The Great Dichotomy of the Solar System

The growth of two planetary embryos. The red embryo is exterior to the ice line and grows to 10 Earth masses while the the green embryo is interior and grows to 0.1 Earth masses. These masses are appropriate for runaway gas accretion in the outer Solar System to make Jupiter and the giant impact stage of terrestrial planet formation to make Earth.

The growth of two planetary embryos. The red embryo is exterior to the ice line and grows to 10 Earth masses while the the green embryo is interior and grows to 0.1 Earth masses. These masses are appropriate for runaway gas accretion in the outer Solar System to make Jupiter and the giant impact stage of terrestrial planet formation to make Earth.

Our paper describing how the Solar System may have obtained its great dichotomy was published online and will be in the September 15, 2015 issue of Icarus. Here is a link to the publication in Icarus and here is a link to the preprint if you don't have access to Icarus.

A one-paragraph summary:

The great dichotomy of the Solar System is the two tiered nature of the planets. Inner planets are rocky and small, whereas outer planets are large and gaseous. Often the ice-line is invoked, since exterior to the ice-line, there is a significant addition in solid mass from ice, but this addition is only a factor of two in mass and not the factor of one hundred necessary to explain the dichotomy. Instead, we think the ice-line is important in a different way. It determines the maximum size of small objects in orbit around the Sun. How efficiently an object is accreted by a planet is dependent on its size. This change in efficiency can be responsible for a factor of a hundred in mass between planets interior and exterior to the ice-line.

More details:

We suggest that the solution is related to the ice-line but in a novel way. Pebble accretion is the process by which planets can grow quickly by collecting material that is just the right size to interact strongly with the gas but also move through the gas. If material in the outer Solar System grows to the bouncing or fragmentation barrier at approximately a decimeter in diameter, then it will be accreted quickly by growing planetary embryos. If these decimeter objects radially drift into the inner Solar System, then the ice inside them will sublimate. We assert that half the mass of these bodies is lost due to ice sublimation, and the silicate material is left in millimeter size fragmentsl, i.e. the chondrule size although we do not assert that they are chondrules. These fragments are much less efficiently accreted by planetary embryos. Thus, there is a dichotomy in growth rates explaining the dichotomy in planet sizes in the Solar System.