SwRI scientists predict that rocky planets formed from ‘pebbles’

Using a new process in planetary formation modeling, where planets grow from tiny bodies called “pebbles,” Southwest Research Institute scientists can explain why Mars is so much smaller than Earth. This same process also explains the rapid formation of the gas giants Jupiter and Saturn, as reported earlier this year.

“This numerical simulation actually reproduces the structure of the inner solar system, with Earth, Venus, and a smaller Mars,” said Hal Levison, an Institute scientist at the SwRI Planetary Science Directorate. He is the first author of a new paper published in theProceedings of the National Academy of Sciences of the United States (PNAS) Early Edition.

The fact that Mars has only 10 percent of the mass of the Earth has been a long-standing puzzle for solar system theorists. In the standard model of planet formation, similarly sized objects accumulate and assimilate through a process called accretion; rocks incorporated other rocks, creating mountains; then mountains merged to form city-size objects, and so on. While typical accretion models generate good analogs to Earth and Venus, they predict that Mars should be of similar-size, or even larger than Earth. Additionally, these models also overestimate the overall mass of the asteroid belt.