The inner solar system rotates much more slowly than the laws of modern physics predict, and a new study could help explain why.
A thin disk of gas and dust, known as an accretion disk, orbits young stars. These disks, on which planets form, contain residual star-forming material that is part of the star’s mass. According to the law of conservation of angular momentum, the inside of the disk should spin faster as the material slowly spirals inward towards the star, much like figure skaters spin faster when they bring their arms closer to their bodies.
However, previous observations have shown that the inner Solar System—the region of the Solar System that extends from the Sun to the asteroid belt and includes the terrestrial planets—is not spinning as fast as the law of conservation of angular momentum predicts. Using new simulations of a virtual accretion disk, scientists at the California Institute of Technology (Caltech) have demonstrated how particles interact in an accretion disk.
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“Angular momentum is proportional to speed times radius, and the law of conservation of angular momentum states that the angular momentum in a system remains constant,” the Caltech researchers wrote in a statement. “So, if a skater’s radius is decreasing because they are bringing their arms together, then the only way to keep the angular momentum constant is to increase the speed of rotation.”
So why isn’t the angular momentum of the inner accretion disk conserved? According to the statement, earlier research shows that friction between regions of the accretion disk, or magnetic fields that create turbulence (and create friction), can slow the speed of rotation of the infalling gas.
“It worries me,” Paul Bellan, a professor of applied physics at the California Institute of Technology and co-author of the study, said in a statement. “People always want to blame turbulence for things they don’t understand. There is now a large cottage industry claiming that turbulence explains the loss of angular momentum in accretion disks.”
To better understand the loss of angular momentum, Bellan studied the trajectories of individual atoms, ions, and gas in an accretion disk and, in turn, the behavior of particles during and after collisions. While charged particles—electrons and ions—are affected by both gravity and magnetic fields, neutral atoms are affected only by gravity.
The researchers used computer models to simulate an accretion disk of 1,000 charged particles colliding with 40,000 neutral particles in magnetic and gravitational fields. They found that the interaction between neutral atoms and a much smaller number of charged particles causes positively charged ions or cations to spiral inward, while negatively charged particles or electrons move outward towards the edge of the accretion disk. Meanwhile, neutral particles lose angular momentum and spiral inward towards the center.
In turn, the accretion disk acts like a giant battery with a positive pole near the center of the disk and a negative pole at the edge of the disk. These terminals generate powerful streams or jets of material that are ejected into space from both sides of the disk.
“This model had enough detail to capture all the main features because it was large enough to behave exactly like the trillions and trillions of colliding neutral particles, electrons and ions orbiting a star in a magnetic field,” said Bellan in research. statement.
According to the statement, computer simulations suggest that when angular momentum is lost, the canonical angular momentum – the sum of the original ordinary angular momentum plus an additional quantity depending on the particle’s charge and magnetic field – is preserved.
“Because electrons are negative and cations are positive, the inward movement of ions and outward movement of electrons caused by the collisions increase the canonical angular momentum of both,” the researchers explained in the statement. “Neutral particles lose angular momentum as a result of collisions with charged particles and move inward, which counterbalances the increase in the canonical angular momentum of charged particles.”
Their findings were published May 17 in The Astrophysical Journal.
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