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The path of our planetary bodies

  1. Jun 26, 2005 #1
    Hello everyone,

    Its good to FINALLY be back! :biggrin: I was wondering why the path of the planets in our solar system is elliptical and not circular?

  2. jcsd
  3. Jun 27, 2005 #2
    The sun is the centre of all forces that provides the planets the necessary centripedal forces to help move around the sun, but the influence of other planets and gravitational attraction due to other planets hinders this perefectly circular path, making it elliptical.
  4. Jun 27, 2005 #3
    How much force do the other planets have on one another? Do the satelites that orbit the planets also have a small effect on the path of the planet?
  5. Jun 27, 2005 #4


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    You do not need the other planets.

    To have a circular orbit both the direction and the magnitude of the velocity of a planet has to be exactly right for the gravitational attraction of the Sun to keep them on a circular orbit.

    In general this is not so and hence the plants' orbits are elliptical.

    The question is: "Why are they nearly circular"?

    I think the answer to that lies in the way the planets accreted from the solar nebula; those not on a nearly circular orbit would have collided or been ejected by now.

  6. Jun 27, 2005 #5
    Another reason could be that the centre of mass of the solar system( sun + 9 planets) lies at one of the foci of an ellipse to maintain the equillibrium or in other words the whole mass of the solar system is distributed in such a manner that the C.M has to lie at one of the foci of an ellipse and not at the centre of a circle...

    Not to mention the planets revolve around the CM (a focus of the ellipse) that lies inside the sun thereby forming an elliptical path
    Last edited: Jun 27, 2005
  7. Jun 27, 2005 #6


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    I'm not sure what sort of answer you're looking for. The simplest one I can think of is that it's because gravity is an inverse square law central force.

    The fact that gravity is a central force means that angular momentum is conserved. This provides one constraint equation. When you add in the fact that the force law is an inverse square one, the law of conservation of energy plus the law of the conservation of momentum from the fact that gravity is a central force completely determines the shape of the orbit.
  8. Jun 27, 2005 #7
    Pervect, the answer you gave make sense to me.

    Panthera, the solar system resting on one foci of an ellipse rather than the center of a circle fits. If we are at one foci of the ellipse, then what is one the other?

    Why don't our planets orbit the sun the way an electron would a nucleus? Is it because the paths would be so unpredictable there would be a high likelyhood of our planet colliding with one of the others?
  9. Jun 27, 2005 #8
    hey i posted 3 times..1st the connection went off, then server problem and now i am tired...well i am posting wait
  10. Jun 27, 2005 #9
    we are not at one of the foci but our(solar system's) CM is....the mass distribution of the solar system is not uniform at one instant of time and its such that the CM lies at one of the foci of an ellipse...for example consider that you put different masses of balls in a bag ..the bag will tilt towards the heavier part (as the gravitational attraction will be more there)and CM wont lie at the center but somewhere at the side (near the massive part) similarly the plane having the sun and the planets have nonuniform mass distribution and the CM atomatically adjusts itself (to maintain equqillibrium) in such a way that it results in an elliptical plane with sun having the CM( its well known fact that the CM lies inside the sun)...and the planets revolve around the sun maintaining that equillibrium set by the mass distribution or in other words in an elliptical orbit..

    hope its clear
    ps- i am really tired and my explanation got modified as this is the 4 th time...
  11. Jun 27, 2005 #10


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    If you are asking why the planets are more-or-less in a plane (the ecliptic), I'd say it's because the sun and planets condensed from a swirling cloud of gas.

    This swirling cloud of gas has an axis around which it swirls. The plane perpendicular to this axis is the plane that most bodies orbit in. There are a few oddball planets like Pluto, which have to be explained.

    See for instance

  12. Jun 28, 2005 #11
    I know how this is. :grumpy: Believe me I have lost a few posts myself because of a sketchy internet connection. I hope your connection this time is more stable. :smile:

    Your post answering my question makes sense. Just to make sure I have it, it's not so much we are on one of the foci of the ellipse but we form the ellipse with the sun as the foci and us traveling in the ellipitcal path around it.
    If you are asking why the planets are more-or-less in a plane (the ecliptic), I'd say it's because the sun and planets condensed from a swirling cloud of gas.

    I don't know much about the formation of the planets. I did know about the swirling cloud of gas, but wasn't there some rock like matter that formed with the gas to create the planets? The plane perpendicular to the axis on which the planet are on now right? So did the planets move from where they were formed to wear they are now? :uhh:
  13. Jun 28, 2005 #12


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    Right! That's one of Kepler's laws, there are also two other parts to the laws, one of which is "equal areas are swept out in equal times".


    Before we get off on a tanget:

    I'm not sure if I understood the rest of your question

    I took a guess and interpreted this as you asking why the planets were all in the same plane. (It wasn't clear to me what you meant by "not like an electron would a nucleus", most pictures of atoms don't have the electrons all in a plane, so I guessed this was what you were asking about). Was that in fact your original question?
  14. Jun 28, 2005 #13


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    I wonder if many exo-solar systems have planets orbiting like the traditional picture of an atom, all in different planes. Most star systems are binary. And if they're wide binaries, with the stars orbiting in different planes than the planets, a few billion years should give the Kozai Mechanism enough time to completely scramble the planets' inclinations.

    I would guess that a wide binary, even if it formed with the stars and planets in the same plane, could have the stars out of plane with the planets, just like Oort Cloud members aren't necessarily in the plane of our Sun's planets. Stellar passages would pull the wide member into a different inclination.
  15. Jun 29, 2005 #14


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    I really haven't thought about the binary star situation very much - thanks for an interesting post, I'll have to think about your ideas more before I can give any sort of coherent answer.

    I do hope we haven't scared Miss Kitty too much :-).

    I did find an interesting illustration of the Oort cloud - it's apparently thought to be non-uniform due to tidal forces, so it's not spherically symmetric, though it's definitely not planar. I'm not sure if we have any actual data on the distrubution of bodies in the cloud, or whether this is all just theory at this point.

  16. Jun 29, 2005 #15
    Ah!! YAY the brain does work during summer holiday! :biggrin:

    You got it. I wasn't sure how best to articulate my thought so I took a a stab at it and hoped someone might get it. :smile: So yes, why are all the planets on one plane with one orbit, but the electrons of an atom are flying around in many different paths? :bugeye:

  17. Jun 29, 2005 #16
    Whoa! There's a few phrases people are tossing out here and I have NO clue what they mean. :bugeye:

    Could someone please explain what these are?:
    Exo-solar system
    Binaries/wide binaries
    Kozai Mechanism

    I briefly skimmed the article Pervect posted with the picture of the Oort cloud and the brief description of what it is. Is there an Oort cloud surrounding our galaxy? How many Oort clouds exist that we know of? How were they discovered, when, and by whom?

    Nah, you guys aren't scaring me with this. Don't worry. :smile:

  18. Jun 30, 2005 #17


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    Planets of other stars. A fair number of them have been discovered in the last few years by astronomers.

    Pairs of stars that orbit each other under gravity. Some are farther apart (wider) than others.

    Don't know this. I'll google it and see what I can add in edit.
    (added in edit) See http://www.spaceref.com/news/viewsr.html?pid=16463, it's a mechanism to explain the fact that most exo solar planets have highly eccentric orbits. That just means their orbits are long thin ellipses instead of round fat ones like the orbit of the Earth. The Kozai mechanism says that perturbations (repeated weak gravity pulls) from a binary companion star to their parent star could make this happen.

    Duh, by Oort. He was a Dutch astronomer who figured out that the orbits of the comets all had an "other end" at similar distances from the Sun. So he figured there was a huge ring of cometary material circling far out beyond the orbit of Pluto (or Neptune!), and this came to be called the Oort Cloud. Oort clouds are harder to see than planets. I don't know if even ours has ever been physically detected in situ. BTW it's around our Solar System - the Sun and the planets, not our galaxy - the milky way with 100 billion stars.
    Last edited: Jun 30, 2005
  19. Jul 1, 2005 #18


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    And although that link doesn't mention it, it can also tinker with the the planet's inclinations BIG TIME (up to 90 degrees).
  20. Jul 1, 2005 #19
    What do you mean it can alter their orbits up to 90 degrees? Some planets will orbit their suns perpendicluar to the other orbits?
  21. Jul 1, 2005 #20


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    Actually, that's what I was wondering. I don't believe we've found any exo solar systems with planets in multiple planes yet.

    Jupiter has dozens of outer moons orbiting it. Many are in highly inclined orbits. But around 60 degrees it ends. There are no moons in polar orbits.

    Under certain circumstances, the Kozai Mechanism causes a periodic exchange between high inclination and high eccentricity. The orbit of a moon in a 90 degree orbit would become so elliptical that it would crash into Jupiter. Slightly less than 90 degrees would cause it orbit at times to become elliptical enough that it would pass through the inner moon region (Io, Europa, Ganymede, Callisto) where it would be gravitationally ejected from the Jovian system, or crash into one of those moons.

    So although the Kozai Mechanism is capable of forcing things into polar (90 degree) orbits, these orbits are fatal.

    I don't see any reason that wide binary star systems should behave any differently.
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