Although the Earth has long been studied in detail, some fundamental questions still need to be answered. One of them concerns the formation of our planet, the origins of which scientists are still unclear. An international research team led by ETH Zurich and the National Center of Competence in Research PlanetS is now proposing a new answer to this question based on laboratory experiments and computer simulations. The researchers published their study in the journal Nature Astronomy.
An inexplicable contradiction
“The prevailing theory in astrophysics and cosmochemistry is that Earth formed from chondritic asteroids. These are relatively small, simple blocks of rock and metal that formed early in the solar system,” explains the study’s lead author, Paolo Sossi, professor of experimental engineering. 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.”
Various hypotheses have been put forward over the years to explain this discrepancy. For example, the collisions of the objects that later formed the Earth were thought to have generated enormous amounts of heat. This vaporized the light elements and left the planet in its current composition.
However, Sossi believes these theories are implausible once you measure the isotopic composition of Earth’s various elements: “The isotopes of a chemical element all have the same number of protons, although different numbers of neutrons. Isotopes with fewer neutrons are lighter and should therefore be able to escape more easily.If the theory of vaporization by heating were correct, we would find fewer of these isotopes of light on Earth today than in the original chondrites. But this is exactly what isotope measurements do not show.”
Cosmic melting pot
So Sossi’s team looked for another solution. “The dynamical models we use to simulate the formation of planets show that planets in our solar system formed progressively. Small grains grew into kilometer-long planetesimals over time by accumulating more and more material through their gravity,” explains Sossi. Like chondrites, planetesimals are also small bodies of rock and metal. But chondrites were heated enough to differentiate into a metallic core and a rocky mantle. “What’s more, planetesimals formed in different regions around the young Sun or at different times can have very different chemical compositions,” says Sossi. The question is whether the random combination of different planetesimals actually leads to a composition that matches Earth’s.
To find out, the team ran simulations in which thousands of planetesimals collided with each other in the early solar system. The models were designed to reproduce the celestial bodies corresponding to the four stone planets of Mercury, Venus, Earth, and Mars over time. Simulations show that a mixture of many different planetesimals could lead to the effective composition of Earth. What’s more, the design of the Earth is even statistically the most likely outcome of these simulations.
Plan for other planets
“Even though we suspected it, we still found this result very remarkable,” recalls Sossi. “Now we not only have a mechanism that better explains the formation of the Earth, but we also have a link to explain the formation of other rocky planets,” says the researcher. For example, the mechanism could be used to predict how Mercury’s composition differs from that of other rocky planets. Or how the rocky exoplanets of other stars might be composed.
“Our study shows how important it is to take both dynamics and chemistry into account when trying to understand planet formation,” notes Sossi. “I hope our findings will lead to closer collaboration between researchers in these two areas.”