The newfound moons are small, between about 1 and 3 kilometers across. Seven of them travel in remote orbits more than 20 million kilometers away from Jupiter, and in the opposite direction from the planet’s rotation. That puts them in the category known as retrograde moons.
The eighth moon stands out because it travels in the same region of space as the retrograde moons, but in the opposite direction
The existence of so many small satellites suggests that they arose from cosmic collisions after Jupiter itself formed, more than 4 billion years ago.
“They did not form with the planet, but were likely captured by the planet during or just after the planet-formation epoch,” says Scott Sheppard, an astronomer at the Carnegie Institution for Science in Washington DC. He and his colleagues announced the discovery on 17 July.
Sheppard’s team typically hunts for objects in the very distant Solar System, out beyond Pluto, and sometimes spots planetary moons during these searches. In 2017, the group reported two additional Jovian moons. They were looking for Planet Nine. Jupiter was in the same part of the sky, so they were able to hunt for moons as well.
The researchers discover new Solar System bodies, and calculate their orbits, by photographing the same part of the sky weeks or months apart. They then look for objects that shift position between the two images, relative to the background stars. The team first spotted most of the new Jovian moons using the Blanco 4-metre telescope at the Cerro Tololo Inter-American Observatory in Chile, and followed up with further observations at other telescopes.