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In the paper https://arxiv.org/abs/1712.00457 about rotation rates of gas giants it says:

"owing to accumulation of angular momentum stored in the source material, a planetary mass object should rotate at or near breakup towards the end of the phase of rapid of gas accretion (irrespective of whether the object formed through core accretion or via gravitational instability). In light of this expectation and the discussion presented above, another mechanism is needed to reduce the rotation rate to values well below break-up, and counteract spin-up due to gravitational contraction and accretion. Because our observations do not show a statistically significant dependence of angular velocities on age, we speculate that the spin-down process (whatever it may be) operates exclusively during the disk-bearing stage of evolution."

Question: What mechanisms would cause the rotation of a gas giant to slow down, assumming that the planet is far enough from its star that tidal interaction with the star won't slow it down?

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    $\begingroup$ Have you read the introduction in the same paper? $\endgroup$
    – AtmosphericPrisonEscape
    Jun 11 at 0:15
  • $\begingroup$ Hmm, it says "This may be due to magnetic coupling with a circumplanetary gas accretion disk, which could provide a channel for young planets to shed their angular momentum". I missed that bit. Thanks. $\endgroup$
    – sno
    Jun 11 at 0:23
  • $\begingroup$ Interesting to note that an analogous problem exists in stellar evolution theory between the stellar core and envelope. $\endgroup$
    – Daddy Kropotkin
    Jun 11 at 23:10
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The paper proposes the magnetic coupling of a central object to the accretion disc from which it is building up its mass. This is thought to be the same route by which low-mass stars are prevented from spinning up to break-up speeds during their formation.

The idea is to imagine a bar magnet in the planets with a dipole field. The field emerges from the planet surface and threads through the surrounding accretion disc.

Now as the planet contracts, or gains material from the accretion disc, conservation of angular momentum demands that it spins up. But if the magnetic field is anchored in the planet and the disc, which will be the case if there is any ionisation in the planet/disc, then the planet can't spin up without dragging the field through the disc material.

The net effect is that the magnetic linkage acts as a "rotostat", keeping the rotation rate of the planet constant by transferring angular momentum to the disc.

Imagine an ice skater trying to go into a fast spin whilst holding out their arms into some surrounding viscous medium.

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The slowdown is due to 'angular momentum transfer' from inner regions of the rotating cloud to outer regions via magnetic fields. You can find a detailed mathematical explanation in terms of a solution of the MHD- equation on this web page http://th.nao.ac.jp/MEMBER/tomisaka/Lecture_Notes/StarFormation/5/node94.html . It may not be easy to fully understand if you are not already familiar with this branch of physics, but it should be intuitively clear that magnetic fields are the only way that mass elements can exchange angular momentum without being in direct contact with each other (provided of course these mass elements consist of ionized material so that they are trapped by the magnetic field lines).

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