Why planets in the solar system lie almost in a plane

  1. planets in the solar system lie almost in a plane. Is this because the sun is rotating on itself ?
  2. jcsd
  3. mfb

    Staff: Mentor

    The whole solar system had a (small) initial angular momentum. Everything else (in particular, motion perpendicular to our disk) could dissipate via collisions, but angular momentum has to be conserved, which gives orbiting things (and most of them roughly in a disk) as solution. Without that angular momentum, everything would have fallen into the sun long ago.
  4. i dunno what the qualification "on itself" means. the Sun rotates.

    i think the reason why most of the solar system is mostly co-planar is similar to why most galaxies are mostly disk-shaped (all stars are close to a common plane). it has to do with the initial primordial turbulence from the big bang. turbulent motion has swirls in it. some swirls are bigger than others. as gravity pulls things together, how might you expect to see these swirls collapse?
  5. Drakkith

    Staff: Mentor

    I don't think this is correct. I don't think the expansion that happened after the big bang could generate turbulent flows because it is an expansion. Nothing is moving through or against anything else. Everything just gets further away from everything else. What did happen was small quantum fluctuations led to areas of high and low density, with the high density regions eventually collapsing under gravity.
  6. Isnt the solarsystem the remnants of an accretion-disc from the sun during its birth? Maybe someone has already mentioned this but I mention it anyways :)

    And what I want to point out by this, if this is the case, is that then it seems logic that all the planets orbit around the sun in much the same plane :)
  7. I think I'm wrong about this, but in GR, are not all the planets lie on different (lower and upper) regions of the "gravitational well" of the sun's spacetime curvature? I might be mistaken so I apologize, but can anybody clear this up?
  8. Drakkith

    Staff: Mentor

    The gravity well extends in 3 dimensions, so there aren't any "lower and upper" regions unless you're talking about the strength of gravity. You aren't thinking of the bowling ball on a trampoline analogy are you? If so that is...barely accurate.
  9. Hahaha oh right, I forgot that analogy is a less-than accurate picture of GR! Yes I admit I was thinking about the ball-trampoline analogy! They show too much of that on documentaries, which I always watch, so it's stuck in my mind. Sorry!

    I always picture that the Earth, or any massive body, is actually pulling spacetime around it towards its center, opposite of that picture of the analogy, to get the better sense of GR, but the analogy gets in my head at times.

    So in any case, what exactly is happening? Are the planets on the same plane? So are there any "gravitational wells" existing at all?
  10. Drakkith

    Staff: Mentor

    No, each planet is on it's own plane that is slightly different. All that means is that the inclination of the orbit is different than other planets. This inclination is constantly changing thanks to the gravitational interaction of everything else on the object though. No body stays in the exact same plane forever.

    From wiki on Gravity Well:

    A gravity well or gravitational well is a conceptual model of the gravitational field surrounding a body in space. The more massive the body the deeper and more extensive the gravity well associated with it. The Sun has a far-reaching and deep gravity well. Asteroids and small moons have much shallower gravity wells. Anything on the surface of a planet or moon is considered to be at the bottom of that celestial body's gravity well. Entering space from the surface of a planet or moon means climbing out of the gravity well. The deeper a planet or moon's gravity well is, the more energy it takes to achieve escape velocity.


    A gravity well is just a model to help understand gravity. It isn't a "real" thing if that makes any sense. It's like having a graph of the force of gravity. The graph is a model on a piece of paper or a computer, it isn't "real" in the sense that the graph actually exists out there in space.
  11. La Place hyposis...
    a large moleculer cloud, once it passes Jeans mass, starts to collapse due to gravity.
    this collapse, starts to cause a rotation , as more collapse occurs it spins faster and forms a disc( circumsteller disc/solar nebula).
    it is form this disc the mass(gas+dust) is accreted to form planets etc.
    so all the material has a prograde rotation. and are mainly in the ecliptic plane.
    there is however the ' the angular momentum problem' within this hypothisis..
    in a collapsing rotating system such as this . the sun should be rotating much faster then it currently is. it is thought that the angular momentum was disipated by viscus drag to the outer part of the circumsteller disc during plantery formation , and this is thought to be why the planets have a greater angular momentum then in a 'simple' rotating sysytem would give them.
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