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How did planets get their Orbits?

  1. Jul 28, 2015 #1
    An orbit is a fixed path in space around a mass. Once an object gets into an orbit, it would stay there until an outside force pulled it out. Thus, there are only two ways to be in an orbit. Either an object was already in orbit, or it inserted itself into orbit. Since a planet cannot intelligently control its motion, it could not have inserted itself into an orbit by changing its speed or direction. As soon as an object enters a gravitational field, without the ability to change its speed/direction, it would already be in some type of orbit, although in most cases the orbit would end up intersecting the object responsible for the gravitational field or leave the field. Thus, the only way a planet could be in its orbit is if it was always in orbit as soon as it entered the gravitational field of the sun. But what are the chances that an object could begin with the perfect speed, direction, and location to be in a stable orbit (not spiral into the sun or leave the gravitational field)? Even when planets formed, mass spiraled and converged into a large object because none of that mass had the correct trajectory to stay in orbit, but somehow the total mass formed into the perfect orbit around a star? Asteroids, meteors, and other objects constantly bombard planets or exit the solar system because they are not at the perfect velocities. Yet somehow planets started out with very high speeds (67,000 mph for Earth, and 100's of thousands more for its motion through the universe) with the perfect direction and location to orbit the sun. And in the case of the Earth, no initial imperfections in its orbit (i.e. not the exact speed/direction required for the given orbit), large objects colliding with it (dinosaur-obliterating asteroids or moon formation), or other masses/planets/moon influencing its orbit has caused us to deviate from our not-too-close, yet not-too-far position from the sun to make way for life, let alone spiraling into the sun. All of this after billions of years!

    How is this possible? Rocket and satellite trajectories have to be carefully planned and designed for a very specific orbit, yet even with the ability to control their speed/direction to an intelligently calculated orbit they have difficulties in staying in orbits for very long, let alone billions of years. Any deviation from a perfect orbit, or small external forces acting on them add up. But somehow there are several planets with several moons in infinitely many solar systems in infinitely many galaxies. Shouldn't all of the mass just spiral into itself eventually?
     
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  3. Jul 28, 2015 #2

    Drakkith

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    No. The orbits of planets are not 'perfect'. They are constantly fluctuating under the influence of multiple bodies. The degree of fluctuating just isn't that large compared to the size of the orbit.

    Also realize that planets and other bodies do not spiral into the objects they are orbiting unless there is something stealing orbital angular momentum from them. These could be other orbiting bodies or dust/gas.

    Rocket/satellite trajectories have to be very, very carefully planned because we want to put them into a very specific orbit. If we just launched a rocket out into space with a random trajectory, it would most likely enter a stable orbit of the Sun (barring close passes to planets and other bodies that could cause the rocket to crash into an object). This orbit could be anything from nearly circular to highly elliptical, with the exact shape depending on the trajectory and speed of the rocket. It's also possible that the rocket enters a hyperbolic orbit, which is an open orbit, meaning that we shot it away from us so fast that it won't stay gravitationally bound to the Sun. But this requires a very high velocity rocket launch. Anything under this velocity will be captured into a closed orbit.
     
  4. Jul 28, 2015 #3
    "The degree of fluctuating just isn't that large compared to the size of the orbit." is pretty close to "perfect". But you still didn't explain how they could have actually entered their orbits without an ability to control their velocity.
     
  5. Jul 28, 2015 #4

    DaveC426913

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    1] Systems (stellar and galactic) are formed from orbiting, infalling gas and dust. So, in a sense it was formed already in-orbit.

    2] Capture events, on the other hand, tend to involve multiple objects. An object on a hyperbolic path encounters an another at just the right moment. One steals energy from the other, so that one is slowed into an orbit while the other is boosted into a higher orbit, or even ejected from the system.

    3] Finally, keep in mind that, in the early chaos, most bodies (small, medium and large) were in highly unstable orbits. What we see now is only the tiny fraction that is left after the vast bulk of them have done all their colliding. If you set a random bunch of marbles going in a funnel, then preferentially removed the 99% in elliptical orbits, you'd be left with just a few that were in fairly circular orbits.

    4] Finally finally, note that - in our own system - 22% (2/9) of our traditional planets are in significantly elliptical orbits (greater than 0.2).
     
    Last edited: Jul 28, 2015
  6. Jul 28, 2015 #5

    e.bar.goum

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    You should consider how solar systems form in the first place, from a protoplanetary disk. Essentially, the planets form with the correct velocity.
     
  7. Jul 28, 2015 #6
    So the same system that made all the mass converge to stars and planets did not make those masses converge to each other?
     
  8. Jul 28, 2015 #7

    DaveC426913

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    Erm. Well, it did what it did.

    To what do you refer? Why there are multiple planets in the Solar System?
     
  9. Jul 28, 2015 #8

    Drakkith

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    Indeed. Stellar and planetary formation is a very complicated process and there are many different effects to take into account, some of which you'd never have imagined. Collisions between gas and dust particles, gravitational interactions, radiation pressure, and many, many more.
     
  10. Jul 28, 2015 #9
    Well the planets and stars were formed by spiraling gases and other masses, so why did the "clumping" of masses stop there? That is what I am referring to.

    All of those complicated processes seem to make it very unlikely that planets could have entered their current orbits, which is what I am having a hard time imagining. It would be like firing a projectile randomly into space; it would most likely just collide with something. And even if you calculated it correctly like a satellite or rocket, those have a hard time staying in orbit for a long time, especially billions of years.
     
  11. Jul 28, 2015 #10

    Drakkith

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    Well they had to form in some orbit. Their current orbits are simply where they ended up after the chaotic formation phase and are stable by necessity. After all, if they weren't stable, then they wouldn't still be in those orbits after 4.5 billion years. Note that an unstable orbit doesn't mean that the orbiting body is in danger of colliding with something, merely that it is unlikely to stay in that exact orbit for a prolonged period of time. Objects in unstable orbits simply drift away into other orbits due to complicated gravitational interactions that made their original orbits unstable in the first place.

    Not at all. A collision would be an extremely unlikely occurrence. You have to make a concerted effort to make one object hit another in space.

    The clumping didn't stop there. The vast majority of the material is locked up in the star, then the planets, then dwarf planets, and so forth, with smaller bodies having up proportionally smaller amounts of the total mass despite being larger in number. There's still gas and dust left over from the formation of the solar system, just a very, very small amount. Am I misunderstanding your question?
     
  12. Jul 29, 2015 #11

    DaveC426913

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    As the system is cleared of unstable bodies in eccentric orbits, the rate of clumping drops off to essentially zero. You are left with a certain few large bodies in orbits that are reasonably distant from each other. "Clumping" still occurs - as witnessed by the recent Shoemaker-Levy impact on Jupiter - but ity has reached a relatively stable level.

    Trust us. ;) the unlikelihood you feel is due to your mental model of how system mechanics works - you're kind of mushing all the mechanics into one. There's a lot going on, both before, during and after a stable system such as Sol. A good short book on Solar Sytstem dynamics might clear it up.

    Do not try to compare rocket missions with solar system dynamics. They're different animals. Almost all probes we launch are designed to orbit VERY closely with bodies - hundreds of miles (i.e. essentially in their outer atmosphere) - not tens of millions of miles.
     
  13. Jul 29, 2015 #12
    In fact 99% of the original nebula DID end up in one clump which is the Sun.
    The other bodies formed as pretty much random eddies within the consolidating gas/dust cloud, and there probably were many more of them early on in the life of the solar system.
    Many of these early planetoids eventually merged to become the major planets, but there still is some smaller scale leftovers out there, ranging from dwarf planets down to insignificantly small asteroids.
     
  14. Aug 2, 2015 #13
    Excellent answer!
     
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