A LONG-STANDING puzzle about how the giant planets of our Solar System collected so many moons has been solved by a team of chemists more at home studying the trajectories of atoms.
鈥淚t鈥檚 very impressive that they can jump fields like this. It鈥檚 a nice piece of work,鈥� says Douglas Hamilton, an astronomer from the University of Maryland, College Park.
In recent years astronomers have been on a roll where moons are concerned. Rapid developments in telescope technology have doubled our Solar System鈥檚 moon count since 1997, to the grand total of 128. Jupiter, being the closest giant planet, claims 60 alone. Scott Sheppard鈥檚 team at the University of Hawaii recently found 43 (Nature, vol 423, p 261), 20 of them just before the paper went to print. Researchers suspect that Jupiter, Saturn, Neptune and Uranus have about 100 satellites each.
Advertisement
Only 8 of Jupiter鈥檚 moons are in regular, near-circular orbits, and were formed at the same time as the planet. The rest are in irregular, elliptical orbits, and astronomers suspect they were once in orbit around the Sun. When the rocks got close to Jupiter they ran into the thick layer of gas surrounding the planet in its early days, slowing the rocks enough to snag them into a new orbit. But no one could pin down the mathematics of how this happened.
So mathematician Stephen Wiggins at the University of Bristol and his chemist colleagues, who were doing research into how atoms split apart from each other during chemical reactions, decided to have a go at the problem. The mathematics for molecules splitting is very similar to the process of moon capture but in reverse, says Wiggins.
Their simulations show that when a rock passes a certain distance from a planet, called the 鈥渃haotic鈥� layer, tiny nudges are enough to change its trajectory dramatically. This helps it stay in the gas long enough to get stuck in orbit (Nature, vol 423, p 264).
The work also shows that rocks orbiting in the same direction as Jupiter鈥檚 spin tend to get caught closer in to the planet, making them likely to be bulldozed by its regular moons. That explains a long standing mystery of why Jupiter, unlike Saturn, has so few 鈥減rograde鈥� moons. 鈥淭hat鈥檚 the really clever bit,鈥� says Hamilton.