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Magnetic forces in planetary orbits

  1. Jan 16, 2012 #1
    This suddenly entered my mind while studying magnetic fields. I know of course that it is the gravitational force that holds the planets in orbit, but do their magnetic fields also have an effect, even by an infinitesimal amount? Do the magnetic fields of the planets have a small influence on each other's orbits?

  2. jcsd
  3. Jan 16, 2012 #2


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    It seems magnetic fields of planets do have an influence on each other's orbits. Here is a quotation from Scientific American:

    “Magnetic fields also muck up planet and star formation, as Susana Lizano of the Universidad Nacional Autonoma de México in Morelia explained. The interstellar clouds of gas and dust out of which planets and stars coalesce are threaded with magnetic fields—weak ones, a thousandth as strong as Earth’s. (Astronomers gauge the field strength by looking at how light from the dust is polarized.) As these clouds collapse, basic theory predicts the field should intensify a billionfold. But if that happened, the field would become powerful enough to stop the collapse. Even leaving this problem aside, as the cloud settles into a swirling disk, the field should bring the swirling motion in the inner part of the disk to a halt. Somehow the fields must dissipate, perhaps through a variant of induction-stove effect.

    Even when the magnetic field weakens, it sculpts the nascent planetary system. It causes gas to revolve around the star more slowly than freely orbiting objects do. Embryonic planets thus experience a drag force and spiral inward. The field also stabilizes the disk, keeping it from fragmenting—further evidence that planets form by step-by-step agglomeration rather than gravitational breakup of the disk. All in all, Lizano built a persuasive case that astrophysicists ignore magnetism at their peril.”

  4. Jan 16, 2012 #3

    D H

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    Read the article again. Magnetism appears to play some role in the initial formation of a star and the protoplanetary disk. The role diminishes as the protostars and protoplanets build mass. Once the star system is formed, the affect on orbits is vanishingly small.

    There is an ongoing magnetic effect: It puffs up planetary atmospheres. Right now the Sun is approaching solar max, and the Earth's atmosphere is puffing as a result. The F10.7 radio flux from the Sun is a fairly good indicator of how "puffed up" the Earth's atmosphere will be. The variations in magnetic flux over the course of the solar cycle has a measurable effect on the atmospheric drag on satellites in low Earth orbit.
  5. Jan 16, 2012 #4


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    Thank you, D H! I reread the article and found that you are correct: once planets form magnetism's effect is vanishingly small.
  6. Jan 16, 2012 #5
    Thanks! That clears up a lot. :smile:
  7. Jan 18, 2012 #6


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    For a magnetic dipole, I believe the field falls of as the cube of the distance so the inverse square of the gravitational field beats it pretty damn soon as you get a long way away - whatever the relative forces / fields are, nearby.
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