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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
    Commented May 15, 2023 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
    Commented May 18, 2023 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
    Commented May 18, 2023 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
    Commented May 18, 2023 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
    Commented May 18, 2023 at 22:03

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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
    Commented May 20, 2023 at 21:23
  • $\begingroup$ Also superfluid liquid Helium. $\endgroup$
    – Zzyzx
    Commented May 20, 2023 at 21:34

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