June 6, 2014 | David F. Coppedge

To Be Habitable, a Planet Needs Inhabitants

In a chicken-or-egg conundrum, astrobiologists are asking whether inhabitants are needed to make a planet habitable.

Which came first, the habitable planet or the inhabitants?  The traditional answer has been that habitability precedes life.  Not so fast, says Jorge Zuluaga, lead author of a new paper submitted to Biogeosciences (see arXiv).  A planet may need inhabitants to be habitable.  PhysOrg explains his thinking:

“It is like trying to design the air conditioning system of a vehicle excluding from the heat and mass balance the effect of passengers inside the cabin; what is the purpose of the air conditioning system if, for designing it, we need to assume that the vehicle is empty?” asks Prof. Jorge I. Zuluaga, leading author of the paper. Although the determinant role of life in the present and past state of Earth’s environment has not been definitively probed, there is a growing amount of evidence supporting the idea that our Planet will not be the same if we remove every single form of life from its surface.

Co-author Juan Salazar adds that biology interacts in complex ways with the geological and atmospheric systems of the Earth.  These interactions may keep the Earth habitable: “powerful, though sometimes subtle interactions between their components continuously maintains the system in a regulated equilibrium state.”  If alien scientists were to evaluate the habitability of the Earth without considering its inhabitants, they might come to a very different conclusion.  How can we be sure, therefore, whether exoplanets are habitable or not?  Space.com may be rushing to judgment when writer Mike Wall headlines, “Found! Oldest Known Alien Planet that Might Support Life” (cf. 6/03/14).  Ditto for Astrobiology Magazine and New Scientist, touting “Earth’s bigger, older brother,” a planet around Kapteyn’s Star with “the right stuff for life.”

The article points out that this idea is not new.  Around 1920, Russian physicist Vladimir Vernadsky introduced the concept of a “biosphere.”  Later, James Lovelock, father of the “Gaia Hypothesis,” developed his thinking around interactions between biology, geology, and atmospherics.  So what’s new?

What Zuluaga and his collaborators are attempting to do in their paper is first to demonstrate that excluding life from the “habitability equation” is unnatural and probably misleads the search for actually inhabited planets. In the second place, the authors present a general conceptual basis supporting the development of habitability models that include life.

They’re doing this because most models up till now have not considered life in the equations.  Life has a special property – namely, information – that can radically alter the interaction of planet and habitability.  “Contrary to abiotic processes, living systems contain and maintain impressive amounts of information that give them unparalleled regulatory capabilities,” Zuluaga says.  For instance, life helps regulate the amount of carbon dioxide in the atmosphere, which can dramatically impact the formation of clouds.

Zuluaga and Salazar seem friendly to the Gaia hypothesis.  Their ideas, therefore, are controversial and not acceptable to all astrobiologists.  Still, they have questioned an assumption that will surely lead to further debates about the criteria for habitability.  “Ultimately,” the article ends, “the truth is that finding inhabited, rather than merely habitable, worlds is the actual goal of our ultimate scientific search for life beyond our pale blue dot.”

The Biblical account pictures a world that started abiotic but became biotic within 3 days, right after the dry land was raised up out of the waters.  This was all God’s creative activity.  There never was a long period of earth history without life.  God designed life and the planet that supports them together.  That’s why we find life here, and so far, nowhere else.  The Biblical worldview does not rule out other inhabited planets, but for theological reasons, it seems unlikely there would be other sentient beings made in the image of God, like humans.

While not endorsing the Gaia-like views of Zuluaga and colleagues, we think their paper raises good questions that have been overlooked by the astrobiology community.  Darwinian imagineers too often equate habitability with inhabited.  Finding a sterile planet in a “habitable zone” will not make it habitable.  The growing number of “earth-like” exoplanets found by the Kepler spacecraft could well be lifeless rocks with deadly atmospheres, for all we know.  Never assume.

If they are right, then we have another requirement for habitability (9/09/13, 9/08/12) that must be considered when looking for extraterrestrials:

  1. Galactic Habitable Zone, where a star must be located (09/29/2009);
  2. Circumstellar Habitable Zone, the right radius from the star where liquid water can exist (10/08/2010);
  3. Continuously Habitable Zone, because too much variety can be lethal (07/21/2007);
  4. Temporal Habitable Zone, because habitable zones do not last forever (10/27/2008);
  5. Chemical and Thermodynamic Habitable Zone, where water can be liquid (12/30/2003);
  6. Ultraviolet Habitable Zone, free from deadly radiation (08/15/2006);
  7. Tidal Habitable Zone, which rules out most stars that are small (02/26/2011).
  8. Stable Obliquity Habitable Zone (1/12/2012)
  9. Stellar Chemistry Habitable Zone (9/08/12)
  10. Stellar Wind Habitable Zone (9/19/13, 6/03/14)
  11. Inhabitants, creating a biosphere that can regulate the atmosphere (06/06/14)



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  • John C says:

    It is the elephant in the room. You can’t have life without life. You can’t have habitability without inhabitants. The Earth was created as an environment for human beings with humans very shortly joining that creation to help regulate and adjust it naturally and artificially for the following generations. How wise and wonderful a Creator to leave this certain sign of His handiwork.

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