The Shrinking Habitable Zone

Otis Graf is a member of Reasons to Believe’s Apologetics Community.  He recently posted a message which deserves to be known to everyone.

I have written often about “the fine-tuning of the universe” (see joshualetter/blog, “The Existence of God – Four Philosophical Arguments,” June 28, 2018 post, chapter II).  Otis presents yet more evidence that our hospitable planet could not have occurred anywhere in the universe by chance, not even once.  Here is Otis’ message, with almost no editing.

A team of five scientists from NASA and several universities has, for the first time, shown that most exoplanets that were thought to be potentially habitable are, in fact, toxic to complex life.  Here, “complex” means oxygen-dependent multicellular life.

Here is the popular article which is a news release from Univ. California Riverside: “New study dramatically narrows the search for advanced life in the universe.  Toxic gases limit the types of life we could find on habitable worlds.”  Here is the paper that was published in the Astrophysical Journal.  “A Limited Habitable Zone for Complex Life

It turns out that for Sun-like stars, most of the “traditional habitable zone” [where water can exist in liquid form] is ruled out because those regions require poisonous concentrations of CO2 in order to remain warm enough to host liquid surface water.  The smaller M dwarf stars will produce atmospheres of toxic carbon monoxide.

The search for intelligent life (SETI) takes a particularly hard hit from this research.  This is a conclusion from the published paper:

One implication is that we may not expect to find remotely detectable signs of intelligent life (“technosignatures”) on planets orbiting late M dwarfs or on potentially habitable planets near the outer edge of their HZs. These CO2 and CO limits should be considered in future targeted SETI searches.

The paper’s authors even invoke anthropic reasoning: “More broadly, limitations on complex life by CO2 and CO may partially address why we find ourselves near the inner edge of the HZ of a G-dwarf star rather than near the center or toward the outer edge of the HZ around one of the much more numerous M-dwarf stars.”

In the UC news release, the lead author of the paper is quoted as saying this:

I think showing how rare and special our planet is only enhances the case for protecting it.  As far as we know, Earth is the only planet in the universe that can sustain human life.

That sums it up.  Earth really is an Improbable Planet.

Otis Graf

Houston, TX

And remember: there are not one but eleven known habitable zones.  For advanced life to be possible, its planet must be in all eleven HZ’s at the same time and must remain there for a long, long time. See



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