By Alexandra Witze
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.
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