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The shape of our solar system's orbits.

  1. Nov 7, 2009 #1


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    What's the shape of the planets orbiting the sun in our solar system according to this website
    they are elliptical. But in a documentary (maybe history channel's "The Universe", but I'm not sure) I heard that our solar system is one of the few known systems with round orbits.Majority of the others have elliptical. So what's the truth?
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  3. Nov 7, 2009 #2
    Isn't an elliptical orbit not rounded?
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  4. Nov 7, 2009 #3


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    I mean "round" not "rounded"?
  5. Nov 7, 2009 #4


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    The truth is that for 7 out of the 8 planets the SS's orbits are very nearly circular.

    Mercury's, Pluto's and Sedna's orbits are more elliptical.

    Here is a list of all extrasolar planets currently known.
    You can even sort on the Eccentricity column (Ecc). Eccentricity is a ratio if ellipticality of the orbit (zero is perfectly circular.)
    Last edited: Nov 7, 2009
  6. Nov 7, 2009 #5


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    Note that a circle is just a special case of an ellipse: all orbits are elliptical.
  7. Nov 7, 2009 #6
    That's what I was trying to get at in my previous post I guess I should have worded it more like: Can't elliptical orbits be round?

    Now that I've reread what I said before it does sound kind of funny haha.
  8. Nov 7, 2009 #7


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    A quick calculation shows that the average eccentricity of our SS's 8 major planets is ~0.0606
    while the average of the 345 extrasolar planets with known eccentricities is ~0.2181
    or about 3.5 times more eccentric.
  9. Nov 8, 2009 #8

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    But does that tell us about the extrasolar planet orbits or about the extrasolar planet detection mechanism?
  10. Nov 8, 2009 #9


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    No doubt that's a factor.

    But there are about 55 or so planets whose eccentricity is listed as unknown. I did not include those ones. Since 'unknown' is explicitly listed, it was a reasonable assumption that the listed values are trustworthy enough for a rough calculation.

    It is a big assumption, I grant.
  11. Nov 8, 2009 #10
    The average does not give the complete picture. If you run the data through a frequency analysis, about 1/3 of the observed eccentricities are less than 0.05 (approximately the solar system average) and around 2/3 are less than 0.2.

    The above doesn't take the distance of an exoplanet from its star into account and its conceivable that planets within different AU ranges could show different means. Furthermore, and this is purely conjecture on my part, it is possible that exoplanets with larger eccentricities are the larger gas giants and thus easier to observe. The data may be inherently biased by our current observational limitations.

    In any case, if 30% of the observed exoplanets match the solar system average, then the solar system cannot be considered to be that unique.
  12. Nov 8, 2009 #11


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    That's why I said "quick calculation" rather than "exhaustive calculation".

    But you're right. The OP's post states "... our solar system is one of the few known systems with round orbits. Majority of the others have elliptical."

    ...which is clearly not what the data is showing when analyzed more carefully than a simple average.
  13. Nov 8, 2009 #12
    It could be. If the you have 30% of the exoplanets match the solar system average, and the eccentricities are random, then the odds of have eight planets all with more or less round orbits is quite low. Apparently there is a resonance between Jupiter and Saturn that keeps the rest of the planets in line.

    I've heard this discussed in terms of the anthropic principle. Why are solar system orbits rather round? Because if they weren't we wouldn't be seeing them.
  14. Nov 8, 2009 #13
    True but only if the eccentricity of each exoplanet is determined independently.

    This argument could actually extend to other systems. Current observations very likely detect the largest object(s) within an extrasolar system. And if 30% of those have low eccentricities, then its possible that other smaller objects within these systems will also be forced into circular orbits.
  15. Nov 12, 2009 #14
    All orbits in this universe are spiral.
  16. Nov 12, 2009 #15


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    Are you just kidding, or are you seriously saying all orbits are spiral?
  17. Nov 13, 2009 #16
    YES. What one isn't?
  18. Nov 13, 2009 #17
    Why shouldn't we report you as a troll? Your statements are unjustified. Do you want to give us more information, or do you just want an argument?
  19. Nov 13, 2009 #18
    All satalites move about primaries that are moving also. If the positions of the two bodies were ploted over time These plots would resemble a spiral. Sorry I live in a universe that has at least four dimentions not two.
  20. Nov 14, 2009 #19


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    Tom is thinking that, from an external reference point, the elliptical orbit coupled with proper motion of the system results in a helical path, which he is then mistakenly calling spiral.

    Tom, the path a planet follows is entirely dependent on the observer's frame of reference. You have chosen the FoR that is from a body in relative motion along the axis of the solar system in which the planet is orbiting. By saying all orbits are [helical] you've mistakenly assumed a FoR reference that is somehow more valid than any other.
  21. Nov 14, 2009 #20


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    Hard to tell what he's thinking. He might be including some very small braking effects, such as the emission of gravitational radiation (which always happens), or Poynting-Robertson drag (which is a larger effect in most circumstances but requires the orbiting body to be warmed by the body it's orbiting so that it emits radiation. Or he might just be being silly - who knows?
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