A compelling new hypothesis could finally explain how the Earth formed

Want to know something funny? We don’t really know how our planet formed. We have a broad general idea, but the finer details are much harder to decipher.

We have a model that is currently accepted as the most likely explanation so far: that the Earth was formed by the gradual accretion of asteroids. But even here there are some facts about the formation of our planet that are difficult to explain.

A new paper combining experimentation with modeling has revealed a new formation pathway that much more closely matches Earth’s characteristics.

“The prevailing theory in astrophysics and cosmochemistry is that the Earth formed from chondritic asteroids. These are relatively small, simple blocks of rock and metal that formed early in the solar system,” said planetary scientist Paolo Sosi of ETH Zurich in Switzerland.

“The problem with this theory is that no mixture of these chondrites can explain the exact composition of the Earth, which is much poorer in light, volatile elements like hydrogen and helium than we would expect.”

There are a whole bunch of question marks about the planet’s formation process, but scientists have managed to put together a general picture. When a star forms from a dense lump of matter in a molecular cloud of dust and gas in space, the material around it arranges itself into a disk that orbits and curls into the growing star.

This disk of dust and gas not only contributes to the waist of a growing star – the small densities within this vortex also collect into smaller, cooler clumps. Small particles collide and stick together, first electrostatically, then gravitationally, forming larger and larger objects that can eventually grow into a planet. This is called the accumulation model and is strongly supported by observational evidence.

But if the rocks that stick together are chondrites, that leaves a big open question about the missing lighter, volatile elements.

Scientists have offered various explanations, including heat generated during the collisions that may have vaporized some of the lighter elements.

However, it doesn’t necessarily track either: the heat would vaporize lighter isotopes of elements with fewer neutrons, according to recent experimental work led by Soucy. But lighter isotopes are still present on Earth in ratios roughly similar to those found in chondrites.

So Soucy and his colleagues set out to explore another possibility: that the rocks that combine to make Earth are not chondritic asteroids from Earth’s common orbit, but planetesimals. These are larger bodies, the “seeds” of planets that have grown to a size large enough to have a differentiated core.

“The dynamical models we use to simulate planet formation show that the planets in our solar system formed progressively. The tiny grains have grown over time into kilometre-sized planetesimals, accumulating more and more material through their gravitational pull,” Soucy said.

“What’s more, planetesimals that formed in different regions around the young Sun or at different times can have very different chemical compositions.”

They ran N-body simulations, changing variables such as the number of planetesimals, under the “Grand Tack” scenario, in which the baby Jupiter moves first closer to the Sun and then back again to its current position.

In this scenario, Jupiter’s motion in the early Solar System had an extremely perturbing effect on the smaller rocks that swirled around, scattering planetesimals into the inner disk.

The simulations are designed to create the inner part of the Solar System we see today: Mercury, Venus, Earth and Mars. The team found that a diverse mixture of planetesimals with different chemical compositions could reproduce the Earth as we see it today. In fact, Earth is the most likely outcome of the simulations.

This could have important implications not only for the Solar System and understanding the different compositions of its rocky planets, but also for other planetary systems elsewhere in the galaxy.

“Although we suspected it, we still find this result very remarkable. Not only do we now have a mechanism that better explains the formation of Earth, but we also have a reference to explain the formation of the other rocky planets,” Sossi said.

“Our study shows how important it is to consider both dynamics and chemistry when trying to understand planet formation. I hope that our findings will lead to closer collaboration between researchers in these two fields.”

The team’s research is published in Natural astronomy.

Leave a Comment

Your email address will not be published.