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What exoplanet atmospheres might support life beside our nitrogen and oxygen atmosphere?

Related question: Could an exoplanet atmosphere be very similar to our own, but contain some component toxic to human life (free chlorine or hydrogen sulfide, for example)?

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closed as primarily opinion-based by Nathan Tuggy, Hohmannfan, TildalWave, kim holder, duzzy Mar 17 '16 at 16:13

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ A water ocean or watery slushy underground like perhaps Mars' recurring slope linaea, without real gaseous atmosphere is a type of environment where Earth's life could survive. In general, life is shaped to fit Earth's environment today. A billion years ago we would've suffocated on Earth, although life was all around. Historic Earth atmospheres. Hydrogen and helium seems to be the most common atmosphere. Would be interesting if biology can deal with that. $\endgroup$ – LocalFluff Mar 17 '16 at 5:33
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    $\begingroup$ The only thing life seems to need is energy, witness the microbes that live in volcanic vents deep on the ocean. These microbes have evolved with very different mechanisms, on our own planet. Why would life need an atmosphere at all? $\endgroup$ – GdD Mar 17 '16 at 8:27
  • $\begingroup$ If there is liquid water on the surface of an exoplanet, there must bean atmosphere above it. Liquid water could not exist below a vacuum. At least water vapour will form an atmosphere. If there are gases like oxygen dissolved in the water, these gases are in the atmosphere too. $\endgroup$ – Uwe Jul 5 '17 at 21:31
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That depends on your definition of life. If you include any self-replicating automata, you probably do not need an atmosphere at all.

So the question should rather be: Which atmospheres do not support the spontaneous development of life. I would put my bet on anything that is either:

  • too dense, i.e. of such a high pressure that no complex molecular structure can surive. In the very extreme, I do not expect life in the "atmosphere" of a neutron star.
  • too empty, i.e. distances between atmons are so large that a merging into more complex molecules is extremely unlikely (think of the moon)
  • too monomorphic, i.e. so pure that there simply are not enough different kind of atoms to form complex molecules. I have no example for that, though.
  • too cold, i.e. there is simply not enough energy to create complex molecules from the existing matter
  • too hot, i.e. there is so much energy that any complex molecule is instantaneously destroyed (e.g. by radiation) again

All these seem to be rather extreme conditions, but in any other case there is at least the possibility of some form of life.

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