For Bbella: transient alien life (Introduction)

by David Turell , Sunday, June 12, 2016, 23:17 (2868 days ago) @ David Turell

Another article on the issue of the difficulty in getting alien life started, and that active biological life stabilizes a young planet:-http://phys.org/news/2016-06-alien-life-exoplanets-dies-young.html-"Many scientists and commentators equate "more planets" with "more E.T.s". However, the violence and instability of the early formation and evolution of rocky planets suggests that most aliens will be extinct fossil microbes.-***-"In research published in the journal Astrobiology, we argue that early extinction could be the cosmic default for life in the universe. This is because the earliest habitable conditions may be unstable.-"In our "Gaian Bottleneck" model, planets need to be inhabited in order to remain habitable. So even if the emergence of life is common, its persistence may be rare.-"Mars, Venus and Earth were more similar to each other in their first billion years than they are today. Even if only one of the planets saw the emergence of life, this era coincided with heavy bombardment from asteroids, which could have spread life between the planets.-"But about 1.5 billion years after formation, Venus started to experience runaway heating and Mars experienced runaway cooling. If Mars and Venus once harboured life, that life quickly went extinct.-"Even if wet rocky Earth-like planets are in the "Goldilocks Zone" of their host stars, it seems that runaway freezing or heating may be their default fate.-"Large impactors and huge variation in the amounts of water and greenhouse gases can induce positive feedbacks cycles that push planets away from habitable conditions.-"The carbonate-silicate weathering cycle, which provides the major negative feedback to stabilise Earth's climate today, was probably inoperative, or at least inefficient, until about 3 billion years ago.-"However, life on Earth may have had the fortuitous ability to create stability by suppressing the positive runaway feedback loops and enhancing the negative feedback loops.-***-"As soon as life became widespread on Earth, the earliest metabolisms began to modulate the greenhouse gas composition of the atmosphere. It is no coincidence that methane, carbon dioxide, hydrogen and water are all potent greenhouse gases and also the reactants and products of metabolic reactions of the earliest microbial mats and biofilms.-"The emergence of life's ability to regulate initially non-biological feedback mechanisms (what we call "Gaian regulation") could be the most significant factor responsible for life's persistence on Earth.-***-"We hypothesise that even if life does emerge on a planet, it rarely evolves quickly enough to regulate greenhouse gases, and thereby keep surface temperatures compatible with liquid water and habitability.-"Maintaining life on an initially wet rocky planet in the habitable zone may be like trying to ride a wild bull. Most riders falls off. So inhabited planets may be rare in the universe, not because emergent life is rare, but because habitable environments are difficult to maintain during the first billion years.-***-"We should not expect technological or spacefaring civilisations because there is no evidence that biological evolution converges to human-like intelligence. And subjective philosophical notions of life in the universe should not inform our estimates of the probability of life beyond Earth."-Comment: Our current atmosphere was provided by biologic activity. The authors have made a very interesting point, and we know it worked on Earth. We are a privileged planet.