Three previously unknown moons found in our solar system

Astronomers have discovered three previously unknown moons around Uranus and Neptune, the most distant planets in our solar system. The find includes one moon spotted orbiting Uranus—the first discovery of its kind in more than 20 years—and two discovered orbiting Neptune.

“The three newly discovered moons are the faintest ever detected around these two icy giant planets using ground-based telescopes,” says astronomer Scott S. Sheppard. “Special image processing was required to identify such faint objects.”

The findings will be useful for missions that may be planned to study Uranus and Neptune more closely in the future, a priority for astronomers since the icy planets were only discovered in detail by Voyager 2 in the 1980s, CNN notes.

The three satellites were announced on February 23 by the International Astronomical Union’s Minor Planet Center.

The newfound moon of Uranus is the 28th to be observed in the ice giant’s orbit, and is also likely the smallest, measuring 5 miles (8 kilometers) across. The moon, called S/2023 U1, takes 680 Earth days to complete one orbit around the planet. In the future, the tiny moon will be named after a Shakespearean character, in keeping with the tradition of Uranian moons being named after literary names.

Sheppard spotted Uranus’ moon in November and December while making observations with the Magellan telescopes at Las Campanas Observatory in Chile. He worked with Marina Brozovic and Bob Jacobson of NASA’s Jet Propulsion Laboratory in Pasadena, California, to determine the Moon’s orbit.

The Magellan telescopes also played a key role in helping Sheppard discover the brighter of Neptune’s two new moons, S/2002 N5. The Subaru telescope, located on the dormant volcano Mauna Kea in Hawaii, helped Sheppard and his colleagues, astronomer David Tholen of the University of Hawaii, astronomer Chad Trujillo of Northern Arizona University, and planetary scientist Patrick Sophia Likavka of Kindai University in Japan, focus on another extremely faint moon of Neptune, S/2021 N1.

Both moons, bringing the total number of Neptune’s known natural satellites to 18, were first discovered in September 2021, but have required follow-up observations from various telescopes over the past couple of years to confirm their orbits.

“Once the orbit of S/2002 N5 around Neptune was determined using observations from 2021, 2022 and 2023, it was traced to an object that was seen near Neptune in 2003 but lost before it was confirmed to orbit planets,” Sheppard said.

The bright moon S/2002 N5 is 23 kilometers across and takes nearly nine years to complete a full orbit around Neptune, while faint S/2021 N1 is about 14 kilometers across and has a long orbit of about 27 years. Both will eventually receive new names that reference the sea goddesses Nereids from Greek mythology. Neptune was named after the Roman god of the sea, so the planet’s moons are named after lesser sea gods and nymphs.

Finding all three moons required dozens of short, five-minute snapshots over three to four hours on different nights.

“Because moons move in just a few minutes relative to the stars and galaxies in the background, single long exposures are not ideal for capturing deep images of moving objects,” explains Sheppard. “When these multiple exposures are stacked on top of each other, stars and galaxies will appear with trails behind them, and moving objects similar to the host planet will appear as point sources, making the moons stand out due to background noise in the images.”

By studying the distant angular orbits of the moons, Sheppard theorized that the moons were placed into orbit around Uranus and Neptune by the gravitational influence of the giant planets soon after their formation. The outer satellites orbiting all the giant planets of our solar system – Jupiter, Saturn, Uranus and Neptune – have similar configurations.

“Even Uranus, which is tilted on its side, has the same number of moons as the other giant planets orbiting our Sun,” Sheppard said. “And Neptune, which likely captured distant Kuiper Belt object Triton—an ice-rich body larger than Pluto in an event that could disrupt its lunar system—has outer moons that appear similar to its neighbors.”

It is possible that some of the moons around the giant planets are fragments of once larger moons that collided with asteroids or comets and broke apart. Understanding how giant planets captured their moons is helping astronomers piece together the chaotic early days of our solar system.