Gas-giant planets may have formed by “pebble accretion”

Nature : Gas-giant planets like Jupiter and Saturn were thought to have formed from the agglomeration of kilometer-sized chunks of material. However, the process would have taken a long time, much longer than the several million years scientists estimate. In 2012 Michiel Lambrechts and Anders Johansen of Lund University in Sweden proposed what has become known as the pebble-accretion scenario, in which the biggest solar-system planets may have first formed from centimeter-sized pebbles of dust and ice. Aerodynamic drag would have allowed the pebbles to more rapidly meld together to form the cores of the gas-giant planets. The theory has one problem, however: Hundreds of Earth-sized objects would have been created, rather than the handful of larger planets that exist. Now, Harold Levison of Southwest Research Institute in Boulder, Colorado, and colleagues propose that as those early embryonic planets orbited the Sun, they gravitationally interacted and the larger ones may have knocked the smaller ones out of the protoplanetary disk and continued growing. According to the Levison group’s models, about one to four gas giants could have formed, similar to Jupiter, Saturn, Uranus, and Neptune.