Chris Peterson wrote:
Ann wrote:On a different note, it's fascinating to note how one thing after another contributes to the habitability of the Earth.
This is, of course, completely true. The Earth is an immensely complex system with all the parts interacting with and changing all the other parts.
Still, we should remember that all these pieces are adaptable. Not much is a requirement in its present form except in the context of all the current pieces. Were this particular carbon recycling component unavailable, others might have evolved. Or, there might simply be more CO2
and life would have evolved to be comfortable with an 80° C climate.
Well, conditions on the Earth have varied enormously over the billions of years that the Earth has been not just "technically habitable", but actually inhabited by biological life forms. Therefore, I quite agree with you that the Earth would almost certainly have remained habitable in an 80o
The Earth would not be habitable to Homo Sapiens
in an 80o
C climate, of course. So take note, Donald Trump.
The possibilities are endless (as we're likely to discover in the near future as we start detecting life in other star systems).
You and I disagree on how common life is on other planets. My belief is that while we are not alone - and how could we ever know that we are alone, even if we are? - complex life forms like, say, earthworms, are extremely rare in the universe. Not only that, but I also believe that simple life is relatively rare. How rare? Let me put it like this. If we have a sample of 100 exoplanets, we are extremely unlikely to find even simple life on even one of them. If we have a sample of 1,000 exoplanets, we are still extremely unlikely to find even simple life on even one of them. If we have a sample of 10,000 exoplanets, we are probably not extremely
unlikely to find even simple life on even one of them. Just unlikely.
I base my reasoning on the assumption that we would be able to detect rather low planetwide levels of simple life forms. If detection requires high levels of simple life forms, the kind of levels that leave quite obvious imprints on the exoplanet's atmosphere, then I think we would still be extremely
unlikely to find even simple life on even one exoplanet out of a sample of 10,000.
Actually I think the figures are even worse. I think we are very unlikely to find easily detectable life on more than one planet in any exo-solar system. So we should talk about the number of exo-solar systems we have found, rather than focusing on the exact number of planets. Therefore, if we have found 10,000 exoplanets, but only perhaps 2,000 exo-solar systems, then the question is whether 2,000 exo-solar systems are enough to have produced at least one planet with easily detectable biological life. I don't think that that number of exo-solar systems is normally enough to find life, at all.
Most stars in the Milky Way are undoubtedly red dwarfs, but I think we underestimate the difficulty for life to establish itself on the surface of a planet orbiting a red dwarf. Red dwarfs are dim and cool and must hug their planets close in order to give them enough warmth to produce liquid surface water. It could well be that it takes, on average, a hundred times the current age of the universe for life to become really successful on the surface of an otherwise suitable planet orbiting a red dwarf. Red dwarfs certainly have time on their side, but that doesn't necessarily mean that they have a lot of life-bearing planets now
It is also very possible that life is extremely
rare on the surface of any planet. Perhaps life is abundant, but only below a protective ice cover of their planet, like on Jupiter's moon Europa or Saturn's moon Enceladus. And that statement doesn't imply that we know that there is life below the surfaces of these two moons in our own solar system, just that we don't know that there isn't
life below the surface of these two moons. If life doesn't "communicate" with its surrounding solar system at all, then how do we know that it is actually there?
I'm not trying to argue with you, Chris, because there is no way that I could prove to you that I'm right. (And when all is said and done, there is no way that I myself could know that I'm right - or wrong! At least not now.) I'm just trying to state my belief. It would be interesting to hear you state yours.