La présence de la vie sur d’autres planètes (abiogenèse) est-elle pratiquement nécessaire ? Autrement dit, comme le nombre de planètes dans l’univers présentant comme la Terre des conditions favorables à l’apparition et au développement de la vie est incalculablement grand, il serait impossible en pratique que l’abiogenèse n’existe que sur la Terre. Il ne s’agirait peut-être pas de vie rigoureusement semblable à ce qu’est la vie sur la Terre, mais elle en aurait les principales qualités. Partout il serait possible de distinguer le vivant du non vivant.
Mais sur le plan des preuves théoriques rien ne permet d’affirmer que de la vie existerait sur d’autres planètes. Une probabilité n’est pas une preuve au sens scientifique .
Daniel Whitmire Daniel P. Whitmire, dans l’article référencé ci-dessous, dont nous publions l’abstract, considère au contraire qu’observer l’abiogenèse sur la Terre n’est pas neutre concernant l’existence de vie sur d’autres planètes semblables à la Terre. La vie ne devrait pas y être un phénomène exceptionnel. C’est bien ce que pense la communauté des exobiologistes. C’est bien pourquoi aussi des sommes considérables sont dépensées pour l’organisation de missions spatiales sur le Lune ou sur Mars à la recherche de traces de vie dans le passé ou le présent de ces planètes..
Référence
Abiogenesis: the Carter argument reconsidered
Published online by Cambridge University Press: 23 September 2022
Daniel P. Whitmire
Abstract
The observation of life on Earth is commonly believed to be uninformative regarding the probability of abiogenesis on other Earth-like planets. This belief is based on the selection effect of our existence. We necessarily had to find ourselves on a planet where abiogenesis occurred, thus nothing can be inferred about the probability of abiogenesis from this observation alone. This argument was first formalized in a Bayesian framework by Brandon Carter. Though we definitely had to find ourselves on a planet where abiogenesis occurred, I argue here that
(1) the Carter conclusion is based on what is known as the ‘Old Evidence Problem’ in Bayesian Confirmation Theory and that
(2) taking this into account, the observation of life on Earth is not neutral but evidence that abiogenesis on Earth-like planets is relatively easy. I then give an independent timescale argument that quantifies the prior probabilities, leading to the inference that the timescale for abiogenesis is less than the planetary habitability timescale and therefore the occurrence of abiogenesis on Earth-like planets is not rare.
Current theories of abiogenesis (AB) vary in extremes from it being thermodynamically favoured (England Reference England2013) and therefore presumably nearly automatic given the same chemical and environmental conditions as existed on early Earth, to an occurrence of less than once in the history of the observable universe (Totani Reference Totani2020).
Historically, the Principle of Mediocrity (along with evidence that life on Earth appeared relatively early) was used to argue that AB is likely given similar chemical and environmental conditions as early-Earth (e.g. Shklovskii and Sagan Reference Shklovskii and Sagan1966).
Subsequently, with the publication of Carter’s anthropic selection principle (Carter Reference Carter1974) and its importance in evolutionary biology (Carter Reference Carter1983; see also Crick Reference Crick1981) it was realized that the assumption that Earth is a random member of the reference class of all early-Earth-like planets was not justified, by virtue of our existence. We necessarily had to find ourselves on a planet where AB occurred.
It is a fact that we (and life on Earth) exist regardless of whether AB is easy or hard or something in between. If there were only a single example of life in the universe then it would necessarily be Earth. The Principle of Mediocrity is still useful but should be applied only after all relevant anthropic selection effects are taken into account (Whitmire Reference Whitmire2020).
For example, Earth is not a typical planet even in our own Solar System, but may be typical of planets that host intelligent observers.
Besides the selection effect of our existence, the occurrence of relatively early AB on Earth might be a selection effect due to the long evolutionary time required for an intelligent species to evolve after AB has occurred (Lineweaver and Davis Reference Lineweaver and Davis2003). A Bayesian analysis of whether early AB on Earth is evidence of AB being easy in general on other early-Earth-like planets has been addressed by Spiegel and Turner (Reference Spiegel and Turner2012; see also Kipping Reference Kipping2020).
Their analysis specifically takes into account the time required for the evolution of intelligent observers. They conclude that although there is some evidence in favour of easy AB it is not significant and that the posterior probability for easy AB depends almost entirely on the assumed prior; and that it is not possible to reject the hypothesis that Earth is the only location of intelligent life in the universe.
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