The early solar system had a mysterious hole where the asteroid belt is today

An MIT study suggests that a mysterious void existed in the solar system’s protoplanetary disk about 4.567 billion years ago and likely shaped the composition of the solar system’s planets. This image shows an artist’s interpretation of a protoplanetary disk. Credit: National Science Foundation, A. Khan

Turn the cosmic clock back a few billion years and our solar system was very different from what it is today. About 4.5 billion years ago, the young Sun shone as it does now, although a bit smaller. Instead of being surrounded by planets, it was nestled in a swirling disk of gas and dust. This disc is called a protoplanetary disc and it is where the planets eventually formed.

There was an obvious gap in the protoplanetary disk of the early solar system, between where " data-gt-translate-attributes="[{" attribute="">March and Jupiter are now, and where is the modern asteroid belt. The exact cause of the discrepancy is a mystery, but astronomers believe it’s a sign of the processes that governed planet formation.

A group of scientists published an article describing the discovery of this ancient gap. The main author is Cauê Borlina, who holds a doctorate in planetary sciences. student in the Department of Earth, Atmospheric, and Planetary Sciences (EAPS) at the Massachusetts Institute of Technology (MIT). The title of the paper is “Paleomagnetic Evidence for a Disc Substructure in the Early Solar System”. It’s published in the magazine Scientists progress.

Thanks to facilities such as the Atacama Large Millimeter/sub-Millimeter Array (ALMA), astronomers are increasingly studying younger solar systems that still have protoplanetary disks and are still forming planets. They often have conspicuous gaps and rings that testify to the formation of planets. But how exactly this all works is still a mystery.

“Over the past decade, observations have shown that cavities, vacancies, and rings are common in disks around other young stars,” says study co-author and planetary science professor Benjamin Weiss. at MIT’s Department of Earth, Atmospheric, and Planetary Sciences (EAPS). “These are important but poorly understood signatures of the physical processes by which gas and dust transform into young suns and planets.”

ALMA TW Hydrae Protoplanetary Disc

ALMA’s best image of a protoplanetary disk to date. This image of nearby young star TW Hydrae reveals the classic rings and gaps that signify planets are forming in this system. Credit: S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO)

Evidence for a gap in the protoplanetary disk of our own solar system around 4.5 billion years ago comes from the study of meteorites.

The magnetic fields of the solar system have had an effect on the structure of meteorites. Paleomagnetism shaped the tiny rocks of the protoplanetary disk called chondrules. Chondrids are molten or partially molten pieces of round rock that have accreted to a type of meteorite called chondrites. And chondrites are among the oldest rocks in the solar system.

As the chondrules cooled, they kept a record of the magnetic fields at the time. These magnetic fields change over time as the protoplanetary disk evolves. The orientation of the electrons in the chondrules is different according to the nature of the magnetic fields of the moment. Collectively, all of these chondrites within all of these chondrites tell a story.

Meteorite NWA 869

This is an image of a chondrite named NWA 869 (North West Africa 869) found in the Sahara Desert in the year 2000. There are both metallic grains and chondrites visible in the cut side. Image credit: H. Raab (User: Vesta), Wikimedia Commons, CC BY-SA 3.0

In this study, the group analyzed the chondrules of two carbonaceous meteorites discovered in Antarctica. They used a device called SQUID, or Scanning superconducting Quantum Interference Device. SQUID is a high sensitivity and high resolution magnetometer used on geological samples. The team used SQUID to determine the ancient original magnetic field of each chondrule in the meteorites.

The study is also based on a phenomenon called the isotopic dichotomy. Two distinct families of meteorites fell to Earth, each with a different isotopic composition, and scientists concluded that the two families must have formed at different times and places during the early solar system. The two types are called carbonaceous (CC) and non-carbonaceous (NC). CC meteorites likely contain material from the outer solar system, while NC meteorites likely contain material from the inner solar system. Some meteorites contain both isotopic imprints, but this is very rare.

The two meteorites studied by the team are both CC type from the outer solar system. When they analyzed them, they found that the chondrules showed stronger magnetic fields than the NC meteorites they had previously analyzed.

This is contrary to what astronomers think happens in a young solar system. As a young system evolves, scientists expect magnetic fields to decrease as they move away from the Sun. Magnetic strength can be measured in units called microteslas, and CC chondrules showed a field of about 100 microteslas, while NC chondrules show a strength of only 50 microteslas. For comparison, the Earth’s magnetic field today is around 50 microteslas.

The magnetic field indicates how a solar system accumulates matter. The stronger the field, the more matter it can attract. The strong magnetic fields apparent in the chondrules of DC meteorites show that the outer solar system was accreting more material than the inner region, which is evident from the size of the planets. The authors of this paper concluded that this was evidence of a great gap, which somehow prevented matter from flowing into the inner solar system.

“Gaps are common in protoplanetary systems, and we now show that we had one in our own solar system,” Borlina says. “It gives the answer to this strange dichotomy we see in meteorites and provides evidence that gaps affect the composition of planets.”

All of this combines into strong evidence of a large, unexplained gap in the early solar system.

ALMA Campaign Offers Unprecedented Views of Planetary Birth

ALMA’s high-resolution images of nearby protoplanetary disks, which are the results of the High Angular Resolution Disk Substructures Project (DSHARP). Credit: ALMA (ESO/NAOJ/NRAO), S. Andrews et al. ; NRAO/AUI/NSF, S. Dagnello

Jupiter is by far the most massive planet, so it’s a good place to start understanding how it all happened in our own solar system. As Jupiter grew, its powerful gravity may have played a role. It could have swept gas and dust from the inner solar system outward, leaving a void between itself and Mars in the evolving disk.

Another possible explanation stems from the disc itself. The first discs are shaped by their own strong magnetic fields. When these fields interact with each other, they can create powerful winds that can move materials and create a vacuum. Jupiter’s gravity and protoplanetary magnetic fields could have combined to create the gap.

But what caused the discrepancy is only a question. The other question is what role did he play? How has it helped shape everything since its formation over four billion years ago? According to the article, the space itself may have acted as an impassable barrier that prevented materials on either side from interacting. Inside space are the terrestrial planets and outside of space are the gaseous worlds.

“Bridging this gap is quite difficult, and a planet would need a lot of external torque and momentum,” lead author Cauê Borlina said in a press release. “So this provides evidence that the formation of our planets was restricted to specific regions of the early solar system.”

Originally published on Universe Today.

Reference: “Paleomagnetic Evidence for a Disc Substructure in the Early Solar System” by Cauê S. Borlina, Benjamin P. Weiss, James FJ Bryson, Xue-Ning Bai, Eduardo A. Lima, Nilanjan Chatterjee, and Elias N. Mansbach, October 15, 2021, Scientists progress.
DOI: 10.1126/sciadv.abj6928

Arline J. Mercier