What makes the Earth habitable?

[Chris Peterson]

I can't help noting that no one here has so far commented on my claim that we have reason to think that the Earth is rare.

I did, at least indirectly. I said that I tend to agree with those who suggest the Universe is teeming with life. My reason for thinking this way is pretty much along the lines of what Art posted: planets seem common, and the history of life on this planet (early life, at least) doesn't suggest any special conditions that shouldn't be common in many other planetary systems.

I don't think intelligent species are common, because I don't think such species are stable. (Sorry, just one data point again.)

[neufer]

When did Devine enter the conversation?

Life in Baltimore is unlikely to be representative of life in the rest of the universe.

[Ann]

I can't help noting that no one here has so far commented on my claim that we have reason to think that the Earth is rare. Personally I believe that the Earth is like a lottery winner, which, just out of pure coincidence, happened to win all the necessary characteristics that work together to make a planet very suitable for life. I also think that this "habitable planet lottery" is a very big lottery, and that there are few winners.

The momentum of discoveries is ALL going in the direction of life being abundant in the universe:

• 1) the radio telescope discovery of complex hydrocarbons in space
  2) the Miller–Urey creation of 22 amino acids "in a test tube"
  3) the discovery of the simplicity of DNA replication
  4) the discovery of prokaryotes 700 million years after the formation of the Earth
  5) the discovery of prokaryotes both deep below the ocean & under the earth
  6) the discovery of more than 100 billion galaxies in the observable universe
  7) the discovery of earth-like planets around other suns

We also have clear evidence that parasites, disease & catastrophes ALL help to speed up the evolution of any life once formed. In fact the development of eukaryotes 2 to 3 billion years ago was probably a direct result of prokaryotes and their parasites benefiting from mutual symbiosis.

Can Ann give even ONE example of any discovery that suggests that life might somehow be rare in the universe?

Thanks for giving me a list of factors to reply to, neufer! ::

Let me start by commenting on your list fist:

1) Being a non-religious person, I of course believe that the necessary building blocks for life can be found naturally in space. I do believe, however, that it takes very, very special conditions to actually put those building blocks together. I find it encouraging that complex hydrocarbons have been found in space. That means I am right that many of the building blocks for life are indeed found in space, and in the few cases where all other conditions are right those building block can be put together to form life on other planets, too.

2) When I was a kid and heard about the Miller–Urey creation of 22 amino acids "in a test tube", when I was still trying very hard to be a religious person, I was shocked. I agree that the experiment is hugely interesting, and it gives us very valuable clues about the formation of life on our planet. So, yes, I truly believe that life can be created through natural processes. However, what Miller-Urey created wasn't life. What does it take to make life out of those amino acids? Probably very favorable conditions.

3) It's a good thing that DNA replication is simple. Creating the DNA helix in the first place seems harder, so I believe that complex hydrocarbons have to "cook" for quite a while under very favorable conditions for DNA to come into existence. I may be wrong about that. If DNA can credibly be found either in space or on a celestial body away from space, where it can't have been put directly by humans, then I will have to rethink my belief that life is rare in the universe.

4) It's impressive that prokaryotes existed on the Earth only 700 million years after the formation of the planet. In my opinion, this means not only that life can be formed through natural processes, but also that conditions were favorable on the Earth extremely early on. How were conditions favorable on the young Earth? Well, 700 million years after its formation the Earth may have had an almost planet-covering ocean due to bombardment by water-rich comets. The Earth had a rich source of internal heat, and there must have been many parts on the bottom of the ocean where heat vents let out a complex hydrocarbon soup into the water. Also, the Earth still had its all-but-circular orbit at a suitable distance from a comparatively "tame" Sun, which wasn't given to huge outbursts. If early Mars had an ocean at this time, too, which it may have had, then the Sun can't have had the kind of outbursts that would boil the oceans off of its terrestrial planets.

5) It is interesting that prokaryotes have been found deep under the oceans and under the earth. I think I'm giving in here, neufer. It is a distinct possibility that life may exist deep in the interior of many extraterrestrial planets. But if life only exists under the surface of those worlds, how will we ever know that it is there?

6) The discovery of more than 100 billion galaxies in the observable universe? So, the tally is 100 billion now? Last I heard, it was only 40 billion. Joking aside, neufer, you may have noticed that I refuse to say that there is no extraterrestrial life in the universe. And the reason why I will never say that is precisely because the number of probable planets out there is so... "overwhelming" isn't the word for it, but you know what I mean. However, neufer. Like I said, my point is not that there is no other life in the universe. Suppose there is, on average, exactly one planet in every galaxy that has life on it. Assuming that there are 100 billion galaxies in the universe, and disregarding questions of exactly "when" that life actually exist(s)/(ed) in relation to us, 100 billion planets with life on them isn't bad. But if the nearest planet with life on it is in the Large Magellanic Cloud, how are we supposed to find it?

7) The discovery of earth-like planets around other suns? I beg to differ. A few earth-sized planets have been found around other suns, yes. So far, we have no reason whatsoever to believe that those planets are the least bit "Earth-like" in such a way that they can support life.

So, neufer, here is my list of things that, when put together, makes the Earth habitable:

• 1) a metal-rich Sun of spectral class G2V, hot enough to save the Earth from a bound rotation
  2) a Sun with a slow rotation, a benign magnetic field and few if any truly large outbursts
  3) a single Sun
  4) a solar system dominated by planets in fairly circular orbits, a Jupiter at a safe distance from us and a Saturn at a safe distance from Jupiter, all contributing to a "calm" and remarkably "stable" solar system
  5) the Earth at a suitable distance from the Sun
  6) an almost circular orbit for the Earth
  7) the fact that the Earth is the most massive and dense of the terrestrial planets, with an iron core, a fairly strong magnetic field and a lot of internal heat
  8) the fact that the Earth's magnetic field protects its atmosphere
  9) the fact that the Earth has probably cooled at more or less the same rate as the Sun has grown brighter
  10) the fact that the Earth probably formed at a distance from the Sun where the young Earth could not be born with a water supply, but that it got that water supply from a bombardment of comets
  11) The fact that the comets likely brought nitrogen to the Earth which could form the bulk of the Earth's atmosphere
  12) the fact that the Earth didn't get a runaway greenhouse effect from the CO2 it received, since the CO2 was mostly incorporated into the Earth's rocks
  13) the fact that the Earth's internal heat has given it plate tectonics, which recycle a lot of life-friendly substances of the soil of the Earth
  14) the fact that the Earth's temperature and atmosphere protects its liquid oceans and gives the Earth a precipitation cycle
  15) the fact that the Earth's Moon, which may be a rest product of the celestial body that gave the Earth its massive core in the first place, stabilizes the Earth's rotational axis
  16) the happy coincidence that some of the Earth's simple life forms started feeding themselves through photosynhesis, thereby releasing the O2 into the atmosphere that large energy-hungry organisms needed to survive
  17) the fact that O2-breathing large creatures made CO2 as a rest product, thereby allowing the CO2 - O2 - CO2 cycle to go on
  18) the fact that the O2 released into the atmosphere turned into oxygen compounds high up in the atmosphere which protect life on Earth from the Sun's ultraviolet light

I'm sure there are more things that ought to be put on this list. Such as, isn't it a happy coincidence that the Sun was likely born in a massive cluster of stars whose birth may have been triggered by a nearby supernova, and yet the Sun managed to escape the chaos of that cluster, get rid of any stellar companions and produce a family of planets in mostly near-circular orbits?

We know so little of the planets we have found around other stars. One thing is clear, however, and that is that chaotic orbits hostile to life seem to be very common out there. What will we find when we learn more about these planets? That apart from their orbits, they are so much like the Earth? That is not what I think, anyway.

Ann

[swainy]

5) It is interesting that prokaryotes have been found deep under the oceans and under the earth. I think I'm giving in here, neufer. It is a distinct possibility that life may exist deep in the interior of many extraterrestrial planets. But if life only exists under the surface of those worlds, how will we ever know that it is there?

Hi Ann.

Is it quite possible, these:

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

Live Here?:

http://www.solarviews.com/eng/europa.htm

I am pretty sure they do.
tc

[bystander]

http://asterisk.apod.com/vie ... 23&t=19601
http://asterisk.apod.com/vie ... 31&t=19684

[Ann]

Thanks for the link to the hypothesis about the Titan-like haze enveloping young Earth, bystander. I note, however, that Titan itself doesn't appear to be particularly warm. I haven't even heard that it is significantly warmer than most other Saturnian moons, despite its cosy covering of haze.

Couldn't the Earth have been warm enough in its youth because the planet simply had more internal heat back then?

Ann

[Ann]

It might be interesting to see what values each of us who are interested in this discussion get when they run the Drake Equation interactive. Mine works out to 798,400 -- a bit on the high side compared to most of you, I imagine. That's mainly because I'm optimistic about the life span of an intelligent species.

For some, this version of the equation might be more suitable.

Rob

I took a look at that Drake Equation interactive. I looked at Frank Drake's own estimates, and my goodness! Talk about unreasonable optimism!

The first factor that goes into the Drake equation, the star formation in our galaxy, is said by Drake to be five stars per year, which seems reasonable enough. The second factor, the number of stars that form planets, is one that we know very little about, but given the fact that all stars are believed to be born out of accretion disks it doesn't seem unreasonable to assume, like Drake, that half of all stars form planets. I think the number may possibly be higher. But the third factor is the average number of planets for each star that could support life, and Frank Drake gives that number as 2! Two! Two habitable planets for each star with planets! On average! That's crazy, if you ask me. In our own solar system we have no reason to regard any other planet than the Earth as habitable, so when it comes to habitability we would do worse than most solar systems in our galaxy!!!! Our solar system would be unusually barren! My goodness!! Just think of the extra-solar planets that have been discovered so far. Do you get the impression that those solar systems with planets in crazy orbits would host an average of two habitable planets each??? That's madness!!!

Frank Drake then says that all the planets that are potentially habitable also host life. Surely we have found more than 200 suns with planets so far? Okay, say hello to 400 planets with life in the universe, 400 planets in solar systems that we have already discovered and can put names and locations to!

(Or maybe not. I think this estimate is pure madness.)

Okay. Frank Drake says that 20% of all planets where life occurs also produce intelligence. That is very optimistic, if you ask me. I think intelligence is a fluke, and it will not be favored by natural selection unless it provides an evolutionary advantage. Dolphins ave very intelligent, but they have hardly become kings of the oceans.

Frank Drake says that 100% of all intelligent species eventually produce interstellar communication. Really? Flipper, when are you going to oink and honk to your dolphin-buddies on other planets?

But anyway, if Frank Drake is right, then there would be life on 400 planets whose solar systems we have already discovered, and there would be intelligence on 80 of them. We should surely start beaming our hellos to them!

Finally, Drake believes that a communicating civilization lasts for an average of 10,000 years. Yes, well, that is possible. And, yes, that means that the 80 civilizations on 80 planets in the solar systems that we have already discovered may have either become extinct or may not have come into existence yet. So I guess there is a chance that no one would return our greeting if we tried to say hello. But surely life itself can be expected to last longer, and surely, if life is as common as Frank Drake says, we must already have discovered other solar systems where there is abundant life on at least one of the planets?

Ann

[Ann]

It might be interesting to see what values each of us who are interested in this discussion get when they run the Drake Equation interactive. Mine works out to 798,400 -- a bit on the high side compared to most of you, I imagine. That's mainly because I'm optimistic about the life span of an intelligent species.

For some, this version of the equation might be more suitable.

Rob

I did the Drake Equation interactive too and got 8 communicating civilizations in our galaxy. I slashed the average number of habitable planets for each star from 2 to 0.1. I think even that may be on high side. I did away with much of the intelligence and the communication, too.

So I guess you are about 100,000 times more optimistic about intelligent life in our galaxy than I am, Rob!

Ann

[Chris Peterson]

I did the Drake Equation interactive too and got 8 communicating civilizations in our galaxy. I slashed the average number of habitable planets for each star from 2 to 0.1. I think even that may be on high side. I did away with much of the intelligence and the communication, too.

I got one. I stuck with Drake's value of 5 for stellar birth rate, and 50% for planet formation. I think he was low with the number of planets in a system capable of supporting life, so I raised it to 5, which is about what we have in the Solar System. In our system 20% of the habitable planets developed life (that we know of), so I put a conservative 10% in for that. Intelligent life I'm inclined to think is much rarer, so I went with 1% there. I assumed that 50% of intelligent species would develop interstellar communications, but figure that such a civilization only has a 100 year lifetime. End result: one civilization like ours in the galaxy at any time.

Throw out the last three terms and you get my estimate for the number of planets with life- a billion or more.

[Ann]

I did the Drake Equation interactive too and got 8 communicating civilizations in our galaxy. I slashed the average number of habitable planets for each star from 2 to 0.1. I think even that may be on high side. I did away with much of the intelligence and the communication, too.

I got one. I stuck with Drake's value of 5 for stellar birth rate, and 50% for planet formation. I think he was low with the number of planets in a system capable of supporting life, so I raised it to 5, which is about what we have in the Solar System. In our system 20% of the habitable planets developed life (that we know of), so I put a conservative 10% in for that. Intelligent life I'm inclined to think is much rarer, so I went with 1% there. I assumed that 50% of intelligent species would develop interstellar communications, but figure that such a civilization only has a 100 year lifetime. End result: one civilization like ours in the galaxy at any time.

Throw out the last three terms and you get my estimate for the number of planets with life- a billion or more.

I didn't even think of the probable average life span for a technological communicating civilization, because I got myself into an uproar over the suggestion that every other star in our galaxy would have an average of two habitable planets each!!! I still find myself frothing with fury at this absolutely outrageous suggestion!

But as for the average life span of a technological communicating civilization, well, you could be right, Chris. Maybe it is no more than a hundred years. But guess what? If I put that value in my personal Drake Equation, I think I'll end up with 0.08 intelligent civilizations in our galaxy, and I can't help thinking that is wrong... because we are here, aren't we? Perhaps I am trying to tell myself that we humans are neither all that intelligent nor all that good at communication?

Ann

[swainy]

I leaned something very interesting today, I did not know, that our galaxy also has a habitable zone.

http://www.daviddarling.info/encycloped ... _zone.html

Thanks for making me go look Ann

tc

[Chris Peterson]

I didn't even think of the probable average life span for a technological communicating civilization, because I got myself into an uproar over the suggestion that every other star in our galaxy would have an average of two habitable planets each!!! I still find myself frothing with fury at this absolutely outrageous suggestion!

Well, we have something like five habitable planets in our own system, so I think it's reasonable to think where there is one, there are a few. As to what percentage of planetary systems actually have characteristics supporting any such planets... well, that's not going to be much better than a guess at this point. In five years, I think we'll be able to put a credible number to it, though.

[neufer]

I didn't even think of the probable average life span for a technological communicating civilization, because I got myself into an uproar over the suggestion that every other star in our galaxy would have an average of two habitable planets each!!! I still find myself frothing with fury at this absolutely outrageous suggestion!

It is most certainly NOT an outrageous suggestion.

Nevertheless, it would certainly be outrageous to state such a thing as fact (or even as a probably scenario).

But as for the average life span of a technological communicating civilization, well, you could be right, Chris. Maybe it is no more than a hundred years. But guess what? If I put that value in my personal Drake Equation, I think I'll end up with 0.08 intelligent civilizations in our galaxy, and I can't help thinking that is wrong... because we are here, aren't we? Perhaps I am trying to tell myself that we humans are neither all that intelligent nor all that good at communication?

The fact that we are here communicating with radio at the present time only means that the probability of such situations is NOT ZERO. It says nothing else about the magnitude of this none zero probability.

[rstevenson]

Okay, having prompted others to try the Drake Equation interactive, I suppose I should back up my outrageous final figure of 498,400.

R - I left the rate of star formation at 5. Current thought suggests it has been as high as 10 in our galaxy but is about 7 at the moment. It changes, so let's be conservative.

Fp - the percentage of stars that form planets? Who knows, but we seem to be finding an awful lot of them. That's not surprising because stars form within accretion disks, so the material is there to make some planets too. So I set it to 50%, which is probably low.

Ne - this is the one that gets people going. How many of those planets that formed could support life? As Chris says, there's about 5 right here in our home neighbourhood, so I think conservatively 2 is about right. Of course that life might be no more than pond scum, or might last only a short time before its home planet takes a turn for the worse, but that's what the rest of the equation is for.

Fl - the percentage of those life-possible planets where life actually forms? I'm an optimist. I think life, of some kind or other, will form if it possibly can, so I set this to 80% -- just to remain conservative.

Fi - the percentage of these where intelligent life arises? A portion of humans are intelligent, so I guess we'd have to admit intelligence has arisen here once (at least.) But if there are 5 possibles in our home system, and only 1 has given rise to intelligence, then that leaves me at 20% for this one. (I know. I estimated 80% of 5 above, but I happen to think we're in a particularly advantageous planetary system, just by the luck of the draw, so I expect we'll find evidence of past life, or current, in all the likely spots we look.)

Fc - the percent of intelligent species that at least try to communicate over interstellar distances, I set to 100%. I suppose it's possible to have an intelligent species that is so paranoid they won't even try, but I happen to think paranoia is counter-productive in the long run, and therefore is mostly weeded out during the evolution of a sentient species.

L - and now the one that kicks my figure way up. I feel that an intelligent species will try very hard to ensure it is immortal. Yes there is danger along the way, with many opportunities for false steps and self-destructive behaviour. But I feel that while civilizations may fall, the intelligent species will not fail in the long run. So I maxed this out, setting it to 1,000,000 years. If I could have pushed the slider further, I would have.

Which gives me an N - the number of communicating species in our galaxy -- of 798,400.

So with so many species out there capable of communicating, why haven't they contacted us? Well they are intelligent, right? I mean, would you go to a sparcely populated part of a large city, a down and out area where gunfire is heard every night, and start yelling, "Here I am! I just want to say Hi!"

Rob

[Henning Makholm]

If I put that value in my personal Drake Equation, I think I'll end up with 0.08 intelligent civilizations in our galaxy, and I can't help thinking that is wrong... because we are here, aren't we?

Doesn't mean anything is necessarily wrong.

What 0.08 means is that there will be an intelligent civilization somewhere in the galaxy roughly one twelfth of the times you look. The fact that we find ourselves to be here each time we look doesn't mean much; that's just because we cannot look during the periods where we're not here.

Depending on your mood you can call that either "selection bias" or the "anthropic principle"; the basic fact is just that you cannot draw valid statistical inferences from your own existence. What the Drake formula can give you is a conditional probability: Given the fact that we're here, the expected number of other civilizations is such-and-such.

[neufer]

Okay, having prompted others to try the Drake Equation interactive,

I suppose I should back up my outrageous final figure of 498,400. ...which gives me an N -
the number of communicating species in our galaxy -- of 798,400.

So with so many species out there capable of communicating, why haven't they contacted us?

We are lousy at math?

Well they are intelligent, right? I mean, would you go to a sparcely populated part of a large city, a down and out area where gunfire is heard every night, and start yelling, "Here I am! I just want to say Hi!"

One of those super intelligent ETs did try to communicate with us once but he was treated rather badly:

IRAN: Ahmadinejad slams Paul the 'psychic' octopus
-- Meris Lutz in Beirut July 28, 2010 | 8:37 am

Like so many troubled stars before him, Paul the 'psychic' octopus was duped by fame into believing that the good times would last forever. But after accurately predicting the outcome of all seven World Cup games, Paul now faces accusations of "spreading Western propaganda and superstition" from none other than Iranian President Mahmoud Ahmadinejad. Ahmadinejad reportedly called Paul a symbol of "decadence and decay" among the enemies of Iran during a speech in Tehran over the weekend. Whether Paul will be able to save his career remains to be seen. Ahmadinejad's comments follow a string of bad reviews and increasingly seedy offers, including death threats from disappointed Germany fans, a Chinese movie called "Kill Paul Octopus," and a bid from a Russian betting firm.

Paul rose to stardom by 'predicting' the outcomes of games by eating a mussel out of a box marked with the flag of the winning team. "Those who believe in this type of thing cannot be the leaders of the global nations that aspire, like Iran, to human perfection, basing themselves in the love of all sacred values," Ahmadinejad said, according to the British Daily Telegraph. Ahmadinejad's comments have inspired a fair amount of snark from Paul fans around the world. Irish journalist Rory Fitzgerald even penned a satirical piece for the Huffington Post in which Paul 'responds' to the 'unprovoked attack.' " 'As an octopus, I can claim some objectivity in my view [of] human affairs,' " a 'visibly shaken' Paul said, adding that he had never before seen such a lack of a sense of humor in a human being, "despite the fact that he lives in Germany." Paul resides at the Sea Life Oceanarium in Oberhausen, Germany, where his handlers will hopefully keep him grounded and safe.>>

[Chris Peterson]

What 0.08 means is that there will be an intelligent civilization somewhere in the galaxy roughly one twelfth of the times you look.

I'd say it means that there is one intelligent civilization in every twelfth galaxy.

[Chris Peterson]

Fc - the percent of intelligent species that at least try to communicate over interstellar distances, I set to 100%. I suppose it's possible to have an intelligent species that is so paranoid they won't even try, but I happen to think paranoia is counter-productive in the long run, and therefore is mostly weeded out during the evolution of a sentient species.

I think you have to allow for the fact that an intelligent species could certainly be non-technological. Big brained animals like whales might develop a complex civilization, with language, art, music, etc, and never develop a single sophisticated tool (or hands to use tools). That's why I went with 50%, although I could easily believe a smaller or larger value. But not 100%.

L - and now the one that kicks my figure way up. I feel that an intelligent species will try very hard to ensure it is immortal. Yes there is danger along the way, with many opportunities for false steps and self-destructive behaviour. But I feel that while civilizations may fall, the intelligent species will not fail in the long run.

Hope you're right, but we don't seem to be setting any such example. The thing is, we don't need to destroy ourselves, only do enough damage to keep setting ourselves back from the level of technology (or wealth) required to engage in interstellar communications. There's good evidence we'll reach that point soon, and we're the only example I have to work with. It sort of makes sense that technology develops exponentially, while biological change is basically linear. Makes it hard to survive if a species develops god-like power while possessing a fairly primitive system of thinking and acting. I also think this is the best explanation for why we don't see intelligent species out there.

[rstevenson]

I suppose I should back up my outrageous final figure of 498,400. ...which gives me an N - the number of communicating species in our galaxy -- of 798,400.

So with so many species out there capable of communicating, why haven't they contacted us?

We are lousy at math?

Nah, just bad at typing. (The latter figure is the one I really came up with.)

Rob

[Ann]

Well they are intelligent, right? I mean, would you go to a sparcely populated part of a large city, a down and out area where gunfire is heard every night, and start yelling, "Here I am! I just want to say Hi!"

One of those super intelligent ETs did try to communicate with us once but he was treated rather badly:

IRAN: Ahmadinejad slams Paul the 'psychic' octopus
-- Meris Lutz in Beirut July 28, 2010 | 8:37 am

Like so many troubled stars before him, Paul the 'psychic' octopus was duped by fame into believing that the good times would last forever. But after accurately predicting the outcome of all seven World Cup games, Paul now faces accusations of "spreading Western propaganda and superstition" from none other than Iranian President Mahmoud Ahmadinejad. Ahmadinejad reportedly called Paul a symbol of "decadence and decay" among the enemies of Iran during a speech in Tehran over the weekend. Whether Paul will be able to save his career remains to be seen. Ahmadinejad's comments follow a string of bad reviews and increasingly seedy offers, including death threats from disappointed Germany fans, a Chinese movie called "Kill Paul Octopus," and a bid from a Russian betting firm.

Paul rose to stardom by 'predicting' the outcomes of games by eating a mussel out of a box marked with the flag of the winning team. "Those who believe in this type of thing cannot be the leaders of the global nations that aspire, like Iran, to human perfection, basing themselves in the love of all sacred values," Ahmadinejad said, according to the British Daily Telegraph. Ahmadinejad's comments have inspired a fair amount of snark from Paul fans around the world. Irish journalist Rory Fitzgerald even penned a satirical piece for the Huffington Post in which Paul 'responds' to the 'unprovoked attack.' " 'As an octopus, I can claim some objectivity in my view [of] human affairs,' " a 'visibly shaken' Paul said, adding that he had never before seen such a lack of a sense of humor in a human being, "despite the fact that he lives in Germany." Paul resides at the Sea Life Oceanarium in Oberhausen, Germany, where his handlers will hopefully keep him grounded and safe.>>

Good to know that those octopuses may still be here when we are gone to give the Earth not only intelligence but psychic insight, too! (Maybe we should ask Paul about the true solution to the Drake equation?)

Ann

[Ann]

Chris wrote:

It sort of makes sense that technology develops exponentially, while biological change is basically linear. Makes it hard to survive if a species develops god-like power while possessing a fairly primitive system of thinking and acting. I also think this is the best explanation for why we don't see intelligent species out there.

That sounds depressingly likely.

Ann

[rstevenson]

I think of it this way: there's a million ways a tool-building species can go wrong, but there's a million and one ways for it to recover. In other words, I feel the positive creativity slightly outweighs the stupidity. And slightly is all we need, though the road can be mighty bumpy along the way.

Rob

[Henning Makholm]

What 0.08 means is that there will be an intelligent civilization somewhere in the galaxy roughly one twelfth of the times you look.

I'd say it means that there is one intelligent civilization in every twelfth galaxy.

Same difference, as long as the time you let pass between samples is long compared to the assumed lifetime of a civilization.

[neufer]

I think of it this way: there's a million ways a tool-building species can go wrong, but there's a million and one ways for it to recover.
In other words, I feel the positive creativity slightly outweighs the stupidity.

And slightly is all we need, though the road can be mighty bumpy along the way.

• http://www.youtube.com/watch?v=1q5S4ncU3Dw

Click to play embedded YouTube video.

http://www.youtube.com/watch?v=TtmLVP0HvDg
http://www.youtube.com/watch?v=ZE4BT8QSgZk

[Chris Peterson]

I think of it this way: there's a million ways a tool-building species can go wrong, but there's a million and one ways for it to recover. In other words, I feel the positive creativity slightly outweighs the stupidity. And slightly is all we need, though the road can be mighty bumpy along the way.

Nope, that explanation doesn't work for me. It's like giving your little kids guns to go outside and play with, since there's a million ways they can kill themselves, but no problem, there's a million ways they can recover from being dead! I think the high technology stage of a civilization (where we are now, with knowledge growing exponentially) may be a trapdoor: fall through, and there's no way back.

Of course, this assumes a species that thinks a bit like ours. Perhaps a species with roots that were highly social (like some herd animals) would not have this problem. I'm not sure that such a species would be likely to develop high intelligence, though.

All very speculative, though. The one thing I'm pretty confident of is that technological civilizations are very rare. We should observe them, otherwise, and I simply don't find any of the explanations that suggest they are present but somehow unseen to be convincing at all.