[ConfusedHermit (OP)]

I’m curious what a planet needs to support life. I’ve got a list of a bunch of answers other people have given on the internet and their own research, but I want to see if this list has anything we can remove from it (or add TO it), and maybe make one final, good-as-we-can-get-it list.

Because otherwise, I'm confused what's true and what isn't :{o~

Ready? Here's the list:
-  A solar system with a single massive Sun than can serve as a long-lived, stable source of energy
- A terrestrial planet (non-gaseous)
- The planet must be the right distance from the sun in order to preserve liquid water at the surface – if it’s too close, the water is burnt off in a runaway greenhouse effect, if it’s too far, the water is permanently frozen in a runaway glaciation
- The solar system must be placed at the right place in the galaxy – not too near dangerous radiation, but close enough to other stars to be able to absorb heavy elements after neighboring stars die
- A moon of sufficient mass to stabilize the tilt of the planet’s rotation
- Plate tectonics
- An oxygen-rich atmosphere
- A sweeper planet to deflect comets, etc.
- Planetary neighbors must have non-eccentric orbits
- Water (all cells are mostly water).
- Atmosphere (to keep the water in a liquid state).
- Energy (from the sun or geothermal/hydrothermal sources).
- Nutrients
- Gravity - to hold it together
- You don't need sunlight. Life lives at the bottom of the sea with no sunlight.
- You don't need oxygen. For a billion years there was life without oxygen.

[CliffordK]

That sounds like a good list.  Some things may be more important than others.

It is good to have not too much, and not too little atmosphere, although I would imagine that life could adapt to the lower pressures at the top of mount Everest, even if we would have difficulty living up there, as well as potentially living in higher pressures (high pressure + heat as on Venus?)

Our atmospheric oxygen was produced by "Life".  In fact, perhaps Carbon Dioxide would be a compound that shouldn't be overlooked in its importance.

If you think of a solvent/solute cellular model as our own.  Water is a very common solvent.  However, there have been questions as to whether other solvents would also support life, which could vastly increase the habitable zone. 

Obviously our computers are engineered with many of the properties of life.  Could a silicone semiconductor evolve independent of engineering?  Can life be attributed to an engineered entity?

[ConfusedHermit (OP)]

That's fascinating that life created oxygen and now scratches its own itch by filling the planet with it.

And I've never thought of considering the definition of 'life' when thinking of a list like this. Hmm... Would it be the absolute-most-fair thing to just look at it from a science-fiction perspective and say 'life can survive anywhere, and ours just happens to do it this way?'

[Don_1]

Some of these requirements are rather 'doubling up' on others. For example, liquid water cannot exist without heat from source. You could say that some attributes are dependant on the existance of others.

As Clifford wrote, our atmosphere was not quite as calm or inviting at first as it became. It was the earliest lifeforms which altered the atmosphere. In fact it is possible, if not probable, that life may not have evolved at all, but for that noxious atmosphere.

But there is one thing in your list which is not so clear cut: 'You don't need sunlight. Life lives at the bottom of the sea with no sunlight. This is true, but much life at the bottom of the oceans does benefit from second hand sunlight. Fish in the surface oceanic waters do benefit from sunlight and eventually die and sink to the murky depths, taking their sunlight legacy with them. There their dead bodies are consumed and thus dwellers of the deep get their little ray of sunshine.

The other problem, again as Clifford hinted, is that you are assuming that any other lifeform must conform to the carbon based pattern of life on Earth. That may not be the case.

Oh yes, and nutrients; what do you mean by nutrients? To the earliest lifeforms on Earth, what constituted a nutient? The dead and decaying bodies of........ Ah! But there were no dead or decaying bodies, since there had been no life. Nutrients to those early lifeforms may have been methane, ammonia, sulphur, nitrogen or any number of pure chemical compounds. One thing is for sure, it wasn't what you might term as 'organic'.

[ConfusedHermit (OP)]

Thank you both for the information on this subject. I was trying to see if there was a definite and simpler list, but because of how many things you have to consider in this, I see there IS no definite and simple list.

I think I'll just settle with the simple 'look, 'life' (however you define it) finds a way to exist no matter what the conditions' thought. It's less work to put into words, and it's technically true!

Hurray, sci-fi! :{D~

[evan_au]

The original list above was oriented around water-soluble hydrocarbon "Life-as-we-know-it", on a planet in the "Golidlocks Zone" for water: the type of planet the Kepler Space telescope is able to start discovering now, after 2-3 years of observation.

Scientists have been amazed at the locations life can persist on our planet - in deep hydrothermal vents as you say, in the bottom of deep goldmines, at the top of clouds, in the coolant water of a nuclear reactor, and within the year we should hear about whether life persists in Lake Vostok, isolated under the Antarctic ice.

We could generalise this to say that the more abundant and active life tends to occur where there is an energy-rich interface, but without "too much" disturbance. Examples on Earth might be:

• Interface between land and air; energy for plants comes from sunlight, and for animals from the plants and other animals
• Interface between sea and air; energy for phytoplankton comes from sunlight, with an ecosystem built on this
• Interface between seafloor and ocean; energy comes down from the surface ecosystem
• Interface between seafloor and igneous rocks at the mid-ocean ridges; energy comes from sulphides of metals, with an ecosystem built on this. This interface is continually moving, so these creatures must continually colonise new, distant "smokers"

Life is characterised by a local decrease in entropy, but a global increase in entropy (compared to a dead environment). This requires a ready source of energy in some form (not necessarily chemical energy).

If we put on our science-fiction hats, and think about where such interfaces may exist, even in our own solar system:

• The surface of the sun (or some of its internal layers): Lots of energy, but immense disturbances
• Surface of solid, airless bodies like Mercury, Moon or asteroids & comets
• Venus: we have no idea what is down there, except that it is hot and acidic!
• Internal layers of the Earth: What about the zones where diamond is the most stable form of carbon, subduction zones in the mantle, or the interface where the iron core is crystallising?
• Mars: Everyone talks about Mars
• Gas giants: Multiple layers, suitable for hydrocarbon-soluble hydrocarbons, or solid/liquid hydrogen
• Water-based moons around gas giants could support water-soluble life, powered by gravitational flexing
• In remote parts of the solar system, sunlight is not a powerful energy source, but superconductivity and superfluidity allows events to occur with very little energy.
• Other stellar systems and more exotic locations like planetary nebula and black holes provide even more extreme interfaces.

Even if we could study these locations, it is likely that any Life would be too slow/fast, big/small, hot/cold hard/insubstantial or just too different for us to recognise.

[CliffordK]

I like your "interface" theory.
Of course, fish are quite 3-dimensional, but they do like to feed either at the bottom or the surface, as well as hiding in the rocks.

Some organisms have been found deep underground.
http://www.universetoday.com/851/bacteria-found-deep-underground/
http://www.csmonitor.com/Science/2011/0603/Worms-from-Hell-How-deep-do-they-dig

I suppose dust would also be considered an interface, but dust can certainly support life on the ground, or airborne.

The surface of Venus may be too harsh to support life as we know it.  Other possibilities?  There are theories that there are layers in the atmosphere of Venus that could support life if there was a balloon or something to hold it in place (and thus the cloud city concept that is common in Sci-Fi).  But, without a surface, it would be difficult for life to evolve and take a foothold there.

Likewise the gas giants likely have levels in the atmosphere that could support life.  And the planets are HUGE, giving more area where it might find a niche.  But, again, it remains to be seen whether life could evolve without a surface.

[TheDoctor]

Planet discoverer Dr. Alex Wolszczan said "We are planet-based beings, and we believe that planets are just the right distance from stars with just the right conditions are the only places life can occur.  So the whole story of searching for extraterrestrial life begins with finding planets around other stars beyond our solar system."

[damocles]

The OP suggested an oxygen-rich atmosphere as necessary for life. That is indeed putting the cart before the horse!

An oxygen rich atmosphere cannot exist or be maintained unless produced and sustained by photosynthesizing life-forms.

Early life on Earth did not have nor need oxygen in its atmosphere, but without oxygen it was restricted to the deep ocean where there was no access for solar radiation. Photosynthesizing life forms were restricted to areas sheltered from direct solar radiation; usually this meant 1-2 metres underwater -- an environment where nearly all of the UV light was scattered away, but plenty of visible light was still available. The land surface of the Earth is a no-go zone: UV solar radiation would randomly break the bonds in important organic molecules. When photosynthesizing life forms had been doing their trick for more than a billion years, they had produced enough oxygen to remove all of the iron(II) and ammonium salts from the oceans, and to build up an atmospheric oxygen concentration around 1-2%. That allowed an ozone layer to form, and life to emerge onto the dry land without too much exposure to solar UV.

It is not the case that life needs an oxygen rich atmosphere; it is the case that an oxygen rich atmosphere can only be produced and sustained by living things. Oxygen gas is a very high energy chemical that could never be produced in quantity by normal geochemical mechanisms.

[shepard150]

Those are swell answers if you want to live on a sterile ball of dirt.   The only really special thing about earth is the iron core creates our magnetosphere, which deflects the most harmful radiation while letting enough through to support evolution.  Bad news for those creating profiles for bodies that could have life.  Just like the little guy from dad that made you on that day, hour and second with that young lady and that 12 pack, life may indeed be a one in a trillion event.       

[CliffordK]

life may indeed be a one in a trillion event.

Quite possible.  However, if there are half a trillion stars in the Milky Way, it isn't very good odds for finding nearby neighbors, but with hundreds of billions of galaxies out there, somewhere there must be another planet that is "just right".

Another thing I've thought about our gas giants (Jupiter, Saturn, also Neptune and Uranus) is that they are HUGE, and they have heat, energy, and probably have the equivalent of liquid oceans, and perhaps even liquid water.  What they may lack is a solid surface that interfaces with the liquid water.  And, perhaps also lacking a bit of stability.  Too much stirring of the pot, and perhaps nothing could self-assemble.  However, we are barely scratching the surface of the exploration of our own solar system.

Even without intelligence, it is possible that "life" could be accidental hitchhikers, developing once, and then spreading to several planets and stellar systems.  Even if no longer "living" in a normal sense, delivering complex proteins and DNA to a planet may help stimulate the evolution of life.

[alancalverd]

The magnetosphere certainly helps deflect charged particles but has no effect on gamma radiation. A few feet of water would suffice to replace it, so a magnetic core isn't essential.

An extraplanetary heat source isn't essential. Life around ocean vents gets all its energy from the grinding of tectonic plates. All that is required is a temperature differential, so the essential interface could be between different phases of a liquid.

Although we have a good idea of the characteristics of living things, we don't have a useful definition of "life"
We don't recognise gaseous reactions as "life forms" - all our definitions of living things require a cell wall, transpiration through that wall, and some level of activity that distinguishes the contents of the cell from its environment. It happens that all the life forms we recognise, involve water. Arguably, however, you could substitute sulfur and hydrogen sulfide for oxygen and water if the conditions were right, and produce long, selfreplicating molecules.

[smart]

The ozone layer is an absolute requirement to support life. https://en.wikipedia.org/wiki/Ozone

[Blame]

An active geology. Lack that and eventually life dumps critical elements at the bottom where it can't get them back. Better an active geology is going to deliver chemical energy to the surface that can power life before photo-synthesis can evolve (if it ever does)

Not convinced that life needs water. A really high pressure gas should work as solvent.

[Tim the Plumber]

The ozone layer is an absolute requirement to support life. https://en.wikipedia.org/wiki/Ozone

Would 6 feet of ice work?

How about life that does not rely upon photosynthesis?

[smart]

The ozone layer is an absolute requirement to support life. https://en.wikipedia.org/wiki/Ozone

Would 6 feet of ice work?

How about life that does not rely upon photosynthesis?

Ozone and oxygen are continuously interconverted, so no lifeform could occur without oxygen and ozone.

See https://en.wikipedia.org/wiki/Ozone%E2%80%93oxygen_cycle

[Tim the Plumber]

The ozone layer is an absolute requirement to support life. https://en.wikipedia.org/wiki/Ozone

Would 6 feet of ice work?

How about life that does not rely upon photosynthesis?

Ozone and oxygen are continuously interconverted, so no lifeform could occur without oxygen and ozone.

See https://en.wikipedia.org/wiki/Ozone%E2%80%93oxygen_cycle

There are life forms whch don't use oxygen. Here on earth.

If you live under a rock you will be protected from UV so no need of ozone.