A mysterious hole in the protoplanetary disk of the solar system
Scientists are finding evidence that the early solar system housed a gap between its inner and outer regions.
The cosmic border, possibly caused by a young Jupiter or an emerging wind, probably shaped the makeup of the nascent planets.
At the start of the solar system, a “protoplanetary disk” of dust and gas circled around the sun and eventually merged into the planets we know today.
A new analysis of ancient meteorites by scientists from MIT and elsewhere suggests that a mysterious hole existed within this disc about 4.567 billion years ago, near where the asteroid belt is today.
The team’s results, published on October 15, 2021, in Scientists progress, provide direct evidence for this discrepancy.
“Over the past decade, observations have shown that cavities, vacancies, and rings are common in disks around other young stars,” says Benjamin Weiss, professor of planetary science in the Department of Earth Sciences, of the Atmosphere and Planets (EAPS) from MIT. “These are important but poorly understood signatures of the physical processes by which gas and dust transform into young suns and planets.”
Likewise, the cause of such a discrepancy in our own solar system remains a mystery. One possibility is that Jupiter may have had an influence. As the gas giant took shape, its immense gravitational pull could have pushed gas and dust towards the periphery, leaving behind space in the developing disc.
Another explanation may have to do with the winds emerging from the surface of the disc. The earliest planetary systems are governed by strong magnetic fields. When these fields interact with a rotating disk of gas and dust, they can produce winds strong enough to blow matter, leaving behind space in the disk.
Whatever its origins, a gap in the early solar system likely served as a cosmic boundary, preventing materials on either side of it from interacting. This physical separation may have shaped the makeup of the planets in the solar system. For example, inside space, gas and dust have clustered into terrestrial planets, including Earth and ">March, while gas and dust are relegated to the other side of the breach formed in more icy regions, such as Jupiter and its neighboring gas giants.
“It’s quite difficult to bridge that gap, and a planet would need a lot of external torque and momentum,” says lead author and EAPS graduate student Cauê Borlina. “So this provides evidence that the formation of our planets was limited to specific regions of the early solar system.”
Weiss and Borlina co-authors include Eduardo Lima, Nilanjan Chatterjee and Elias Mansbach of MIT; James Bryson of the University of Oxford; and Xue-Ning Bai from Tsinghua University.
A split in space
Over the past decade, scientists have observed a curious split in the makeup of meteorites that have reached Earth. These space rocks originally formed at different times and in different places as the solar system took shape. Those that have been analyzed show one of two isotopic combinations. Rarely have meteorites been found to present 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.
Weiss’s group analyzes 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. Weiss’s group specializes in measuring chondrules to identify the ancient magnetic fields in which they originally formed.
In previous work, the group 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. Weiss’s group had previously identified the old magnetic field in samples from this nearby region.
A meteorite shift
In their new study, the researchers 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 said to originate further in the solar system.
They analyzed chondrules, each measuring about 100 microns, from two carbonaceous meteorites discovered in Antarctica. Using the Superconducting Quantum Interference Device, or SQUID, a high-precision microscope from Weiss’s lab, the team determined the ancient and original magnetic field of each chondrule.
Surprisingly, they found that their field strength was stronger than that of the nearer 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, Borlina and his colleagues found that distant chondras had a stronger magnetic field, of about 100 microteslas, compared to a field of 50 microteslas in the nearest chondrules. 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 chondrums, 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,” 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.”
Reference: “Paleomagnetic evidence for a disk 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
This research was supported, in part, by ">Nasa, and the National Science Foundation.