In the beginning of the solar system, Jupiter was a solo traveller. It was so big that it had carved out a path clear of dust and debris around the sun, leaving no material to build the massive moons that it has today. Then, Saturn began to form, and it may have tossed some rocks Jupiter’s way that later formed moons.
Jupiter now has 69 moons, but its four biggest – Io, Europa, Ganymede, and Callisto – likely formed around Jupiter when the solar system was young, rather than being captured from elsewhere in the solar system like most of its other smaller moons.
Those four Galilean moons likely formed from a ring of dust and rocks at the outer edge of Jupiter’s orbit. After Jupiter had formed out of the dense disc of gas, dust, and pebbles around the sun that coalesced into all our solar system’s planets, it ploughed through the material in its path, leaving an empty orbit in its wake. But the sun’s gravity may have pulled some space rubble into an orbit just out of the giant planet’s reach.
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Throwing stones
at Aix-Marseille University in France and his colleagues used computer simulations of Saturn’s formation to investigate how it would affect this rocky reservoir. They found that an infant Saturn could have disturbed the orbits of those rocks. Some of them may have been thrown into orbit around Jupiter, where they smashed together to form the Galilean moons.
Saturn could have formed from the rocks just outside of Jupiter’s orbit or started further out and then migrated inward due to the sun’s gravitational pull – but either way, according to the researchers’ simulations, it would have resulted in displaced debris that eventually became Jupiter’s moons.
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Over the next 100,000 years or so after Saturn formed, these boulders would have smashed together and grown to become the Galilean moons. Then, the researchers say, this same process could have occurred as Uranus and Neptune formed just outside of Saturn’s orbit, forming Saturn’s large moons Titan and Iapetus.
This could have implications for moons around planets in other stellar systems, none of which have been definitively spotted yet despite years of searching. “It would mean that massive moon systems like Jupiter’s would form preferentially – or even only – in systems with multiple large planets,” says Ronnet. This could help us decide where to point our telescopes in the hunt for distant moons.
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Read more: Neptune’s other moons were normal until Triton crashed the party
