[syhprum (OP)]

I have always believed the black body temperature of the earth to be -18°C too cold for liquid water although of course our water vapour and CO2 atmosphere pushes this up.
Is probability of an atmosphere considered when defining the habitable zone.

[alancalverd]

Since every concept of life involves water, some kind of atmosphere is inevitable if an environment is to be considered habitable.

[CliffordK]

For Earth, it may well be that life itself has helped maintain Earth's position in the habitable zone. 

If one thinks of Venus, it is a lot closer to the sun, but there may be an element of carbon being throughout the planet during initial formation, then carbon oxides rising to the surface, and being slowly released into the atmosphere over time where they get stuck without some method to reduce the oxides and allow them to be incorporated back into the crust.  Thus, without life, the planet may slowly warm. 

Estimates indicate that the sun probably has gotten brighter (more solar radiation) over time.


This would seem to mean that one would expect the temperature to be slowly increasing on Earth.  Again, life may have inadvertently helped maintain the liquid water phase by reducing the atmospheric CO₂.  But, for humanity to exist, that may have been a requirement.

I have wondered if a planet like Jupiter or Saturn does, or could support liquid water, in which case, the "habitable zone" could be far larger than one might otherwise expect, at least with respect to ocean life.

[evan_au]

Here we are talking about "Life As We Know It On Earth", and ignoring science-fiction favorites like computer chips which become sentient, or patches of solar corona plasma held together by magnetic fields...

For Life As We Know It On Earth, we need:

• A liquid phase: We have a soup of organic and inorganic chemicals floating around in a water solution.
  
• A solid phase: To distinguish inside from outside, and provide a source of chemicals.
  
• A gas phase: For many types of life (eg birds, humans, trees & whales), the gas is a fluid which distributes oxygen around the Earth, and carries away wastes like CO₂ and SO₂ from our cars. But for some other types of life (eg fish and seaweed), liquid water is a fluid which carries out this function quite efficiently.
   

Now think what happens if we don't have these three phases:

• If we have no liquid phase because all the water has frozen into a solid: The chemicals are frozen in place, and can't interact, so the organism is effectively dead (or at least in suspended animation).
  
• If we have no liquid phase because all the water has boiled into a gas (due to high temperature or low pressure): The gas has very low density, and cannot dissolve large organic molecules. Organic molecules can't interact, so the creature would die. (Temperature itself is not a barrier; the organic chemicals humans use are unstable above about 60C, but some bacteria can survive in high-pressure nuclear reactors, well above 100C.)
  
• If we have no solid component: Everything will float around. This is effectively the situation inside some bacteria which don't have a membrane around the nucleus. So life is possible, but it would not be very recognisable by us. In particular, it would be hard to distinguish individuals.
  
• If we have no gas component, because it is all liquid: Some types of life like fish and bacteria can still  survive, since they don't interact directly with a gas phase. Nutrients and waste products would need to be carried around by the liquid phase, rather than the gas phase.
  
• There are some "half-way" phases, such as near the triple-point of a fluid, or in high-density gases on Jupiter or Saturn which could be dense enough to carry out the same functions as both air and liquid water do on Earth.
   

We can see some places in the solar system where such a habitable environment might be possible, like the ice moons Enceladus and Europa. These are so cold that they are surrounded by a layer of ice many miles thick. But there is thought to be a core of liquid water and rock, which is heated by tidal effects. So potentially this could be a liquid/solid environment (with no gas) which could theoretically harbor life as we know it on Earth.

Recent drilling into ice-covered lakes in Antarctica has searched for life isolated from the atmosphere.

I think that the major limitation here is not presence of a gas phase, but having enough high-grade energy in the form of solar photons, which allows photosynthesis on Earth. This is a much better energy source than low-grade tidal heating. It is photosynthesis which allows oxygen production on Earth, supporting multicellular creatures with high metabolic rates like humans, reptiles, insects, fish and trees. Without this, we would have an anaerobic environment, with a much less active form of life.

It is this Solar input which allows life on Earth (and coincidentally produces the atmosphere), rather than the atmosphere which directly allows life.

[evan_au]

In most environments, I expect that some form of membrane will be required to enclose a fluid-encased life-form, separating an "outside" environment from a more controlled "inside".

Otherwise, the components of that life-form will diffuse away through the bulk of the surrounding fluid; they will become so diluted that the desired chemical reactions which go to make up life will almost stop. However, unwanted reactions which break down the chemicals of life will still continue, and the organism will die (indeed, some antibiotics work by punching holes in the cell membrane).

Another source of energy apart from solar photons is decay of radioistopes, which can ionise water. This is suspected to be an energy source for bacteria found deep in gold mines, and for a gamma-ray eating fungus found growing in the carcass of the Chernobyl nuclear reactor.

At this point in the Solar System's history, this radiation is fairly diffuse (except inside a nuclear reactor), and potentially extremely destructive to living tissue, so it leads to a somewhat precarious existence.

[syhprum (OP)]

Here is a link to a recent paper on this subject

http://arxiv.org/abs/1411.5564v1