Earth

Earth Might Have Formed in Just a Few Million Years

Earth formed about 4.6 billion years ago. That simplistic statement is common, and it’s a good starting point for understanding our planet and our Solar System. But, obviously, Earth didn’t form all at once. The process played out for some period of time, and the usual number given is about 100 million years.

New research suggests that Earth formed more quickly than that in only a few million years.

The 100 million-year number is part of a general scientific narrative describing Earth. Earth cooled, an atmosphere coalesced, then a magnetic shield, plate tectonics, single-celled life in the oceans, etc. Another significant part of the narrative suggests that after it formed, it acquired its water through chance collisions with asteroids and comets.

But just because we can express these happenings in a matter-of-fact way doesn’t mean it was matter-of-fact. It’s taken an enormous amount of scientific investigation to uncover these things, including the Apollo missions and the lunar samples they returned, and the work isn’t finished yet. Earth’s long, detailed history is still rife with mystery, and the further back we look, the more the mystery deepens.

In a new paper, a team of researchers from Denmark’s University of Copenhagen presents evidence that might upset the common narrative describing Earth’s formation. The paper is “Silicon isotope constraints on terrestrial planet accretion,” and it’s available in the journal Nature. The corresponding author is Isaac Onyett, a Postdoctoral Researcher at the Centre for Star and Planet Formation at the University of Copenhagen. 

It might seem counterintuitive that Earth’s formation is based on tiny, millimetre-size dust grains. But that’s how rocky planets get their start. This new research doesn’t disagree with that.

But it does add something to it: Earth’s water is a result of how it formed from those grains, rather than from incidental collisions with water-rich asteroids and comets.

This is an artist’s conception of asteroids or comets bearing water to a proto-Earth. This new research suggests that may not have been how Earth got the bulk of its water. Credit: Harvard-Smithsonian Center for Astrophysics

“We show that the Earth formed by the very fast accumulation of small millimetre-sized pebbles. In this mechanism, the Earth was formed in just a few million years. Based on our findings, it appears that the presence of water on Earth is a byproduct of its formation,” says Martin Bizzarro, who is a Professor at Globe Institute and one of the researchers behind the new study.

This research focuses on chemical elements like silicon (Si) and its common oxide silica, also called silicon dioxide (SiO2.) Si is a major component of planets like Earth, and is the second most abundant element in the crust, second only to oxygen. Scientists track the distribution of these chemicals from their time of formation in a supernova or other massive dying star, through their decay chains, into their presence in Solar System bodies. The researchers examined samples from more than 60 different meteorites and planetary bodies and analyzed their isotopic compositions. This is called cosmochemistry, and it’s an extraordinarily complex and detailed branch of science and one that’s critical to understanding Earth’s formation history.

“The cosmochemistry of Si, a major planetary component, provides a novel perspective on the accretion history of terrestrial planets, emphasizing the role of early-formed, inner Solar System differentiated asteroids as major planetary building blocks,” the paper states. 

Artist’s impression of a Type II supernova. Supernovae are one of the original sources of heavy elements in the Universe. Researchers can track these elements and their decay products in Solar System bodies to help untangle how Earth formed. Image Credit: ESO

The research relies on the Solar System’s nucleosynthetic isotopic variability. Scientists use this method to try and trace the development of planets in the Solar System by measuring the presence and abundance of isotopes in different objects. Silicone is an abundant refractory element in Solar System bodies that can serve as a nucleosynthetic tracer. Using it to track the formation of Earth is a significant step forward, according to the authors.

“Thus, a major step forward towards understanding the nature of the precursor material to terrestrial planets is developing a nucleosynthetic tracer that is a major planetary building block,” the authors write. “We present a high-precision nucleosynthetic isotope analysis of silicon (Si), the most abundant refractory Solar System element.” 

In the current widespread model of Earth’s formation, only chance collisions with water-rich objects transformed Earth into the life-supporting planet it is. “If that is how Earth was formed, then it is pretty lucky that we have water on Earth. This makes the chances that there is water on planets outside our solar system very low,” says Martin Schiller.

The authors’ evidence showed that once young Earth reached a certain size from dust accumulation, it ‘vacuumed’ up more and more dust from the protoplanetary disk. “There was a disk around the young sun where the planets were growing. The disk was filled with small dust particles. Once a planet reaches a certain size, it sort of acts like a vacuum cleaner, sucking up all that dust very quickly. And that makes it grow to the size of Earth in just a few million years,” says Ph.D. student Isaac Onyett, the corresponding author of the study.

This artist’s concept shows a protoplanetary disk around a young star. If this new research is accurate, then rocky planets can form quickly with water as part of the process. There’s no requirement for water-rich impactors to deliver it after the planet has already formed. Credit: NASA/JPL-Caltech

But part of the dust sucked up by the young, still-forming Earth was ice grains. Those ice grains are the source of much of Earth’s water, according to the researchers, rather than post-formation impacts from comets and asteroids. “The disk also contains many icy particles. As the vacuum cleaner effect draws in the dust, it also captures a portion of the ice. This process contributes to the presence of water during Earth’s formation, rather than relying on a chance event delivering water 100 million years later,” says Isaac Onyett.

“People have debated how planets form for a long time. One theory is that planets are formed by the gradual collision of bodies, progressively increasing their size over 100 million years. In this scenario, the presence of water on Earth would need a sort of chance event,” says Associate Professor Martin Schiller, who is also behind the new study.

Will this study end the debate? There are still some questions, especially about the Solar System’s frost line. If Earth formed in its current location from dust grains and ice grains, then Earth’s relation to the frost line must have been different. There’s evidence that the frost line was at about 3 astronomical units (au) earlier in the Solar System’s history, while it’s at about 5 au now. In the early days, the Sun was a T-Tauri star and was much cooler. Could the frost line have been even closer to the Sun in the distant past? Could Earth’s orbit have been different in the past? Neither of those things might have been necessary.

This is not the first paper by some of the same authors to show that Earth formed much more quickly than thought. In 2020, authors Schiller and Bizzarro from this paper, as well as Julien Siebert from the Paris Institute of Planetary Physics, Université Sorbonne Paris, published a paper in Science Advances. Its title was “Iron isotope evidence for very rapid accretion and differentiation of the proto-Earth.

That paper also presented evidence showing Earth formed quickly, and it was based on iron isotope variability between Solar System objects. That research showed that the thermal processing of dust close to the young Sun affected ice grain availability at the time Earth was formed, which isn’t a controversial statement. But in their paper, the authors also explain that there were two separate reservoirs of material available to the young Earth. One was thermally processed (ice-poor) material from the inner protoplanetary disk, and the other was ice-rich material from the colder regions of the Solar System that was funnelled into the Earth’s ‘feeding ground.’ Some of the “… isotope composition of terrestrial planets reflects mixtures of two reservoirs, namely, the thermally processed (and reduced) inner disk material and the pristine, volatile-rich CI-like envelope material,” that paper states.

It’s deeply fascinating how researchers are constructing a narrative of Earth’s formation based on the evidence available to them in chunks of asteroids and other meteorites that have fallen to Earth. Since it’s our planet and every step and breath we take results from Earth’s formation as a terrestrial planet with ample water, their effort touches us all. But the results also extend to exoplanets and how many of them might have liquid water, the primary factor in habitability.

“This theory would predict that whenever you form a planet like Earth, you will have water on it. If you go to another planetary system where there is a planet orbiting a star the size of the sun, then the planet should have water if it is in the right distance,” says Martin Bizzarro.

Scientists are on the cusp of gathering more robust evidence on exoplanets, their water, and their potential habitability. Upcoming giant telescopes will begin to image exoplanets directly and in far more detail. The Giant Magellan Telescope (GMT) might crank exoplanet science up to 10 when it gets going.

This illustration shows what the Giant Magellan Telescope will look like when it comes online. Each of its mirror segments is a 20-ton piece of glass. Image: Giant Magellan Telescope – GMTO Corporation

The GMT will have ten times the resolving power of the Hubble Space Telescope, and four times that of the James Webb Space Telescope. Once the GMT gets going in the 2030s, its Large Earth Finder instrument and its Near-Infrared Spectrograph will start to transform our understanding of Earth-size planets in other solar systems. “It’s got two instruments perfectly positioned to do direct imaging of planets like our own Earth to find potentially habitable planets,” said GMT Chief Scientist Rebecca Bernstein in an article in Forbes.

Will the GMT be able to confirm how quickly Earth and other rocky planets form and if the process means water is a part of it all rather than a happy accident from collisions?

Stay tuned.

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Evan Gough

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