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I was idly thinking what states of matter are common, which are rare overall. The commonest states of matter in the universe, by total mass, would be gases, solids (mostly as dust), and plasma (mostly as stars). Quite rarer would be the states of matter of white dwarfs, neutron stars, and black holes (if you can call it matter). Liquids, if I am not mistaken, would be the rarest, occurring only on certain planets, and only within a narrow range of interior depths and atmospheric heights. One might argue for further dividing liquids into more exotic high-pressure states (e.g. metallic liquid hydrogen, as in Jupiter), beside the familiar low-pressure ones. This would make the latter, such as water, even rarer from a cosmological point of view.

Do I have this right? Have I missed any rarer states of matter?

Ed.: I have seen Fukugita and Peebles's paper. Since planets overall form the smallest fraction of baryonic matter in the universe, the issue clearly becomes finding the rarest state of matter within planets.

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    $\begingroup$ I am counting plasma as a separate state of matter, but in any case, non-ionized gas and plasma are very abundant, so the choice wouldn't affect figuring out which is the rarest state of matter. $\endgroup$
    – Zzyzx
    May 15 at 18:45
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    $\begingroup$ @uhoh, no, because I'm looking for the rarest, not the most abundant matter. Fukugita and Peebles estimate 23% of the total energy in the universe is dark matter rest energy, and all baryonic matter rest energy adds up to 4.5% (of which liquids are a tiny fraction). The distribution of different forms of matter at the high end doesn't affect that at the low end. $\endgroup$
    – Zzyzx
    May 18 at 0:47
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    $\begingroup$ @Zzyzx Sorry, I'm blaming my misunderstanding on cosmic rays penetrating the aluminum foil in my hat. I've deleted my comments. I'll add a bounty instead. Thanks for your patience! $\endgroup$
    – uhoh
    May 18 at 1:15
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    $\begingroup$ I'm thinking about the matter in neutron stars. It may be a superfluid of neutrons, and I don't know if that counts as "liquid" or not. As a mass fraction, it may be on the same order as solids. "Regular" liquids are, I have no doubt, are the least common regular state of matter, but it's hard to source that fact. $\endgroup$
    – James K
    May 18 at 21:13
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    $\begingroup$ @JamesK Electron- and neutron- degenerate matter is usually described as a Fermi gas. Its density varies with pressure, whereas liquids are (relatively) incompressible fluids. $\endgroup$
    – PM 2Ring
    May 18 at 22:03

2 Answers 2

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No, by far the rarest state of matter in the universe is Bose-Einstein Condensate. As far as we know BECs do not naturally occur anywhere in the Universe, and have only been realized under lab conditions.

BECs may not even the rarest, there might be exotic states of quark matter at low temperature and high chemical potential that are simply never realized in nature, and also (currently) cannot be realized in a lab. This part of the QCD phase space is also not well understood theoretically. It is not exactly clear what phases may exist, nor where exactly the phase transitions happen. Some of these phases may naturally occur in neutron stars, but it is possible that some of them are well out of reach. Those would be even rarer than BECs.

TL;DR There may be some extremely exotic states of matter that exist only in a theoretical sense.

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  • $\begingroup$ Good point. I was thinking of naturally-occurring states of matter. Supercritical fluids, which also don't occur in nature AFAIK, are also rarer than liquids (though not as rare as BECs). $\endgroup$
    – Zzyzx
    May 20 at 21:23
  • $\begingroup$ Also superfluid liquid Helium. $\endgroup$
    – Zzyzx
    May 20 at 21:34
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If protons do not decay, ALL atomic matter behaves like a liquid and becomes spherical for a certain period of time under its own gravity – until the cold liquid spheres of stuff decay to cold spheres of iron-56 (which also behaves like a liquid), which then collapse into more compact spherical neutron stars. (On such time scales, stars fade out long before this happens, and they too, together with proto-stellar objects, transform into spheres as liquid. Planets fall onto their stars or become gravitationally unbound.)

So on such a medium time scale, the liquids are not the rarest state of matter because they are the only known matter (of course, black holes put aside because the best guess so far is that they might be a singularity). If you wait longer (as time approaches infinity), there's no matter at all (because all of it collapses into black holes, which eventually radiate away).

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    $\begingroup$ thanks, and so to the question "Are liquids the rarest state of matter in the universe?" is your answer Yes or No? I still can't tell. $\endgroup$
    – uhoh
    May 18 at 14:25
  • $\begingroup$ @uhuh On a medium time scale, they are not. (On the long time scale, i.e., when time goes to infinity, there's no matter at all because all of it collapses into black holes, which eventually radiate away.) $\endgroup$
    – AlMa0
    May 19 at 19:48
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    $\begingroup$ I don't understand what you mean. Gas condenses to protostellar objects which become stars. At no point in time is any part of any of these liquid. $\endgroup$
    – Zzyzx
    May 20 at 21:28
  • $\begingroup$ @Zzyzx “No point in time” is wrong. Only so far no part of a star is a liquid. After the star burns out completely and cools down thereafter, it behaves in the long run like a liquid due to quantum tunneling effects. The probability that any atom manifests itself at a different location than the current one is extremely low but unequal zero. The atoms of the star have the least potential energy together if the star is an absolute sphere. Therefore, the atoms will eventually tunnel to this least-energy state. The dead star behaves like a very viscous liquid slowly taking a spherical form. $\endgroup$
    – AlMa0
    May 22 at 17:00
  • $\begingroup$ @Zzyzx Of course, if you object that “behaves like a liquid” and “is a liquid” are two different things, you may be technically right, but the observable difference in our case is only the duration of the process that transforms an object into a sphere. $\endgroup$
    – AlMa0
    May 22 at 17:01

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