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Why can dark matter make our planets move in same velocity?

  1. Oct 3, 2015 #1
    I read dark matter makes the stars in a galaxy to move about a same orbital velocity.
    Why isnt this applicable to our solar system?
     
  2. jcsd
  3. Oct 3, 2015 #2

    mfb

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    The amount of dark matter in our solar system is negligible.
    Dark matter doesn't interact via electromagnetism, so there is nothing that would let it clump together on such a "small" scale.
     
  4. Oct 3, 2015 #3

    phinds

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    It IS applicable to our solar system. Why do you think it is not? Our solar system (the Sun being "the star" in question) orbits the center of the Milky Way in a way that is different than what it would do if there were no dark matter. Of course, this is totally aside from the fact that if there were no dark matter there most likely wouldn't BE a Milky Way, but that's a separate issue.

    EDIT: I see that mfb interpreted your question differently (and more likely in the way you meant) and he is also right. Look again at what you read. It does not say that Dark Matter affects the orbits of planets in a solar system, it say that it affects STARS in a galaxy, which is the point I was addressing.
     
  5. Oct 3, 2015 #4
    Then does a heavy object with more mass like black holes warp more dark matter towards it?
     
  6. Oct 3, 2015 #5

    phinds

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    Black holes attract all matter, dark or otherwise. As far as is known, dark matter acts gravitationally no differently than normal matter.
     
  7. Oct 3, 2015 #6
    Then why is dark matter is evenly distributed in galaxies when they should be found more in the centre where a supermassive black hole is believed to exist?
     
  8. Oct 3, 2015 #7

    mfb

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    The density is expected to be slightly higher there, but the same as for our solar system applies: dark matter gets attracted, makes a single pass close to the black hole and flies away again as there is nothing that would brake it.
    The black hole is tiny compared to the galaxy, so direct impacts are very rare.
     
  9. Oct 3, 2015 #8
    I know dark matter is unaffected by electromagnetism..But shouldnt it interact with gravity and thereby get attracted to the black hole?
    And dark matter is present everywhere so why isnt direct impact possible?
     
  10. Oct 3, 2015 #9

    phinds

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    Dark matter is not evenly distributed, it exists more in the "halo" at the outer edges of a galaxy than it does near the center, black hole or no black hole.

    The reason for this is that dark matter is collisionless and that mean that unless it actually hits the event horizon of a black hole, or comes very close to it, it just keeps on going. If you look at the travel curve showing distance from the center, it's a sine wave, with the speed near the black hole large and the speed out in the halo small. That means the dark matter particles spend more time in the halo than in the center of a galaxy.
     
  11. Oct 3, 2015 #10

    phinds

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    Regular matter, when it comes near a black hole, collides with other matter and forms an accretion disk before falling in. Dark matter doesn't do that.

    EDIT: there are stars orbiting the black hole at the center of the Milky Way. Using your logic, they should fall into the black hole, but they don't. You seem to believe there's something different about black holes than there would be for the same amount of mass that was not in the form of a black hole.

    As far as anything well outside the black hole is concerned, the fact that it's a black hole is irrelevant. All that matters is that there's a lot of mass with a strong gravitational attraction.

    EDIT: EDIT: "why isnt direct impact possible" You need to look up the characteristics of dark matter. No impact is one of the characteristics.
     
  12. Oct 3, 2015 #11
    Thank you mfb and phinds for your valuable info.
     
  13. Oct 3, 2015 #12

    Drakkith

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    Direct impact of dark matter with a black hole is certainly possible. It's just that the overwhelming majority of dark matter that falls towards a black hole simply 'misses' it since the black hole is very small compared to the vast distances we're talking about here. Most dark matter simply comes in, swings around the black hole, and then flies right back out, much like how most comets come into the inner solar system, swing around the Sun, and then fly back out to the outer reaches. Only dark matter with a very specific velocity and direction would fall at just the right path to hit the black hole directly.
     
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