The formation of our solar system involved the accretion of gas and dust into a preplanetary disk, with meteorites providing important insights into its early composition.
Four and a half billion years ago, our solar system was a cloud of gas and dust swirling around the Sun, until gas began to condense and accumulate along the dust to form asteroids and planets. What did this preplanetary disk look like, and what was its structure? Astronomers can use telescopes to "see" preplanetary disks far from our much older solar system, but it's impossible to see what our solar system looked like in its early days—only an alien billions of light years away would be able to see it as it once was.
Luckily for us, space threw up some clues - fragments of objects that formed early in the history of our solar system and fell through the Earth's atmosphere - meteorites. The composition of meteorites tells us stories about the formation of the solar system, but these stories often raise more questions than answers.
A team of planetary scientists from the US reports in a paper that refractory metals, which condense at high temperatures, such as iridium and platinum, were more common in meteorites formed in the outer disk, which was cold and far from the sun. These metals had to form close to the sun, where the temperature is much higher. Was there a pathway that transported these metals from the inner to the outer disc?
Most meteorites were formed in the first millions of years of the solar system. Some meteorites, called chondrites, are unmelted inclusions of grains and dust left over from the formation of planets. Other meteorites were subjected to enough heat to melt while their parent asteroids were forming. When these asteroids melted, the silicate part and the metallic part separated due to their difference in density, similar to how water and oil don't mix.
Today most asteroids are in a thick belt between Mars and Jupiter. The scientists think that Jupiter's gravity disrupted the orbits of these asteroids, causing many of them to collide and break up. When pieces of these asteroids fall to earth and are found, they are called meteorites.
Iron meteorites originated in the metallic cores of the earliest asteroids, and they formed before any other rocks or celestial bodies in our solar system. The iron contains molybdenum isotopes that point to many places in the protoplanetary disk where these meteorites formed. This allows scientists to know what the chemical composition of the disk was like in its early life.
Previous studies have found many disks around other stars that resemble concentric rings, like a target plate. The rings of these planetary discs, such as HL Tau, are separated by physical gaps, so such a disc could not provide a pathway to transfer these refractory metals from the inner to the outer disc.
The new paper claims that our Sun's disk did not have a ring structure at the very beginning but looked like a doughnut, and asteroids with metallic grains rich in iridium and platinum moved into the outer disk as it rapidly spread.
More of the topic in Hayadan:
