Scientists think 'planetary pebbles' were the building blocks for the largest planets

Researchers at Southwest Research Institute (SwRI) and Queen's University in Canada have unraveled the mystery of how Jupiter and Saturn likely formed. This discovery, which changes our view of how all planets might have formed, will be published in the Aug. 20 issue of Nature. Ironically, the largest planets in the solar system likely formed first. Jupiter and Saturn, which are mostly hydrogen and helium, presumably accumulated their gasses before the solar nebula dispersed. Observations of young star systems show that the gas disks that form planets usually have lifetimes of only 1 to 10 million years, which means the gas giant planets in our solar system probably formed within this time frame. In contrast, the Earth probably took at least 30 million years to form, and may have taken as long as 100 million years. So how could Jupiter and Saturn have formed so quickly? The most widely accepted theory for gas giant formation is the so-called core accretion model. In this model, a planet-sized core of ice and rock forms first. Then, an inflow of interstellar gas and dust attaches itself to the growing planet. However, this model has an Achilles heel; specifically, the very first step in the process. To accumulate a massive atmosphere requires a solid core roughly 10 times the mass of Earth. Yet these large objects, which are akin to Uranus and Neptune, had to have formed in only a few million years. In the standard model of planet formation, rocky cores grow as similarly sized objects accumulate and assimilate through a process called accretion. Rocks incorporate other rocks, creating mountains; then mountains merge with other mountains, leading to city-sized objects, and so on. However, this model is unable to produce planetary cores large enough, in a short enough period of time, to explain Saturn and Jupiter. http://phys.org/news/2015-08-scientists-planetary-pebbles-blocks-largest.html