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Is it possible that all dark matter is made of rogue planets (free-floating planet)? (and other stuff like asteroids or meteoroids)

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it has been envisaged in the 90s that some of the dark matter in halos was made of brown dwarfs; this was later ruled out through lack of indirect detection via light deflection (known as micro lensing). – chris Mar 28 '14 at 20:57
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To expand on @chris' comment about dense, cold planet like object in interstellar space of our Galaxy add up to not more than 3% of the excess mass density needed to explain the galactic rotation curves of similar galaxies. These things are out there, but they aren't the answer to the puzzle. – dmckee Mar 31 '14 at 4:08
up vote 15 down vote accepted

First of all I'll start with a few ideas:

  1. Baryonic Matter: Baryons are elementary particles made up of 3 quarks. This includes protons and neutrons, and the term baryonic matter refers to matter made of baryons, such as atoms. Examples of non-baryonic matter includes neutrinos, free electrons and other exotic matter.
  2. Things like planets, stars, dust, etc. are all made of atoms, and so are classified as baryonic matter.

Now, how do we know that dark matter is present in the universe?

Astronomers measure the gravitational pull of galaxies and galaxy groups/clusters based on how objects behave when interacting with these objects. Some examples of this include tidal gas/dust stripping, the orbit of stars in a galaxy and gravitational lensing of distant light from a large cluster. Using this they determine the mass of the galaxy (or galaxy group). We can also determine the mass of a galaxy or group by looking at it and adding up the mass of all the objects (like stars, dust, gas, black holes, and other baryonic matter). While these methods both give us approximations, it is clear that the gravitational mass of galaxies and groups exceeds the baryonic mass by a factor of 10-100.

When astrophysicists first found this phenomenon they had to come up with a plausible explanation, so they suggested that there is some new, invisible matter called dark matter. (Aside: some astrophysicists also came up with other explanations like modified gravity, but so far dark matter does the best job at explaining observations).

Okay, so now how do we know dark matter is not any sort of baryonic matter?

There are a few reasons astrophysicists know that it is extremely unlikely that dark matter is baryonic. First of all if all the stars in a galaxy shine on an object it heats up, this heat causes the release of radiation, called thermal radiation, and every (baryonic) object above zero kelvin (or -273.14 deg celcius) emits this radiation. However, dark matter does not emit any radiation at all (hence the name dark!)

If dark matter were baryonic it would also mean that it could become light emitting. If we got a clump of baryonic matter* and put it in space it would gravitationally contract, and would eventually form a star or black hole** - both of which we would be able to see.

So, because of these reasons the dark matter in galaxies and in galaxy groups/clusters cannot be baryonic, and so cannot be planets, dead stars, asteroids, etc. It would definetely not be planets as there is no way 10-100 times the mass of the stars in a galaxy would be planets, as the mechanism for making planets relies on supernovae, and the number of supernovae needed for the that many planets would be far too high to match our observations. I hope that this answered your question!

*provided the clump of baryonic matter was large, and the amount there is in galaxies definitely is!

** we don't observe black holes directly, but can see radiation from their accretion disks.

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Thanks for taking the time to write a clear explanation. I have a question on one bit I am trying to follow. Am I right to understand that in the thought experiment on a "clump of dark matter" that equally if it is not baryonic it should also gravitationally contract too? This follows from its existance from your first paragraph, unless it is so exotic that it can affect the orbits of stars around galaxies but does not interact with its own kind. Does this make sense? – Puffin Feb 4 at 21:41
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@Puffin I'm not sure I completely understand what you are asking, but dark matter does interact with other dark matter - however this interaction is purely gravitational. Baryonic and dark matter also only interact through gravity, but baryonic matter interacts with other baryonic matter through gravity, electromagnetism, nuclear forces, etc. Since baryonic matter interacts in this way it can "lose energy" through radiation and other means to contract, but as dark matter has no way to "lose energy" it can't contract as efficiently. Does this answer your question? – Robbie Feb 5 at 1:11
    
Thank you. Your answer covers it well, I think. My knowledge is rather flaky here and am having to take a large leap. Are you saying that, for example, gravitational waves would provide a means for matter to lose orbital energy and thus between the two types of matter, baryonic and dark, and would thus allow conventional matter to form stars and galaxies whilst dark matter remained more distributed? – Puffin Feb 5 at 12:20
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Yes, gravitational waves are one way for matter to lose energy (although it is very minor). As a gas cloud contracts it heats up, and this heat radiates away, cooling the gas and allowing it to contract more and more. This is why matter can form stars and planets and cool stuff like that but dark matter can't. – Robbie Feb 6 at 10:57
    
OK, thank you, its much clearer with the heat example. – Puffin Feb 6 at 11:01

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