Scientists have a new theory about how our planet formed.
Besides answering the mystery of how our planet got here, the theory would explain Earth’s peculiar chemical composition. And it can help tell the story of other planets like ours.
“The prevailing theory in astrophysics and cosmochemistry is that the Earth formed from chondritic asteroids. These are relatively small, simple blocks of rock and metal that formed early in the Solar System,” explains Paolo Sosi, Professor of Experimental Planetology at ETH Zurich.
“The problem with this theory is that no mixture of these chondrites can explain the exact composition of the Earth, which is much poorer in light, volatile elements like hydrogen and helium than we would expect.”
Researchers have put forward numerous ideas over the years to explain this, suggesting that the collisions of the raw materials that formed the Earth generated enormous amounts of heat and vaporized the lighter elements.
However, the isotopic composition of the Earth seems to suggest otherwise: “All isotopes of a chemical element have the same number of protons, although different numbers of neutrons. Isotopes with fewer neutrons are lighter and therefore should be able to escape more easily,” Professor Soucy said.
“If the heating vaporization theory were correct, we would find fewer of these light isotopes on Earth today than in the original chondrites. But that’s exactly what the isotopic measurements don’t show.’
Researchers began looking for a better answer. Planets in the Solar System are thought to have formed over time, with smaller grains growing into planetesimals—small bodies of accreted gas and dust—by accreting material through their gravitational attraction.
Unlike chondrites, planetesimals were heated enough to create a separation between their metallic core and rocky mantle; moreover, planetesimals formed in different regions around the Sun or at different times can have strikingly different chemical compositions.
The team ran simulations of thousands of planetesimals colliding to see if they could produce bodies similar to Mercury, Venus, Earth and Mars. The simulations show that not only could a mixture of many different planetesimals have formed Earth, but that a planet with Earth’s composition is statistically the most likely outcome.
“Although we suspected it, we still find this result very remarkable,” says Professor Soucy.
“Now we not only have a mechanism that better explains the formation of Earth, but we also have a reference to explain the formation of the other rocky planets,” says the researcher.
“The mechanism can be used, for example, to predict how the composition of Mercury differs from that of other rocky planets. Or how rocky exoplanets can be composed of other stars.
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