Is it possible that all dark matter is made of rogue planets (free-floating planet)? (and other stuff like asteroids or meteoroids)
First of all I'll start with a few ideas:
- 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.
- 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.
The fact that there is not enough luminous matter to explain the gravitational properties of galaxies and clusters of galaxies does not inexorably lead to the conclusion of weird dark matter that does not emit or absorb light.
For a long time in the 70s, 80s and 90s, there was very much a search for small, dark objects that could make up this missing mass. This included things like cold white dwarfs, low mass brown dwarfs and "rogue planets". None of these searches - and specifically the microlensing surveys, which continue today - turned up anything like the numbers of objects required to solve the "dark matter" problem.
A more fundamental problem for the "rogue planets" idea is that we are now quite sure that whatever dark matter is, it is not in the form of everyday "baryonic matter" that emits and absorbs electromagnetic radiation (a k.a. light).
Measurements of the primordial abundances of helium and deuterium, created in the big bang, combined with measurements of the universal expansion rate, give a direct estimate of the amount of baryonic matter in the universe. Whilst the inferred amount is greater than can be counted up in the luminous matter of the universe, it is insufficient by a factor of 6 compared with what is required to explain the dynamics of galaxies, clusters of galaxies and the universe as a whole.
Further indirect evidence of the non-baryonic nature of this matter comes from simulations of structure formation in the universe. The "dissipationless", slow-moving nature of non-baryonic matter is what is required to explain (i) the relatively tiny fluctuations in the cosmic microwave background, emitted when the universe was 380,000 years old; and (ii) how this then develops into the rich set of galaxies and galaxy clusters that we seen in the universe today.
In other words, most of the "dark matter" must be non-baryonic and so cannot be made up of rogue planets.
Yes, it's definitely possible. For our solar system, you'd need about 18,000 Saturns of "rogue planets". That seems pretty doable for that amount of matter to be between us and the closest star, and for us to not observe it.
Just to put it in perspective, if earth was the size of a dime, a Saturn rogue planet would be a bit smaller than a volleyball. With that same scale, the nearest planet would be over 50,000 km away; that would make a 2D area (3D makes it even bigger) about the size of 60 Pacific Oceans. Seems fairly easy to have 18,000 volleyballs floating unobserved in 60 Pacific Oceans.
… I am confused. “Dark Matter”, by definition, is “matter we cannot see”. You are assuming that we would be able to “see” a planet-sized chunk of baryonic matter at stellar distances. I am not referring to nebulas and such — those are relatively low mass, highly diffuse, clouds of light elements with very free electrons with which to produce high amounts of photons/em radiation. Not everything behaves as such. Photons are photons — ever take a picture of a face with the light behind the subject? The light behind will cause any foreground light to be washed out; it is also why we can’t see into our own galactic center all that well.
That “volleyballs in an ocean” comment above really puts that into context. Thank you, kind individual. That would be, what, one volleyball per over 200,000 square miles? I have lost soccer balls in public parks before, let alone losing one within a city, but those are state sizes/scales.
Come on astronomy community, you are not that arrogant, are you?