> Basically, we were discussing about habitable planets, how to get them, and what the stellar
> implications were. My basic position was that for a planet to have an nitrogen/oxygen
> atmosphere (i.e. atmosphere codes 2-9;D-F, or at least 4-9) requires some kind of biosphere
> that took a really, really long time to develop.
Generally this seems to be true. On Earth, oxygen didn't start building up in the atmosphere until it had essentially dissolved to capacity in the oceans and reacted with sediments and got locked up in whatever rocks it could bind with too.
> If a world had no life, the oxygen wouldn’t be in the atmosphere.
That isn't necessarily true. There is another possible way for oxygen to show up - photodissociation of water vapour in the atmosphere by solar UV radiation (UV hits H2O, H2 is lost to space, O hangs around in the atmosphere). On Earth there isn't really enough water vapour in the atmosphere for it to build up, but an panthalassic/ocean world where H2O may be a significant percentage of the atmospheric composition, it's possible that enough of it could dissociate over time to build up O2 in the atmosphere. Someone's probably done more exact calculations, but I would think you'd only get a breathable atmosphere like this in rare cases (the O2 has to dissolve in a lot more water, for one. And having that much water in the atmosphere would end up with a 'moist greenhouse39; that wouldn't exactly add to the world's habitability).
> Since that level of oxygen takes so long and takes at least some kind of life, that means
> such a planet really needs a reasonably stable star, meaning likely one of F G K M main
> sequence.
Generally, yes. Late F V to mid K V is probably best for an earthlike planet (lower mass stars would tidelock planets in their habitable zones).
> Obviously, the ancients can influence this, but there *were* only 400 of them (even if they
> controlled billions of subjects, there were only 400 of them), and even then there is only so
> much they can do over their 10,000 – 20,000 years of activity.
And even if they did anything, 300,000 years is enough time for any changes that they made to decay back to their natural state.
> - Is the above even remotely close to correct?
At least some of it is, as best we know. :)
> - Granted, we only have a sample of 1, but is it feasible for primitive life to generate an
> appreciable oxygen atmosphere over 300,000 years? Or would it take millions of years?
On a planetary scale? No way. It'd take hundreds of millions of years.
> - If a world had a type 6 atmosphere, but lost its entire biosphere, how long would it take
> to lose its oxygen?
If all life of every kind just magically keeled over and died and nothing else arose in its place? I don't know. I'm not sure what would happen to all the dead organic material of the biosphere if all the bacteria etc that caused organic matter to decay died out too. Would the oxygen in the atmosphere chemically react with it somehow? Weather presumably would still happen, so the remaining oxygen could be used up in the fires caused by lightning strikes. At a guess, I'd say... maybe a million years? Possibly less? Oxygen's a pretty reactive gas after all.
> - Assume a world with a thick layer of frozen volatiles outside the a star’s habitable zone.
> Assume it is as ideal for this situation as possible. When the star enters its giant phase and
> the planet finds itself in the habitable zone, would there be enough time for life and oxygen
> to develop before the star burns out?
If there's a few billion years of stability during a giant phase, maybe (only likely with K V stars, and even the oldest of those haven't had time since the formation of the universe to evolve off the main sequence yet). It probably won't happen during the subgiant or red giant branches, but the Horizontal branch is the most stable of the post-MS stages, so possibly then? But again, I can't see that happening for stars that are 1 solar mass or more massive.
> implications were. My basic position was that for a planet to have an nitrogen/oxygen
> atmosphere (i.e. atmosphere codes 2-9;D-F, or at least 4-9) requires some kind of biosphere
> that took a really, really long time to develop.
Generally this seems to be true. On Earth, oxygen didn't start building up in the atmosphere until it had essentially dissolved to capacity in the oceans and reacted with sediments and got locked up in whatever rocks it could bind with too.
> If a world had no life, the oxygen wouldn’t be in the atmosphere.
That isn't necessarily true. There is another possible way for oxygen to show up - photodissociation of water vapour in the atmosphere by solar UV radiation (UV hits H2O, H2 is lost to space, O hangs around in the atmosphere). On Earth there isn't really enough water vapour in the atmosphere for it to build up, but an panthalassic/ocean world where H2O may be a significant percentage of the atmospheric composition, it's possible that enough of it could dissociate over time to build up O2 in the atmosphere. Someone's probably done more exact calculations, but I would think you'd only get a breathable atmosphere like this in rare cases (the O2 has to dissolve in a lot more water, for one. And having that much water in the atmosphere would end up with a 'moist greenhouse39; that wouldn't exactly add to the world's habitability).
> Since that level of oxygen takes so long and takes at least some kind of life, that means
> such a planet really needs a reasonably stable star, meaning likely one of F G K M main
> sequence.
Generally, yes. Late F V to mid K V is probably best for an earthlike planet (lower mass stars would tidelock planets in their habitable zones).
> Obviously, the ancients can influence this, but there *were* only 400 of them (even if they
> controlled billions of subjects, there were only 400 of them), and even then there is only so
> much they can do over their 10,000 – 20,000 years of activity.
And even if they did anything, 300,000 years is enough time for any changes that they made to decay back to their natural state.
> - Is the above even remotely close to correct?
At least some of it is, as best we know. :)
> - Granted, we only have a sample of 1, but is it feasible for primitive life to generate an
> appreciable oxygen atmosphere over 300,000 years? Or would it take millions of years?
On a planetary scale? No way. It'd take hundreds of millions of years.
> - If a world had a type 6 atmosphere, but lost its entire biosphere, how long would it take
> to lose its oxygen?
If all life of every kind just magically keeled over and died and nothing else arose in its place? I don't know. I'm not sure what would happen to all the dead organic material of the biosphere if all the bacteria etc that caused organic matter to decay died out too. Would the oxygen in the atmosphere chemically react with it somehow? Weather presumably would still happen, so the remaining oxygen could be used up in the fires caused by lightning strikes. At a guess, I'd say... maybe a million years? Possibly less? Oxygen's a pretty reactive gas after all.
> - Assume a world with a thick layer of frozen volatiles outside the a star’s habitable zone.
> Assume it is as ideal for this situation as possible. When the star enters its giant phase and
> the planet finds itself in the habitable zone, would there be enough time for life and oxygen
> to develop before the star burns out?
If there's a few billion years of stability during a giant phase, maybe (only likely with K V stars, and even the oldest of those haven't had time since the formation of the universe to evolve off the main sequence yet). It probably won't happen during the subgiant or red giant branches, but the Horizontal branch is the most stable of the post-MS stages, so possibly then? But again, I can't see that happening for stars that are 1 solar mass or more massive.