Refractory metals in meteorites suggest non-uniform structure of early solar system disk

Four and a half billion years ago, the nascent solar system was a chaotic cloud of gas and dust. This primordial mix, swirling around the newly formed , began to condense and coalesce into solid bodies, giving rise to asteroids and . This initial stage of planetary formation occurred within a structure known as a protoplanetary disk. But what did this cosmic nursery look like, and how was it structured?

Modern astronomers can observe protoplanetary disks around distant young using powerful telescopes. However, directly observing what our solar system's disk looked like in its infancy is beyond our reach—only an observer from a distant could witness it as it was billions of years ago. Fortunately, space has left us clues in the form of meteorites, which are fragments of early solar system bodies that have survived their journey through Earth's atmosphere.

Meteorites are invaluable to scientists because their composition holds records of the solar system's . A recent study published in the Proceedings of the National Academy of Sciences by a team of planetary scientists from UCLA and Johns Hopkins University Applied Physics Laboratory sheds light on one of the mysteries revealed by these cosmic messengers. The researchers found that refractory metals, which condense at high temperatures—such as iridium and platinum—were more abundant in meteorites formed in the cold, distant regions of the protoplanetary disk. This finding is puzzling because these metals are expected to condense close to the sun where temperatures were much higher.

This unexpected distribution raises questions about the processes that could have transported these metals from the inner to the outer regions of the disk. Most meteorites formed within the first few million years of the solar system's history. Some, known as chondrites, are conglomerations of unmelted grains and dust that date back to the time of planet formation. Others underwent significant heating, causing them to melt; during this process, the denser metallic components separated from the silicate materials, similar to how oil separates from water.

The majority of today's asteroids reside in the asteroid belt between Mars and Jupiter. Jupiter's immense gravity is believed to have influenced their trajectories, causing collisions that shattered many of these bodies. Fragments from these collisions that reach Earth are identified as meteorites. Among these, meteorites, which originate from the metallic cores of the earliest asteroids, are particularly , offering a window into the solar system's formative years. These meteorites contain isotopes of molybdenum that suggest they originated in diverse locations within the protoplanetary disk, providing insight into its early .

Previous observations using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile have revealed that many protoplanetary disks around other stars resemble concentric rings, akin to a dartboard. These rings are separated by gaps, which seemingly preclude the migration of refractory metals from the inner to the outer disk.

The new research proposes a different scenario for our solar system's disk. Instead of a neatly ringed structure, the early solar disk likely resembled a doughnut. As the disk rapidly expanded, asteroids containing metal grains rich in iridium and platinum migrated outward, dispersing these metals throughout the outer regions.

This doughnut-like model offers a plausible explanation for the surprising distribution of refractory metals in meteorites and enhances our understanding of the dynamic processes that shaped the early solar system. By studying meteorites and drawing parallels with observations of distant protoplanetary disks, scientists continue to unravel the complex history of our cosmic origins, piece by piece.

Source: University of California, Los Angeles