Before forming a flattened disk, the distribution of dust and rocks had more in common with a doughnut than a pancake. Scientists came to this conclusion after studying iron meteorites in the outer Solar System, and found that they could only be explained if the Solar System was once toroidal in shape. This is information that could help us interpret other emerging planetary systems and determine their order of assembly.

The formation of a planetary system around a star begins in a molecular cloud of gas and dust drifting through space. If a portion of the cloud becomes dense enough, it will collapse under the influence of its own gravity, spinning as it moves, and becoming the seed of a newborn star. As it spins, matter from the surrounding cloud is drawn into the circumstellar disk that feeds the protostar.

Smaller clusters form within this disk and become protoplanetary seeds; these either continue to develop into full-fledged planets or (which seems to be much more common) stop their development and remain as a smaller object such as an asteroid. We have repeatedly seen these disks around other stars, with gaps formed by planets breathing dust as they move. But iron meteorites found in our solar system tell another part of the story.

According to a research team led by planetary scientist Bidong Zhang of the University of California, Los Angeles, the composition of asteroids in the outer solar system requires the cloud of material to be a toroidal disk, rather than a series of concentric rings in a flat plane. This suggests that the early stages of the system’s coalescence were toroidal.

These iron meteorites (pieces of rock that made the long journey from the outer Solar System to Earth) contain more refractory metals than those found in the inner Solar System, such as platinum and iridium, and their formation can only occur in a very hot environment, such as near a forming star.

This is a bit confusing because these meteors come from the outer part of the Solar System, not the interior, meaning they must have formed near the Sun and moved outward as the proto-planetary disk expanded. According to simulations carried out by Zhang and his colleagues, these iron objects will not be able to penetrate the gaps in the proto-planetary disk.

According to their calculations, migration would occur most easily if the protoplanetary structure had a toroidal shape. This would cause metal-rich objects to move toward the outer limits of the Solar System formation. Then, as the disk cooled and planets began to form, the rocks would be unable to pass through the gaps in the disk, acting as a very effective fence, preventing them from migrating back toward the Sun under the influence of gravity.

“When Jupiter formed, it probably opened a physical gap that trapped the metals iridium and platinum in the outer disk, preventing them from falling to the Sun,” says Zhang. “These metals were then incorporated into asteroids formed in the outer disk. This explains why meteorites formed in the outer disk (carbonaceous chondrites and carbonaceous iron meteorites) have much higher iridium and platinum contents than their inner disk counterparts.”

It’s amazing what you can learn from a boneless piece of metal rock. The study was published in the Proceedings of the National Academy of Sciences.