A new paper, combining experimentation and modeling, reveals a new formation pathway that fits Earth’s properties more precisely.
Planetary scientist Paolo Susi, of ETH Zurich in Switzerland, said: ‘The prevailing theory in astrophysics and cosmology is that Earth formed from cartilaginous asteroids. These are relatively small clumps of rock and metal that formed early in the solar system. The problem with this theory is that no There is a mixture of these chondrites that could explain the exact composition of the Earth, which is much poorer in light and volatile elements such as hydrogen and helium than we expected.”
There is a whole host of question marks about the planet formation process, but scientists have managed to piece together a general picture. When a star forms from a dense mass of matter in a molecular cloud of dust and gas in space, the surrounding matter forms into a rotating disk and droops into the growing star.
And this disk of dust and gas contributes not only to the star’s growing surroundings—the small densities within that vortex also collect into smaller, cooler clumps. Small particles collide and stick together, first with static electricity, then with gravity, forming larger and larger objects that can eventually grow into a planet. This is called the accretion model, and it 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 and volatile elements.
Scientists have put forward various explanations, including the heat generated during the collisions that could vaporize some of the lighter elements.
However, that doesn’t necessarily follow either: heat will vaporize lighter isotopes of the elements, with fewer neutrons, according to recent experimental work. But lighter isotopes are still found on Earth in proportions roughly similar to those of chondrites.
So the researchers set out to investigate another possibility: that the rocks that came together to make Earth weren’t cartilaginous asteroids from Earth’s general orbital neighborhood, but rather small planets. These are larger bodies, the “seeds” of planets that have grown to a size large enough to have a distinct nucleus.
They ran an N-body simulation, altering variables such as the number of minor planets, along a “Grand Tack” scenario, in which Jupiter first moves closer to the sun, then returns back to its current position.
Under this scenario, the motion of Jupiter in the early Solar System had a very disturbing effect on the smaller rocks orbiting it, scattering the smaller planets into the inner disk.
The simulations are designed to produce the inner solar system we see today: Mercury, Venus, Earth and Mars. The team found that a diverse mixture of minor planets with different chemical compositions could reproduce the Earth as we see it today. In fact, the Earth was the most likely outcome of the simulations.
This could have important implications not only for the solar system, and for understanding the changing compositions of its rocky planets, but for other planetary systems elsewhere in the galaxy.
The team’s research is published in the journal Nature Astronomy.