In the early days of our solar system, the proto-Earth took much less time to form than we previously thought. The study, developed by scientists from the Centre for Star and Planet Formation (Starplan) at the Globe Institute of the University of Copenhagen, Denmark, concluded that the Earth formed in just five million years, extremely fast on an astronomical scale. Compared to a 24-hour period, the proto-Earth formed in one and a half minutes.
These Starplan results break with the traditional theory that proto-Earth originated from random collisions between ever-larger planetary bodies over several tens of millions of years, equivalent to approximately five-year 15 minutes of those fictitious 24 hours mentioned above.
The findings support, on the contrary, a more recent alternative theory about the formation of planets through the accumulation of cosmic dust. It has to be recognized that at this point in the 21st century, we are still not quite sure how planets form.
“Essentially, it all begins from cosmic dust. Objects the size of a millimetre, all together, raining down on the growing body and making the planet once again,” explains Martin Schiller, leader of the study published by Science Advances magazine. "This implication of the rapid formation of the Earth is not only interesting for our solar system. It is also interesting to assess the likelihood of planets forming elsewhere in the galaxy,” he continues.
When it comes to planet formation, small pieces of dust and rock in the disc are thought to stick electrostatically. Then, as they increase in size, so does their gravitational force. They begin to attract other groups, through casual interactions and collisions, increasing in size until they eventually become an entire planet.
Isotopes of iron
Up until now it was believed that it had taken tens of millions of years for Earth to form, however, the iron isotopes on the Earth’s mantle suggest otherwise. Scientists made the most accurate measurements of iron isotopes to date and, by studying the isotopic mixture of the metal element in different meteorites, found only one type of meteoritic material with an Earth-like composition: the 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’s like the bulk element of the solar system.
This dust process combined with gas channelled through a circumstellar accumulation disc lasted approximately five million years and our planets were made of material in this disc. Now, researchers estimate that the ferrous core of proto-Earth was also formed during this period.
The fact that the composition of our planet is only comparable to IC dust suggests a different model 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 iron early. When the solar system cooled, after its first hundreds of thousands of years, the farthest IQ dust could migrate inward to where the Earth was forming. It spread all over the planet, basically overwriting the iron in the mantle.
"This added IC powder overlaid the iron composition on the Earth’s mantle, which is only possible if most of the former iron was removed from the core," Schiller explained.
This conclusion leads us to believe 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