Astronomers have confirmed three new irregular outer moons circling Uranus and Neptune. One orbits Uranus. Two orbit Neptune. The discoveries raise Uranus’ known moon count to 28 and Neptune’s to 16.
The new Uranian moon is provisionally called S/2023 U1. The two Neptunian moons are S/2021 N1 and S/2002 N5.
Carnegie Science announced the discoveries on February 23, 2024, after observations led by Scott Sheppard and collaborators. The International Astronomical Union’s Minor Planet Center issued the official announcement that same day for three moons: S/2021 N1, S/2023 U1 and S/2002 N5. Months later, the team’s results appeared in The Astronomical Journal, which set out the measurements and orbital interpretation in formal paper form.
What was found
Carnegie reports the new Uranus moon S/2023 U1 is about 8 kilometers wide, probably making it the smallest known moon of Uranus. It takes 680 days to orbit the planet. The journal paper estimates S/2023 U1 at about 7 kilometers in diameter based on its brightness. The moon has an apparent magnitude of 26.6 in the r-band.
The brighter of the two new Neptune moons, S/2002 N5, is about 23 kilometers wide. It has an apparent magnitude of 25.9 in the r-band. The Minor Planet Center orbital elements give its orbital period as about 8.6 years.
The fainter Neptunian moon, S/2021 N1, is about 14 kilometers wide with an apparent magnitude of 26.9 in the r-band. Its orbit takes almost 27 years.
All three are small, faint outer moons with distant, tilted and elongated orbits.
The new total counts
The discoveries bring Uranus to 28 known moons and Neptune to 16. The journal paper confirms these totals. Before these announcements, Uranus had 27 known moons and Neptune had 14.
The journal paper says Uranus now has 10 known outer irregular satellites and Neptune now has eight known outer irregular satellites. These newly confirmed objects belong to those small, far-flung outer populations.
How the team found them
The survey used very deep observations from the Subaru 8.2-meter telescope and the Magellan 6.5-meter telescopes. Instead of relying on one long exposure, the team took many shorter images over several hours and then shifted and stacked them at the apparent motion of the planet.
That made stars and galaxies blur into trails while moons moving with the planet stayed as compact points.
Carnegie says the search reached as faint as 26.9 magnitude around Uranus and 27.2 magnitude around Neptune in the r-band. These images went deeper than previous observations near Uranus and Neptune.
A Uranus moon hidden in old images
S/2023 U1 was first spotted on November 4, 2023. After the team estimated its path, Sheppard was able to trace it back into older images from 2021 taken at Magellan and Subaru.
S/2023 U1 has an r-band magnitude of 26.6. The journal paper says it has a distant, eccentric and inclined retrograde orbit similar to Caliban and Stephano. The authors interpret this similarity as a sign that these satellites are fragments of a once larger parent satellite.
That longer observational arc helped secure the moon’s orbit.
Two Neptune moons, one lost for years
Both new Neptune moons were part of the 2021 to 2023 observing campaign, but their timelines were not identical.
S/2021 N1 was first seen in September 2021. It was so faint that the team needed later observations with the Very Large Telescope and Gemini to secure its orbit.
S/2002 N5 was confirmed with observations from 2021, 2022 and 2023. Under the IAU’s provisional designation convention, “2002” refers to the year of first accepted observation. The MPC record shows S/2002 N5 was first observed on August 14, 2002. Once the orbit was determined from the 2021–2023 data, the object was matched to that earlier observation, which is how it earned its 2002 designation.
Small groups around big planets
The paper argues that both Uranus and Neptune show dynamical groupings of outer moons similar to those already recognized around Jupiter and Saturn.
Around Uranus
S/2023 U1 has an orbit similar to Caliban and Stephano. The authors say that similarity implies these satellites are fragments from a once larger parent satellite.
Around Neptune
S/2021 N1 has a retrograde orbit similar to Neso and Psamathe, indicating a dynamical family. S/2002 N5 forms a family of distant prograde satellites with Sao and Laomedeia.
Carnegie says these orbital groupings suggest larger parent moons were broken apart by past collisions, leaving fragments behind in similar orbits.
What the survey likely completed
The journal paper says the survey mostly completes the known outer satellites of Uranus down to about 8 kilometers and Neptune down to about 14 kilometers in diameter.
The paper notes this is a completeness estimate under assumed albedos and detection limits for r-band observations. It doesn’t mean every smaller moon is now known. Fainter fragments may still lie outside current detection limits. The survey pushed deep enough to make stronger comparisons across the giant planets.
What happens next
The names are still provisional.
S/2023 U1 should eventually get a name from a Shakespeare character. S/2021 N1 and S/2002 N5 should get names based on one of the 50 Nereid sea goddesses (sea nymphs) of Greek mythology. Neptune’s irregular satellites are named after the Nereids, daughters of the sea god Nereus.
For now, the main result is simple. Uranus has one more known moon. Neptune has two more. Both planets show outer-moon groupings similar to those already recognized around Jupiter and Saturn.
References
Carnegie Science. (2024, February 23). New moons of Uranus and Neptune announced. https://carnegiescience.edu/new-moons-uranus-and-neptune-announced
Minor Planet Center. (2024, February 23). MPEC 2024-D112: S/2021 N 1. https://www.minorplanetcenter.net/mpec/K24/K24DB2.html
Minor Planet Center. (2024, February 23). MPEC 2024-D113: S/2023 U 1. https://www.minorplanetcenter.net/mpec/K24/K24DB3.html
Minor Planet Center. (2024, February 23). MPEC 2024-D114: S/2002 N 5. https://www.minorplanetcenter.net/mpec/K24/K24DB4.html
Sheppard, S. S., Tholen, D. J., Brozovic, M., Jacobson, R., Trujillo, C. A., Lykawka, P. S., & Alexandersen, M. (2024). New moons of Uranus and Neptune from ultradeep pencil-beam surveys. The Astronomical Journal, 168(6), 258. https://doi.org/10.3847/1538-3881/ad7fed
