Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why is it that Mercury rotates at a faster rate than say Pluto

  1. Mar 15, 2017 #1
    If dark matter is responsible for the inner stars within a galaxy rotating at the same rate as the outer stars, then why is it that murcury rotates at a faster rate than say pluto.This posses the question why are we searching for evidence of dark matter deep in the earth when there seems to be no evidence of it present in our solar system.
     
  2. jcsd
  3. Mar 15, 2017 #2

    phinds

    User Avatar
    Gold Member
    2016 Award

    The density of dark matter is such that its total effect on the solar system is that of a modest sized asteroid. In other words, basically zero. The galaxy is MANY orders of magnitude larger.
     
  4. Mar 15, 2017 #3

    Bandersnatch

    User Avatar
    Science Advisor
    Gold Member

    Do you mean the underground detectors? They're placed underground not because dark matter is expected to be more abundant there, but because Earth provides good shielding. You want your detectors to be free from interference from radiation, while being able to catch the hypothetical dark matter particles. The latter don't interact with baryonic matter, so the Earth is transparent to them.
     
  5. Mar 15, 2017 #4
    Yes I realise why the detectors are underground to filter out the unwanted particles down from hundreds an hour to just a few a week but Dark matter makes up 85% of our universe,carrys mass and is present through out our galaxy and beyond locking stars in fixed orbits but not so with our solar system.
     
  6. Mar 15, 2017 #5
    Ahhhh just read cdms Berkceley.
     
  7. Mar 15, 2017 #6

    Drakkith

    User Avatar

    Staff: Mentor

    The distribution of dark matter is such that it forms a spherical "halo" around the galaxy. This halo is made up of dark matter which is simply on the outermost portion of its orbit, the portion where it is traveling the slowest (just like how Pluto's orbital velocity is much smaller than Mercury's because of its increased distance from the Sun). From there it falls inwards, gaining speed and making a pass through the galaxy before slowing down again on the other side. Because the speed of its orbit is so much slower on the outer portions, dark matter spends most of its time out there. So there is relatively little dark matter present in the inner portions of the galaxy (including within our solar system) and its effect on the orbits of the planets around the Sun is negligible.
     
  8. Mar 16, 2017 #7

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    This is not correct. Dark matter has a distribution that is typically much denser towards the center of the halo. It would be true if all dark matter particles had the same energy, but they do not. Still, at solar system scale, luninous matter is dense enough to dominate completely.
     
  9. Mar 16, 2017 #8

    Drakkith

    User Avatar

    Staff: Mentor

    I'm not quite seeing how that contradicts my post. While the density is higher, isn't the amount of dark matter near the center only a small percentage of the whole halo?
     
  10. Mar 16, 2017 #9

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    I think your post makes it sound as if there is essentially a spherical shell of dark matter outside Earth's position and that dark matter will be more abundant in the outer regions. That the halo is spherical just means that it is spherically symmetric. That the dark matter in the center is a minor part of the whole halo does not mean that it is not at its densest there and the number you get depends on how you define "near the center".
     
  11. Mar 16, 2017 #10

    Drakkith

    User Avatar

    Staff: Mentor

    Ah, I see your point.

    Alright, so it would be better to describe it as spherically symmetric instead of a spherical halo, and that the density increases as you move inwards.
     
  12. Mar 16, 2017 #11

    phinds

    User Avatar
    Gold Member
    2016 Award

    @Orodruin:
    First, I don't understand "center of the halo" ... sounds like a contradiction of terms.
    Second, I was under the impression that the density of dark matter was higher IN the halo than in the center of the galaxy because of what Drakkith pointed out in post #6. Subsequent posts have not clarified for me why that is not the case. You even say that there is less DM at the center but "does not mean that it is not at its densest there". Why is it more dense in the center? I'm just not getting it. Can you clarify?

    EDIT: the link that Sean provides in the post directly below says the same thing you are saying but offers no justification so I'm no closer to understanding WHY it should be more dense in the center.
     
    Last edited: Mar 16, 2017
  13. Mar 16, 2017 #12
  14. Mar 16, 2017 #13
    There is even theorys that black holes are made of dark matter, this throws doubt on my personal view that black holes contain universes. Or does it .
     
  15. Mar 16, 2017 #14

    phinds

    User Avatar
    Gold Member
    2016 Award

    It is irrelevant whether BHs are made of dark matter although to think that most of their contribution was dark matter has been debunked here many times and the thought that BH's contain universes is just silly.
     
  16. Mar 16, 2017 #15
    Throughout the universe most things work out dense towards the centre I surpose.
     
  17. Mar 16, 2017 #16

    phinds

    User Avatar
    Gold Member
    2016 Award

    You are missing the point Drakkith made in post #6, which has been made on PF many times before and explains why there is a lot more DM in the HALO.

    Now that I think about it though, I do see that the argument does not directly imply that it is more DENSE in the halo. I'll wait to see what Orodruin has to say.

    LATER: Thinking about it more, I think I'm beginning to see how/why it can be (probably is) more dense in the center even though there's much more in the halo. First, obviously there is WAY more volume in the halo but that just supports the "more in the halo", not "more dense in the halo". Second, even though the particles are traveling very fast in the center and very slow in the halo, still, they ALL go though the center whereas they are enormously spread out in the halo. @Orodruin, have I got this right? @Drakkith what do you think?
     
    Last edited: Mar 16, 2017
  18. Mar 16, 2017 #17
    I think the calculations and observations show that the main force in our solar system is our sun and that all the dark matter in our solar system does not constitute a mass greater than our sun where as dark matter is more dense towards the centre of our galaxy creating a force upon the stars.This is how im seeing it let me know if thats wrong.And if it is wrong then my question still stands.Or is it that we just live in an area where dark matter is scarce.
     
  19. Mar 16, 2017 #18

    phinds

    User Avatar
    Gold Member
    2016 Award

    see post #2. That has nothing to do with the discussion of the last several posts which are about the density of DM throughout the galaxy.
     
  20. Mar 16, 2017 #19

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    The dark matter halo is not something surrounding a galaxy, it is a background that extends inside and beyond the galaxy and has the highest density in the center. Typical models for the dark matter density include the NFW and Einasto profiles. The NFW profile is essentially a broken power law diverging as 1/r towards the center. This is essentially what is expected from cold dark matter, but not very well in line with observations that suggest a more cored profile (density going towards some constant towards the center, the Einasto profile essentially describes such a situation.
    See: https://en.wikipedia.org/wiki/Navarro–Frenk–White_profile
    and: https://en.wikipedia.org/wiki/Einasto_profile
    That halos seem to be cored more than cusped is one of the small scale structure problems (called the core vs cusp problem - very imaginative).

    This image is taken from http://inspirehep.net/record/1332709/
    Figures_CDM_zoom.png
    and is essentially what you would expect from the halo of a dwarf galaxy in the CDM case. (Red represents higher density.)
     
  21. Mar 16, 2017 #20

    phinds

    User Avatar
    Gold Member
    2016 Award

    Ah ha. So at least part of my problem is that I've been using a mistaken definition of "halo". Thanks.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Why is it that Mercury rotates at a faster rate than say Pluto
  1. Faster than light? (Replies: 12)

Loading...