The proto-Earth formed in a cosmic minute and a half

Our planet originally formed much faster than previously thought, researchers report.

This finding provides new insights into both planetary formation and the likelihood of water and life existing elsewhere in the universe.

The precursor to our planet, the proto-Earth, formed within approximately five million years, the study shows. On an astronomical scale, this is extremely fast, the researchers say.

From dust to proto-Earth

If you compare the solar system’s estimated 4.6 billion years of existence with a 24-hour period, the new results indicate that the proto-Earth formed in what corresponds to about a minute and a half.

The results break with the traditional theory that the proto-Earth formed through random collisions between larger and larger planetary bodies throughout several tens of millions of years—equivalent to about 5-15 minutes out of the above-mentioned fictional 24 hours of formation.

Instead, the new results support a more recent, alternative theory about the formation of planets through the accretion of cosmic dust.

“The other idea is that we start from dust, essentially. Millimeter-sized objects, all coming together, raining down on the growing body and making the planet in one go,” says lead author Martin Schiller, an associate professor from the Centre for Star and Planet Formation (StarPlan) at the Globe Institute at the University of Copenhagen.

“Not only is this implication of the rapid formation of the Earth interesting for our solar system. It is also interesting to assess how likely it is for planets to form somewhere else in the galaxy.”

Metals from meteorites

The key to the new finding came in the form of the most precise measurements of iron isotopes that scientists have published so far.

By studying the isotopic mixture of the metallic element in different meteorites, the researchers found only one type of meteoritic material with a composition similar to Earth: so-called CI chondrites.

The researchers describe the dust in this fragile type of meteorite as our best equivalent to the bulk composition of the solar system itself. It was dust like this combined with gas that a circumstellar accretion disk funneled onto the growing sun.

This process lasted about 5 million years and our planet was made from material in this disk. Now, the researchers estimate that the proto-Earth’s ferrous core also formed already during this period, removing early accreted iron from the mantle.

Other meteorites, from Mars for example, tell us that at the beginning the iron isotopic composition of material contributing to the growing Earth was different. Most likely due to thermal processing of dust close to the young sun, the researchers say.

After our solar system’s first few hundred thousand years it became cold enough for unprocessed CI dust from further out in the system to enter the accretion region of the proto-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. That is why the core formation must have happened early,” Schiller explains.

“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. You would get a mixture of everything,” he adds.

The search for life

Based on the evidence for the theory that planets form through the accretion of cosmic dust, the researchers believe that the same process may occur elsewhere in the universe.

This means that also other planets may likely form much faster than if they grow solely from random collisions between objects in space.

The theory makes it more likely that the ingredients of life, as we know it, exist elsewhere in the universe

This assumption is corroborated by the thousands of exoplanets—planets in other galaxies—that astronomers have discovered since the mid-nineties, explains coauthor Martin Bizzarro, a professor in and leader of StarPlan.

“Now we know that planet formation happens everywhere, that we have generic mechanisms that work and make planetary systems. When we understand these 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,” he says.

“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.”

The research appears in Science Advances. The Danish National Research Foundation and European Research Council supported the study.

Source: University of Copenhagen

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