Holy moly this a very interesting answer here! O and B type stars are absolutely out of the question, A type stars are extremely rare to have life probably. So this means F type stars and higher class G stars are what I am looking for. Now in your list there it shows F5 stars, but later class F stars might be a better bet. Like F7, 8 and 9. 9 would be the best most likely option here. 

How long would F7-9 stars last? 5-8 billion years? That sounds like a decent amount of time for life to evolve on a planet around them. Though for a civilization around such a star, they would only probably have 100 maybe a billion years at best before their oceans evaporate...which would be interesting.

O, B, and A stars would have different colors of light which would be uncomfortable to live under. F and higher G class stars would have a similar light color output.

This is really only the good image I could find that shows this.


http://www.xenology.info/Xeno/5.4.2.htm

This here is also an interesting read. Worlds around F stars would have blue skies, much like ours but more so.

This here is also an interesting read. Worlds around F stars would have blue skies, much like ours but more so.

Very interesting indeed!

How long would F7-9 stars last? 5-8 billion years? That sounds like a decent amount of time for life to evolve on a planet around them.

Not so much I think. Even if thermal habitable zones and UV habitable zones overlap for many F-type stars, that doesn't mean that the overlapping region isn't moving while the star evolves. As stated in page 8 of this paper, the part of the evolution of F-type stars where we should expect stable conditions for our planet is between 2 and 4 billion years at best. Still, plenty of time to have life I suppose but shrink your expectations in that sense.

Also consider that the orbit of the planet should not be placed in some random part of the overlapping region of both habitable zones. Many orbits inside the overlap are going to end outside it in less than a billion years while the regions displace. The best orbit inside the overlap would yield some billions of years of stability as I said, but this orbit is very fine-tuned; for example, for an F-star with 1.5 solar masses the climate stability range is for orbits between 2.535 and 2.566 AU. To have a rocky planet in that very specific region means to hope for fewer possibilities to find habitable F-stars systems.

So, yeah it is possible but we have to keep optimism controlled here.

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While writing the last post about O, B, A and F stars habitability prospects I thought something that even if unrelated I would like to share here. Those who know me know that I'm not so convinced life is a common emergence in the universe.
The first argument pointed by everyone (even if it's not he strongest in scientific terms) is an argument from numbers. "In a universe with trillions of galaxies containing hundreds of billions of stars with their planets life has to exist in millions of places at least" Ok, that's fair. We know life can exist (it sounds stupid but it's an important fact that we can take for granted thanks to our own existence) and we know there are around 10²⁵places in the universe to possibly call home. Wow, a 10.000.000.000.000.000.000.000.000 possible planetary surfaces in the universe. But what usually gets disregarded is the probability of life emerging. It is true that life here on Earth appeared very rapidly and also true that we know for many possible mechanism that would easily form vesicles and self-replicating polymers, meaning that there are possibly few conditions to accomplish to have life, but we still really know very little about the probability of the emergence of life, we still have a habitable planet that formed life just once (as far as we know) in 5 billion years of history, we still can't recreate the most primitive life-forms in the lab using Earth conditions and despite the possible mechanism hypothesized for early biochemistry we could be experiencing a bias due to the idea that every-day simple chemical reactions were in place (an idea necessary to start the search) instead of a complex chain of serendipious events that led to a highly improbable situation (which would be for sure more difficult to study and prove). So, we know that there are a lot of possibilities in terms of places for life but we don't really know much about the probability for life in each one of those. What if life is a 1 in 10²⁵possibility? What if the puny numbers for this probability are as staggering as the planet count in the universe?

Think about lottery. Winning the jackpot in the Mega Millions multi-state lottery in the United States is a 1 in 258.890.850 possibility. If we granted a ticket for every planet in the universe there would be nearly a thousand winners in just our own galaxy. What if we played again? The probability of a planet winning the lottery twice would become 1.5 x 10^-17. Each galaxy would have then a 0.0006% chance to have a planet like this. You would need to search between hundreds of thousands of galaxies to have a reasonable chance of finding a twice-lottery-winning planet. But hey, there would still be 152 million planets like that in the universe. Then, what if we needed a three times winner of the lottery? Well, then the probability for any particular planet would be 6 x 10^-26 . Think about that. There would be a 60% chance that there is a single planet in the entire universe with those characteristics. If the probability for life on a planet is at most as winning the lottery three times then we are hopeless, we would probably the only biosphere in the entire cosmos. Now the question is Do you think it is easier to form life than to win lottery three times? Maybe I should realize how difficult in fact it is to win the lottery but for now my intuition (the same at play when one says "look at how large the universe is how can't there be any aliens?") tells me that it shouldn't be as hard as the formation of life, if it was easier why the Earth (that have brought a lot of tickets in comparison to other planets with harsh environments) has only experienced the formation of life once?

Planets around F stars would be unlikely to host life as we know it F8 and F9 would be the best one of those to look for possible Earth like planets in habitable zones. The star in the sky would look very bright, twice as bright most likely so you would go blind alot quicker by looking at it. Sunburn would be a horrible experience on these worlds.

For G class stars though, how about G0, G1, and brighter G2V stars like Alpha Centauri A? I assume those would be better off for life as we know it. 

_

Thats a rather interesting point of view there! Though for me personally I view it is too soon for us to guess about such things, heh.

In this discussion I think it's useful to distinguish between marine life and land life.  Whlist the environment at the surface may be too hostile to support life as we know it, oceans may still have benign conditions.

if it was easier why the Earth (that have brought a lot of tickets in comparison to other planets with harsh environments) has only experienced the formation of life once?

How sure are you that Earth formed life only once?  Suppose that life on Earth succeeded in developing independently tens, or even thousands of times.  In what ways would this scenario lead to differences from what we observe, and how easily should we expect to be able to find them?  How confident are you in your answer?  (These are fundamental questions and challenges for the study of shadow biospheres, and abiogenesis).

On the other track, suppose life on Earth did develop only once.  What would that imply for the probability of a habitable planet developing life in general?  To be quantitative, let's set the probability at an optimistic 1%.  What fraction of planets then would then be expected to have two completely independent origins of life on them?  Three?  Ten?  In this optimistic scenario, is Earth having one development of life considered an outlier or the norm?  How does your answer change if the probability is instead a pessimistic 1 in 10²⁰?



I agree that there is little value in trying to conclude what value nature sets for the probability of potentially habitable planets going on to develop life, and I hope the above questions further help illuminate that.  Historically this is a debate between pessimists and optimists -- those who think the life is rare and give reasons why the probability must be small, and those who think life is common and give reasons why the probability must be large.  I think we lack sufficient knowledge, even in terms of the development of life on this planet, to obtain that solution yet.

What I find more interesting is that we do now know the value that nature sets for the frequency of potentially habitable planets forming.  We have good data for the star formation rate, the fraction of stars with planets, and the fraction of those planets that are terrestrial-sized and exist in the habitable zones for geologically significant time periods.  So although we don't know the values for the "biotechnical factors" of the Drake equation (how frequently life develops, how frequently it becomes technologically advanced, etc), we do know the "astrophysical factors", and thus can establish hard limits for "how pessimistic we would need to be" to conclude that life is rare in the universe, or that we are the only technologically advanced civilization that exists or has ever existed.

What's further interesting is that this knowledge of exoplanets mostly arose just in the last 20 years.  The next 20 will be even more revealing, especially as we begin to characterize the atmospheres of terrestrial exoplanets! We're in a golden age for finding answers toward what is possibly one of the oldest and most human of questions.

What's further interesting is that this knowledge of exoplanets mostly arose just in the last 20 years.  The next 20 will be even more revealing, especially as we begin to characterize the atmospheres of terrestrial exoplanets! We're in a golden age for finding answers toward what is possibly one of the oldest and most human of questions.

Indeed.  However, 30 years ago there was much less debated whether planets around stars are common than there is debated today whether life is common.  But that makes the coming decades even more exciting.  In my opinion our own solar system looks pretty dead besides Earth, so we really should rechannel efforts to find biological signs to the exoatmospheres.

And one of the most fascinating aspects of astronomy is in my opinion how much there is to learn from studying light.  Think of it, how amazing is it that we can discover what the atmospheres of the sun, the stars and soon planets lightyears away are made of without sending probes, but instead observe direct evidence of the behaviour of electrons, stuff at the quantum scale, at such distances?

How sure are you that Earth formed life only once?  Suppose that life on Earth succeeded in developing independently tens, or even thousands of times.  In what ways would this scenario lead to differences from what we observe, and how easily should we expect to be able to find them?  How confident are you in your answer?  (These are fundamental questions and challenges for the study of shadow biospheres).

On the other track, suppose life on Earth did develop only once.  What would that imply for the probability of a habitable planet developing life in general?  To be quantitative, let's set the probability at an optimistic 1%.  What fraction of planets then would then be expected to have two completely independent origins of life on them?  Three?  Ten?  In this optimistic scenario, is Earth having one development of life considered an outlier or the norm?  How does your answer change if the probability is instead a pessimistic 1 in 1020?

You are right in these considerations. I think we talked about this in the past. It's true, probably life emerged independently several times on Earth but just at the begining when the first cells were forming. For what we know currently though all the biosphere is traceable back to a single organism, meaning that our life-forms not only dominated population but sterilized all traces of possible alternate life forms on this planet since then (and have been an attenuating factor for the emergence of life since then).
I should have said that. And yeah I also agree with the fact that this is a debate of pessimists vs optimists that should be adressed with more data and understanding. My fear is that the debate is always inclining to the optimistic view-point as pushed by human folklore instead of actual evidence. Rare Earth hypothesis has very strong arguments that are always dissregarded in popular science. A tendency to this is even common in modern scientific literature. There are many ways to make an exotic world habitable but there are much more to make it uninhabitable, and the fact we have this small tendency to research for our conclusions instead to aproach this without any conclusions at first is something to be aware of and to educate. I'm not saying that this happens a lot in science and even when it happens the skepticism and a speculation advise are put foward but there is a general tendency to favor optimism and we could end creating a wrong cosmovision on the matter. I don't really think life is non-existent in the rest of the universe (just a feeling in the end) but the idea of life emerging once in the universe is for me as fascinating as the reverse.

What I find more interesting is that we do now know the value that nature sets for the frequency of potentially habitable planets forming.

I usually see this from the complementary perspective. We think about how our future discoveries on the origin of life could bring us accurate probabilities of finding extraterrestrial life, but do not forget the situation where we discover extraterrestrial life first and we use that fact to understand that the prebiotic chemical reactions that led to life on Earth are more probably due to simple phenomena rather than unfrequent and complex ones. I think many in the field take the search for extraterrestrial life as a search for our origins. This is for sure a very interesting topic.

we do know the "astrophysical factors", and thus can establish hard limits for "how pessimistic we would need to be" about the development of biology to conclude that life is rare in the universe, or that we are the only technologically advanced civilization that exists or has ever existed.

This is also very true. There's no compelling astrophysical factor to be hardly pessimistic and ignore the possibility of life (even if was there I think science should commit to this search). We know stars like our Sun are frequent, and the habitable zone of our galaxy (if that ends to be important) is full of stars, planets like Earth are probable also, a big moon may be a more improbable situation but we are not certain that this would be explicitly necessary for life (even if it was a key element for the emergence of life on Earth), the heavy late bombardment and the inyection of water on Earth might be usual mechanisms on other young systems, when life emerges natural selection comes into play and this mechanism alone could generate intelligent life just in a matter of time, This is all true.

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Another unrelated thing! We usually see scale models in space dimensions but we see little scale models in time. I found a spectacular video on which the entire history of the universe is reduced to 10 minutes. Each second corresponds to 22 million years and Homo sapiens sapiens has the last frame of the video for itself (even if the real timespan should be a fraction of that frame). Usually we tend to see natural history with some sort of an inverse proportionality; more recent events are better studied so we pay more atention to them and use more of our time to analyze them and events like the formation of the first galaxies usually take just a minute or even a single phrase in a classroom. I like this video because even if we know that things happened this way it still has the ability to put thing into perspective. The long and slow sequence from the big bang to Earth while things keep going at 22 Myr/sec just relaxes your nerves, also unicellular life has a fairly long existence as protagonist of the video, but then in a matter of seconds dinosaurs come and go. The emergence of intelligence has such an exponentiall growth that you can't even react when it arrives. The last part of the video is... well.. even scary. The universe has generated something incredible here that has evolved extremely quickly. Immagine other lifeforms out there. Immagine what just a few seconds more would give them in terms of technological advantage and complexity (if they have a similir path to our own). Intelligent life emerges like a violent force in a matter of two frames. Even then, think about the fraction of that million year frame that civilization occupies. I really would love to see a version of this video but for two hours, then we could see maybe a frame of us building the international space station.

[youtube]TBikbn5XJhg[/youtube]

I wonder if it is possible that the dark matter is in the hidden dimensions of string theory. Matter that would be in these dimensions would interact with nothing except gravitation.
These hidden or rolled-in dimensions can not be infinitely small according to quantum theory; they must be at least as large as the Planck length. And they also can not be completely empty. There should be something like quantum fluctuation in them.

I wonder if it is possible that the dark matter is in the hidden dimensions of string theory.

I too wonder the same. This article makes me think we might have evidence of such.
Is the Eridanus Supervoid Actually Evidence of the Multiverse?
Well, the idea of a multiverse only exists in the imagination of theoreticians!

The giant voids we see between galaxies is however a very real thing. It would be nice to say something interesting about this observation. To say that a region with a lower temperature also has a lower density (although not by much) is a natural thing, since density fluctuations and temperature fluctuations are proportional to each other in the standard view about the formation of inter-galactic structures. In the Topological Geometrodynamic or TGD framework one considers flux tube networks and that the formation of these mega-structures is based on different mechanisms, but the correlation between density and temperature fluctuations still remains true. Cosmic strings - flux tubes containing Dark Matter - would generate the visible matter by a transformation of Dark Matter to ordinary Matter by h_eff reducing phase transition. Long Strings could form knots via temporary re-connections giving rise to spiral and elliptic galaxies (for these re-connection would then form a separate closed flux tube). Stars could be formed as sub-knots of galactic knots in the same manner. Could this formation mechanism be able to explain large regions with slightly lower density?

What is new in TGD cosmology as compared to GRT based cosmology? In TGD the counterpart of inflationary period is the period during which cosmic string dominated phase (no space-time in conventional sense as space-time surface having 4-D M^4 projection in M^4xCP_2 but 2-D string world sheets as M^4 projections). One can say that space-time in a conventional sense emerged during this period.

The basic prediction is that the transition from the string dominated phase to the radiation dominated phase need occur at precisely the same time everywhere. However, there are fluctuations. If the transition occurred for a given region earlier than for the environment, cosmic expansion also started earlier. Hence the density of matter at later times would be lower than in the surrounding regions. Could this explain the finding? Could the typical size scale for the fluctuation initiating the transition ot radiation dominance be larger than the typical size scale of density fluctuation in standard cosmology?

The giant voids we see between galaxies is however a very real thing. It would be nice to say something interesting about this observation.

They are a natural consequence of gravitation in an expanding universe.   

As a universe filled with nearly uniform mixture of gravitating particles (matter and dark matter) expands, localized regions with slightly higher than average density collapse, increasing the gravitational field and thus bringing more matter into them from the surrounding space.  This is what leads to formation of the sheets and filaments of the cosmic web, while the regions that were lower than average density become the voids.  Simulations of the universe's evolution with the Lambda-CDM model do a good job of reproducing this structure:

[youtube]74IsySs3RGU[/youtube]

To say that a region with a lower temperature also has a lower density (although not by much) is a natural thing, since density fluctuations and temperature fluctuations are proportional to each other in the standard view about the formation of inter-galactic structures.

Some corrections are needed here.

The fluctuations in the CMB are a measure of density of the universe at that time, and it is true that they are also often described in terms of temperature, but we have to be careful in understanding what that means.  The latter is related to the radiation, not the actual temperature of the material that it came from, and actually the relationship between density and temperature is backwards.  Higher density regions are associated with colder temperature CMB radiation.

Explanation:
When we look at the CMB, we are seeing a thermal (blackbody) distribution of photons that were released when the universe became transparent.  The expansion of the universe has redshifted that light, stretching out its wavelength by about a factor of 1100.  Redshifting a thermal spectrum is equivalent to making it have a colder temperature, and we say this radiation now has a temperature of about 2.7 Kelvin.  But there is another effect overlaid on that.  If those photons were released from a region that was slightly denser than average, then they have climbed out of a gravitational field associated with that over-density.  Climbing out of a gravitational field also redshifts photons, so radiation from those denser regions looks slightly colder than average.  Whereas photons released from a lower density region were blueshifted, making it look like a higher temperature.  Again relative to the average.

So the CMB anisotropies are variations in the effective temperature of the radiation (the temperature that you would fit to the thermal spectrum), which are a consequence of the light being redshifted or blueshifted by gravity as it moved from regions that were higher or lower in density, respectively.  

Added:
Confusingly, the data visualizers who made the CMB anisotropy maps chose to give the cooler redshifted radiation a blue color, and the warmer blueshifted radiation a red color, because I guess people associate warm with red and cool with blue?

¯\_(ツ)_/¯

Cosmic strings - flux tubes containing Dark Matter - would generate the visible matter by a transformation of Dark Matter to ordinary Matter by h_eff reducing phase transition. Long Strings could form knots via temporary re-connections giving rise to spiral and elliptic galaxies (for these re-connection would then form a separate closed flux tube). Stars could be formed as sub-knots of galactic knots in the same manner. Could this formation mechanism be able to explain large regions with slightly lower density?

The concern I have over this hypothesis is that it seems unnecessary.  The formation of the cosmic web, voids, galaxies, and stars are all pretty well understood by current knowledge of astrophysics and cosmological models.  I don't think there is a need to develop new physics to explain them. 

Here for example is the formation of a spiral galaxy in Lambda-CDM cosmology.

[youtube]hVNuwAtnKeg[/youtube]

I wonder if it is possible that the dark matter is in the hidden dimensions of string theory. Matter that would be in these dimensions would interact with nothing except gravitation.

There are a lot of people who wonder if dark matter and dark energy could be explained in other ways, some more wild and speculative than others.  In all cases, in order to begin to pursue such a model, it must be able to be worked into a rigorous mathematical form, such that it can make predictions that we can test against observation.  

What we currently know about dark matter is that it is an additional source of gravitation that is not uniform in the universe, and is also not related to the known distribution of matter.  It is something separate, which moves at slow enough speeds such that it can be bound up in the gravitational fields in clusters of galaxies.  Generally it is modeled in the context of a collection of massive but weakly interacting particles, which so far is making the right predictions.  What those particles actually are is anyone's guess.

It's possible that dark could be something more exotic, but whatever we model it as must be consistent with the same observations, which essentially means it will be equivalent to massive weakly interacting particles, anyway.

I wonder if it is possible that the dark matter is in the hidden dimensions of string theory. Matter that would be in these dimensions would interact with nothing except gravitation.

I just wanted to say that. If one day it turns out that it is true and a Nobel Prize is awarded for it, I already announce my claim.
(Of course, only if I'm still alive. As far as I know, the Nobel Prize can only be awarded to living people.)

Dark matter being connected to hidden dimensions of string theory would definitely be a Nobel Prize worthy revelation in physics, but having the idea wouldn't be enough -- the Nobel Prize is given to those who demonstrate it rigorously.  That can be by either the development of a theory that explains observations (e.g. Einstein explaining the physics behind the photoelectric effect, which showed that light energy comes in discrete packets), or the experimental confirmation of a theory (e.g. Higgs Mechanism).  

I know that. But I can dream ... 

I'm mainly trying to imagine how hidden or wrapped up dimensions could be shown experimentally.  I don't know how it would work, but if it did it would surely be a fascinating experiment.

I think it used to be speculated that the gravity of an object would have to decrease by more than the square of the distance, if the extra dimensions actually exist. But perhaps, as they are rolled up, possibly to point size, this decrease may not be measurable.
Not yet.