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Early Formation Of Earth Took Less Time Than Previously Thought

clockFeb 21 2020, 19:15 UTC
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Artist's illustration of the later stages of the formation of planetary systems in protoplanetary discs surrounding a star. Pat Rawlings/ NASA

Research from the University of Copenhagen’s Centre for Star and Planet Formation (StarPlan) has put a new timescale on the formation of the precursor to our planet, the proto-Earth. By measuring iron isotopes in a selection of meteorites, researchers were able to deduce that the proto-Earth was established within approximately 5 million years.

In astronomical scales, that is pretty quick. If the solar system’s estimated existence of 4.6 billion years was mapped to a 24-hour period, then the new research suggests the proto-Earth was formed in about 90 seconds.

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Previous formation timescales that equate to between 5 and 15 minutes of the 24-hour period are based on the traditional theory of how Earth was formed. This argues that larger and larger objects randomly collide over a period of tens of millions and years. However, StarPlan’s research, published in Science Advances, supports a more recent and alternative theory where planets are formed from the accretion (accumulation) of cosmic dust.

The latest evidence for this theory came from the “most precise measurements published” of iron isotopes in different meteorites. Earth’s isotopic composition was only comparable to one other meteorite, the so-called CI chondrites. The “fragile dust” they are comprised of is described as our best equivalent to the “bulk composition of the solar system.”

The isotopic composition of Earth was only similar to one type of meteoritic material, CI chondrites, which were only within the terrestrial planet-forming region in the solar system's first 5 million years of existence. University of Copenhagen

“If the Earth’s formation was a random process where you just smashed bodies together, you would never be able to compare the iron composition of the Earth to only one type of meteorite,” Associate Professor Martin Schiller, the study's lead author, said in a statement. “You would get a mixture of everything.”

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The fact that this one type of meteoritic material was a CI chondrite is also of importance when looking at the timescale of proto-Earth’s formation.

“The only epoch in the history of the solar system when the CI-like material is readily available within the terrestrial planet-forming region is during the lifetime of the protoplanetary disk,” the researchers wrote in their research article. This period lasting around 5 million years represents the time when CI-like dust combined with gas that was then accreted onto the growing Sun. The planets were made from the remaining material in the disc.

However, before the CI-like dust was accreted onto Earth, as suggested by the alternative planet formation theory and presence of the material in the analysis of Earth’s isotopic composition, the researchers deduced proto-Earth’s core must have already formed.

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Other meteorites from rocky planets, such as from Mars, told the researchers that the iron isotopic composition at the beginning of Earth’s formation was different. They believe that this early-accreted iron was transported from the Earth’s mantle into its core. After a few hundred thousand years of the solar system’s existence, the protoplanetary disc became cool enough for CI dust from the outer reaches of the system to accrete (or “rain down”) onto Earth.

“This added CI dust overprinted the iron composition in the Earth’s mantle, which is only possible if most of the previous iron was already removed into the core,” Dr Schiller explained. “That is why the core formation must have happened early.” In this case, much earlier than the traditional collision formation theory predicted.

The team’s evidence for this alternative planet formation could mean that other planets elsewhere in the universe may form faster than expected as well.

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“When we understand these [planet forming] mechanisms in our own solar system, we might make similar inferences about other planetary systems in the galaxy. Including at which point and how often water is accreted,” co-author of the study, Professor Martin Bizzarro, explained.

“If the theory of early planetary accretion really is correct, water is likely just a by-product of the formation of a planet like the Earth – making the ingredients of life, as we know it, more likely to be found elsewhere in the universe.”


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