Based on typical estimates of the galaxy’s dark-matter composition (and assuming it’s not terribly lumpy), how much would there be within the solar system - say within the orbit of Jupiter? Would it be the mass of a planet? Asteroid? Mustard seed?
About 6 times of detectable matter.
? Please rephrase.
Nowhere close to that. The peak estimates for dark matter density near the solar system are about 0.0087 solar masses per cubic parsec:
Jupiter’s orbital radius is about 2.42e-5 parsecs, and thus has a volume (assuming a sphere) of 5.93e-14 pc3. That’s 5.16e-16 solar masses, or 1.03e15 kg. Deimos, the smaller moon of Mars, is 1.8e15 kg.
Excellent! Thank you - that puts it in perspective.
Dark matter really only starts to add up when you consider volume of a galaxy well outside its visible bounds–including above and below (the dark matter halo around the Milky Way is spherical, whereas the stars are roughly in a plane). Even if you count the volume of the Solar System all the way out to the Oort cloud, the contribution of dark matter is less than a percent of the Sun’s mass.
Maybe I misunderstood, but the numbers I have read were in this range :
from : Dark Energy, Dark Matter - NASA Science
So 27% divided by 5% is about 6. So I glibly said 6 times detectable matter.
I was not aware that the distribution of dark matter is different in the solar system compared to the universe.
Some confusion here. Dark energy is, as best we can tell, spread evenly throughout the universe. Dark matter is clustered in halos around galaxies. It is very smooth on the scale of, say, the solar system. Just not on the scale of the universe.
What is very much not smooth is the distribution of ordinary matter. As empty as it seems, the matter inside the orbit of Jupiter is incredibly dense compared to that of our solar system as a whole. Our solar system is dense compared to our galactic neighborhood. And our galactic neighborhood is dense compared to the universe.
That’s what’s throwing off the ratio–not the density of dark matter, but the fact that there’s so much ordinary matter nearby.
Thank you. Learnt something new.
Yep, sure thing. Dark energy is the really weird one. It appears to be constant per unit volume, all throughout the universe. Even though the universe is expanding, the dark energy doesn’t thin out like you’d expect. It just keeps growing with the volume of the universe.
Dark matter is pretty mundane in comparison. It’s probably just a hard to detect particle, a little like a neutrino. It spreads out because it doesn’t interact much with normal matter, just via gravity.
Yes, I think it’s hard to fathom the amount of empty space between stars.
When we see a galaxy as a mist of stars it’s really hard to appreciate how big the spaces between stars are compared to the sizes of the stars themselves*. (And indeed how much space there is between planets compared to the sizes of the planets – I’m assuming that most here will have seen If the moon were a pixel). So even a very diffuse dark matter, as long as it is filling that space is going to add up to a lot of mass.
* But of course this depends on where we are in the universe. I was tempted to start a thread not so long ago on just why stars are so far apart, but on googling found that they aren’t: if you’re near the center of a galaxy or in a globular cluster.
In the middle of a globular cluster the next star may be as close as Pluto is to us, and the night sky would be spectacular. (However it is thought that planets are unlikely to form in such an environment)
There’d be no “night sky”. Nightfall would never come.
Only once in two thousand and forty-nine years.
By “night sky” I just mean facing away from the nearest star.
It would be bright for sure, but facing the nearest star may still be much brighter.
I suspect a star as close as Pluto would create at least a dim daytime.
But I was making a reference to a famous short story.
Isn’t there a theory that dark matter was basically the scaffolding of the universe, and regular matter coalesced to areas with higher dark matter? I hope I’m remembering correctly.
Possibly, but once baryonic (what we would call “normal”) matter started coalescing, the irregularities would be increasingly due to the baryonic matter, and less due to the dark matter.