New research suggests some planets could have formed from tiny "pebbles," and the findings could help explain why Mars is so much smaller than Earth.
Models of the newly proposed process could also help solve the mystery of how gas giants such as Jupiter and Saturn form so rapidly, the Southwest Research Institute reported.
"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 the Proceedings of the National Academy of Sciences of the United States (PNAS) Early Edition.
Mars is only about 10 percent of the mass of Earth, and researchers have always wondered why this is. The standard model of planetary formation that assumes the planets in our solar system formed through accretion (a gradual merging of materials to form larger and larger objects), but this model predicts Mars should be the same size as Earth or larger; these models also tend to overestimate the overall mass of the asteroid belt.
"Understanding why Mars is smaller than expected has been a major problem that has frustrated our modeling efforts for several decades," Levison said. "Here, we have a solution that arises directly from the planet formation process itself."
The solution is based on a planetary formation process called Viscously Stirred Pebble Accretion (VSPA), in which dust eventually grows into "pebbles" that can gravitationally collapse to form asteroid-sized objects. These asteroids can then "feed on" other surrounding pebbles to grow larger. The researchers noted the position of the primordial asteroid determined its ability to grow and the speed at which it does. For example, the largest asteroid in the asteroid belt Ceres would have grown extremely quickly in Earth's current position, but near Mars the aerodynamic drag is too weak for pebble capture to occur.
"This means that very few pebbles collide with objects near the current location of Mars. That provides a natural explanation for why it is so small," said co-author Katherine Kretke. "Similarly, even fewer hit objects in the asteroid belt, keeping its net mass small as well. The only place that growth was efficient was near the current location of Earth and Venus."
The finding could help tell the story of the asteroid belt's history. Past estimates have suggested the belt contained a couple of Earth-masses' worth of material and the planets originally formed there. This new model suggests this mass never existed in the asteroid belt.
"As far as I know, this is the first model to reproduce the structure of the solar system -- Earth and Venus, a small Mars, a low-mass asteroid belt, two gas giants, two ice giants (Uranus and Neptune), and a pristine Kuiper Belt," Levison said.
