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I was watching the new Cosmos and Neil DeGrasse Tyson said something that made me go hmmm...

He mentioned that the only reason that the Earth did not experience the runaway greenhouse effect like Venus is that life had evolved to the point at which it could feed on carbon and take it out of the atmosphere and deposit it in solid form as limestone.

The question is, if we were to find a planet in the Goldilocks zone, how likely is it that it would be habitable for life without already harboring life? Would we expect that a similar scenario would have had to taken place on that planet to curb the greenhouse gasses in the atmosphere, thereby turning it into another Venus? Can a carbon cycle exist without life?

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  • $\begingroup$ Venus is an unknown planet. Our nearest and most similar. And the planet about which we know the least of all. It might well be the opposite to that old "run away greenhouse gas"-myth, that Venus instead formed with intense water vapor as a greenhouse gas, and later cooled as the water went off into space. Venus, not Pluto, is in fact the planet which existence the IAU denies. $\endgroup$
    – LocalFluff
    Commented Oct 15, 2014 at 22:53
  • $\begingroup$ The sun's lower luminosity (about 75% of current) 3.5 billion years ago likely also played a role in reducing CO2 (or H2O) greenhouse effect back when the first CO2 fixing organisms appeared. dnva.no/geomed/solarpdf/Nr_10_Hessen.pdf scientificamerican.com/article/… Of course once self-replicating organisms got started they naturally evolved to handle whatever environmental legacy left by previous generations of replicants. That organic process allows for some slop in placement of life bearing planets within the Goldilocks zone. $\endgroup$
    – Wayfaring Stranger
    Commented Oct 16, 2014 at 1:20
  • $\begingroup$ This is Neil DeGrasse Tyson's speculation. We know that life on Earth started immediately after the Late Heavy Bombardement and therefore has had to evolve much earlier. At that time Venus and Mars were like the Earth, habitable planets apart form the bombardment by asteroids, some of which large enough to vaporise all of the oceans. To me this suggests that the Late Heavy Bombardment itself played an import role in the origins of life, perhaps by moving essential molecules from planets to space and back. Venus, Mars and early Earth could all have contributed to life here on Earth. $\endgroup$
    – Count Iblis
    Commented Oct 16, 2014 at 2:55

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To answer your question directly, it is quite unlikely that planets would be habitable without already having life. It would have to be in a near-perfect condition, which would be statistically unlikely. In case of planets already having life, the existence of life provides robustness to small changes and makes habitability possible in a larger range of conditions.

I am assuming 'habitable' here means habitable for man, trees and an ecosystem etc. Remember that there are extremophiles (or certain other hardy species) which completely change the perspective on habitability, and are often the key in making those places habitable for other forms of life.

This is also true for parts of earth which undergo a sudden change and are not 'habitable' for most forms of life till some hardy species takes over and changes the conditions, and consequently allows other species to be able to survive. See, for example, ecological succession (https://en.wikipedia.org/wiki/Ecological_succession), esp. primary succession and pioneer species (follow the links within the article).

Also, this is a long process. In case of the earth, there was also evolution that needed to happen, so it probably won't take as long as a few billion years, but even on earth, it takes a few years to make small disturbed microhabitats habitable. The atmosphere and other factors (soil, moisture etc., for the plants) need to be made suitable for living. Check this interesting article on terraforming: https://microbewiki.kenyon.edu/index.php/Terraforming

Hope this was helpful. I don't really know if a carbon cycle is possible outside of living organisms in some way.

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Maybe he was thinking about the Great Oxidation Event? It was produced by early life forms (such as cyanobacteria) and, even if it caused an extinction at the time, it is thought that it allowed subsequent, more complex life forms to emerge.

Can a carbon cycle exist without life?

(This question might get better answers at Earth Science SE maybe?)

To answer this part: yes, a carbon cycle can exist without life. I don't know for the atmospheric carbon cycle, but there is also a deep carbon cycle, where carbonates precipitate at the bottom of oceans, reach the mantle through subduction, and eventually go back to the surface as CO$_2$ through volcanism. Many carbonates are formed by life, but they can also be formed by non-biological processes.

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