Scientists have a controversial theory for how – and how fast – Earth formed


More than 4.5 billion years ago, Earth began forming from a blend of dust and gas that was around our young Sun. Eventually, it grew larger and larger until it became similar to the planet we live on today – a process scientists now say happened much faster than they once thought. That formation, they say, also included water, a detail indicating that finding another habitable planet is not out of the question.

In a new study released in Nature this week, researchers state that Earth formed within just 3 million years. That’s notably faster than previous estimates placing the timeline up to 100 million years.

Millions of years may not seem quick to mere humans, but on an astronomical scale, it’s incredibly fast. In the 4.6 billion years of our solar system’s existence, 3 million years is like a blink. That is the equivalent of less than a minute in a 24-hour day. (If Earth had formed over tens of millions of years, that would be equivalent to 5 to 15 minutes in a day.)

“Planets can go from their infancy to the size of Earth and Mars within just a few million years, which is really, really fast compared to the hundreds of millions of years that was previously thought,” said Isaac Onyett, lead author of the study and Ph.D. candidate at the University of Copenhagen. “We can also predict that if other planets formed … by the same mechanism, then the ingredients required for life such as water, should be present on other planets and other systems, so there’s a greater chance that we have water worlds elsewhere in the galaxy.”

The authors assert that this rapid genesis occurred through a theory called pebble accretion. The general idea, according to co-author and cosmochemist Martin Bizzarro, is that planets are born in a disk of dust and gas. When they reach a certain size, they rapidly attract those pebbles like a vacuum cleaner. Some of those pebbles are icy and could provide a water supply to Earth, thought of as pebble snow. This would have led to an early version of our planet, known as proto-Earth, that is approximately half the size of our present-day planet. (Our current rendition of Earth likely formed after a larger impact about 100 million years later, which also led to the formation of our moon.)

This pebble accretion theory for Earth’s formation is a controversial one, though.

Scientists agree that this theory explains the formation of gas giants in our solar system, such as Jupiter and Saturn – pebble accretion is the only way for a giant planet to grow fast enough before its planetary disk dissipates. But it’s disputed for their terrestrial counterparts. Others have pointed out issues of where did the pebbles around the terrestrial planet come from or why the planets aren’t bigger.

A more traditionally accepted theory is that terrestrial planets, like Earth, formed through a sequence of collisions from asteroids that got progressively larger and larger, said Onyett. That process would mean it took Earth around 100 million years or more to form. In this mechanism, the occurrence of water on Earth may have been lucky, perhaps delivered by an asteroid or comet.

Co-author Martin Schiller said he and his colleagues want others to rethink how terrestrial planets form. “Yes, of course, pebbles are available everywhere. Of course, terrestrial planets will also grow by pebble accretion,” said Schiller, also a cosmochemist at the University of Copenhagen.

Bizzarro added this study provides “the strongest evidence” that terrestrial planets formed via pebble accretion.

The team determined the time scale of Earth’s formation by looking at silicon isotopes from more than 60 meteorites and planetary bodies in the vicinity of Earth, which represent the rubble leftover after planet formation. Silicon is a main element in rocks and a vital building block of a planet, akin to what carbon is for life. Because it’s so abundant in the universe, it is produced easily and commonly and can act as a good tracer for planetary formations.

By analyzing the silicon compositions in samples of different ages, Onyett said they can piece together a time sequence of what was happening in the disk of dust before Earth formed. They found that, as the samples increased in age, the composition of the asteroids changed toward the composition of the cosmic dust that was being accumulated by Earth.

“That’s very strong evidence that this dust was also being swept up as it was drifting inwards towards the Sun,” said Onyett. “It would have been swept up by Earth as it was growing by accretion.”

Birger Schmitz, an astrogeologist at Lund University who was not involved in the research, said these results are “very compelling” and could shift how we think about our planet’s formation.

“If these interpretations hold (which I think they will) this represents a major paradigm shift in our understanding of Earth’s formation,” Schmitz, a research associate at the Chicago Field Museum, wrote in an email. “Paradigm shifts always come as big surprises. . … how wrong we were for such a long time!”

Most importantly, he said the results show there is nothing special about our water-carrying planet. “It is just a very ordinary planet in our galaxy. This is important in our attempts to understand how common higher forms of life are in the universe.”

Isotope cosmochemist Francois Tissot, who was not involved in the study, said the new research presented a fascinating analysis of the isotopic composition of silicon across so many planetary materials. But he isn’t sure that pebble accretion is the simplest explanation for the trends in the silicon data. He said additional analysis of how the new Si isotope data fits, doesn’t fit, into other models is needed.

Nonetheless, “these are exciting results that will bring key new constraints to our understanding of Earth’s formation,” said Tissot, based at the California Institute of Technology. “It is an exciting time for the community, and an undeniable step forward.”



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