Why Is Earth Bigger Than Mars: Planets Were Formed From Pebbles, New Study Finds

Researchers are now able to explain why the Earth is so much bigger than Mars, after using a new process in planetary formation modeling, where planets grow from miniature bodies called pebbles. The new technique also enabled the researchers to determine the extensive formation of gas giants Jupiter and Saturn.

"Understanding why Mars is smaller than expected has been a major problem that has frustrated our modeling efforts for several decades," said Hal Levison in a news release, who is the first author of the study and Planetary Science Directorate at Southwest Research Institute.

"Here, we have a solution that arises directly from the planet formation process itself. This numerical simulation actually reproduces the structure of the inner solar system, with Earth, Venus, and a smaller Mars," Levison said.

Mars has about ten percent of the mass of Earth. The standard model of planet formation found that similar sized objects go through a process known as accretion, where rocks incorporate other rocks, creating mountains, then mountains merge to create massive supersized objects. Similar accretion models have helped in the formation of Earth and Venus, thus researchers assume that Mars should have been the same size as the Earth, or even bigger. The model estimates exaggerate the overall mass of the asteroid belt, according to the researchers.

The majority of asteroids in the solar system are located in a region of the solar system out beyond Mars, forming the asteroid belt, which is in an area between the orbits of Mars and Jupiter.

"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," Levison said.

The researchers' calculations follow the growth and evolution of a system of planets, which shows that the structure of the inner solar system was created from a natural outcomes of a new model of planetary growth known as Viscously Stirred Pebble Accretion (VSPA).

The VSPA dust particles grows into pebbles, which are a few diameters, some of them eventually develop to form asteroid-sized objects. These asteroids could feed on remaining pebbles and they can merge with planetary bodies. The asteroids could eventually become oversized planets in a short period of time. However, not all pebbles go through the accretion process to later create massive planets and asteroids.

"This model has huge implications for the history of the asteroid belt," said Bill Bottke, a researcher of the study. "This presents the planetary science community with a testable prediction between this model and previous models that can be explored using data from meteorites, remote sensing, and spacecraft missions."

This study reveals that pebbles can form the cores of the giant planets, and it examines the structure of the outer solar system.

The findings of this study were published in the Proceedings of the National Academy of Sciences of the United States (PNAS) Early Edition.

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