Dark Matter is Denser in the Solar System

Article Updated: 24 Dec , 2015
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Dark matter was theorized to exist relatively recently, and we’ve come a long way in understanding what makes up a whopping 23% of our Universe. Our own galaxy is surrounded by a halo of dark matter that adds to its mass. A recent paper on the dark matter closer to home – right here in our own Solar System – reveals that it is denser and more massive than in the galactic halo.

Dark matter is just plain weird stuff. It doesn’t give off light, has mass and reacts gravitationally with “normal” matter – the stuff that we and our planet and the stars are composed of. Just like normal matter, it “clumps” up, or accretes, because of this gravitational attraction; we find more dark matter near galaxies than in the vast expanses between them.

Dark matter isn’t just far off in the Milky Way or somewhere on the other side of the Universe, though: it’s right here at home in our Solar System. In a recent paper submitted to Physical Review D, Ethan Siegel and Xiaoying Xu of the University of Arizona analyzed the distribution of dark matter in our Solar System, and found that the mass of dark matter is 300 times more than that of the galactic halo average, and the density is 16,000 times higher than that of the background dark matter.

Over the history of the Solar System, Xu and Siegel calculate that 1.07 X 10^20 kg of dark matter have been captured, or about 0.0018% the mass of the Earth. To get a handle on this number, the mass of Ceres – the largest object in the asteroid belt between Mars and Jupiter – is about 9 times this amount.

Siegel and Xu calculated how much dark matter the Solar System has swept up over it’s 4.5 billion-year lifespan by modeling the composition of the background dark matter halo in the orbit of the Solar System around the galaxy, and calculating just how much dark matter would be trapped by the Solar System as it moves through this halo. They ran this calculation for the Sun and each one of the eight planets separately, giving the distribution of the matter throughout the Solar System, as well as the total amount captured.

Much like when you drive your car through a light snowfall, dark matter “sticks” to the Solar System when it is gravitationally bound by the Sun and planets. Just as some of the snow melts on your windshield (hopefully), some doesn’t stick to the hood and most just flies right by, dark matter isn’t distributed evenly throughout our Solar System, either. Some planets have more dark matter surrounding them than others, depending on where they are. Shown below is the density distribution of the dark matter in the Solar System

The first spike is Mercury, and the next two spikes are Venus and Earth (Mars doesn’t show up). The next is Jupiter, followed by a small bump from Saturn and finally Uranus and Neptune combined create the last small bump.

How does the local dark matter effect interactions in the Solar System? Well,it doesn’t have a large effect on the orbits of the planets, nor does it slow down the Solar System in its orbit around the galactic center appreciably.

“Planetary orbits, if there were enough dark matter present, would have their perihelia precess faster than if there were no dark matter. The amount of dark matter allowed from these observations is considerably greater than the amount I predict. The errors on the measurements of perihelion precession are in units of hundredths of an arc second per century…Even if you assume the dark matter is at rest with respect to the galaxy that the Solar System moves through (which is the extreme example), the Sun is of order 10^30 kg; capturing a 10^20 kg clump of dark matter will slow you down by about 20 microns/second over the lifetime of the Solar System. So that would be small.” – Ethan Siegel in an email interview.

And, alas, the mystery of the Pioneer anomaly is not going to be solved by this revelation, as the mass of the captured dark matter is not enough to explain the odd motions of that spacecraft.

The discovery of a higher density and mass of dark matter in our neighborhood may aid in the study and detection of dark matter, though. Knowing the mass and density distribution of the local dark matter – and thus knowing how much and where to look for it – will provide astronomers looking into solving exactly what it’s made up of with more information .

“Our determination of the local dark matter density and velocity distribution are of great importance to direct detection experiments. The most recent calculations that have been carried out assume that the properties of dark matter at the Sun’s location are derived directly from the galactic halo. By comparison, we find that terrestrial experiments should also consider a component of dark matter with a density 16,000 times greater than the background halo density,” wrote Xu and Siegel.

Source: Arxiv, email interview with Ethan Siegel


39 Responses

  1. LLDIAZ says:

    Correct me if I’m wrong but I thought we had already observed dark matter. Remember the picture with the big ring around the galaxies it said something about there being a cosmic collision of galaxies in which dark matter created this ring like structure.

  2. RL says:

    Has anyone actually measured or deduced the presence of dark matter in our solar system? I thought that the prseence of dark matter was detected in regions outside of galaxies. Not local. Is this model just the product of a scientists “What if” model or is it based on observations? I’m confused on this point. Any help is appreciated.

  3. Adam says:

    “The first spike is Mercury, and the next two spikes are Venus and Earth (Mars doesn’t show up).”

    I don’t see a spike at or near 1au. It looks like mercury, venus, and jupiter or saturn, nothing else.

  4. Bill says:

    This is highly conjectural and totally math-based. This is not credible research.

  5. Ethan Siegel says:

    RL, this is theoretical work. It’s mine, so I know a little about it. The spike, Adam, comes at about 60% of the planet’s distance from the Sun, so Earth’s spike is first evident at 0.6 AU.

    This is the amount of dark matter that *definitely* gets captured by the Solar System; it works out to about 700 kilograms/second. An interesting question, which our next paper will seek to answer, is just how much of this dark matter remains in our Solar System, as opposed to how much gets kicked out after it gets captured.

  6. James says:

    I’m still really confused as to how we know anything about dark matter if all it does is react gravitationally with normal matter.
    How do we even know it exists, and furthermore, how can such figures be drawn as to its whereabouts?
    I also don’t understand how we can say how much of the visible universe is composed of normal matter, dark matter, and then dark energy. How do we know how much ‘junk’ is in the universe, anyway? What if all we see is all there is?
    If what we say we know about dark matter is true, though, maybe it’s a piece of the puzzle in understanding why gravity exists.

  7. John Mendenhall says:

    Ethan, when you say theoretical, do you mean “If there is dark matter, and if it is captured locally, then these are the amounts that should be (have been?) captured?”

    I like the work, it’s just that that is a big “if” right at the start.

  8. RL says:

    Thanks Ethan. I hope to read your paper one day. Its great to hear from one of the authors.

    Is there a way to test your theory by possibly predicting the location of dark matter and attempting observations of that region or even designing a probe to go out and find it. It seems to me that this research could be a very useful tool for determining what dark matter is and its properties. And for settling any debate on whether it really exists.

  9. If dark matter exists within our solar system, would there be a way for us to send a space craft and “pick some up?”

  10. Damo says:

    A really basic question for Ethan or anyone who understands this stuff. At the top of this article the author says dark matter has mass & reacts gravitationally with matter.

    So, what is the gravitational reaction? Does “normal” matter exert a force on dark matter? And how do you measure it or how do you deduce it?

    What do you measure to say that at 0.6 AU there is higher concentration of dark matter than somewhere else?

  11. Death From The Skies! says:

    “I’m still really confused as to how we know anything about dark matter if all it does is react gravitationally with normal matter.”

    It may not exist. It just happens to be the leading theory to explain the problem of so much missing mass in the universe.

    An alternate theory suggests an unknown particle referred to as “gravitons” as the solution.

  12. Ayti says:

    Does dark matter occupy the same space as normal matter simultaneously? Perhaps we don’t need to send a spacecraft anywhere maybe DM is touching us right now.

  13. John Mendenhall says:

    Questioners above, there is a long string of ” ifs ” in dark matter. Try the WIKI article for a quck summary, with the usual caveats about WIKI.

    There are plenty of people, including Y.T., with a lot of reservations about DM. But it’s the best explanation going for a lot of problems.

  14. sundukan says:

    cool , so what can u do with dark matter??

  15. One Skunk Todd says:

    Professor Farnsworth uses it to power starship engines.

  16. John Mendenhall says:

    LDIAZ, the notorious ‘picture’ is of where the dark matter has to be, if there is such a thing, to produce the observed lensing. It is NOT a picture of DM.

  17. JamesB says:

    Dark matter isn’t even a theory, it’s a hypothesis which makes it not just uncertain but unlikely to exist.

    It’s one attempt to explain certain phenomenon that has been observed but not understood. The more phenomena the hypothesis explains the closer to a theory science gets. And the more complete the explanation provided by the hypothesis, the closer to a theory science gets as well.

    Anything you see on the subject of dark matter at this point in time is “hypothetical” and not “theoretical”, and although entertaining should be taken with a grain of salt.

  18. LBFay says:

    JamesB, it’s reminding me to Darwin’s theory of evolution (or I should say “hypothetic”?)

  19. Chuck Lam says:

    Dark energy, dark matter or the something out there we really don’t understand may be stuff of the “big bang.” Forget the math and paradigms for a moment. The BB could simply be dark energy morphng or condensing in some strange way into possibly dark matter which in turn morphs into the visible universe over a lengthy period of time. This morphing, for lack of a better word, BB could be going on right now on a time scale we can’t measure. The classic BB theory of matter “bursting forth in an instant from a small point” may be totally wrong.

  20. Graham says:

    Yes, that’s the point about science and the journey from hypothesis to theory – and the aspect that politicians and others get wrong. In science we start with a hypothesis. It accumulates evidence and the probability of it being correct is seen to increase. We start to talk about “belief” in the theory. We never mean belief in a religious sense (ie that perceived probability never equals 1); there should always be that pinch of salt. The ability to behave as if a theory were correct, but at the same time have a gremlin sitting on one shoulder whispering “rubbish, this is a house of cards” is critically important. Quite difficult to square with a public desire for certainty and a successful career in science though – groupthink can be powerful.

  21. Don Cox says:

    The distribution of Dark Matter seems to closely match that of Stupidity.

    Could they be the same thing?

  22. Chuck Lam says:

    To James, Yes! I am having fun with science. Hmm . . . didn’t Einstein admit he was wrong abour a few astrophysical things? Also . . . isn’t the “big bang” just another abstract thought in an infinite number of theories? To R2K, Keep in mind that speculation is among the first steps of progress. .

  23. Tony Trenton says:

    All +ve energy is attractive. This is fundamental.

    Everything is scaleable from infinitely small & dense to infinitely large & rarified & every condiion in between.

    All +ve matter are condensed portions of the overall Electromagnetic field. Created during the early moments of the Big Bang.

    The dark energy IS the overall rarified Electromagnetic field.
    If this became matter it would no longer be dark!

    Gravity waves are the variations in the density of the EMF on a very large scale.
    Such as those delineated by the spiral arms
    of galaxies.

    Tthis is consistant with the nature that we observe.

    There are two fundamental forces:

    The Electromagnetic and Turbulence

    That is all we nead .

  24. R2K says:

    So sad to see astronomy go from very elegant and rock solid work on the nature of stars and the evolution of the universe… to lots of speculation on things we simply dont understand yet. Speculation is great, but people are way too confident in their work sometimes.

    Something isnt rotating the way it should? Invent something called dark matter that must be there, rather than saying “We dont understand this. We dont know yet. Lets keep trying.”

    Having said that, if dark matter is real, that mass makes it sound like comets are to blame. Thus dark matter, and dark energy (comets pulling probes in the direction of the oort cloud) are caused by comets. QED and Quid Pro Quo.

  25. CookieMonster says:

    I agree with R2K. Didn’t Einstein come up with the “cosmological constant” to explain why his relativity theories didn’t work set against an expanding universe?

    Dark matter / energy is a fudge theory, because we don’t have any other explanations for something we don’t understand.

    It’s also very comforting to know that we don’t understand everything, and that there are fundamental parts of fabric of the universe which are completely unknown.

    The laws of physics are not complete yet.

  26. James says:

    I think Graham said some pretty important words.
    Chuck, are you an astrophysicist? Since you posted the theory here, I’m going to assume you’re not. Your theory, while interesting, could possibly already be disproved with things we already know. Things that make logical sense to you usually don’t work out in the real world of physics, unless you’re Einstein.
    Sure, you completed step one of Graham’s scientific method, but can you do the rest? Throwing out random hypotheses is really just having fun with science. There are approximately an infinite number of theories like that. “Maybe we’re wrong and it’s actually…” Yeah, maybe.
    R2K, if you think about it, dark matter makes a little more sense than just saying ‘I’ll invent something out of the blue’ I mean, if the mass is there, but you can’t see it, matter that only reacts gravitationally makes sense. It solves several problems, so they’re trying to prove it now. Really, that’s all science has ever been. It’s never been ‘rock solid.’ Since when wasn’t it speculation on things we simply don’t understand yet?
    Basically, there are some pretty fundamental flaws in physics as we know it. Dark matter solves some of those issues better than just about every other theory (except maybe string theory, which is more like a belief than a science?). I think researching more into dark matter is a good idea. If you don’t, you’ll need to say a little more convincing argument than ‘I don’t think it’s real.’ Because it could be.

  27. Tyler Durden says:

    “R2K, if you think about it, dark matter makes a little more sense than just saying ‘I’ll invent something out of the blue’ I mean, if the mass is there, but you can’t see it, matter that only reacts gravitationally makes sense. It solves several problems, so they’re trying to prove it now.”

    In a way it is the most plausible theory, at least in the framework of multiple dimensions beyond the 4 measureable in spacetime.

    It’s entirely possible that the majority of the mass in the universe exists solely in the other 5-7 dimensions (depending on who you ask) and can only interact with the observable universe through gravity.

    But there’s still too many variables that are entirely unknown (and unnamed) in this situation and until they can pin at least some of them down it’s more practical to simply refer to the issue as “missing mass” just as everyone used to do.

  28. quantum_flux says:

    Ethan Siegel,

    I am curious if dark matter interacts at all with regular matter in terms of electromagnetic or nuclear forces. Saying it is the local dark matter in our solar system is some 16,000 times denser than the background dark matter leads me to believe that there should be some other up-close forces involved besides just gravitation because something must be responsible for the capturing mechanism (or perhaps the dark matter energy is redshifted by the expansion of the universe and thus is captured).

  29. Ed2 says:

    Ethan Siegel:

    Assuming your speculation is correct, can you estimate the density of dark matter at sealevel on planet Earth? Do you think/believe that dark matter are elctron-neutrinos?

  30. Chuck Lam says:

    To Ed2, I did the math based on the most current theories surrounding dark matter and it appears the density to be about “one of something the weight of a proton” in every cubic mile at sea-level. This seems like a lot of matter for this small a cube, but that’s what the math is showing. Considering what we are looking for, I don’t believe we have anything that can detect this “something” called dark matter if it actually exists.

  31. bob says:

    DM cannot be electon neutrinos because it does not move at relativistic speeds. The only way neutrinos could be DM would depend upon the possible existence of primordial degenerate neutrinos gathering at the galaxy clusters’ center of masses. Degenerate matter puts up a fight against against gravity in the case of white dwarfs and neutron stars and while it can shake at relativistic speeds, it is unlikely displaced through space relative to matter at relatvistic speeds. They are locked at energy levels that they cannot rise and fall from, disabling them to give off light or interact with it. Do degenerate neutrinos fail to oscillate? Is this their form of locking? Do they group into a mass that moves slowly? More experiments are needed to verify this.

    Science is not based upon finding rock solid truths. Science does not seek the truth. Science is based upon falsification that realizes beliefs that are less false. This makes science dynamic. Seeking truth is to seek absolutes and absolutes are brick walls that attempt to resist change.

    The CMB shows spots at the center of masses of galaxy clusters where photons from the CMB are being scattered. When one crunches out the numbers one can calculate the amount of mass that does the scattering and it comes to about 3 to 4 times the mass of all the visible matter in the cluster.

    Independently, if one uses X-ray telescopes and aims them at those spots, one sees X-rays emitted from each of them caused by neutral hydrogen that has gravitationally clustered and is colliding at those spots, emitting the X-rays we see. One can observe the change in the amount of X-rays across each of those regions to calculate the amount of mass of neutral hydrogen responsible for the emissions and they match the calculations in the preceding paragraph….3 to 4 times the mass of the visible matter in the cluster.

    However, it is still not enough to account for the rotation curves observed. DM is simply the best of the competition right now. Even Jacob Beckenstein, a member of MOND, had to introduce DM and DE in order to make sense of the MOND hypothesis. The whole reason MOND formed was to disprove the existence of DM and DE so Beckenstein’s publication simply enraged many who then broke off and formed a spinoff called MOG.

    The only way a ne

  32. Astrofreak says:

    Oh man, there isn’t enough time in a lifetime to begin even talking abot what crap all the “dark matter'” theories are. Of course there has to be dark matter [dark energy, worm holes, gravity waves et. al] because are theories tell us they are needed and lots of science classes, textbook sales, grants and careers are at stake. Hmm, the world used to be flat, earth was the center of the universe and atoms were indivisible. Oh yeah, don’t forget the ever popular “expansion of the universe is speeding up!”

  33. bob says:

    Cookie Monster,

    No. Einstein’s general relativity does not fail to predict expansion of the universe at all. It does predict it and Einstein thought that the universe was static and only introduced the cosmological constant to fudge up GR and make it fit a static state.

  34. Richan says:

    It is mean we are going to changes and changes,because everything must be “alive”.
    We wait but not afraid ,we dare because some one care.
    Count the date until we dead,and than we will relize that we still alive.

  35. Tyler Durden says:

    ^^ Was that in English?

  36. Lightning Bug says:

    Observations may be consistent with there being at least two distinct but related forms of matter.

    The first and most interesting being as described by Newton and an apple falling to Earth; the material of which we are all composed.

    The second being locked to the structure of the universe but attracted to the mass of freely moving bodies; resulting in a pucker or concentration of the raw universal mass.

  37. Bill says:

    It looks like the material referred to as normal matter is not what is normal for matter in the universe.

    Guess it’s too hard to change now. People got real upset about Pluto so I can guess the level of outrage that would occur if astronomers said we humans are not made of normal matter.

  38. RedSparrow says:

    Astrofreak, this is for you:

  39. Scott says:

    Can black matter be the remnants of a black hole? If black matter does exist, it’s obvious that its mass must be significant.

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