The inner solar system rotates much more slowly than the laws of modern physics predict, and new research may help explain why.
A thin disk of gas and dust – known as the accretion disk – spirals around the young stars. These disks, where planets form, contain remnant star-forming material that is part of the star’s mass. According to the law of conservation of angular momentum, the inner part of the disc should spin faster as the material slowly spirals inward toward the star, similar to how figure skaters spin faster when they bring their arms closer to their bodies .
However, previous observations show that the inner solar system – the region of solar system which extends from sun to the asteroid belt and includes terrestrial planets — is not rotating as fast as predicted by the law of conservation of angular momentum. Using new simulations of a virtual accretion disk, scientists at the California Institute of Technology (Caltech) have demonstrated how particles in the accretion disk interact.
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“Angular momentum is proportional to radial velocity, and the law of conservation of angular momentum states that the angular momentum in the system remains constant,” the Caltech researchers wrote in declaration. “So if the skater’s radius decreases because they’ve pulled their arms out, then the only way to keep the angular momentum constant is to increase the spin rate.”
So why is the angular momentum of the inner accretion disk not conserved? Earlier research suggested that friction between regions of the accretion disk or magnetic fields generating turbulence (and creating friction) could slow the rotation rate of the infalling gas, according to the statement.
“It’s troubling to me,” Paul Belan, a professor of applied physics at Caltech and co-author of the study, said in the statement. “People always want to blame turbulence for phenomena they don’t understand. There is currently a large cottage industry claiming that turbulence explains the removal of angular momentum in accretion discs.’
To better understand the loss of angular momentum, Belan studied the trajectories of individual atoms, ions and gas in the accretion disk and, in turn, how the particles behave 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 results in positively charged ions, or cations, spiraling inward and negatively charged particles, or electrons, moving outward toward the edge of the accretion disk. Meanwhile, neutral particles lose angular momentum and spiral inward toward the center.
In turn, the accretion disk acts like a giant battery with a positive terminal near the center of the disk and a negative terminal at the end of the disk. These terminals generate powerful currents or jets of material that are fired space on both sides of the disc.
“This model had just the right amount of detail to capture all the essential 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 magnetic fieldBelan said in the statement.
Computer simulations suggest that while angular momentum is lost, canonical angular momentum — the sum of the original ordinary angular momentum plus an additional amount that depends on the charge of a particle and the magnetic field — is conserved, according to the statement.
“Because electrons are negative and cations are positive, the inward movement of ions and the outward movement of electrons, which are caused by 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 balances the increase in canonical angular momentum of the charged particles.”
Their findings were published on May 17 in The Astrophysical Journal.
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