How Earth Was Formed: Collisions With Tiny Planets Over Hundreds of Millions of Years Explain Why It's So Big

Collisions with tiny planets and large moons would have flung material away from Earth but also helped grow the planet. For hundreds of millions of years after the moon formed, Earth was bombarded by what scientists call planetesimals, nicknamed the building blocks of planets. Southwest Research Institute/Simone Marchi

Scientists have always known that Earth has a long history of withstanding celestial violence, but now they think that very same ordeal explains why our planet is as large as it is. That's according to a new study published in the journal Nature Geoscience, which argues that the way scientists have been calculating what Earth gained from that violence was flawed.

The mess here began after the moon formed, during some kind of very large, chaotic event. (Scientists are still evaluating a few theories about precisely what happened.) As if that weren't traumatic enough for a young planet, for hundreds of millions of years after the moon formed, Earth was bombarded by what scientists call planetesimals, nicknamed the building blocks of planets.

Those guilty parties, which are still found throughout the universe, are a sort of an intermediate stage: Dust in a very young solar system lumps together to create them, and then they stick to each other, eventually forming planets if they end up growing large enough. Planetesimals are considered to be fairly large when they hit about 600 miles across, larger than an asteroid.

Until now, scientists chalked up just 0.5 percent of the mass of our planet to all the planetesimals that hit it during those early days. That's based on analyzing how much of certain elements—heavy metals like gold and platinum, which Earth receives exclusively from space, through cosmic collisions of all sizes—are in the liquidish level below Earth's surface, the mantle. (This fall, scientists confirmed that those same elements are originally created not just from anything in space, but from the giant collisions between superdense neutron stars.)

Related: How did life start? Meteorites crashing into Darwin's warm little ponds may have been trigger

But now, the scientists behind the new paper think that model might have been a little bit off, underestimating how much of those exotic metals could have sunk all the way down to Earth's deep core or been flung back out into space.

Recalculating based on their new estimates of those proportions, the authors say that planetesimals may have been responsible for up to five times more of Earth's mass than the old estimate had suggested.

And that would also explain a mystery scientists had struggled with in very old rock samples, which don't display the precise chemical fingerprint they anticipated and which was complicating some of the debate around the moon's formation. The missing celestially born elements the new paper tracks down could be the solution to both puzzles.