Does the existence of life on Earth tell us anything about the probability of abiogenesis — the origin of life from inorganic substances — arising elsewhere? That question has confounded scientists and anyone else inclined to ponder it for some time.
The Carter argument
A widely accepted argument from Australian-born astrophysicist Brandon Carter argues that the selection effect of our existence puts constraints on our observation. Since we had to find ourselves on a planet where abiogenesis occurred, nothing can be inferred about the probability of life elsewhere based on this knowledge alone. At best, he argued, the knowledge of life on Earth is of neutral value.
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Another way of looking at it is that Earth can’t be considered a typical Earth-like planet because it hasn’t been selected randomly from the set of all Earth-like planets.
Old evidence problem
However, a new paper by Daniel Whitmire, a retired astrophysicist who teaches mathematics at the University of Arkansas, argues that Carter used faulty logic. Though Carter’s theory has become widely accepted, Whitmire argues that it suffers from what’s known as “The Old Evidence Problem” in Bayesian Confirmation Theory, which is used to update a theory or hypothesis in light of new evidence. After giving a few examples of how this formula is employed to calculate probabilities and what role old evidence plays, Whitmire turns to the conception analogy.
As he explains:
“One could argue, like Carter, that I exist regardless of whether my conception was hard or easy, and so nothing can be inferred about whether my conception was hard or easy from my existence alone.”
In this analogy, “hard” means contraception was used. “Easy” implies no contraception was used. In each case, Whitmire assigns values to these propositions. Whitmire continues:
“However, my existence is old evidence and must be treated as such. When this is done, the conclusion is that it is much more probable that my conception was easy. In the abiogenesis case of interest, it’s the same thing. The existence of life on Earth is old evidence, and just like in the conception analogy, the probability that abiogenesis is easy is much more probable.”
Existence of life on other planets
In other words, the evidence of life on Earth is not of neutral value in making a case for life on similar planets. According to Whitmire’s explanation, other observers could exist on other planets similar to Earth. Life is more likely to emerge on other Earth-like planets — maybe even on the recent “super-Earth” type planet, LP 890-9b, discovered 100 light years away.
Those with a taste for math can read Whitmire’s paper, “Abiogensis: The Carter Argument Reconsidered,” in the International Journal of Astrobiology.
Provided by the University of Arkansas [Note: Materials may be edited for content and length.]