Jupiter formed more than 4 billion years ago. Caption: NASA

Astronomers have discovered 10 small moons orbiting Jupiter, bringing its total to 79 — by far the most moons known around any planet. One of the finds is an oddball that moves in the opposite direction from its neighbours.

Together, the moons help to illuminate the Solar System’s early history. 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. Last year, the group reported two additional Jovian moons. In this case, the scientists were looking for a putative unseen massive planet popularly known as 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.

Space debris

These images show the movement of the Jovian moon dubbed Valetudo (labelled in yellow) relative to the background stars.Credit: NASA

All the newfound moons are small, between about 1 and 3 kilometres across. Seven of them travel in remote orbits more than 20 million kilometres 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 (that is, in the same direction as Jupiter’s spin). Its orbit is also tilted with respect to those of the retrograde moons. That means it could easily smash into the retrograde moons, pulverizing itself into oblivion. It may be the leftovers of a bigger cosmic collision in the past, Sheppard says.

Jupiter’s moons are named after gods with connections to the mythological Jupiter or Zeus. Sheppard has proposed naming the oddball Valetudo, after one of Jupiter’s descendants, the Roman goddess of hygiene and health.

The ninth and tenth newfound moons orbit closer to Jupiter, moving in the same direction as the planet.

Had all these small moons formed at the same time as Jupiter, they probably would have been captured by the gas and dust still swirling around the newborn planet, and have been engulfed. Their existence suggests that they are leftovers of later collisions between space rocks that left the debris encircling Jupiter.

If astronomers can work out the history of these collisions, they could also determine the sizes of satellites pulled into the orbit of a young Jupiter. “That's the big question, and that's what makes these ten new moons interesting," says Douglas Hamilton, an astronomer at the University of Maryland in College Park. “How can we link all this to how planets formed?”

Sheppard says there might still be a few more moons of Jupiter to discover — as yet unseen because they were hiding in the Sun’s glare when the scientists were looking. Saturn, the runner-up to Jupiter in the moon competition, has 62 known satellites.