A recently discovered tiny moon orbits Neptune and Uranus

Astronomers have discovered new moons around Uranus and Neptune for the first time in 10 years. These are the faintest moons ever discovered orbiting a planet, confirming a long-held idea about moons in the outer solar system.

Scott Shepherd from the Carnegie Institution for Science in Washington, D.C., discovered these moons using the Magellan Telescope in Chile and confirmed them using several other large telescopes around the world. “We looked about four times deeper than anyone has ever looked,” Shepherd said. “These satellites are at the edge of our capabilities. They’re just faint, faint points of light.”

Typically, when looking for the moon, you can only get a maximum exposure of about 5 minutes before it becomes overexposed and the moon’s movement renders it useless. Shepard and his team got around this problem by taking many of these five-minute images in quick succession, observing them for hours, and then combining the darker parts of the images. This allowed them to find dim points of light shining from the faintest moons ever discovered, as well as the smallest moons ever discovered around each planet.

The new moon around Uranus is tentatively named S/2023 U1, but will eventually be given the name of a Shakespearean character, along with the planet’s other moons. It is only about 8 kilometers in diameter and orbits once every 680 Earth days.

One of the new moons around Neptune is called S/2021 N1, and we await its official name from Greek mythology. With a diameter of about 14 kilometers, it takes about 27 Earth years to orbit the planet, making it the farthest moon from its host planet ever discovered. This is also the darkest moon ever discovered.

The brighter, larger moon discovered orbiting Neptune is called S/2002 N5. As its name suggests, this satellite was first discovered more than 20 years before, but was lost before astronomers could confirm its orbit. “The moon can get lost really easily,” Shepard says. “Basically, you need really good weather, your telescopes need to work perfectly, and everything needs to go well to detect these satellites.” If something goes wrong and a planned observation is lost, the satellite moves out of orbit and becomes very difficult to find again.

Each of the three new moons has an orbit similar to the other two moons in its planetary system, and these fellow travelers form small groups that orbit together. This means that each of these groups likely formed together when larger moons broke up during the early solar system chaos.

“Until now, it was unclear whether Uranus and Neptune had a group of exomoons like Jupiter and Saturn,” Shepard said. “We believe these are debris from satellites that were once much larger, but we’ll probably find many more smaller satellites.” Unfortunately, we’re reaching the limits of what we can discover with current technology, he says it may take even longer before these smaller moons are discovered around Uranus and Neptune.