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What keeps the planets in their consistant orbit?

  1. Mar 6, 2006 #1
    Even the Moon is moving away from the Earth each year by approximately 1 inch. What keeps the planets in their alignment? Why aren't they either being drawn into the Sun or pushed away(like our moon to Earth)?
  2. jcsd
  3. Mar 6, 2006 #2
    it got to do with the centrifugal force and the gravitational force. i.e. if an object goes in a circle, there is a pseudo force called centrifugal force that tries to push it away. for ex. if you attach a piece of string string to a stone and swing it, it will start going in a circle. if you let go of the string, u can observe the path it takes. so what was keeping it in the circular track while you were holding it by the string? the force you apply by holding the string exactly cancel out the centrifugal force on the stone. so it stays in track. in space the centrifugal force is compensated by the gravitational force. which exists between objects that have mass. this is a attractive force. so the gravitational force (attractive) between sun and the planet balance out (almost) the centrifugal force of that planet which in-turn keeps it in track.

    why they are aligned is a matter of minimizing the potential energy. if you drop a ball, it fall under gravity? why? because it tries to minimize its gravitational potential.
    same thing. but in large scale. since all planets are massive, for ex. earth is in the gravitational field of sun, moon, Saturn, ...all the planets + universe.
    so it's resultant of all that.

    (you can also think in terms of Newtons law: an object is either remains at rest or moves at a constant velocity unless acted upon by a net resultant force.)
  4. Mar 6, 2006 #3
    Understood but........

    ...the model you gave me works in a circular orbit. What about an elliptical orbit like Pluto?

    Why then is the Moon slowly leaving us? Since its creation, why hasn't it reached a point along the way where it is balanced or almost balanced (centrifugal force v. gravitational force.)? Will it ever reach a point?
  5. Mar 6, 2006 #4


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    If one considers their motions to that kind of precision (inches per year), then the planets are certainly not on constant orbits. In fact, the planetary orbits are unpredictable (chaotic) on timescales greater than something like a million years, if I'm remembering correctly. The discrepant motions aren't as simple as the one you're describing for the moon because they result from perturbations from all of the other planets rather than a single tidal interaction.

    Despite these motions, however, the planets do seem to stay relatively well aligned over billions of years. The reason for this is still, I think, a matter of debate, but it appears that we lucked out -- that is, the other planetary systems we've observed so far are considerably less organized and convenient for the formation of life.
  6. Mar 6, 2006 #5


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    I like the simple, anthropic principle answer: the planets are in orbits that are stable on the timeframe of at least 4 billion years because if they weren't, they'd have already collided with other objects and wouldn't be here.

    That's more descriptive than it at first seems - in the early years, the solar system actually was much more of a cosmic pinball machine. What remains is the parts that were more stable.
    Ehh, maybe its just wishful thinking, but I'd like to wait a while on that - for now a big part of the reason we're finding solar systems that aren't like ours is that we aren't capable of finding solar systems like ours.
  7. Mar 6, 2006 #6


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    I'm prone to agree, though it's an unpopular answer in the scientific community because it's not really testable and discourages one from exploring alternatives. Interestingly, though, we often take for granted that the properties of our home planet are special due to anthropic selection.

    Mostly true, and maybe I'm exhibiting some bias here, but I would be very surprised to find a plethora of solar systems as large and organized as ours. From a consideration of the dynamical stability of many-body systems and the anthropic principle, one wouldn't naively expect a lot of such systems. I would tentatively say that the eccentricities of extrasolar planets tend to be more extreme than in our solar system:

    http://jilawww.colorado.edu/~pja/planets/extrasolar.html" [Broken]

    Of course, the extrasolar planets will be typically more massive than our inner planets, so it's not a direct comparison.

    Anyway, my statement that we "lucked out" is not yet a consensus view in the community -- I don't mean to give the wrong impression.
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  8. Mar 6, 2006 #7


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    Most stars are members of binary (or greater) systems. I could see how this would disrupt the planets. The Kozai mechanism will make nearly-circular orbits non-existant in such systems.

    Although we don't have the technology yet to observe systems like our own, there are some clues in our own solar system as to the consistency of orbital systems; the moons.

    Look at how vastly different the moon systems are in our solar system. Mercury and Venus have none. I'd guess that their small Hill Spheres are primarily responsible. Lots of stars that formed in crowded clusters may share their fates.

    Earth has one moon that is appreciable in mass compared to its planet. Perhaps the Earth once had more moons but as the Moon spiraled outward it destabalized their orbits. Might this be the fate of tight binary star systems?

    Jupiter is like a miniature solar system, with 4 large moons, 3 that orbit in resonance with each other. It has its own version of an Oort cloud with a large collection of moons orbiting in a myriad of directions. Its got a collection of small interior moons as well.

    The systems of Mars, & Saturn - Pluto seem to highlight the fact that every moon system in our solar system is remarkably different from the others. I don't see any reason to believe that star systems should follow some basic template.
  9. Sep 19, 2010 #8
    I understand how planets are 'held' to keep their shape of orbit, but what force is keeping it pushing forward?
  10. Sep 19, 2010 #9

    Jonathan Scott

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    No force, just inertia (see Newton's first law).
  11. Sep 19, 2010 #10


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    Well Inertia is a great answer, but I think since this is a system of planets that are interacting with other objects in the universe that enter into it, its more of a conservation of energy and angular momentum. If you had a giant asteroid enter our solar system it would have an effect on everything, and even though the objects had an inertia, they had a conserved inertia and angular momentum prior to the huge mass entering the equation.

    Once a huge asteroid enters our solar system it will disrupt this conservation and affect every planet and the Sun, and if it was massive enough it could collapse everything into it, or if it was fast enough and massive enough it would just leave our solar system and pull other planets off their path and 'cavitate' them into each other over millions/billions of years, depending on mass and velocity of said mass entering the system.
  12. Sep 19, 2010 #11
    Ok I'm a bit closer to understanding it (I'm an Electrical Engineer not a Physicist). I've read the inertia article, so the planets are in inertial motion now yes? So did something have to accelerate them initially?
  13. Sep 19, 2010 #12


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  14. Sep 19, 2010 #13
    They formed from smaller particles already in orbit around the Sun, so nothing accelerated the planets as such. How did the stuff end up in orbit to start with, you may wonder? Conservation of angular momentum meant not all the original collapsing cloud of gas and dust could directly collapse into a star, so the rest ended up in orbit around most of it which had collapsed.
  15. Sep 19, 2010 #14
    Ok, doesn't seem enough power there to move planets, but a guy demonstrated how pulling mass in increases the spin speed, but still don't see how was the dust spinning at the start? Maybe I'll stick to Electro-Magnetism!

    I wrote this before seeing the previous answer
    Last edited: Sep 19, 2010
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