Jupiter now has only four large moons, but in the early days of the solar system it may have had 20 or more (Image: JPL / NASA)
THE four giant "Galilean" moons orbiting Jupiter are the last survivors of at least five generations of moons that once circled the gas giant.
"All the other moons - and there could have been 20 or more - were devoured by the planet in the early days of the solar system," says Robin Canup of the Southwest Research Institute in Boulder, Colorado.
The four Galilean moons have played a key role in the history of science - their discovery by Galileo 400 years ago provided irrefutable evidence that not all bodies orbited the Earth. But until recently, nobody had suspected that Jupiter had once had many more moons.
Astronomers have long been aware of a mystery thrown up by simulations of the way Jupiter and its moons formed, says Canup. These models indicate that the mass of the debris disc around Jupiter, from which the moons formed, was several tens of a per cent of the mass of giant planet. And yet only 2 per cent is enough to make the moons we see today.
Now Canup and her colleague William Ward believe they know why. The extra mass can be explained if other moons formed while the debris disc was still present (www.arxiv.org/abs/0812.4995). "A key process is therefore the interaction between the growing moons and the disc material still flowing in from the solar system," says Canup. This interaction would have caused the early moons to spiral in towards Jupiter and eventually be "eaten".
This would explain the discrepancy in the earlier simulations, says Canup: as one set of moons was swallowed, a new set immediately began to form. "There could have been five generations of moons," she says. "The current Galilean moons formed just as the inflow of material into the disc from the solar system choked off, so they escaped the fate of their unfortunate predecessors."
According to Canup and Ward, in each generation the total mass of the moons was the same, but the number of moons could have varied. "We think something similar happened around Saturn, where the last generation contained one giant moon - Titan," says Canup.
This could have implications for the solar system as a whole. Rocky planets may take as long as 10 million years to aggregate, chunk by chunk. The process continues long after the debris disc around the sun has blown away, so these planets would not have been at risk of spiralling inwards.
In contrast, the gaseous cores of gas giants like Saturn and Jupiter condense out of the solar debris disc very quickly via gas shrinkage. This means they would have had time to interact with the debris disc. John Papaloizou of the University of Cambridge says it is entirely conceivable that the sun may have swallowed numerous gas cores before the current stable configuration of the solar system emerged.