than 8 km in diameter around Uranus and smaller than 14 km around Neptune.