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I like the idea of Oort Cloud objects all being crusted with a thin layer of hydrogen snow, though what little information I've managed to find on the topic seems to imply that that is unlikely, although I'm really not sure, since I can't tell what information is definitely applicable, and I haven't found a direct analysis of this issue. That being said, even if that isn't true, it does seem possible that hydrogen ice might exist on the surface of some objects.

I know that 2.725 Kelvin is the theoretical coldest temperature without refrigeration, since that is the temperature at which the black-body radiation from an object balances out the Cosmic (Microwave) Background radiation (CMB). This paper ( https://arxiv.org/pdf/2005.12932.pdf , pg. 4) about 'Oumuamua mentions the sublimation temperature of H2 (in a vacuum?) as 6K. From other sources I could only glean that H2 freezes at around 10K at pressures 1% that of Earth, which is down from 14.01K at one atmosphere. If the 6K number is correct, then one would expect it to be possible for solid hydrogen to be stable on the surfaces of objects without those objects being refrigerated, if maybe only in cosmic voids.

That being said, anywhere inside a galaxy (or between galaxies for that matter) is also inundated with starlight. I've read that interstellar gas clouds only get down to 10~20K, and I even have one source specifically saying, "No interstellar cloud in a galaxy can every [sic] get cold enough for Hydrogen or Helium to go through a phase change from gas to solid (or liquid). Light from stars in our and other galaxies, and even light from the Big Bang itself, is sufficient to keep the H and He warm enough to stay in the gas phase." ( https://www.vanderbilt.edu/AnS/physics/astrocourses/ast201/snowline.html ) Despite this, I've also read claims on several pop-sci and news sites that the object 'Oumuamua (MPC#: 1I/2017 U1) might have been made of frozen hydrogen, which is possible because "such temperatures exist in the coldest cores of giant molecular clouds ... said Laughlin and lead author Daryl Seligman, who was a Yale grad student but is now at the University of Chicago." ( https://www.space.com/oumuamua-interstellar-visitor-hydrogen-ice.html ) The paper the people they're quoting wrote speaks of temperatures of 4K and 3K being detected along "filaments" in the cores of some "Giant Molecular Clouds" ( https://arxiv.org/pdf/2005.12932.pdf , pg. 6 and 7). It describes how 3K in such dense environments is cold enough for tiny grains of carbon and silicon compounds to act as seeds for large objects of mostly solid dihydrogen to form in tens of thousands of years, which can last for millions of years, at least, in galactic orbits. This discussion implies that normal interstellar space is too hot for such things and that it would even slowly evaporate such objects, but not so fast that they couldn't be spread out across much of the galaxy.

Here is a related thread on this stack exchange, though it deals mostly with the temperature of the interstellar medium itself, not of large objects in interstellar space, and those that did broach the topic only considered the CMB, ignoring starlight: How cold is interstellar space?

This all seems to indicate that the interstellar medium is usually too hot for mostly-hydrogen objects to form, except maybe deep inside nebulae, though I would like to know numbers if anyone knows where to find them. I'm also not sure if that means hydrogen couldn't accrete on the surfaces of large objects made from other substances at more normal interstellar temperatures (either from normal interstellar medium or from nebulae if the object were inside one), since I don't know whether size affects the thermodynamics. Based on the vanderbilt.edu source, I'm guessing that it is em radiation that's heating the gas, but it did cross my mind that there might also be other processes that wouldn't affect large solid bodies. There also is the possibility of chemical reactions or surface features (of certain geometry?) on solid objects that could trap hydrogen, although that might not really be hydrogen forming on the surface then, but rather hydrogen compounds.

More importantly, theoretically, a large enough object would would be able accrete a non-trivial amount of gaseous hydrogen, which would increase the pressure and thus raise the freezing point of Hydrogen. This would be much easier in interstellar space, since it would be much colder and other gravity sources would be much further away. I know that the interstellar medium is very thin, so I wouldn't be surprised if any accretion that did happen would be totally insignificant over the lifetime of the universe at densities like those in our Oort cloud, even if the theoretical equilibrium point was significant (though it should be be noted that even a layer just a few molecules thick could significantly change an object's appearance, including it's albedo, and might form on bodies with almost no craters or other geological activity). However, this could obviously be different inside of molecular clouds, since even things as big as stars can accrete in those, and such objects might easily leave these clouds after forming and might even be present in our Oort cloud or elsewhere in interstellar space.

Now, even if the interstellar medium is hot enough that any solid hydrogen will evaporate over time, if it's very cold, I would think that evaporation might be rather slow (as the 'Oumuamua paper suggests). Since the one place we're really sure planets and other solid objects form is near protostars, it does seem reasonable to discuss ejected planets and planetoids in this issue, and what state hydrogen on their surfaces might be like. Larger objects are more likely to form with more hydrogen and to lose it slower, but they also tend to generate more internal heat from things like gravitational compression and radioactive decay, even if we assume lone objects with no tidal heating. This means it's difficult to be sure what type of object would be most likely to have hydrogen ice on it's surface if it were kicked out into interstellar space.

I think that all the hydrogen in our gas giants (close to the sun as they are) is in some kind of fluid form, be that H2 or metallic, but even if there is some solid hydrogen deep inside, that is presumably under lots and lots of supercritical and/or liquid hydrogen, so I won't count that as the "surface". I know that gas giants create a lot of internal heat — enough that we would expect to be able to see them in infrared even if they weren't reflecting sunlight — and since those heating processes have obviously been going since the solar system formed, I find it difficult to believe that gas giants could have gotten all that cold by now, even if they were as old as the first stars, but they do have as much hydrogen atmosphere as you could want, so maybe small ones that formed with few radioisotopes could get cold enough that liquid and even solid hydrogen might exist on the surface without above any supercritical fluid. One problem I see with this is that most, if not all, gas giants probably have enough helium to reach supercritical pressures if it were separated and covered the surface, and such separation might happen if the temperature got cold enough for hydrogen to start condensing out of the air in the upper atmosphere. I also don't know if there are any electromagnetic refrigeration effects that might cool down gas giants (or, conversely, effects that would likely heat them up).

Perhaps a more likely candidate for solid hydrogen on its surface would be a smaller, mostly solid, world, that had only a small amount of hydrogen, either from accretion in formation, which would also imply helium, or perhaps mostly from some chemical reactions and/or volcanism. If such a planet (or smaller object?) had few enough radioisotopes-per-unit-surface-area, maybe it could get cold enough for everything to freeze except for a modest hydrogen-helium atmosphere, just thick enough to allow some of the hydrogen to freeze at interstellar temperatures (not that I'm really sure what those temperatures are in this context). I know some estimates say that there are likely to be many more rogue planets than planets in star systems, so it's possible that objects like this could be very common in the universe.

As I said near the beginning, even if all or some of this is impossible for objects in or near a galaxy, it is also worth considering whether or not they might exist in intergalactic space, including even in cosmic voids, if only for curiosity's sake. All such places ought to have at least some planet- and asteroid-sized objects, if only ones ejected from star systems ejected from galaxies.

One big issue with all of these objects from an observation standpoint is that, by their very nature, they would be very dim, since their black-body radiation would be so cold and they would too far from any stars to reflect much light. Another is that any that did happen to pass by the sun (or any other star) would likely loose much or all of their hydrogen before they were close enough for us to notice them, especially if they had only thin layers of hydrogen; although, as I have pointed out, 'Oumuamua may be an exception to this rule.

I'd love to hear people's thoughts about these ideas and their likelihoods, especially if those thoughts include more exact and/or better sourced information and/or math than the various hand-wavy musings I put here.

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    $\begingroup$ Is this useful? iopscience.iop.org/article/10.1088/0004-637X/736/2/91 $\endgroup$ – James K Jul 4 at 9:34
  • $\begingroup$ I think that's related, yes. $\endgroup$ – H. H. Sep 27 at 18:45
  • $\begingroup$ Feel free to read, consider, summarise and self-answer! $\endgroup$ – James K Sep 27 at 19:12
  • $\begingroup$ Honestly, I feel like I mostly answered myself when I wrote the question. It wasn't originally going to be as long as it is. That paper seems to have info that might change what I thought somewhat, but I don't actually understand it very well and still haven't read that much of it. I'm not entirely sure what to do in this situation. $\endgroup$ – H. H. Sep 27 at 21:18
  • $\begingroup$ Perhaps your best option is to try to identify what specfically you don't understand and ask a short question (that refers to the paper and your prior research). I think many people will be put off by a question that is more than three paragraphs, so you need to be succinct. You could leave or delete this question. $\endgroup$ – James K Sep 27 at 21:21

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