Ann wrote:... If M-type red dwarfs are equally good hosts for life as G-type stars like the Sun, then we have indeed beaten the odds by being in orbit around a G-type stars like the Sun instead of an M-type red dwarf like Gliese 876. I think it is just too improbable that an unusual star like the Sun would be our star, instead of a "dime a dozen" star like Gliese 876, if both are equally good hosts for life. ...
That's not at all how I see it. Life clearly developed here, and if you rule out panspermia and seeding by aliens, life arose here. Since it happened here I have no problem imagining it's happened in a lot of similar situations. We don't know enough about conditions in which life can arise and exist to say whether it can happen on red dwarf planets or not, but if it can then surely there are scads of red dwarf planets with life. And in no way does life existing here rule out life existing around a red dwarf; in fact I see no way in which life existing here could impact the probability of life existing around a red dwarf. They are independent events, at least for practical purposes.
The point I was trying to make is that an M-type star has its habitable zone very close to itself. Remember that the the heat and light that a star emits is not linearly proportional to its mass. Proxima Centauri, the very nearest star apart from the Sun, has a mass about one tenth that of the Sun, but its visual light output is about one part in eighteen thousand that of the Sun. It does a bit better when it comes to infrared light, but even there it only produces one part in 600 that of the Sun.
Conclusion? In order to receive enough heat from Proxima Centauri, a planet would have to cosy up very close to that star. But if the planet was so close to Proxima Centauri, the planet would feel the gravity of its red dwarf sun much, much more than we feel the gravity of the Sun. Remember that while Proxima only produces one part in 600 the heat of the Sun, it holds one tenth the mass of the Sun.
The gravity of Proxima Centauri would cause the planets inside its habitable zone to become tidally locked, or at the very, very least to rotate extremely slowly. That might lead to very strange weather on that planet.
I also think that Proxima Centauri has had a couple of bright ultraviolet flares from its "surface". A planet that orbited very close to Proxima Centauri would be strongly affected by those flares.
My point is that any star is dangerous, so that you don't want to orbit too close to it. But a red dwarf star has such a small habitable zone so close to itself that a planet inside the red dwarf's habitable zone will have to orbit very close to that star.
Like Chris said, we don't know how life elsewhere might adapt to its conditions. It could well be that alien life forms might thrive in living conditions that seem absolutely impossible to us.
Nevertheless, I remain unconvinced that life just adapts to whatever conditions it meets. Perhaps bacterial life can adapt to an incredibly wide range of circumstances, but I doubt that complex life can repeat that trick. Consider Mars. Very many scientists seem to believe that there might be bacterial life on Mars. That is because we can see that there is water on Mars, and we have good reasons to believe that underground water on Mars might very well offer acceptable living conditions for bacteria.
But what about more complex life forms? I have never heard a single astrobiologist suggest that there might be little green men on Mars after all, but in caves underground. No one seems to believe that large complex life forms - as large as earthworms, say - can survive on Mars, or below the Martian surface. What reason do we have to think that large complex life forms can thrive on a planet that has to cosy up very close to its red dwarf star and be subject to that star's strong gravity (strong when you are close to it) and its various outbursts?