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I think LIFE itself is common, but sentience is not. For example, I am 75% sure there is life on Europa, but life there will never become intellegent. There will be a lot of microbial diversity, but there will be no higher life, because there is no light, no photosynthesis in the depths of the Europan seas. This is where I think we will find that most life in the universe is in the form of chemosynthetic microbes under the ice of Europa-type worlds.
Earth-like planets that stay habitable are rare in the universe. Venus is a prime example of this, I think it was once truely earthlike world, with oceans, plate tectonics, and life. If we when back 2 billion years Earth and Venus would probably truely look like twins, both with complex communities of microbes and newly oxidized atmospheres; but Venus was in trouble. As with all stars, the sun has gotten more luminouis over time. Sol was 30% dimmer 4 billion years ago than it is now. By 2 billion years ago, geo-biological feedback mechinisms had sucked down the atmospheric CO2 to keep the planet cool, but the Level of CO2 had reached 0, the cooling feedback loop was done. It was the beginning of the end for venus. It kept getting hotter and hotter. Oceans boiled away, ending plat tectonics since crust needs to be hydrated for subduction to occur. Carbonates decomposed, freeing 90 atmospheres of CO2, turning it into the hell it is today. Earth will follow her sister in this fate less than a billion years from now, we have 100 atmospheres worth of CO2 trapped in carbonate rocks, and our plants are in a constant state of CO2-starvation. If Earth would have been 5 million miles closer to the sun, we wouldent be here.
We also should thank good ol' Luna for keeping the earth's axis stabilized.
Now biology comes into play, We come from a long line of faliures. We are Tetrapods, whose lobe-finned ancestors barely sneaked past the ray-finned and shark hordes. We are Mammals, who got lucky when the better designed dinos went bye-bye. We are Primates, who were sent packing to Africa from their North American late Cretaceous birthplace 35 million years ago by the cooling forests and the rodents. We are apes, which barely survived the transition from forest to grassland 5 million years ago. Finnaly, our australopithicine ancestors barely sqweeked through the start of the ice age 2.5 million years ago to give rise to Homo.
So, I think there are, at most, 4 other sentient species in our galaxy.
Earth-like planets that stay habitable are rare in the universe. Venus is a prime example of this, I think it was once truely earthlike world, with oceans, plate tectonics, and life. If we when back 2 billion years Earth and Venus would probably truely look like twins, both with complex communities of microbes and newly oxidized atmospheres; but Venus was in trouble. As with all stars, the sun has gotten more luminouis over time. Sol was 30% dimmer 4 billion years ago than it is now. By 2 billion years ago, geo-biological feedback mechinisms had sucked down the atmospheric CO2 to keep the planet cool, but the Level of CO2 had reached 0, the cooling feedback loop was done. It was the beginning of the end for venus. It kept getting hotter and hotter. Oceans boiled away, ending plat tectonics since crust needs to be hydrated for subduction to occur. Carbonates decomposed, freeing 90 atmospheres of CO2, turning it into the hell it is today. Earth will follow her sister in this fate less than a billion years from now, we have 100 atmospheres worth of CO2 trapped in carbonate rocks, and our plants are in a constant state of CO2-starvation. If Earth would have been 5 million miles closer to the sun, we wouldent be here.
We also should thank good ol' Luna for keeping the earth's axis stabilized.
Now biology comes into play, We come from a long line of faliures. We are Tetrapods, whose lobe-finned ancestors barely sneaked past the ray-finned and shark hordes. We are Mammals, who got lucky when the better designed dinos went bye-bye. We are Primates, who were sent packing to Africa from their North American late Cretaceous birthplace 35 million years ago by the cooling forests and the rodents. We are apes, which barely survived the transition from forest to grassland 5 million years ago. Finnaly, our australopithicine ancestors barely sqweeked through the start of the ice age 2.5 million years ago to give rise to Homo.
So, I think there are, at most, 4 other sentient species in our galaxy.
I consider that equation a bunch of over-optimisstic crap.
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