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Scientists work out why the inner solar system seemed to break the laws of physics


Scientists believe they have solved the mystery of why the inner solar system spins much more slowly than the laws of physics suggest it should.

The inner ring of the solar system contains thin layers of gas and dust known as accretion disks, which spiral around young stars heading slowly inwards.

When this spirally happens, the inner part of the disk should spin faster according to the laws of angular momentum – like a figure skater spinning faster when their arms are in than when they are out.

While observations found that the inner part of an accretion disk does spin faster, it does not move as fast as predicted.

Researchers have produced many possible explanations for this behaviour: whether that is friction between the inner and outer rotating parts of the accretion disk, or magnetic fields generating a “magnetorotational instability” that produces gas and magnetic turbulence that slows down the rotational speed of inward spiralling gas.

Neither of these explanations satisfied Paul Bellan, professor of applied physics at Caltech, because calculations show that accretion disks have negligible internal friction and turbulence was “concerning”.

Professor Bellan started analysing the trajectories of individual atoms, electrons, and ions in the gas that made up the disk, to see how the particles behaved when colliding and how they moved in between collisions.

Using a simulation of around 40,000 neutral and about 1,000 charged particles that could collide with each other, as well as gravity and magnetism.

“This model had just the right amount of detail to capture all of the essential features,” Professor Bellan says, “because it was large enough to behave just like trillions upon trillions of colliding neutral particles, electrons, and ions orbiting a star in a magnetic field.”

The simulation showed that collisions between neutral atoms and charged particles would cause positively charged ions to spiral inwards, while negatively charged elections spiral outwards. Analysing this behaviour showed that angular momentum is not conserved, but a force called “canonical angular momentum” is.

Canonical angular momentum is the sum of original ordinary angular momentum plus an additional quantity that depends on the charge on a particle and the magnetic field. For neutral particles, there is no difference between these forces, but charged particles can change drastically because of the large magnetic quantity.

The difference in charge increases the momentum of both positive and negative particles increases their canonical angular momentum, with neutral particles losing angular momentum and moving inwards.

This small distinction has a huge ripple effect on a scale as large as the solar system, with only one in a billion particles needing to be charged to explain the observed loss of angular momentum of the neutral particles.

This motion makes the disk becoming akin to a giant battery – with a positive terminal near the centre and a negative terminal at the edge. This creates huge electric flow and powers astrophysical jets that shoot out in both directions; these have been observed by astronomers for over a century, without them ever knowing the force behind their origin.

The research, ‘Neutral-charged-particle Collisions as the Mechanism for Accretion Disk Angular Momentum Transport‘, was published in the Astrophysical Journal.


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