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Throwaway_97534 t1_iu9gtyo wrote

Reply to comment by forte2718 in Is dark matter orbiting galaxies with the same speed as normal matter? by taracus

If we were in space at relative rest and there was an earth-mass chunk of dark matter that wandered into our path, does that mean we would suddenly find ourselves falling toward it for seemingly no reason?

I'm assuming we haven't observed dark matter at smaller than galactic scales, but I'm wondering if the current theories and observations allow for smaller amounts as well.

Can we run into planet-sized bits of dark matter just like we can run into planet-sized primordial black holes? One of the theories is that we haven't observed a 9th planet in the solar system that's shepherding trans-neptunian objects because it may actually be a primordial black hole... Could it also be a small bit of dark matter?

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forte2718 t1_iu9iwcn wrote

>If we were in space at relative rest ...

At relative rest with respect to what? Remember ... all speeds are relative to other things! :)

>... and there was an earth-mass chunk of dark matter that wandered into our path, ...

Dark matter doesn't really clump up the way ordinary matter does. The only way you would get this is if dark matter happens to be in the form of "MACHOs" (massive compact halo objects) such as black holes, but that hypothesis has almost been completely ruled out except for a range of masses that excludes things anywhere near the Earth's mass. So, this just isn't an actual possibility, I'm afraid!

>... does that mean we would suddenly find ourselves falling toward it for seemingly no reason?

We would feel its gravitational effects, yes. If you had something like an Earth-mass MACHO, it could do things like disrupt orbits, to the extent that something of Earth mass can do so (naturally something like the Sun or Jupiter wouldn't be significantly affected, but smaller planets would).

>I'm assuming we haven't observed dark matter at smaller than galactic scales, but I'm wondering if the current theories and observations allow for smaller amounts as well.

Unfortunately, not really ... at least, not also in grouped clumps that are within a few orders of magnitude of the Earth's mass. You can have smaller amounts if it is very diffuse (like axions or neutrinos or some other particulate form that doesn't interact electromagnetically) but not if it clumps up due to an interaction of some kind.

>Can we run into planet-sized bits of dark matter just like we can run into planet-sized primordial black holes?

That's a hard no, unfortunately. The observational evidence (at least that which I am aware of) excludes as a form of dark matter MACHOs including primordial black holes in the mass range of 10^(-8) Earth masses or higher. [1]

>One of the theories is that we haven't observed a 9th planet in the solar system that's shepherding things because it may actually be a primordial black hole... Could it also be a small bit of dark matter?

It and things like it couldn't be a significant form of dark matter, no. I don't see why it couldn't be a black hole though, whether primordial or otherwise.

Hope that helps,

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