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Do scientists know why planets orbit stars on the same dimentional plane?

  1. Jul 18, 2012 #1
    Pretty straight forward. The same generally holds true with moons belonging to the same planet. If we don't have concrete reason why this occurs, are their any strong theories?
  2. jcsd
  3. Jul 19, 2012 #2

    Simon Bridge

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    The strongest theory has been that the planets and their stars formed from eddies in a spinning cloud ... so you'd tend to get the orbits all in about the same plane and more likely than not to have them all in the same direction.

    The trouble is that extra-solar planets seem to come in all sorts of combinations of orbits. Need more data to sort it out.
  4. Jul 19, 2012 #3
    To amplify Simon Bridge's answer, moons are dragged toward the plain in which the planet they orbit orbits its star. So moons outside the elliptic are either recently captured or had a recent near collision or collision with another object. The rings of Saturn are a sufficient sample to "prove" this.

    As for planets of non-multiple stars, we have no idea. In the solar system we can probably blame the innermost planets being in roughly the same plane on Jupiter. It is not a star, so the sun is not part of a double star, but it is probably close enough to act on the orbits of the inner planets. More likely it is the tidal effect from Jupiter, plus the fact that the sun is an oblate spheroid (fatter at the equator) that gets the inner planets to behave. Take the asteroid belt and compare to Saturn's rings. The asteroids inclinations are all over the place.

    Going out in the solar system, Uranus famously rolls on its side, and Pluto and the other Kupier belt objects have the same sort of scatter (or more) compared to the asteroid belt.

    What about all those exoplanets that have been discovered? The two main techniques used to find exoplanets do not, in general, tell us about the inclination to the parent star, or to other exoplanets in the system. That data gets winkled out later if at all. But already some exoplanets have been discovered that orbit the parent star in the direction opposite to the star's rotation.
  5. Jul 20, 2012 #4
    One way of thinking about it is that if you have gas and dust in wildly different inclinations they will collide and gravitationally interact with each other until they fall into the same plane.

    It's pretty standard for gravitationally bound objects to form disks, you see this in galaxies and in gas disks around black holes.
  6. Jul 20, 2012 #5
    Collisions will not result in particles or planets ending up in the same plane. Just the opposite. First if a particle collides with a disk, conservation of momentum means that the momentum normal to the disk will still be there after the collision. What if two co-planar objects collide? Both can/will now leave the plane of the disk. The net momentum normal to the disk is zero, but that doesn't prevent the two particles (or protoplanets) from carrying away large amounts of momentum normal to the disk, even though the sum must be zero.

    So how does the accretion disk around a black hole form? The gas is hot enough that most of the energy/momentum of a collision is carried off by photons, from infrared well up into the gamma ray spectrum. Note that near the black hole, it is impossible for a normal matter particle to collide with the accretion disk. The radiation is so intense that matter not in the disk gets accelerated away normal to the disk. (See jets from black holes.) Note that it is possible for matter inside the event horizon to still become part of the jet. Depends on the geometry, but the event horizon is the point at which a particle, even traveling at the speed of light can't escape. But the boost from disk photons can constantly add energy and momentum to such a particle.
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