Is a protoplanet magnetic?

Somewhere on a forum I read that planets also rotate because magnetic fields played a role.

This was immediately rejected with the argument that the magnetic fields of planets were not strong enough, but in physics I learned that elementary magnets in iron cancel each other out. If protoplanets are also magnetic, then colliding protoplanets can more or less cancel each other out.

Asker: Matt, 15 years old

Answer

It is not correct that planets rotate because magnetic fields play a role, but rather the other way around: the rotation of a planet contributes to the dynamo effect that generates the magnetic field.

Why does a planet rotate? Why does a star rotate? Because the star, around which planets form, is created by the contraction of a large thin cloud. If something rotates and it contracts, it will spin faster. This is called the law of conservation of torque. This can be checked with a swivel chair and some weights: make the chair you are sitting on rotate, hold the weights and extend your arms, the rotation will be slowed down; pull your hands back in, and it will spin faster again. In stars that form, the contraction is enormous: the radius of the star ends up being about ten million times smaller than the radius of the cloud; that immediately means that it will rotate about 10 million times faster. A very small initial rotation is greatly magnified; to end up with no rotation, the initial state must have been a near-perfect standstill, and there is no such thing. It is therefore fatal that a star has to rotate, and that the matter that clumps together into planets benefits from that rotational movement.

Incidentally, an important problem with star formation is that it is expected that the star would rotate much too quickly, and would therefore not be able to stay together. A lot of rotational movement must therefore be lost during the contraction. And there it is thought that magnetic fields play a role. The whole cloud is magnetic, not much, but enough to extract rotational motion from the cloud through the magnetic accelerations, so that a stable star can form.

Is a protoplanet magnetic?

Answered by

prof. Christopher Waelkens

Astronomy

Catholic University of Leuven
Old Market 13 3000 Leuven
https://www.kuleuven.be/

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