Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Marika on 28/06/2018 09:34:59
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Donald wants to know:
How much dark matter is in our solar system? Upper and lower limits. How do we know?
What do you think?
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There is a lot we don't know about Dark Matter. But most physicists currently think it might be some form of Weakly Interacting Massive Particle (or WIMP). Assuming this is true...
There is still a lot we don't know about the properties of these hypothetical Dark Matter Particles, especially their mass. I heard one scientist suggest that if you have a 1 liter water bottle on the table, there are probably a few Dark Matter particles inside it at any given moment.
But they won't stay there for long. Meteorites normally hit Earth's atmosphere at 11-30 km/second, because they are orbiting the Sun in the same general direction as the Earth, and mostly in the same general plane as the Earth.
However, the Dark Matter particles are not orbiting the Sun, they are orbiting the Galaxy. And they are not orbiting in the plane of the galaxy, but in all possible planes (including the opposite direction). So they could pass through the Earth at up to 1 million km/h.
Although Dark Matter is thought to make up 4x the visible mass of the galaxy, it is not concentrated in a thin disk, but is spread out through a very large spherical volume surrounding the galaxy.
- A quick, back-of-the-envelope calculation:
- The galactic disk is about 1,000 light years thick
- The Sun orbits at a radius of about 26,000 light years from the galactic center
- Considering the volume within 26,000 light years of the galactic center:
- The Galactic Disk makes up 1/30 of this volume
- But teh Galactic Disk makes up 1/5 of the mass
- So the density of Dark Matter in the Galactic disk is about 1/6 of the density of visible matter in the Galactic disk
This is just an order-of-magnitude calculation. The uncertainties are large, so I can't provide upper or lower limits.
See: https://en.wikipedia.org/wiki/Dark_matter_halo
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There is a lot we don't know about Dark Matter. But most physicists currently think it might be some form of Weakly Interacting Massive Particle (or WIMP). Assuming this is true...
There is still a lot we don't know about the properties of these hypothetical Dark Matter Particles, especially their mass. I heard one scientist suggest that if you have a 1 liter water bottle on the table, there are probably a few Dark Matter particles inside it at any given moment.
But they won't stay there for long. Meteorites normally hit Earth's atmosphere at 11-30 km/second, because they are orbiting the Sun in the same general direction as the Earth, and mostly in the same general plane as the Earth.
However, the Dark Matter particles are not orbiting the Sun, they are orbiting the Galaxy. And they are not orbiting in the plane of the galaxy, but in all possible planes (including the opposite direction). So they could pass through the Earth at up to 1 million km/h.
Although Dark Matter is thought to make up 4x the visible mass of the galaxy, it is not concentrated in a thin disk, but is spread out through a very large spherical volume surrounding the galaxy.
- A quick, back-of-the-envelope calculation:
- The galactic disk is about 1,000 light years thick
- The Sun orbits at a radius of about 26,000 light years from the galactic center
- Considering the volume within 26,000 light years of the galactic center:
- The Galactic Disk makes up 1/30 of this volume
- But teh Galactic Disk makes up 1/5 of the mass
- So the density of Dark Matter in the Galactic disk is about 1/6 of the density of visible matter in the Galactic disk
This is just an order-of-magnitude calculation. The uncertainties are large, so I can't provide upper or lower limits.
See: https://en.wikipedia.org/wiki/Dark_matter_halo
To take this a bit further, the density of the Solar system is much higher than that of the galactic disk as a whole.
In the vicinity of the Earth, the Interplanetary medium density has a density of ~ 5 particles/ cm^3 If we took the entire mass of the Sun and all the planets out to Neptune and spread it out evenly over a spherical volume encompassed by Neptune's orbit, it would work out to be ~3 e12 particles per cm^3 . This still works out to be a very high quality of vacuum.
The estimated dark matter density in the region of the solar system works out to be ~ 1 particle/ 3cm^3.
Put another way, the total of dark matter inside the orbit of Neptune works out to be ~2.0e 17 kg, or the equivalent to the mass of one of Jupiter's smaller moons.