Define Habitable

by Dr. Elizabeth Mitchell on June 2, 2012
Featured in News to Know

Define habitable. It’s harder than it sounds!

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The question of whether alien life exists moved from the realm of science fiction to a real search for extraterrestrial life long ago. Space is a big place, so scientists want to know where to look—hence the need to define habitable. Just where would alien life be, if it exists?

Of course the search for alien life is driven by the belief that life evolved by random processes and therefore could have evolved anywhere conditions were right. The “right” conditions would depend on the characteristics of such a life-form, so scientists have long tried to determine the sorts of organisms that could evolve. Only then can they know where to look. (“Look” here is a somewhat metaphorical or wishful term, since our ability to actually “look” for alien life in most of the galaxy is limited by great distances.)

As data from the Kepler project and other telescopes have become available, the number of planets deemed “habitable” has become truly astronomical.

As data from the Kepler project and other telescopes have become available, the number of planets deemed “habitable” has become truly astronomical. At least 770 exoplanets have already been discovered. Now that more star survey data have been collected, astrobiologist Caleb Scharf says, “We could be in a situation where there are a billion objects that conceivably match expectations for habitability—the right-sized planet around the right sort of star. How do you categorize them in a tree of possibilities for life?”

Scientists are devising indices to denote the likelihood of a planet being habitable. There are multiple ways to categorize habitability. Some are deduced from astronomical assessments such as size, orbit, and luminosity of the associated star. Others relate to supposed stellar and planetary history, thereby being based on unverifiable assumptions about the distant past. Even astronomical observations are limited by present technology, it not being possible for instance to be certain of planet composition and of any atmosphere so far away.

“Goldilocks planets” are those deemed to be neither too hot nor too cold (a la the porridge from the “The Three Bears”/Goldilocks fairy tale) to support liquid water, which is necessary for life as we know it. Dr. Abel Mendez of Puerto Rico’s Planetary Habitability Laboratory has codified these parameters as the Habitable Zone Distance scale. So far, four known planets appear habitable by this index, although scientists note that conditions can fluctuate over time, ending “evolutionary experiments” with life.

To take into account these changing conditions, a “dwell time index” purports to compare how long a planet has actually been in the habitable zone with the time required for life to evolve on earth. This index involves two major areas of unverifiable speculation, since neither the stellar history “out there” nor the evolution of life “down here” can be evaluated in the present day by experimental science. Uncertainty about stellar history afflicts even cosmology based on conventional evolutionary assumptions because “many of these alien worlds confound conventional theories of planet formation and solar system development.”

The Earth Similarity Index rates planets “by their resemblance to our own planet, on a scale from zero to one, based on their mass, radius, density and surface temperature.” According to Mendez, “This index is based on life as we know it—and that is Earth.” Compared to earth’s “1,” 0.66 for Mars sounds promising, and GJ667Cc at a distance of 0.85 seems to be the highest yet. At a distance of 22 light years, however, both diplomacy and specimen collection are out of the question.

Once again, scientists are assuming that life actually evolved, and then assuming that any evolving life would have followed earth’s evolutionary pattern.

Once again, scientists are assuming that life actually evolved, and then assuming that any evolving life would have followed earth’s evolutionary pattern. While evolutionary scientists readily admit that unusual life-forms quite unlike earth’s could exist, the baseline assumption that life evolved at all cannot be falsified or verified by experimental science. It is pure imagination.

In view of the assumptions underlying the expectations for finding extraterrestrial life, two Princeton scientists have advised caution. Their study, published earlier this year in the Proceedings of the National Academy of Sciences, evaluates the great expectations reflected in so many habitability headlines. By using a Bayesian statistical analysis designed to filter out the effect of assumptions on scientific conclusions, astrophysicist Edwin Turner and exoplanet expert David Spiegel conclude the expectations of finding extraterrestrial life have been built “more on optimism than scientific evidence.”1 Turner explains, “Our paper really just calls into question one of the few pieces of evidence we use for life elsewhere. The fact that life developed on Earth quickly has encouraged people to think that the probability of it happening elsewhere is quite high. It’s consistent, but it’s not really strong evidence. In fact there is no evidence.”2

The Princeton scientists point out that many scientists assume life would have evolved as quickly elsewhere as on earth, and other scientists note the tendency to assume life would evolve along the same lines as on earth. However, no one (in the evolutionary camp) seems to be noticing the greatest assumption of them all—the idea that life randomly evolved from nonliving components. Abiogenesis is now assumed as a fact of earth’s past by evolutionary scientists, yet it has never been observed. If admitting the other assumptions suggests a caveat even in the evolutionary community, then factoring in the fact that the foundational concept of the whole habitable planet hunt is itself a mere assumption would cripple the entire operation.

The starting assumptions about the origin of life determine the final interpretation of data. Turner says, “If scientists start out assuming that the chances of life existing on another planet as it does on Earth are large, then their results will be presented in a way that supports that likelihood.”

Origins science attempts to peer into the untestable, unobservable, unrepeatable past, thereby necessitating many starting assumptions. God shared His eyewitness account of our origins in the Bible, and accepting His eyewitness account is the basis of a biblical worldview. How much more would a biblical worldview that God created all that is—as He said He did—about 6,000 years ago (and without evolution) overturn present evolutionary dogma, not to mention the hunt for E.T.’s home?

Read more about what the Bible does and does not say about the possibilities of life in outer space at “Kepler’s Mission: To Boldly Seek Out Where Life Could Have Evolved.”

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  1. Morgan Kelly, “Expectation of Extraterrestrial Life Built More on Optimism than Evidence, Study Finds,” News at Princeton, April 26, 2012,
  2. Wendy Plump, “Princeton Scientists Say, If Found, Alien Life Won’t Resemble Familiar Forms,”, April 28, 2012,


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