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Is There Life Out There? Another Step Toward Its Improbability


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Given all the UFO talk lastly I thought this might shead some light on the subject

Dr Michael G Strauss

June 23 2019


When I give talks on complex life in the universe I will often quote a quantitative estimate of the probability of finding such life made by the astrophysicist Hugh Ross. In 2004, Ross compiled a list of 322 parameters that are necessary for a truly "earth-like" planet to exist,4 and in 2009, he released another list of 676 parameters that are necessary to sustain uni-cellular life for 3 billion years on a planet.5 These estimates are what scientists often call "back of the envelope" or "order of magnitude" calculations. They are not meant to be exact but to give a best approximation based on known quantities. In his calculation, Ross includes correlations and the expected number of planets in the visible universe. He determines that the probability for finding any planet in the visible universe that meets the requirements to be 1 in 10282 for the first case and 1 in 10556 in the latter case. With such low probabilities it is basically impossible that any other complex life exists in the visible universe. (It should be noted that as a theist I believe that since God created at least one planet with life he could have created many planets with life despite the odds. A discovery of another planet with life may actually increase the evidence for a creator if these low odds continue to hold up to more scrutiny.)

I am often criticized for taking Ross's estimation seriously. Critics argue that Ross is being too conservative in his estimation, and those who have looked at his calculation in detail claim that his numbers are not well documented. (As to the latter criticism, I agree that although Ross lists references for all of his numbers, it is nearly impossible to ascertain the individual probabilities from the references as presented in his table. It would be helpful for him to footnote each individual probability with its references.) Regarding the first criticism, I believe that Ross's calculation only seems to be conservative because other scientists have not taken the effort to compile a comprehensive list of the conditions necessary to support complex life, and because scientists too easily accept the rhetoric to "follow the water" expecting that life can thrive given only a few simple ingredients rather than a long list of necessary requirements.

However, a recent publication takes another step that narrows the probability for finding complex life and provides more validation for the low probability of finding life that Ross has calculated. In this recent paper, five scientists have examined the extent of the habitable zone around a star, considering not just the presence of liquid water, but also whether or not certain gases in the planet's atmosphere, such as CO and CO2, would be toxic for any life more complex than simple microbial life.6 The new paper shrinks the habitable zone for simple life by a factor of two and for complex life like humans to one-third of the previous estimate.


3Brownlee, Donald, and Peter D. Ward, Rare Earth: Why Complex Life is Uncommon in the Universe, (Copernicus Books: New York, NY), 2000.
6Schwieterman Edward, et.al., "A Limited Habitable Zone for Complex Life," The Astrophysical Journal, June 10 (2019)

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