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Thread: Can a star magnetic field make rotate a planet ?

  1. #1
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    Can a star magnetic field make rotate a planet ?

    We just learned that Planets around red dwarfs are common. Some are orbiting their star from a very short distance , may be inside the habitable zone. But there is problem about their habitability , they are probably tidally locked to their star , like our moon is locked to Earth.

    So I wandered if there were possibly an other factor able to keep these planets turning. And I found one possible suspect : the magnetic field.

    A planet in the habitable zone of a red dwarf is about 100 times nearer that its star than Earth , so the magnetic filed should be 10,000 stronger . (Inverse square law).

    Is it possible that the interaction of the star magnetic field , with the planet magnetic field , keeps it running , even at a slow rate ?

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    Assuming that both the Red Dwarf and the Sun start off with the same magnetic field, do you have a sense of how those fields would apply torque to the planet? ... also you might check to see if magnetic field intensity really drops off as inverse square.
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    I would think a rotating magnetic field (of a rotating star) would exert a rotating torque on a magnetic or magnetised planet either directly or as a magnetic equivalent of "tidal" given the scale.

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    Quote Originally Posted by antoniseb View Post
    Assuming that both the Red Dwarf and the Sun start off with the same magnetic field, do you have a sense of how those fields would apply torque to the planet? ... also you might check to see if magnetic field intensity really drops off as inverse square.
    I assumed that , same magnetic field. About the drop off I was too pessimistic , far from the dipole it drop off like 1/R^3. SO the magnetic filed near the star could be very strong.

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    ...do you have a sense of how those fields would apply torque to the planet? ...
    Perhaps simply through "inheritance," where the torque of one body is transferred to another (in this case, lesser body). If the "child" body is said to be derived from the "parent" body, then torque becomes a "trait" and any interactive magnetic fields the result, rather than the cause of the relationship?

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    Quote Originally Posted by galacsi View Post
    I assumed that , same magnetic field. ... SO the magnetic filed near the star could be very strong.
    See if you can Google around and find how many Watts the Sun's magnetic field adds to Earth rotational energy. I think you'll find that even multiplying by a billion won't get you into the right ballpark to beat the Moon's tidal influence. But I didn't look it up, and you have a chance to find surprising results.
    Forming opinions as we speak

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    Doesn't that mean the planet must also have a magnetic field or an odd distribution of charge?

    I'm thinking it would try to flip the planet, like uranus. If everything starts off with poles that are parallel or antiparallel.

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    All I could find is rather fuzzy :
    " Earth's rotation is slowing. Earth has to shed both energy and angular momentum for this to happen. The chief mechanism for that is the tidal effect which transfers angular momentum to the Moon and energy to the oceans. A lesser mechanism involves the fact that the Earth rotates in the Sun's magnetic field, transfering angular momentum to the sun and losing energy by eddy currents."
    and
    "The US Naval Observatory estimates that the Earth's rotation is slowing at about 1.4 milliseconds per solar day per century which roughly agrees with the rate of rotation of the Earth has actually slowed down since 1820."
    That's does not answer my question but give some indications . If the sun magnetic field can slow down Earth , one can imagine an other planet in other conditions could be accelerated no ? I found also in Wikipedia that the dipolar magnetic field of the sun is about 10 - 4 and it is about 10-9 at the Earth orbit . So a 105 increase of the strength of the field is not impossible.

    Back of the envelope computation indicate my idea is not completely stupid . I cannot go any further . May be somebody more versed in geophysics than me can give it a try ?

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    According to the book Terrforming, there was this plan to spin up Venus. Don't remember how that was supposed to work.
    http://www.amazon.com/Terraforming-E...4010702&sr=1-3

    I seem to remember a quote from this book that said, that--if you could instantly replace Venus CO2 Atmosphere with water vapor--it would actually get hotter.

    More:
    http://www.amazon.com/Terraforming-C...4010702&sr=1-1

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    the way you can look at it is in the following, but this is hardly a complete description.
    Magnetized Earth rotates, making in the equatiorial plane vertical magnetic field move "backward" on the day side ad "forward" on the night side.
    Then you have to take the solar wind magnetic field (which is averaged over long times a Parker spiral, but can for all purposes be replaced with a simple dipole field) with some vertical component too.
    Then you will have to see how this backward moving magnetic field interacts with the solar wind and the forward moving part, now there is a project for a rainy sunday afternoon (or two or three or ...)
    This will give you an estimate of the torque on the Earth through B-B interaction.
    Naturally, you can also calculate the accelerating/decelerating force of the solar dipole sweeping over the Earth, creating a current as a unipolar inductor, which will create a force with the magnetic field (another sunday afternoon project )
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    Any spinning magnet will apply a torque to any remote object with suitable magnetic properties, depending on the angle of the pole alignment of course, if displaced laterally it will also apply a force as the magnetic field sweeps through. If the magnet is not spinning there would just be a drag force

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    Quote Originally Posted by quotation View Post
    Perhaps simply through "inheritance," where the torque of one body is transferred to another (in this case, lesser body). If the "child" body is said to be derived from the "parent" body, then torque becomes a "trait" and any interactive magnetic fields the result, rather than the cause of the relationship?
    It is difficult to respond to this message because it uses words in a way that are not commonly seen in scientific circles. The term "inheritance" has some meaning in biology, and in computer science, but not in physics. Perhaps you are trying to convey the idea of a conservation law -- maybe the conservation of angular momentum. The word "trait" is another word that is completely absent from the vocabulary of physics, so I can't understand what you are saying in that case.

    "Torque" is a term commonly used in physics. It refers to an influence on a body which causes the body's angular momentum to change -- either by changing its rotation rate, or the direction of its rotational axis, or both. "Torque" is a rotational analog to "force." However, neither torque nor force are properties of a body; they cannot be transferred from one body to another. Linear momentum and angular momentum, on the other hand, are properties of bodies, which can be transferred from one to another.

    It would help me to understand your posts if you could adopt the same jargon as scientists use in the fields of physics and astronomy. There are many good textbooks at the high school and college level which explain these terms.

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    Sorry SM, sometimes it just pops out of me. Too much exposure to object-oriented programming and a desire to look at things in new ways, so sometimes (always actually) I try out the exchange of mechanical metaphors for biological ones (when I can come up with them) and if I told you it stemmed from a life goal of trying to democratize corporatism, that probably wouldn't make sense either, but there it is. Sometimes it works, other times (as in this case) not so much

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