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Klown Kar planet

  1. Sep 30, 2015 #1
    Hi all;

    Here's the sit: I'm writing a novel which involves colonization of other systems. I want to have a weirdo planet (thus the title) and I decided on a planet that doesn't quite complete a rotation. There's a thread here: https://www.physicsforums.com/threads/how-the-last-days-look-like-just-before-a-tidal-lock.790409/ that discusses it.

    Anyway, the point is a planet close enough to its primary to be subject to eventual tidal locking. This planet will be mostly solid, and will have a significant mass inhomogeneity that will make it act like an unbalanced bicycle wheel. At some point, the rotation slows down until it can't quite make it around on the last day. After that, it will swing back and forth through 350 degrees or whatever, every "day".

    So, two questions I'm interested in: 1) What would be a plausible range for the period of the swing, and how long would this state last until tidal forces finished locking the planet?

    I don't need rigorous math proofs, this is fiction. Just something that won't make the reader want to throw the book at the wall.
     
  2. jcsd
  3. Sep 30, 2015 #2

    DaveC426913

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    Not sure I follow. It would swing back and forth through 10 degrees, not 350.
     
  4. Sep 30, 2015 #3
    I doubt that this could happen but I think it is quite a clever idea, so go for it. If you want to be semi-realistic, then make the period quite long, like a millenium. If it could happen at all it would seem that it would go on for a very long time, like a million years.
     
    Last edited: Sep 30, 2015
  5. Sep 30, 2015 #4

    DaveC426913

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    Maybe it's just me, but I think you need to lay the order of events out more explicitly to ensure this makes sense.

    At some point, tidal forces slow the planet until its day is almost as long as its year. It's revolving around the sun in, say, 7 million hours, but rotating on its axis in, say, 6,900,000 hours.

    What next?
     
  6. Sep 30, 2015 #5
    (I think it's important to re-stress that the planet is not homogenous. I'm assuming a significant off-center mass concentration. Otherwise this doesn't work.)

    Actually, I think 350 (some amount slightly smaller than 360) is wrong. I think it might be some amount slightly smaller than 180. Possibly.

    Understand that I'm viewing this intuitively, not from a rigorous math POV, but I visualize it like a satellite with a large boom, with a weight at the end. This uses simple orbital mechanics to ensure that the boom is always pointed toward the Earth. If you force a rotation on the satellite, and assume for the sake of argument that tidal braking would be effective, then the satellite would gradually slow and would eventually stop rotating with the boom once again pointing towards the Earth (could it stop with the boom pointing away from the Earth?). But it wouldn't stop with the boom perpendicular to the Earth, because that's not a stable orientation.

    But I'm interested in the part just before it stops rotating. Before this point, the rotation would be uneven in that it would slow down and speed up depending on the orientation of the boom. Think of an unbalanced bicycle wheel. As it approaches a 90 degree attitude, it slows down, and as it crosses the 90 degree point it speeds up again. Eventually the satellite will be slowed down enough so that the boom isn't quite able to make it past the 90 degree point. It stops, then swings back, passing though the nadir, then makes it almost all the way up to 90 degrees again on the other side. Rinse, repeat. I can think of no mechanism or reason why it would just come to a stop at nadir without some period of swinging back and forth.

    So, additional question: Is an orientation with the boom pointed straight away from the Earth stable? Or would it spontaneously flip? If stable, then Klown Kar planet will only swing through 180 less a bit. If not stable, then KKP would swing through 360 less a bit.
     
  7. Sep 30, 2015 #6

    DaveC426913

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    As am I.

    I am not sure this is true. it takes a LOT of energy to slow down and speed up the rotation of a planet's rotation. I don't think this can occur over the year time frame you're proposing, let alone happened cyclically.



    The reason why a bicycle wheel would bob before stopping is because its inertia (keeping it rotating) is large compared to gravity (trying to stop it). In the case of a planet, over a much longer time period, I think the tidal force from the sun is larger than the planet's rotational inertia. Once it reaches tidal lock, it's not going to over rotate.

    I am not certain, but my gut tells me so.
     
  8. Sep 30, 2015 #7
    Also, you'll have a difficult time constructing a lopsided planet. It certainly couldn't form that way, a planets like hydrostatic equilibrium. Your planet might have to be very small in order to have a lopsided shape. I'm not sure how you could make a large planet be so off-center. The moon is denser on the side that faces the earth, but not by much, and we're a pretty big gravity well considering how close the early moon was to the planet.
     
  9. Sep 30, 2015 #8
    Right, but it won't have reached tidal lock yet. Look at it this way. Assuming tidal braking is linear, then if you start at time A with a rotation X, and you know that the planet will be slowed at Y milliseconds per year, you can calculate the date Z when it becomes locked. Given a homogenous sphere, it's pretty straightforward. But given a non-homogenous sphere (which brings back the satellite-boom metaphor), at some point before Z the planet is going to be rotating pretty slowly. As with the bicycle wheel, the slower the rotation, the more pronounced the effect of an unbalance will be. The big unknown, of course, is the magnitude of the inhomogeneity compared to the mass of the planet.
    If the ratio is too low, of course, the slowing down approaching the 90 and 270 points will be detectable only with sensitive instruments and might never stop a rotation "short", so to speak. But if the ratio is high enough, it might.
    Fortunately, I'm writing fiction, so the ratio is high enough because I say so :biggrin: . But you confirm my thought that if it did work out, it wouldn't be anything like a 36 hour period or anything like that. (I initially came up with a period of 1.43 days, but then realized I was using the entire planet's mass in the pendulum period calculation)
    I'm still a little out to lunch about whether such a behavior would be based on a 180 or 360 degree (less a bit) swing. If 180, there would be a small slice of the planet that would never see the sun. Hmmm.

    Anyway, I'm quite prepared to be beaten up over this. The planet doesn't get introduced in detail until book 2, and I have a more prosaic alternative available. And if anyone has a weirder idea than this, I'm all ears.
     
  10. Sep 30, 2015 #9
    Also, the interior has to be solid or nearly so, or the inhomogeneity would just sink to the center.

    Of course, our heroes can speculate all they want, but they won't know for sure what happened. For whatever reason, there will be a large mass concentration at one point on the planet, not too far below surface level. Keeping it as near the surface as possible keeps it away from the still-fluid core and means maximum moment-arm for the pendulum effect.

    If the planet was still rotating fairly quickly, it might actually be a prolate spheroid rotating on its minor axis. However, by this point, I think it'll be generally spherical.
     
  11. Sep 30, 2015 #10

    DaveC426913

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    True, but you as the writer have no such luxury. You must know.
     
  12. Oct 1, 2015 #11
    The inhomogeneity WILL sink to the center. It would have to be a foreign body that by some very strange chance collided at low speed. The collision would have to be recent enough that it hasn't sunk to the center, but not so recent that everything is too hot ie. molten. It would have to be of heavy metals in order to be denser than the rest of the planet.
     
  13. Oct 1, 2015 #12
    Does this lopsidedness have to be natural? An artificial gravity device dropped on the planet by an alien species in the distant past could cause your weird planet. Mining on a planetary scale could as well.
     
  14. Oct 1, 2015 #13

    DaveC426913

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    Kind of takes the fun out of an emergent scenario though. Once you introduce advanced gravity control, it might as well just make the planet move in figure-8's or spell out 'Eat at Joes'.

    Heh. I like this idea.
    When our slag heap outsizes our planet, it's time to move on.
     
  15. Oct 1, 2015 #14
    One more thing: the planet's core would have to be stone like Venus, not heavy metal iron like Earth. Otherwise the inhomogeneity could not be denser than the core.
     
  16. Oct 1, 2015 #15
    :eek: I've actually got an alien species in the book that does drive-by mining. However, what they leave behind isn't generally colonisable.
     
  17. Oct 1, 2015 #16
    Does Venus not have a metal core? I was under the impression that Venus has had some fairly recent volcanism, so it should at least still be molten.

    You brought up a thought with this comment. The planet has to be non-homogenous in order for a pendulum effect to be possible, but the mass concentration doesn't have to be heavier than average. A large concentration of low-mass material on one side would automatically shift the center of mass. It also wouldn't tend to sink to the center, another plus. I think it would create sufficient non-uniformity to give tidal forces something to latch onto, to create the pendulum effect.

    Alternatively, a large concentration of basaltic rock on one side of the planet, with an equivalent concentration of granitic on the other side (maybe a Pangea?) would create an imbalance. Would have to be one helluva continent, though. OTOH, having the imbalance right at the surface, and reinforced antipodally, would maximize the effect.

    I'm just spitballing, now.
     
  18. Oct 1, 2015 #17
    Just to be clear, I'm pretty sure Venus's core is molten iron, just like Earths (am I wrong?) I was under the impression that the interior of Venus is very similar to Earth, I think you're confused by the fact that it has a minimal magnetic field. A hot molten iron core doesn't make a magnetic field by itself, it has to be spinning. Venus spins VEEEEERRRY slowly.

    I could see a Type II civilization extracting huge chunks of a solid planet in order to build megastructures nearby. I imagine them just taking trillions of tons of material all at once and leaving a planet shaped like a this kind of clown car.

    Is1uC.gif
     
  19. Oct 1, 2015 #18

    Venus has a stone core, and no magnetic field. Stone doesn't conduct EM. The Moon is stone too.

    By heavier you mean denser.

    Planets of sufficient mass have powerful forces making them (almost) round. But change in shape tends to be slow. Did you know that Canada is still rebounding from the weight of glaciers that melted away 10,000 years ago?

    But planets also have powerful forces that would make a large dense mass sink to the center. Basaltic/granitic sounds good. If the denser mass were spread out thin and flat it wouldn't sink. The planet would change shape until it was balanced, but it would take quite a long while.
     
  20. Oct 1, 2015 #19
    Whoops, I guess I was wrong about Venus. That's what I get for trusting informal communications. The Moon is stone though, so a stone planet seems possible.

    By heavier you mean denser.

    Planets of sufficient mass have powerful forces making them (almost) round. But change in shape tends to be slow. Did you know that Canada is still rebounding from the weight of glaciers that melted away 10,000 years ago?

    But planets also have powerful forces that would make a large dense mass sink to the center. Basaltic/granitic sounds good. If the denser mass were spread out thin and flat it wouldn't sink. The planet would change shape until it was balanced, but it would take quite a long while, especially if it were solid.
     
  21. Oct 2, 2015 #20
    I'm really dying to know why you want these details. I mean, it would be practically impossible to live on the surface anyway, right?
     
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