In the early days of our solar system, proto-Earth took much less time to form than we previously thought. The study, developed by scientists from the Center for the Formation of Stars and Planets ( StarPlan ) at the Globe Institute of the University of Copenhagen (Denmark), concludes that the Earth was formed in just five million years, extremely fast on an astronomical scale. If we compare it with a 24-hour period, the proto-Earth was formed in a minute and a half.
These StarPlan results break with the traditional theory that proto-Earth originated from random collisions between increasingly large planetary bodies over several tens of millions of years, which is equivalent to about 5-15 minutes of those fictitious 24 hours mentioned above.
Alternative theory
The findings support, on the contrary, a more recent alternative theory about the formation of planets through the accumulation of cosmic dust. And, it must be recognized that at this point in the 21st century, we are still not entirely sure how planets are formed.
“We start from cosmic dust, essentially. Objects the size of a millimeter, all together, raining down on the growing body and making the planet at once ”, explains Martin Schiller, leader of the work that publishes the journal Science Advances. “This implication of the rapid formation of the Earth is not only interesting for our solar system. It is also interesting to evaluate the probability that planets will form elsewhere in the galaxy,” he continues.
When it comes to planet formation, it is believed that small pieces of dust and rock in the disk begin to adhere electrostatically. Then, as they grow in size, so does their gravitational force. They begin to attract other groups, through casual interactions and collisions, increasing in size until they finally become an entire planet.
Iron isotopes
For Earth, we believed that this process had taken tens of millions of years. But iron isotopes in the Earth’s mantle suggest otherwise. Scientists made the most accurate measurements of iron isotopes to date, and by studying the isotopic mixing of the metallic element in different meteorites, they found only one type of meteorite material with a composition similar to Earth: so-called CI chondrites. The interesting thing about these meteorites is that they have a composition similar to the solar system as a whole, let’s say it is like the bulk element of the solar system.
This process of dust combined with gas channeled through a circumstellar accumulation disk lasted approximately five million years and our planets were made of material in this disk. Now, the researchers estimate that the ferrous core of proto-Earth also formed during this period.
The fact that our planet’s composition is only comparable to CI dust suggests a different pattern of formation: through a shower of cosmic dust, a process faster than the accumulation of larger rocks. During this time, the iron core was formed, absorbing the early iron. When the solar system cooled, after its first hundreds of thousands of years, the furthest CI dust was able to migrate inward to where Earth was forming. It spread across the planet, basically overwriting the iron in the mantle.
“This added CI dust superimposed the iron composition in the Earth’s mantle, something that is only possible if most of the previous iron was removed in the core,” Schiller explained.
This conclusion leads us to think that if this is how the Earth was finally formed, it is possible that there are other planets in other parts of the universe that could have formed in the same way.
Reference: Martin Schiller, Martin Bizzarro, Julien Siebert. Iron isotope evidence for very rapid accretion and differentiation of the proto-Earth. Science Advances, 2020; 6 (7): eaay7604 DOI: 10.1126 / sciadv.aay7604
