Astronomers Discover New Moons around Uranus and Neptune

The three newly-discovered moons — S/2023 U1, S/2002 N5 and S/2021 N1 — are the faintest ever found around Uranus and Neptune using ground-based telescopes.

The discovery image of the Uranian moon S/2023 U1 using the Magellan telescope on November 4, 2023. Image credit: Scott Sheppard.

Provisionally named S/2023 U1, the new Uranian moon was first spotted on November 4, 2023, by Carnegie Institution for Science astronomer Scott Sheppard using the Magellan telescopes at Las Campanas Observatory.

At only 8 km (5 miles), it is probably the smallest of Uranus’ moons. It takes 680 days to orbit the ice giant.

S/2023 U1 will eventually be named after a character from a Shakespeare play, in keeping with the naming conventions for outer Uranian satellites.

Its discovery brings the ice-giant planet’s total moon count to 28.

Dr. Sheppard also used the Magellan Telescope to find S/2002 N5, the brighter of the two newfound Neptunian moons.

The moon is about 23 km (14.3 miles) in diameter, and takes almost 9 years to orbit the ice giant.

The fainter Neptune moon was discovered in by Dr. Sheppard and his colleagues using the Subaru telescope.

Named S/2021 N1, it is about 14 km (8.7 miles) accross, and has an orbital period of almost 27 years.

Both S/2002 N5 and S/2021 N1 were first seen in September 2021.

They will both receive permanent names based on the 50 Nereid sea goddesses in Greek mythology.

“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,” Dr. Sheppard said.

S/2023 U1, S/2002 N5 and S/2021 N1 have distant, eccentric, and inclined orbits that suggests they were captured by the gravity of these planets during or shortly after Uranus and Neptune formed from the ring of dust and debris that surrounded our Sun in its infancy.

All of the giant planets in our Solar System have similar configurations for their outer moons, regardless of their size or the process by which they formed.

“Even Uranus, which is tipped on its side, has a similar moon population to the other giant planets orbiting our Sun,” Dr. Sheppard said.

“And Neptune, which likely captured the distant Kuiper Belt object Triton — an event that could have disrupted its moon system, has outer moons that appear similar to its neighbors.”

The new moons also show there are dynamical orbital groupings of outer moons around Uranus and Neptune, like those found around Jupiter and Saturn.

At Uranus, S/2023 U1 has a similar orbit as Caliban and Stephano.

At Neptune, S/2021 N1 has a similar orbit as Psamathe and Neso while S/2002 N5 has a similar orbit as Sao and Laomedeia.

These groupings suggest once larger parent moons have been broken apart by past collisions, probably with comets or asteroids, leaving the broken fragments behind in similar orbits as the original larger moon.

Many smaller moon fragments likely exist in these groupings, but they are generally too faint to efficiently observe with current technology.

These moon groupings show the early Solar System was a very chaotic place with the movement and collisions happening between various objects all of the time.

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