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Inspired by Are there any short-term survivable locations in the solar system, outside of Earth? I'd like to look beyond our solar system and ask if there are there any short term habitable candidates for human life anywhere outside of it?

Somewhere where the average person in their average clothes could comfortably survive indefinitely, provided they had some magic supply of food and water. I.e., it doesn't get so cold helium freezes at night/at the apogee; the atmosphere is relatively breathable at ground level; and I'm not too concerned about fitness of the soil for agriculture or fauna for hunting.

Obviously the universe is large and this coincidence of factors would likely be very far away and hard to confirm if it exists, so I'd also be very happy to hear about vague "well probably"s based on nothing but spectral analysis or what have you.

(I don't imagine it has a crew compartment, but Voyager 1 doesn't count).

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    $\begingroup$ The answer would at the moment be: maybe... Ask this question again in a few years when we have atmospheric data of many more planets. We're now just beginning to be able to research that. $\endgroup$
    – TrySCE2AUX
    Sep 20 at 4:51
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    $\begingroup$ Do you mean anywhere, or close enough we may conceivably get to sometime in the future? $\endgroup$
    – GdD
    Sep 20 at 9:34
  • $\begingroup$ I mean anywhere at all, but the question is poorly worded - I'd like to hear about a "well probably" based on something other than Fermi-esque reasoning. Something like "we have this measurement from this planet" or "this cluster shows a higher than average spectral line for oxygen so it's more likely here" or something. $\endgroup$ Sep 20 at 9:52
  • $\begingroup$ So what you are asking is if there are any candidates based on observation rather than probabilistic reasoning. It's a fair question, I think the answer is 'we aren't there yet'. $\endgroup$
    – GdD
    Sep 20 at 10:02
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    $\begingroup$ So it would be useful to reduce the expectations at least to "being able to walk around without any special suit using only oxygen mask". You may actually encounter such conditions on Venus, not on the surface but about 50 km above it. $\endgroup$
    – OON
    Sep 20 at 10:29

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I think the answer is that no observations have yet been achieved of objects outside the solar system providing sufficient information to suggest habitability for any period of time at all.

The only candidates are planets. A large, dense gas cloud would either have so little gravity it would quickly dissipate so as not to be breathable, or enough gravity to collapse into a gas planet. Realistically some rock would get included to and you would have a gas giant or ice giant.

Many exoplanets with masses comparable to the earth have been discovered. A fair number of those are thought to be rocky and close enough to a red dwarf star so that the temperature, at least, should be reasonable. Ones in the habitable zone of sun-like stars are less known because it takes longer to observe enough orbits to detect them.

As far as I can tell, spectroscopic analyses of exoplanet atmospheres is only just beginning. I believe discovery of oxygen in an exoplanet atmosphere would be big news, and I have not heard of it happening. For instance, this NASA site only mentions detections of helium and water vapor.

Even if oxygen were detected, the atmosphere might be much too thin. It is almost impossible to estimate surface atmospheric pressure of a rocky exoplanet; I think you would have to directly observe the thickness. We are not close to being able to image exoplanets with that kind of detail.

A medium-mass, low density planet can be assumed to have a thick atmosphere that would have an earth-pressure level. If oxygen were detected spectroscopically on such a planet, then it would be a candidate.

To summarize, the best candidates would be an earth-sized rocky planet with known oxygen but unknown pressure, or a larger low-density body with known oxygen and a likely altitude with suitable pressure, but no surface to stand on. But no such objects have been found. Yet.

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  • $\begingroup$ One of the problem is that with current technology we would not be able to detect earth like planets around sun like stars. But this will probably change within the next one or two decades. $\endgroup$
    – TrySCE2AUX
    Sep 22 at 4:50
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    $\begingroup$ Are you sure that's true? Surely some G-type stars are currently under long-term observation looking for periodic transits? I mean, if Kepler had gotten lucky it could have found some in the 4 years it had, since it just takes three transits? Or is there some reason we couldn't detect the transits? $\endgroup$ Sep 22 at 4:56
  • $\begingroup$ My Comment was incomplete. Sorry. Earth like planet around sun like star in earth like orbit. We've found smaller planets than earth but mostly much closer orbits. Many around red dwarfs which seem to have a tendency for a mean solar storms that bathe the surroundings in radiation. So I'd ay we're really close but not quite there. $\endgroup$
    – TrySCE2AUX
    Sep 22 at 5:21
  • $\begingroup$ For a breathable atmosphere, the partial pressure of oxygen should be between 0.16 and 0.4 bar. The total pressure should be between about 0.2 and 5.0 bar. There should be no toxic gases like chlorine or narcotic gases like xenon. $\endgroup$
    – Uwe
    Sep 22 at 14:12
  • $\begingroup$ If the Corona virus got really BAD then we could theoretically be wearing things like gas masks or astronaut suits. Then the atmosphere would not really matter on the other planets because ordinarily street clothes would be enough to survive. $\endgroup$ Sep 22 at 15:11

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