Recent research has pointed out that the early shape of our Solar System was more like a doughnut than the flat disk we recognise at present. The discovery comes from an in-depth study of iron meteorites originating from the outer reaches of the Solar System.

What does it mean?

The implications of this finding are significant for understanding the formation of other emerging planetary systems and the sequence in which they develop.

Development of planetary system

Planetary system formation begins in a molecular cloud of gas and dust drifting through space. When a portion of this cloud becomes sufficiently dense, it collapses under its own gravity and starts spinning, forming the seed of a nascent star. The surrounding material pulls into a rotating disk which feeds into the growing protostar.

Within this disk, smaller clumps form, evolving into protoplanetary seeds. These seeds either grow into full-fledged planets or remain as smaller objects, such as asteroids, if their development is arrested.

Observations of other stars reveal these disks are often with gaps carved by planets consuming dust as they orbit. 

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A team led by planetary scientist Bidong Zhang of the University of California, Los Angeles, found that the composition of asteroids in the outer Solar System suggests a toroidal cloud of material rather than a series of concentric rings in a flat disk during the early stages of the Solar System’s formation.

The iron meteorites in question, which have travelled to Earth from the outer Solar System, are richer in refractory metals like platinum and iridium. These metals can only form in very hot environments, close to a forming star. Notably, these meteorites originated not from the inner Solar System but the outer regions. They must have formed close to the Sun and moved outward as the protoplanetary disk expanded.

The scientific models indicate that these iron objects could not have crossed gaps in a protoplanetary disk. Instead, the latest calculations suggest that migration would have been more feasible if the protoplanetary structure was toroidal. This shape would have directed metal-rich objects toward the outer edges of the forming Solar System.

As the disk cooled and planets began to form, gaps in the disk would have prevented rocks from migrating back towards the Sun, acting as an effective barrier.

“Once Jupiter formed, it very likely opened a physical gap that trapped the iridium and platinum metals in the outer disk and prevented them from falling into the Sun,” Zhang said in a statement and added, “These metals were later incorporated into asteroids that formed in the outer disk, explaining why meteorites from this region have higher iridium and platinum contents than their inner-disk counterparts.”

The research has been published in the Proceedings of the National Academy of Sciences.

(With inputs from agencies)

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