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

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Go back the cosmic clock a few billion years ago and our solar system was very different from what it is today. About 4.5 billion years ago, the young Sun shone like it does now, albeit a little smaller. Instead of being surrounded by planets, it was surrounded by a swirling disc of gas and dust. This disc is called a protoplanetary disc and it is where the planets finally formed.

There was a clear gap in the protoplanetary disk of the early Solar System, between where Mars and Jupiter are now, and where the asteroid belt is today. What exactly caused the gap is a mystery, but astronomers believe it is a sign of the processes that governed the formation of the planets.

A group of scientists published an article describing the discovery of this ancient loophole. The main author is Cauê Borlina, doctor 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 article is “Paleomagnetic evidence for a disk substructure at the start of the solar system”. It is published in the journal Science Advances.

Using facilities like the Atacama Large Millimeter / sub-Millimeter Array (ALMA), astronomers are increasingly studying younger solar systems that still have protoplanetary disks and still form planets. They often have clearly visible gaps and rings which are evidence of the formation of planets. But how exactly it 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 Benjamin Weiss, study co-author and professor of planetary science. in the Department of Earth, Atmospheric and Planetary Sciences at MIT (EAPS). “These are important but poorly understood signatures of the physical processes by which gas and dust transform into young suns and planets.”

This ALMA image of the protoplanetary disc around the neighboring young star TW Hydrae reveals the rings and gaps in the young discs. Credit: S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO / AUI / NSF); ALMA (ESO / NAOJ / NRAO)

Evidence of 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 disc called chondruses. Chondrules are melted or partially melted pieces of round rock that have accumulated into 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 of 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 in all of these chondrites tell a story.

This is an image of a chondrite named NWA 869 (North West Africa 869) found in the Sahara Desert in the year 2000. Metallic grains and chondruses are visible on the cut face.  Image Credit: by H. Raab (User: Vesta) - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=226918
This is an image of a chondrite named NWA 869 (North West Africa 869) found in the Sahara Desert in the year 2000. Metallic grains and chondruses are visible on the cut face. Image credit: By H. Raab (User: Vesta) – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=226918

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, high resolution magnetometer used on geological samples. The team used SQUID to determine the original old magnetic field for each meteor chondrule.

The study is also based on a phenomenon called isotopic dichotomy. Two separate 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 at the start of the solar system. The two types are called carbon (CC) and non-carbon (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 fingerprints, but this is very rare.

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

This is contrary to what astronomers think is happening in a young solar system. As a young system evolves, scientists expect magnetic fields to decay with distance from the Sun. Magnetic force can be measured in units called microteslas, and CC chonders showed a field of about 100 microteslas, while NC chonders show a force 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 more powerful the field, the more material it can suck. The strong magnetic fields apparent in the chondrules of CC meteorites show that the outer solar system was accumulating more matter than the inner region, which is evident from the size of the planets. The authors of this article concluded that this is evidence of a large discrepancy, which somehow prevented matter from flowing into the inner solar system.

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

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

ALMA’s high resolution images of nearby protoplanetary disks, which are the result of the High Angular Resolution Disk Substructures (DSHARP) project. 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 to understand how it all played out in our own solar system. As Jupiter grew, its powerful gravity may have played a role. It could have driven gas and dust from the inner solar system to the periphery, leaving space between itself and Mars in the evolving disk.

Another possible explanation comes 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 space. Jupiter’s gravity and magnetic fields in the protoplanetary may have combined to create the gap.

But what caused the gap 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 document, 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 space are the gas worlds.

“It is quite difficult to bridge that gap, 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 limited to specific regions of the early solar system.”

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