The Early Solar System Had a Gap Where the Asteroid Belt is Today

If you turn the cosmic clock back just a few billions years, your Solar System will look very different from it today. The young Sun shined much the same way it does today, but it was smaller around 4.5 billion years ago. Instead of being surrounded with planets, the young Sun was enveloped in a swirling disc of dust and gas. This disk is known as a protoplanetary disc and it's where the planets eventually formed.
A conspicuous gap was found in the early Solar Systems protoplanetary disc, where Mars and Jupiter now reside, and where today's asteroid belt is located. Astronomers believe the gap to be a sign of processes that governed the formation of planets. However, it remains a mystery as to what caused it.

This paper was published by a group of scientists that describes the discovery of this gap. Cau Borlina, a Ph.D. student at the Massachusetts Institute of Technology (MIT) in Planetary Science, is the lead author. The paper's title is Paleomagnetic evidence of a disk substructure within the early solar system.

Thanks to facilities like the Atacama Large Millimeter/sub-Millimeter Array (ALMA), astronomers are getting better at looking at younger solar systems that still have protoplanetary disks and are still forming planets. These systems often show evidence of the formation of planets, such as rings and conspicuous gaps. It is still not clear how it all works.

Research has shown that cavities, gaps, and rings have been observed in the disks of young stars over the past decade, according to Benjamin Weiss, co-author of the study and professor of planetary sciences at MIT's Department of Earth, Atmospheric and Planetary Sciences. These signatures are crucial but not well understood signs of the physical processes that gas and dust undergo to become the young suns and planets.

This ALMA image shows the rings and gaps found in young disks. It is located around the young star TW Hydrae. Credit: S. Andrews, Harvard-Smithsonian CfA; B. Saxton, NRAO/AUI/NSF); ALMA. (ESO/NAOJ/NRAO).

Meteorites provide evidence that there was a gap in the protoplanetary disk of Solar Systems, some 4.5 billion year ago.

The structure of meteorites was affected by the Solar Systems magnetic fields. The protoplanetary disk's tiny rocks, called chondrules, were shaped by paleomagnetism. Chondrules are pieces of molten, or partially molten, round rock that were accreted to a meteorite type called chondrites. Chondrules are the oldest rocks in our Solar System.

The magnetic fields of the chondrules were kept as they cooled. These magnetic fields can change as the protoplanetary disc evolves. The magnetic field at the time can affect the orientation of electrons in chondrules. All those chondrules tell a story when taken together.

This image shows a NWA 869 (Northwest Africa 8699) chondrite that was discovered in the Sahara Desert in 2000. The cut face shows both metal grains as well as chondrules. Image Credit: By H. Raab (User: Vesta) Own work, CC BY-SA 3.0,

The group studied chondrules of two carbonaceous meteorites found in Antarctica. SQUID is the Scanning superconducting Quantum Interference device. SQUID, a magnetometer with high-resolution and high-sensitivity that is used to analyze geologic samples, is high-resolution and has high sensitivity. SQUID was used by the team to determine the original ancient magnetic field of each chondrule found in meteorites.

This study also relies on an isotopic dichotomy phenomenon. Scientists concluded that two distinct families of meteorites fell to Earth with different isotopic compositions. They must have formed at different places and times in the early Solar System. These two types of meteorites are known as carbonaceous and non-carbonaceous. CC meteorites are likely to contain material from outer Solar System, while NC meteorites may contain material from within the Solar System. However, some meteorites may contain both isotopic fingerprints. This is very rare.

Both the outer Solar System CC types were used in the study of the two meteorites. They found that the magnetic fields of the chondrules were stronger than those of NC meteorites.

Astronomers believe that this is what happens in a young solar systems. Scientists expect that magnetic fields will decrease as a system grows older. The magnetic strength can be measured in microteslas. The CC chondrules had a field of approximately 100 microteslas. NC chondrules have a strength of 50 microteslas. The Earth's current magnetic field is approximately 50 microteslas.

The magnetic field is a measure of how solar systems produce material. The stronger the field, the more material it can draw in. Based on the size of the planets, the strong magnetic fields evident in the chondrules from the CC meteorites suggests that the outer Solar System was accreting a greater amount of material than its inner region. This paper concludes that there is evidence of a large gap which prevented material from flowing into and within the inner Solar System.

Borlina states that gaps are common in protoplanetary system and now we can prove that one existed in our solar system. This provides the solution to the strange dichomy that we see in meteorites and evidence that gaps can affect the compositions of planets.

All of this evidence is strong evidence for an unexplained large gap in the early Solar System.

ALMAs high resolution images of nearby protoplanetary discs are the result of the Disk Substructures at High Angular Resolution Project. Credit: ALMA (ESO/NAOJ/NRAO), S. Andrews et al. ; NRAO/AUI/NSF, S. Dagnello

Jupiter is the largest planet in the Solar System. This makes it a great place to begin to understand how this all played out. Jupiter's powerful gravity might have played a part in Jupiter's growth. It may have swept dust and gas from the inner Solar System to the outskirts, creating a gap between Mars and it in the developing disk.

Another possibility is the disk itself. The magnetic fields that formed early disks were powerful enough to shape them. These magnetic fields can interact with each other to create strong winds that can move material and create gaps. The gap could have been created by Jupiter's gravity and the protoplanetary's magnetic fields.

The gap itself is only one question. Another question is: What role did it play in creating the gap? It is a question of how it has shaped the world since its formation over four billion years ago. The paper suggests that the gap may have been an impassable barrier, preventing material from interfacing. The terrestrial planets are located on the inside of this gap, while the gaseous worlds are found on the outside.

This gap is difficult to cross, and a planet would require a lot external torque and momentum, according to Cau Borlina, the lead author. This is evidence that our planets formed in specific areas of the early solar system.

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