Scientists say the shape of the solar system has changed dramatically: ScienceAlert

The solar system was once shaped a little more like a dough.

Before it arranged itself into a flattened disk, the distribution of dust and rocks had more in common with a doughnut than a pancake. This is the conclusion scientists reached after studying iron meteorites from the outer solar system, finding that they could only be explained if the solar system's shape was once toroidal.

This is information that can help us explain other emerging planetary systems and determine the order in which they formed.

The formation of a planetary system around a star begins with a molecular cloud of gas and dust drifting through space. If a part of the cloud becomes dense enough, it will collapse under its own gravity, becoming the seed of a growing baby star as it spins. As it spins, material in the surrounding cloud is pulled into a swirling disk that leads into the protostar.

Within that disk, small clusters form, becoming protoplanetary seeds, which either continue to grow into full-fledged planets, or – which happens more often – their growth stops, and they remain as small objects, such as asteroids.

We've seen these disks repeatedly around other stars, with gaps created by planets carrying dust with them.

But iron meteorites found in our own solar system tell another side of the story.

According to a team led by planetary scientist Bidong Zhang of the University of California Los Angeles, the structure of asteroids in the outer solar system requires the cloud of material to be donut-shaped, not a series of concentric rings in a flat disk. This suggests that the system's first stages of fusion are toroidal.

The iron meteorites in question – pieces of rock that have made their long journey from the outer solar system to Earth – are richer in refractory metals than those found in the inner solar system. These are metals such as platinum and iridium, which can only be formed in very hot environments such as close to a forming star.

This is a little tricky, because those meteoroids came not from the inner solar system, but from the outer solar system, which means they would have formed closer to the sun and moved outward as the protoplanetary disk expanded. However, according to the modeling done by Zhang and his colleagues, these iron objects would not have been able to cross the gaps in the protoplanetary disk.

According to their calculations, this migration would have been most likely to have occurred if the protoplanetary structure had been toroidal in shape. This would have led to metal-rich bodies moving toward the outer edges of the solar system.

Then, as the disk cooled and planets began to form, the inability of rocks to cross the gaps in the disk would have acted as a very effective fence, preventing them from being swept back toward the Sun under the influence of gravity.

“When Jupiter formed, a physical gap probably opened up that trapped the iridium and platinum metals in the outer disk and prevented them from falling into the Sun,” Zhang says.

“These metals were later incorporated into asteroids that formed in the outer disk. This explains why meteorites that formed in the outer disk — carbonaceous chondrites and carbonaceous-type iron meteorites — have much higher amounts of iridium and platinum than meteorites that formed in the inner disk.”

It's amazing what you can learn from a pitted, metallic pile of rock.

This research has been published in the Journal of Neurology. Proceedings of the National Academy of Science,

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