Want to know something funny? We do not know how our planet was formed. We have a broad general idea, but the finer details are much harder to discover.
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. Even here, however, there are some facts about the formation of our planet that are challenging to explain.
A new paper, combining experimentation with modeling, has revealed a new formation pathway that fits much better with 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 Sossi 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, unstable elements like hydrogen and helium than we would expect.”
There are many questions about the planet’s formation process, but scientists have been able to create a general picture. When a star forms from a dense clump of matter in a molecular cloud of dust and gas in space, the material around it organizes into a disk that spins and spins into the growing star.
That disk of dust and gas doesn’t just contribute to the waistline of a growing star—tiny densities within that swirl also clump 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 growth model and is strongly supported by observational evidence.
But if the rocks sticking together are chondrites, that leaves a big open question about what lighter, unstable elements are missing.
Scientists have put forward various explanations, including heat generated during collisions that could have vaporized some of the lighter elements.
However, this does not necessarily mean: the heat would have vaporized lighter isotopes of the elements, with fewer neutrons, according to recent experimental work led by Sossi. But lighter isotopes are still present on Earth in ratios roughly similar to those found in chondrites.
So Sossi and his colleagues set out to investigate another possibility: that the rocks that coalesced to create Earth were not chondritic asteroids from Earth’s general orbital neighborhood, but planetesimals. These are larger bodies, the “seeds” of planets that have grown to a size large enough to have a differentiated core.
“Dynamic models with which we simulate planet formation show that planets in our Solar System formed progressively. Tiny grains grew over time into kilometer-sized planetesimals by accreting more and more material through their gravitational pull. ,” Sossi said.
“Furthermore, planetesimals that formed in different areas around the young Sun or at different times can have very different chemical compositions.”
They ran N-body simulations, varying variables such as the number of planetesimals, along the “Grand Tack” scenario, in which a baby Jupiter first moves closer to the Sun and then returns to its current position.
According to this scenario, the motion of Jupiter in the early Solar System had an extremely disturbing effect on the smaller rocks that orbited it, scattering planetesimals into the inner disk.
The simulations were designed to produce the inner 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 was the most likely outcome of the simulations.
This could have important implications not only for the Solar System and understanding the different compositions of the rocky planets in it, but also for other planetary systems elsewhere in the galaxy.
“Although we had suspected it, we found this result very remarkable. We now not only have a mechanism that better explains the formation of the Earth, but we also have a reference to explain the formation of other rocky planets,” Sossi. said.
“Our study shows how important it is to consider both dynamics and chemistry when trying to understand planetary formation. I hope our findings will lead to closer collaboration between researchers in these two fields.”
The team’s research was published in Astronomy of Nature.