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Rings and The Solar System

  1. Mar 18, 2014 #1


    Staff: Mentor

    Apologies for a long preamble. There is a simple question at the end of this post. Wikipedia and the PF archive were not forthcoming on this subject.

    Saturn's rings seem sharply bounded in the vertical dimension. If I got it right, the initial orbits of debris were centered on the orbits of the originating protoplasmic clouds and/or moons. But the orbits may have been scattered among many axes. However collisions between particles is the negative feedback that eliminates vertical components of momentum. In a ring, the relative velocities of particles is nearly zero. A gravitational close encounter with a large object, might disturb the orbits of particles, but the feedback mechanism would return most of them to a ring.

    In the early solar system, similar processes could apply. However, after planet formation and formation of Earth's moon, the system's population is sparse and planetary collisions don't happen. If the planetary orbits were disturbed by a gravitational close encounter, then I see no feedback mechanism to return them to approximately coaxial planes (right?). Yet, Wikipedia says that the orbits of the planets are coaxial to within 6 degrees.

    Can the small 6 degree spread in planetary orbit axes infer that no gravitational close encounter strong enough to disturb planetary orbits, has happened since planetary formation?
  2. jcsd
  3. Apr 9, 2014 #2

    Ken G

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    There are complicated possible feedbacks, you don't get collisions of planets that much, but there is always their gravity, especially Jupiter. Maybe Jupiter has a tendency to corral the planets into a plane, or maybe it ejects those that are not at the right orbital distances and in the plane, I don't know. But if not, then your supposition seems reasonable.
  4. Apr 13, 2014 #3


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    Wouldn't the considerable variation in axial tilt among the planets tend to indicate some disturbance since formation? For instance, Earth's tilt is 23°, Uranus is rotating "on its side" at 97°, and Venus rotating upside down at 177°, in retrograde. Is it to be assumed that the planets formed with these irregular tilt angles, or that something has greatly disturbed them from initial uniformity?
  5. Apr 13, 2014 #4

    Ken G

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    I don't think that's known. Take Venus, for example-- is it more plausible to form a planet that essentially doesn't spin, or to have a collision just fortuitously end up with a nearly unspinning planet? As for the direction of the spin axis, there are relatively weak and subtle forces that can act for a long time that can change that, like can happen to a top, without any violent or dramatic encounter. For example, it is often said that the presence of our Moon acts to stabilize the direction of our axis, from which it follows that without a Moon, our axis might have moved around a lot.
  6. Apr 13, 2014 #5
    One of the problems in understanding unusual rotations such as the examples you posted is that you need to know the planets entire history. Uranus rotation for example is said to be caused by a collision, The Earth is also often considered a result of the same collision that caused the formation of the moon. In rotation it comes down to preservation of angular momentum due to influences that impart momentum. As Ken mentioned tidal locking due to gravity also plays a big part in it.
  7. Apr 13, 2014 #6

    D H

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    Possibly. The Earth may have been impacted my a Mars-sized body. That is the widely accepted explanation of the Earth's Moon. Whether Venus was is dubious. Venus' orientation can be explained solely due to interactions between its thick atmosphere, the Sun, and Jupiter.

    Even if those axial tilts did result from impacts, it's a whole lot easier to change a planet's rotation than it is to change its orbit. Almost all (> 99.999%) of the angular momenta of those three planets is due to their orbits about the Sun rather than their rotations about their axes.

    Another way to look at it: A good *whack* from some impacting body can make big changes in a planet's rotation axis / rotation rate but will only make a small change in the planet's orbit.
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