Mysterious hole existed in solar system’s protoplanetary disk, new research finds

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About 4.567 billion years ago, our solar system housed a gap in the protoplanetary disk, near where the main asteroid belt is today, and likely shaped the makeup of the planets, according to one study conducted by scientists at MIT.

This is an artist’s impression of the protoplanetary disc around HD 107146. Image credit: A. Angelich / NRAO / AUI / NSF.

“Over the past decade, observations have shown that cavities, vacancies, and rings are common in disks around other young stars,” said Professor Benjamin Weiss, a researcher in the Department of Earth Sciences, of the atmosphere and planets at MIT.

“These are important but poorly understood signatures of the physical processes by which gas and dust transform into young Suns and planets.”

Over the past decade, planetologists have observed a curious division in the composition of meteorites.

These space rocks originally formed at different times and in different places as the solar system took shape.

Those which have been analyzed exhibit one of two isotopic combinations. Rarely have meteorites been found to exhibit both – a conundrum known as the isotopic dichotomy.

Scientists have proposed that this dichotomy could be the result of a discrepancy in the disc of the early solar system, but such a discrepancy has not been directly confirmed.

Professor Weiss and his colleagues analyze meteorites for signs of ancient magnetic fields.

When a young planetary system takes shape, it takes with it a magnetic field whose strength and direction can change depending on various processes within the evolving disk.

As the ancient dust gathered into grains called chondrules, the electrons inside the chondrules aligned with the magnetic field in which they formed.

Chondrules can be smaller than the diameter of a human hair and are found today in meteorites.

Researchers specialize in measuring chondrules to identify the ancient magnetic fields in which they originally formed.

In previous work, they analyzed samples from one of two isotopic groups of meteorites, known as non-carbon meteorites.

These rocks are believed to originate from a reservoir, or region of the early solar system, relatively close to the Sun.

They previously identified the old magnetic field in samples from this nearby region.

In the new study, they wondered if the magnetic field would be the same in the second isotopic and carbon group of meteorites, which, judging by their isotopic composition, are believed to originate further into the solar system.

They analyzed chondrules, each measuring about 100 microns, from two carbonaceous meteorites discovered in Antarctica.

Using the Superconducting Quantum Interference Device (SQUID), they determined the ancient and original magnetic field of each chondrule.

Surprisingly, they found that their field strength was stronger than that of the nearby non-carbon meteorites they had previously measured.

As young planetary systems take shape, scientists expect the strength of the magnetic field to decrease with distance from the Sun.

In contrast, the authors found that distant chondras had a stronger magnetic field, around 100 microteslas, compared to a field of 50 microteslas in the nearest chondra. For reference, the Earth’s magnetic field today is around 50 microteslas.

The magnetic field of a planetary system is a measure of its rate of accretion, or how much gas and dust it can attract to its center over time.

According to the magnetic field of the carbonaceous chondrules, the outer region of the solar system must have accumulated much more mass than the inner region.

Using models to simulate various scenarios, the team concluded that the most likely explanation for the mismatch in accretion rates is the existence of a gap between the inner and outer regions, which could have reduced the amount of gas and dust flowing towards the sun from the outer regions.

“Deficiencies are common in protoplanetary systems, and we are now showing that we had one in our own solar system,” said Cauê Borlina, a graduate student in the Department of Earth, Atmospheric and Planetary Sciences at MIT.

“It gives the answer to this strange dichotomy we see in meteorites and provides evidence that the gaps affect the makeup of the planets.”

The results were published in the journal Scientists progress.

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Cauê S. Borlina et al. 2021. Paleomagnetic proof of a disk substructure at the start of the solar system. Scientists progress 7 (42); doi: 10.1126 / sciadv.abj6928


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