Question about a fictional planet - star system

[brdavis]

So... the atmosphere needs to have H2O in it.

Not bloody much of it. On the day side, there will be next to no water vapor in the atmosphere because while it is hot, there's also no source. On the dark side, the equilibrium vapor pressure will be very low due to the temperature. So, yes, water vapor in the atmosphere, but *not* remotely uniformly distributed, and not high levels (for some atmospheres).

When the planet is formed, that H2O will not be immediately sequestered in ice sheets on the dark side (if there is even a dark side early in formation).

Nope, you're right - early on the planet would likely not be a synchronous rotator. So you are likely to end up with a warm wet atmosphere. Perhaps not unlike the Earth. Note that Earth isn't remotely in a runaway (or even moist) greenhouse state; due to the tropospheric cold trap, Earth holds onto water just fine at Earth-normal insolation. That's exactly the type of system the paper in question was studying; Earth-like sized worlds with Earth-like scale insolations. Those do not have problems with runaway greenhouse effects.

Its climate may be stable later with massive ice sheets on the dark side sequestering all that H2O... but would those massive ice sheets form in the first place with that H2O free?

Almost certainly. Take a look at the Earth. We have massive ice sheets at the poles (and have had still more massive ones in the past) which are location that are *not* in shadow at all times. Yep, if the Earth became tidally locked, it would build up a big ice sheet on the night side as well… remarkably rapidly actually (do the thermal analysis for the Earth and you find you could have effective cooling over the course of *months*, not years or decades under those conditions).

Feedback loops often lead to stable "on" or "off" conditions.

Combinations of positive and negative feedbacks can… they can also lead to stabilizing systems (the carbonate-silicate feedback system on Earth, for a canonical example).

If something is "off" it takes a large perturbance to switch it "on", and vice versa.

It might take a larger than normal perturbance. That's why the GCM was run with the presence of starspots. To make sure that such a perturbance did *not* drive the climate state into an atmospheric collapse state.

I'm asking if there is a viable model of planetary evolution that reaches that condition.

A reasonable question. So look at it this way:

A Earth-sized world forms close enough to a red dwarf that it experiences Earth-normal heating. The result is, not surprisingly, and Earth-normal environment… possibly including plate tectonics, oceans, and all the rest. Over time the rotation rate slows, so "nights" get longer and longer… finally getting long enough that significant portions of the water cycle become trapped in "temporary" (month to year long 'nighttimes') on night side. The planet is still rotating, but now increasing amounts of water vapor are being trapped in ice sheets. Even while the day side becomes locally warm enough to loose its tropospheric cold trap, the amount of water vapor in the atmosphere is dropping… sharply dropping, since more and more of the water is being locked up, gradually, on the night side. Worse yet, those large sheets of ice (if we are talking "normal" oceans depths, piling the Earth's water on one hemisphere could produce a 4+ km thick hemispheric ice sheet) make tidal locking easier, and the process continues.

Eventually a tidal lock occurs, with the climate having shifted fairly smoothly from Earth-normal to a locked state with a N2-dominated atmosphere and almost all the water locked up on the night side.

Additionally, it takes some time for CO2 atmospheres to form, no? from volcanic outgassing?

Not very long, actually… *especially* for an Earth-sized world. Remember the models didn't require thick CO2 atmospheres, but fairly thin ones to prevent atmospheric collapse. Earth's natural variation in CO2 is more than enough to pop levels up to 30 mb in a few million to tens of million of years, much much shorter than the tidal locking timescales in this situation.

Might it be that the outgassing rate was never sufficiently high, and it just accumulated on the dark side as it outgassed (again, this depends on how quickly the planet becomes tidally locked)?

As near as we can tell (based, again, on things like the responsiveness of the carbonate-silicate cycle here on Earth), the characteristic time for CO2 in the Earth's atmosphere is on the order of 10's of thousands of years; unless the tidal locking timescale is shorter than this (which it could be around *very* dim stars), that seems unlikely to be a problem.

I think a model that doesn't account for an early steam atmosphere is incomplete.

See the above evolution; if the Earth, with Earth-normal insolation, avoids this problem, there's no reason to think a tidally-locked Earth, with Earth-normal insolation would have a different one.

I'm not ready to say that based on one paper... You haven't linked the paper…

Sorry, take a look here:

http://crack.seismo.unr.edu/ftp/pub/gillett/joshi.pdf

As a molecular biologist... I'm used to a whole lot more controls, and looking for other variables that could affect the outcome.

As a physicist who's done a lot of research about planetary science… yes, I'd like to have a bunch of controls too . Planetary science might not have the same standards as molecular biology does in this regard. But read the paper; they *do* consider a number of different atmospheres, including N2 dominated ones.

(regarding life starting around deep-sea hydrothermal vents…)

thats sort of the pop-culture interpretation, which is a little out of date. As per the RNA world hypothesis, when looking for the first life, we look for the first self replicating RNAs. We've made a number of RNA directed RNA polymerase Ribozymes (that is, RNA that copies other RNAs).

No, I understand the RNA world hypothesis. I was referring to the bioenergetic origin of a some of the basic energetic cycles. I mean I am NOT a molecular biologist, but I found this review-ish article in the Royal Transactions rather good:

http://rstb.royalsocietypublishing.org/content/368/1622/20130088

It turns out, cryogenic conditions are actually much better for this to occur...

In that case, the presence of a large ice sheet that would flow towards the dayside while sublimating and perhaps melting should be rather handy for kickstarting life… not to mention the number of sub-ice-sheet lake and rivers that would form (throw 4 km of ice over a whole hemisphere and think about what a little bit of geothermal heating will do.. or go to Antarctica and look for yourself).

The best conditions for the synthesis of the precursors (such as nucleotides of the RNA bases from formamide) often need heat, but then the best conditions for those bases to form a self replicating RNA seems to be a lot colder (so maybe they form in one set of conditions, and diffuse to another location with other conditions).

I agree a high thermal gradient can be useful for a lot of bioenergetic processes. I'm not sure why you think such things are less likely to exist on a tide-locked world than on a non-tidally locked one however.

(Snipped a lot of other VERY good biochemistry… not because it wasn't interesting, it WAS, and it was great! Thank you!)

If the day/night cycle is needed for abiogenesis, then the tidal locking would have to occur slow enough for life to arrise first.

True. So how long did it take for life to arrive on Earth? Rather quickly as I understand it? Around a billion years? So, perhaps you can form life if tidal locking takes more than a billion years? So that would specify a "near limit" based on tidal locking time, and constrain the likely stellar size based on that and its luminosity, assuming that diurnal cycles are needed.

One also has to consider that when photosynthesis happened on Earth. In the Devonian, as Plants colonized the land, and evolved woody proteins that couldn't yet be effectively degraded, CO2 concentrations collapsed.

No, they dropped; precipitously. And if that happened on a hypothetical tide-locked world, the CO2 level would drop, driving down temperatures (and also potentially starving the carbon-fixation pathways in those early plants), which would stop plant growth… whereupon CO2 levels would build back up due to things like volcanoes. Very similar to how the carbonate-silicate cycle worked on Earth as a stabilizing climate feedback and pulled us out of the the nearly "icehouse Earth" scenario.

How would a tidally locked planet handle such an event?

Pretty much the same as Earth: again, start with Earth and evolve it into a tide-locked state. I'm not sure I see any significant problem. Not even for life (although it would certainly become rather oligotrophic in the process).

What of buffer gasses? You need some Nitrogen in the atmosphere, its hard to imagine life without nitrogen (its not as important as carbon, but still very important).

You can have N2 dominated atmospheres; N2 isn't in *any* danger of atmospheric collapse under these circumstances. And while terrestrial life may do well with N2 as a (relatively) inert gas, it's not like it has a significant biological role (Nitrogen? Absolutely… but there's nothing that points to requiring an atmospheric source vs something else. Phosphorus is pretty critical to terrestrial biochemistry too, sometimes being the limiting factor, but it certainly doesn't stop life much that it doesn't exist in an incredibly difficult to fix form that forms 3/4's of the atmosphere).

As I said... I'm still deeply skeptical.

You've got every right to be. It's a hard problem. My point was it's a problem that has been known about, and worked on, in planetary science for decades… literally. Not just because planetary scientists like to think up weird things (that's SF writers), but because the statistics keep pointing in that direction.

PS- Yeah, I'm long-winded. Apologies to all the TL;DR crowd, but this stuff is really interesting to some of us. If we spirits, have offended…

[AngelLestat]

I guees the first step it would be set all the planet conditions, and then you can think in a story for the star system formation (or planet capture) to validate those conditions that the planet has.

True, but I don't have a good reference for the oceanic circulation of a global sea on a one-faced world . Yes, it would help.

I find something better

The confirmation of this theory:

http://www.exoclimes.org/wp-content/uploads/2014/02/Hu_Yongyun_exoclimes3.pdf

It seems that tidal locked cases would have a lot of sense orbiting dwarf stars, I was reading about them and there is many things that can be used to an story.

http://en.wikipedia.org/wiki/Habitability_of_red_dwarf_systems

The first 1billion years from a dwarf star formation are very intense in variability on the energy flux. If the planets form at the same time, it may be difficult to keep its atmosphere due erosion. There are many way to avoid this, but if we wanna keep it simple, the planet could be in at higher orbit at first, and then capture into a lower orbit.

Hmm. Not sure I follow - the super rotation of the upper atmosphere is a product of the heating, but also the slow rotation of the planet (like Venus). Being far from the star isn't going to help much (unless you are so far it's too cold or not tide-locked).

First you dont need a 100 atm atmosphere. It can have 2 or 5 or 10 atm.

If we keep the dwarf case, then the world can be far enoght from the star, but due how dense the atmosphere is, it will trap enoght heat to have human temperature at the surface.

Also less energy receive from the star, the super rotation winds (in case you dont have much water) would be weaker. A thicker atmosphere helps against radiation (in case the flux variability of the star is not completely finished), and it will not have strong magnetic fields with low rotation speeds.

So playing with the amount of water, gravity (influences the pressure and thick of the atmosphere), distance to the dwarf star and how thick is the atmosphere; you can find any scenery you want for an story.

I made in other topic a set of parameters-conditions (just as a guide, I do not pretend to be super accurate) on pressure, gravity, temperature, radioactivity, etc. where humans can survive and reproduce.

http://forum.kerbalspaceprogram.com/threads/108772-Living-at-other-worlds-A-paradigm-shift

Edited January 30, 2015 by AngelLestat

[magnemoe]

That is right in a rocky planet, and if you have a thick atmosphere then you also need more distance to the star to avoid higher temperatures on the surface.

Being far from the star also reduce a little how strong the super rotation winds will be. To avoid higher pressure then your gravity needs to be lower with a magnetic field.

But I have a better idea.

A tidal locked world with a Huge Sea. Water is very effective to absorb heat and carry that to different places.

Take a look at england, they should have very cold weather, however they live in a constant warm myst due the main sea current.

The heat capacity of the oceans are 1000 times greather than our atmosphere. And this taken into account that you use just the first 30 meters deep of that capacity.

Density and thermal capacity of water is a lot higher than gasses.

So with this scenary, you can control how much water this world has, to make the story you want.

The water would control how much difference in temperature you might fine, it would also control the winds at surface level, they would not be so strong, but at higher altitudes you would have constant faster winds that can be used by airships to fly from one point of the planet to another.

Yeah that documentary is pretty inaccurate.

Yes, an wet tidal locked planet would be habitable as the water would heat the backside stopping all the water from ending as ice on the backside.

An thicker atmosphere also help, an uneven orbit and a wobble would also help a lot however this would probably also be reduced by tides over time.

[KerikBalm]

Not bloody much of it. On the day side, there will be next to no water vapor in the atmosphere because while it is hot, there's also no source. On the dark side, the equilibrium vapor pressure will be very low due to the temperature. So, yes, water vapor in the atmosphere, but *not* remotely uniformly distributed, and not high levels (for some atmospheres).

Note much once the ice sheets form... but before that, there would be some.

Nope, you're right - early on the planet would likely not be a synchronous rotator. So you are likely to end up with a warm wet atmosphere. Perhaps not unlike the Earth. Note that Earth isn't remotely in a runaway (or even moist) greenhouse state

Early on, you mentioned a CO2 concentration as low as 30 millibar (though wouldn't you want some other unit than bar, higher temperatures acheive the same pressure, with less CO2, wouldn't it be better to measure the mass... anyway...). On Earth, at 1 bar, its ~300 ppm. So that would give a partial pressure of 300 µbar if I'm not mistaken. You are talking about 30,000 µbar.... 100x the concentration of CO2 on Earth... (of course, in the past, the CO2 was much much higher, but there is also the question of where Earth's water came from, because many models assume the Early water was lost)

Almost certainly. Take a look at the Earth. We have massive ice sheets at the poles (and have had still more massive ones in the past) which are location that are *not* in shadow at all times. Yep, if the Earth became tidally locked, it would build up a big ice sheet on the night side as well… remarkably rapidly actually (do the thermal analysis for the Earth and you find you could have effective cooling over the course of *months*, not years or decades under those conditions).

Well, many places are in shadow for 6 months... and in Earth's past, the poles were often ice free, at CO2 concentrations far less than 30 millibar.

A reasonable question. So look at it this way:

A Earth-sized world forms close enough to a red dwarf that it experiences Earth-normal heating. The result is, not surprisingly, and Earth-normal environment… possibly including plate tectonics, oceans, and all the rest. Over time the rotation rate slows, so "nights" get longer and longer… finally getting long enough that significant portions of the water cycle become trapped in "temporary" (month to year long 'nighttimes') on night side. The planet is still rotating, but now increasing amounts of water vapor are being trapped in ice sheets. Even while the day side becomes locally warm enough to loose its tropospheric cold trap, the amount of water vapor in the atmosphere is dropping… sharply dropping, since more and more of the water is being locked up, gradually, on the night side. Worse yet, those large sheets of ice (if we are talking "normal" oceans depths, piling the Earth's water on one hemisphere could produce a 4+ km thick hemispheric ice sheet) make tidal locking easier, and the process continues.

The problem with this is that your model is assuming an atmosphere at a minimum of roughly 30 parts per thousands CO2, whereas CO2 in our atmosphere since there have been oceans and such seems to have been on the order of hundreds of parts per million...

Thus I have my doubts that the same outcomes could be reached when the requisite CO2 concentration is much much higher.

Earth's natural variation in CO2 is more than enough to pop levels up to 30 mb in a few million to tens of million of years, much much shorter than the tidal locking timescales in this situation.

When was that last time Earth's atmosphere reached 30 millibar?

In that case, the presence of a large ice sheet that would flow towards the dayside while sublimating and perhaps melting should be rather handy for kickstarting life… not to mention the number of sub-ice-sheet lake and rivers that would form (throw 4 km of ice over a whole hemisphere and think about what a little bit of geothermal heating will do.. or go to Antarctica and look for yourself).

I think you'd want the flow going the other way compounds forming in the warm areas going to cold areas and starting to freeze. Geothermal may work. There are still a lot of unknowns for the start of life. We can try things like the Miller-Urey experiment to form some compounds... but so far there doesn't seem to be 1 set of conditions where you form everything you need. Some compounds would form readily in drying tidal pools exposed to UV light, some for readily in conditions mimicking hydrothermal vents... etc.

It may be that you needs a these conditions and an ocean to mix them...

So many variables, and an scale (in time and physical size) beyond what one can do in the lab (much like planet formation) - I'm not saying its impossible... I'm just that we always hear from the "glass-is-half-full" ideas of these planets. If someone says it seems possible... the pop-sci hype tends to very much become optimistic about there being life there.

-This is one reason that I would REALLY REALLY REALLY like to see a subsurface Europa mission. If they funded some stupid manned mars mission instead, I would be very very very disappointed.

Europa has water and an energy gradient... does it have life? that could help us constrain our estimates about what conditions abiogenesis may occur (since there is no reason to assume that places capable of supporting life = places where abiogenesis is possible)

True. So how long did it take for life to arrive on Earth? Rather quickly as I understand it? Around a billion years? So, perhaps you can form life if tidal locking takes more than a billion years? So that would specify a "near limit" based on tidal locking time, and constrain the likely stellar size based on that and its luminosity, assuming that diurnal cycles are needed.

Earth... 4.5 billion years old... life... 3.7 or 3.8 billion (probably)... but tidal locking is not a binary state. 48 hour days are probably fine... 1 week days? 1 month days?

I would imagine that the slower the rotation rate, the slower tidal locking takes place (ie, the rate at which rotational energy is lost is related to the rate the planet rotates, no?)

No, they dropped; precipitously. And if that happened on a hypothetical tide-locked world, the CO2 level would drop, driving down temperatures (and also potentially starving the carbon-fixation pathways in those early plants), which would stop plant growth… whereupon CO2 levels would build back up due to things like volcanoes. Very similar to how the carbonate-silicate cycle worked on Earth as a stabilizing climate feedback and pulled us out of the the nearly "icehouse Earth" scenario.

"dropped; precipitously" vs "collapsed" "to-may-toh", "to-mah-toh"

This reminds me of an argument I had with a climate change denier... the whole CO2= plant food thing. This guy made some claim that CO2 on earth was close to the lower limit for plants, and that more CO2 was good. So I went into debunking that....

The ability of plants to draw down CO2 concentrations is quite impressive. They can survive in CO2 levels much much lower than what Earth has now (of course, its less than optimal). They could easily still be drawing down CO2 levels when they reach this critical collapse point of 30 millibar. Of course, as temperatures drops, they may freeze... but this isn't earth. On Earth, as temperatures drop, the amount of area plants can grow decreases.

If on this planet, they only grow in a ring around the terminator, they would simply more "sunward" to warmer climes (where the isolation area they can make use of would be higher, no?). I imagine the temperature gradient would be quite high, and the distance they'd need to move would be quite low (so insolation area wouldn't really start to change until they've moved significant distances from the terminator).

I could easily see photosynthesis drawing the CO2 concentration down too far (below the collapse threshhold) before a negative feedback would limit their growth sufficiently.

And while terrestrial life may do well with N2 as a (relatively) inert gas, it's not like it has a significant biological role (Nitrogen? Absolutely… but there's nothing that points to requiring an atmospheric source vs something else. Phosphorus is pretty critical to terrestrial biochemistry too, sometimes being the limiting factor, but it certainly doesn't stop life much that it doesn't exist in an incredibly difficult to fix form that forms 3/4's of the atmosphere).

Phosphorus is often the limiting factor much more often than nitrogen. Having solubile sources of P certainly helps the P cycling, and a tidally locked planet won't have a planetwide ocean that helps a lot of the nutrient cycling.

It's a hard problem. My point was it's a problem that has been known about, and worked on, in planetary science for decades… literally. Not just because planetary scientists like to think up weird things (that's SF writers), but because the statistics keep pointing in that direction.

Yes, of course. Its good to have actual calculations on the atmosphere collapse threshholds, rather than just a vague idea that the atmosphere could all freeze on the dark side.

But when you get into questions of "life" a lot of variables arise that planetary scientists may not be equipped to handle.

Its certainly a good start to tell us where we might find liquid water though...

[KerikBalm]

So there are many things about the way that paper is written that I do not like.... and I also notice that its nearly 20 years old..... but anyway...

One factor that will affect the mean T0 of a planetary atmosphere is CO2 recycling. When liquid water is present, CO2 will be recycled into carbonates, cooling the climate. If liquid water does not exist on the surface, the rate of carbonate formation decreases, and CO2 can build up in the atmosphere, increasing its greenhouse effect

and possibly, even raising T0 to the point where liquid water is once more stable (Kasting 1988)

It seems hard to have oceans if the water condenses in the dark side.... so even if you've got an atmosphere where its not going to collapse... is that CO2 concentration stable, or will it keep increasing until it looks like Venus? (Venus, given its extremely slow rotation rate, might as well be tidally locked, and it was very Earth like in initial composition - even if it was moved out the the orbit of Earth, that thick atmosphere would render it uninhabitable and still too hot)

This effect will act to raise p0 to 1000–1500 mb of CO2 at S = 1 and to higher pressures at lower values of S. Thus, if CO2 partial pressure is controlled by the carbonate–silicate cycle, 1000–1500 mb of CO2 will be the minimum surface pressure of a habitable synchronously rotating planet.

1 to 1.5 atms of CO2 now... minimum... I'm not liking this... atmosphere collapse, and liquid water formation are not the same thing... It seems to me the margin between habitable and venus is shrinking.

The minimum p0 of 1000–1500 mb will be higher if CO2 is a minor component of an otherwise optically thin atmo-

increase.

Great.... so if there is N2, you need an even denser atmosphere.

Also, regarding CO2's collapse point, and the addition of nitrogen...

One reason why this surface pressure is so low is that as pressure decreases, the frost point temperature Tc also decreases. Tc (1000 mb) is about 195K, whereas Tc(30 mb) is approximately 160 K.

So addition of N2 means that CO2 will freeze out at a higher temperature (but the N2 won't contribute to a greenhouse effect, although it will aid in temperature circulation).

Then of course, we should consider the spectrum of a Red Dwarf is also less than ideal for photosynthesis... the primarily IR outpout won't reach the surface if the atmosphere is mostly CO2.

Also the habitable area and area with accessible sunlight would be quite small, I suspect biodiversity on such worlds would be small compared to that of Earth.

[cantab]

If there is life there, it's most likely prokaryotic, since prokaryotes were dominant for billions of years on Earth.

If there is life in any other star system, it is almost certain to be unlike Earth life. To use any of the technical and specific language created for life on Earth is misleading. Even to talk of "bacteria" on other planets is wrong.

[KerikBalm]

Note: There are instances of multicellularity in prokaryotes.

Also, many prokaryotes do have "nucleoids" which are highly organized structures where they keep their DNA... but its not membrane enclosed...

I agree... lets not use the term (I think it also comes off as condescending towards prokaryotes)

[brdavis]

(Edit: never mind, while I was out it seems you read the paper; let me catch up here for a moment "Hmm… I think the best way to address this might be to read the paper I linked to. It addresses a lot of your points in ways I seem to be doing poorly. Try reading it; it's not too difficult, and might address some of the points you and I seem to be sticking on.")

It seems hard to have oceans if the water condenses in the dark side.... so even if you've got an atmosphere where its not going to collapse... is that CO2 concentration stable, or will it keep increasing until it looks like Venus?

Good question; I'm not sure we know. Can you have a carbonate-silicate cycle without plate tectonics and a nearly global ocean? Perhaps not. You actually won't have one if you have too much ocean as well (no climate-controled influx of things like Ca++). So here the question might be just how much of an ocean do you need, and do you enter a runaway greenhouse anyway (note that Mars, for example, hasn't, despite lacking a carbonate-silicate cycle for a very long time).

1 to 1.5 atms of CO2 now... minimum... I'm not liking this…

Why not? what's wrong with a significant CO2 atmosphere and potential habitability?

Great.... so if there is N2, you need an even denser atmosphere.

Again, this is a problem… how? Are we assuming that for habitability you need an Earth-normal atmosphere? For a planetary scientists, habitability is perhaps much broader - can.potentially, support liquid water.

Then of course, we should consider the spectrum of a Red Dwarf is also less than ideal for photosynthesis... the primarily IR outpout won't reach the surface if the atmosphere is mostly CO2.

It's not ideal for terrestrial-style photosynthesis… which is not surprising to me as it didn't evolve to cope with that.

Also the habitable area and area with accessible sunlight would be quite small…

True, very true.

I suspect biodiversity on such worlds would be small compared to that of Earth.

I'm not sure if that's true or not. Yes, the area available may be small… but the diversity of environments is plenty large. And by far most of the biodiversity of life on Earth is microscopic.

We may be arguing different things here. I'm pointing out that among all the possible planets in the galaxy, it seems likely that the majority of potentially habitable ones (that could support life, not terrestrially-evolved life) may be tide-locked. It's not that they are 'better' than Earth, or even 'as habitable'… it's that they are not less likely, and that by virtue of their much greater potential number they are therefore a higher probability.

The CO2 limit is interesting. It was my understanding that C3 plants limit at around a pCO2 of 0.1 mb, while C4 can still support growth down to pCO2s as low as 0.01 mb (if memory serves here). What sort of … ignorant fool … would argue we're near any sort of reasonable lower limit for plants with regard to pCO2?!? Then again when you are already denying well-understood science… sigh…

Thank you for reading the paper. Did you have a chance to look at the bioenergetics one, dealing with an origin at alkaline hydrothermal vents? I'm curious what your take on that is as well (to me it was really really interesting… but I admit outside my field. Significantly outside my field

Edited January 30, 2015 by brdavis

[KerikBalm]

I did read it... see the 2nd of my back to back posts... I was quoting directly from it.

- I could have read it closer.... but again, I think its not written very well. There are many references to "run 1" or "run 2", but its not very clear what the conditions are (and there's a 1 run for 1 figure, and a run 1 for another), I the figures aren't labeled so well... etc..

- The problem with dense atmospheres that I see, is, of course, the venus scenario.

There's going to be a range between too little, and too much. Venus has too much, and it doesn't matter if it rotates fast, or rotates slow.

This tidal locking scenario is driving up the "too little" condition, but I don't see how it affects the "too much" condition.

Thus the acceptable range (goldilocks range?) is being driven to be smaller and smaller than a non-locked planet, no?

[Kibble]

Even to talk of "bacteria" on other planets is wrong.

Not nessecarily. The Moon, and all the other words for it in other languages used to be a proper noun of a specific object, but it has been generalized to mean any object in the Solar System whose primary is not the Sun. Right now Bacteria is a taxon which refers to a specific set of terrestrial organisms, but if we find creatures like it somewhere else, we'll likely call them bacteria too.

[brdavis]

I did read it... see the 2nd of my back to back posts... I was quoting directly from it.

Yep, my bad; my first post was too quick, and your second post hadn't propagated out to me yet; I just left my mistake "visible" because I really hate retconning reality

The problem with dense atmospheres that I see, is, of course, the venus scenario.

Then you move it out further. Seriously, the outer edge of the habitable zone would be populated (for habitable planets) with planets with dense atmospheres. Those aren't a problem - in the habitable zone doesn't mean "habitable", it means "possible" in this context. So you could have a habitable planet close in with a thin atmosphere, or a habitable planet further out with the thick atmosphere. Look at some of Kasting's work on the continuously habitable zones around main sequences stars (Kasting is another planetary atmospheres guy).

Venus has too much, and it doesn't matter if it rotates fast, or rotates slow.

Venus has too much only for its current insolation. Move it out further, and it will be cooler. Move it out far enough, and that 90 bar CO2 atmosphere will be cooler still.

Thus the acceptable range (goldilocks range?) is being driven to be smaller and smaller than a non-locked planet, no?

Not by much. Other papers I've read show that the habitable band in terms of insolation extends out to Mars and beyond… but with dense CO2 rich atmospheres of something like pCO2 of 8 bars. And while those calculations are generally done for roughly solar-type stars, the shift in frequency of the light really isn't very significant (our Sun is at 5000 K, while the coolest M stars are around 3000+ K… about the same temperature, and therefore color balance, as an incandescent lightbulb. It's still "short wave", both to your eye, and things like the CO2 IR window).

Edited January 30, 2015 by brdavis

[KerikBalm]

Then you move it out further. Seriously, the outer edge of the habitable zone would be populated (for habitable planets) with planets with dense atmospheres. Those aren't a problem - in the habitable zone doesn't mean "habitable", it means "possible" in this context. So you could have a habitable planet close in with a thin atmosphere, or a habitable planet further out with the thick atmosphere.

Well it was listing 1-1.5 atmospheres of pure CO2 for earth level insolation... farther out, more gas is needed...

So then I have to wonder what the feedback loops are that will make CO2 atmospheres orders of magnitude thicker than that on Earth, but stop orders of magnitude thinner than that of Venus (Venus, even if moved out to the orbit of Earth, would still be way too hot now.

I don't doubt that there could be a limited point in time where such a planet would have rather nice conditions... but would those conditions be stable on the order of a billion years?

When Earth had a thick CO2 atmosphere, the sun was about 30% dimmer, and most models have it losing its water (hence the debate over the source of Earth's water) - now I imagine that red dwarfs have their luminosity change at a much lower rate... but there are all kinds of changes that will happen a billion years after accretion. Volcanic activity would initially be high, and would decrease over time - this would be even more dramatic than on Earth because the tidal locking process would heat the interior, no?

Its going to need to be able to mantain conditions within a certain range, or recover when that range is exceeded (as earth recovered from snowball episodes, and times when there was no ice at the poles).

The narrow habitable band of the surface , and large gradient suggests to me that life would be relatively unaffected by climate changes, as it would simply mean movement to the stellar/antistellar point with very little change in the total habitable area (assuming required nutrients do not have a strong gradient at the terminator) . Movement towards the solar point would even seem to increase the amount of photosynthesis (assuming the life does that... some of these proposals have atmospheres that are so thick, I have my doubts photosynthesis would be viable)

So I doubt there would be a helpful biological feedback like we see on Earth.

I'm not saying its impossible... and I know in academic discussions "habitable zone" means "potentially habitable on or near the surface" -> and "potentially" really just means they lack a clear convincing reason why it wouldn't have a habitable surface (ie, this is the zone where we can't rule out the possibility of water-based life on the surface)

Its just after things like this, the "glass is half full" people take (IMO) very optimistic figures and plug them into the drake equation.

As red dwarfs are by far the most common type of star, the issue of how many of them have habitable planets is very relevant to the Fermi paradox. I think the simplest solution to the Fermi paradox, is that we've just been wildly overoptimistic about many of the variables.

Consider Earth only has half a billion years left before the suns luminosity grows too much... bigger stars have shorter lifespans...

Likely we can throw out all the O/B/A/F class stars as candidates for hosting worlds that are habitable for long enough periods.

I suspect this tidal locking issue and a few others may allow us to throw out the M class stars as well... leaving just the G and K type stars... which puts on a good start towards resolving the fermi paradox.

It may not be that no red dwarf has a habitable planet... just that they are so extremely rare that the far less numerous G and K type stars actually have more habitable planets.

If popular opinion was more negative towards extraterrestrial life (ie, less people expecting a galaxy as full of life as in star trek/wars), I'd be playing Devil's advocate on the other side.

These stars may have habitable worlds... but when I see that this gets assumed, and press releases say that they expect there to be "billions and billions" of earth like planets based on extrasolar planet surveys + the number of red dwarfs.... I feel a need to make counter arguments for more conservative numbers.

Edited January 31, 2015 by KerikBalm

[CoolVikingCo]

Since planets rotate around their axis while also orbiting a star, then I would say it is possible, just unlikely that there would be life as WE know it. Maybe some other form of life out there uses hydrogen as oxygen.

Edited January 31, 2015 by CoolVikingCo