Factors a planet needs for suitability of life; perhaps

There are many notions about what life might be like on other worlds.^[001]  However, from the limited examples of what we know about life, it would seem to us that there is a preference for life that is based on carbon and water.  As the study On the probability of habitable planets says,

...exploring the wide field of modern chemistry and challenging the most open-minded chemists reveals that with our present knowledge it is difficult to imagine any alternative chemistry approaching the combination of diversity, versatility and rapidity afforded by liquid water-based biochemistry. This results from the unique ability of carbon to form complex species, and the unique characteristics of water as a liquid solvent...^[002]

Another factor is that that carbon and the molecules that are formed from carbon seem to be very common in our galaxy, being found in interstellar space, other planets, comets, asteroids and space dust.  Organic material seems to be everywhere.^[002]  There is a nebula that is practically made of alcohol.^[003]

Perhaps are search for exoplanet life needs to extend beyond simply looking for water.  Maybe our search should include crosschecking with a search for carbon.

Concept illustration of Kepler-22b, which may be a good
place to search for life

According to On the probability of habitable planets, there are four types of habitability on planets that may harbor life in some form.

• Class I - Habitats where conditions allow for water on the surface, and where energy is primarly provided by the planet's sun.  This is the most Earth-like class.
• Class II - Habitats where the planet may have had water on the surface early on, but conditions did not allow the planet to retain that water.  This is most Mars-like class.
• Class III - Habitats where significant water exists below the surface, and where such underground oceans can interact with a silicate-rich core.  This planets may be too far from their sun to have surface liquid water, but via some process, such as geothermal heating, liquid water is present within the planet.  This is the most Europa-like class.
• Class IV - Habitats where a lot of liquid water exists above an icy layers. Oceans may actually be sandwiched between ice layers.  Ganymede and Callisto may represent this class.

Is complex and even intelligent life possible on any of these classes?  It seems that the most likely class that would have complex life is Class I.  But, of course that is based on assumptions and biases born from our own example.  Classes II, III and IV may extend the limits of what is considered to be the Habitable Zone around a star.

Another factor is the CO₂ cycle.  Perhaps the CO₂ content of a planet will allow that planet to retain more heat from its sun.  

It turns out that a thick CO₂ atmosphere may be one of the most efficient solutions for keeping a planet warm. This is not only due to the properties of the CO₂ gas itself.

However, taking into account the radiative effects of the CO₂ ice clouds, which tend to form in such thick CO₂ atmospheres allows further increases in the warming of the surface thanks to a cloud “scattering greenhouse effect”.  Taking into account this process, the outer edge of the habitable zone has been extended as far as 2.5 AU.^[002]

In other words, CO₂ in the right mixture within a planet's atmosphere may extend the outer limit of how far a way a planet can be from its sun and still be warm enough to support life.  But, other factors must be explored.

[A planet] staying in the habitable zone is obviously not sufficient for a planet to continuously maintain liquid water on its surface: it must have an atmosphere which keeps the surface pressure and the surface temperature (through its greenhouse effect) in the right range, for billions of years.^[002]

In addition to forming the correct atmosphere necessary to support life, a planet must also be able to keep that atmosphere for a very long time. Also, that atmosphere may need to change over time in order to adjust to changes in stellar output.  For example, a planet has to be large enough (or have enough gravity) to keep its atmosphere from escaping, not just as a result of simply drifting away, but also to counter the effect of stellar wind and other star related phenomenon.^[002]

Plate tectonics is another factor that may be important to a planet's ability to support life.  Plate tectonics manage planetary cycles, such as CO₂.^[004]  The process of how a planet develops plate tectonics on a global scale is not well understood.  However, when examining the two examples of planets of similar size within our own solar system, Earth and Venus, the key difference appears to be water.  Perhaps the higher water content of Earth enables plate tectonics.  How special is Earth, after-all?^[002]  

Would an equivalent to plate tectonics be necessary on class III and IV planets?  For those same classes, atmospheres may not be a factor at all, since oceans would be underground.  What other cycles would be necessary in such classes?  How many class I planets with a long term atmosphere and plate tectonics are in Habitable Zones?  There's a lot of open questions.  Another question I have, would we be able and willing to seed Terran lifeforms on these other classes planets (and moons), even within our own solar system, even if we do not intend to colonize them for ourselves?

Pirmary reference:
F. Forget. International Journal of Astrobiology, 13, Issue 3, July 2013, pp. 177-185, arXiv:1212.0113 [astro-ph.EP], On the probability of habitable planets

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Article Series:

• Limited lifespan of Habitable Zones around other stars [and a loosely held secret finally revealed about me]
• Small stars may have stable Habitable Zones, but habitable planets might not be common there
• Habitable Planets around White Dwarfs
• Habitable Worlds Around Binary Star Systems might not match Sci-fi
• How many Earth-like planets are orbiting Sun-like stars?
• First round of life in the Universe might have been possible extremely early
• Factors a planet needs for suitability of life; perhaps
• "Goldilocks zone of metallicity" on a galactic scale
• Maybe we are the first