The Earth may have formed from two separate rings around the Sun

The inner solar system may have formed differently than we have long thought it must have formed. For decades, scientists thought the rocky planets formed from a single disk of dust and debris in the early solar system, but new simulations suggest there may have been two separate disks of material.

Models with a single disk or ring of material around the young Sun tend to be unable to recreate several elements of the Solar System as we observe it. For one thing, Earth appears to be made of two different types of rock, which wouldn’t make sense if they all came from the same ring. Also, single-ring models tend to end up with Mercury and Mars too large, Venus and Earth too close together, and Earth and Mars compositions too similar.

Bill Bottke at the Southwest Research Institute in Colorado and his colleagues created a series of detailed simulations of the different ways planets could form from a single reservoir of material and then evolve, but problems persisted.

“We spent six months on the computer, nothing was working, so we played around desperately. We thought, why don’t we try another cartridge?” Bottke said at the presentation this job at the Lunar and Planetary Science Conference in Texas on March 16. “It turned out that this model not only did a great job of creating terrestrial planets, but also explained quite well some of the things that had been bothering us.”

The best-fitting model had two separate disks, one at about half the current Sun-Earth distance and the other at about 1.7 times the Sun-Earth distance. This simulation finished with all the planets at the correct sizes and distances from each other.

It also fits the composition of the Earth, Moon and Mars. “We think the Earth was made mostly of [inner solar system] material and only the last bit came from the outer solar system,” he said Jan Hellmann at the Max Planck Institute for Solar System Research in Germany during another presentation on the same day. If the Earth formed from an inner disk with a modest contribution from the outer disk, as predicted by Bottke’s model, it would be consistent with these expectations. Mars, on the other hand, would form mostly from the outer disk, which explains the differences in the composition of the two planets.

There is some concern that the model requires very specific initial conditions to correctly reproduce the inner solar system, and it is not 100% clear why these conditions would have the required values. “Minor changes in the shape of the disk can give you big differences in where the terrestrial planets move,” Bottke said.

Scientists are now working to refine their model and explore its further implications for the solar system. “We use a lot of time on the supercomputer to try every reasonable option,” Bottke said. If it works, this new explanation could explain all sorts of solar system mysteries, from strange asteroids to unexplained rocks on the moon’s surface.

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