SETI and the Drake Equation

[SkepticLance]

Just reading the latest edition of Skeptic journal. Vol. 14 no. 2 from page 28.

 

An article by Dr. Michael Shermer in which he refers to the basis for the Search for Extra Terrestrial Intelligence (SETI). Apparently the Drake Equation is very popular with SETI researchers. This was proposed by Dr. Frank Drake in 1961 and popularised by Dr. Carl Sagan. It refers to the number of intelligent, technological, and potentially contactable civilised species in our galaxy. The number of these is obtained by multiplying together the following.

 

1. The number of stars in the galaxy (E11)

2. The fraction of these stars with planets (usual guess 50%)

3. The number of Earth like planets per stellar system (usual guess 10% ??)

4. The fraction of these planets with life (usual guess 10% of item 3. ??)

5. The fraction of these planets with intelligent life (usual guess 10% of 4. ??)

6. The fraction of them with communicating technology(10% of 5. ??)

7. The average lifetime of communicating species

 

Dr. Shermer spends a lot of time discussing the last of these. Apparently SETI researchers have more difficulty with this than the other factors. They are happy to slot in their best guess for the numbers of other factors, but have no idea of how long a civilised species might survive. Estimates of lifetime range up to 10 million years, and average 50,000. And the result of their best guess calculations range up to many millions of civilised, communicating alien species.

 

Dr. Shermer comes up with his own version of the lifetime factor. He argues that we have a number of civilisations in our own past that give figures for the probably lifetime of our hypothetical species. He produces a list of 60 past Earth civilisations, from ancient Sumeria and Egypt to the British Empire, and an average lifetime of 420 years. Plugging this number into the Drake equation gives an estimate of just over 3 alien civilisations in our galaxy.

 

My own view is that Dr. Shermer has introduced and artificial and unlikely number into the equation. I see all Earth civilisations as part of one single progression towards today's world. The ancient Hittites invented iron smelting. We still use that and it is a part of the current society. When the Hittite civilisation died, civilisation as a whole did not. In my view, this destroys Shermer's thesis. The true number for Earth is of the order of 3,000 years.

 

Mind you, I seriously disagree with using 10% for factors 3 to 6 in the equation. That is a blatant guess, and probably utterly renders the whole thing meaningless.

 

What do you think?

[Gilded]

What's the definition of "Earth like"? Has water and is in the green zone? The version of the equation on Wikipedia is rather different.

[Sayonara]

I agree with the civilisation point. There is a difference between considering a pre-industrial sub-civilisation, and considering a fully-fledged species level civ which has the capacity to communicate with other worlds.

[Pangloss]

(This thread might actually be a good candidate for a "speculations" thread that actually has a positive context instead of the usual flim-flamery. Just a random thought -- no need to move it, IMO.)

 

I think it's a fascinating question, and one which science fiction authors have wrestled with as well. One thing I'm a little confused/surprised by is the inclusion of "communicating civilizations" that have no electromagnetic control (e.g. radio). That's what that category is actually supposed to be -- species that can transmit and receive, not species that have reached some arbitrary level of "civilization". Sure, the ancient Sumerians could write on clay tablets, but they couldn't have received an Arecibo Message even if someone had been aiming one at them.

 

In that sense the Earth number would be more like 150, or perhaps even less. And not only are we lacking data on other intelligent species, we don't even know how long our own "communicative" capability will last.

[SkepticLance]

To Gilded

 

The article did not include a definition of 'Earth-like'. From other reading I have done, I assume it means being in that orbit in which water is liquid.

 

I have some difficulty with this one. The estimate of 10% of planets being 'Earth-like' seems way over the top to me. The 200 odd planets outside our solar system so far discovered nearly all had strongly elliptical orbits. From that data, it seems that, if 'Earth-like' includes having a nearly circular orbit, like Earth, then the proportion is way lower than 10%. If the orbit is strongly elliptical, then it would move in and out of the zone in which water is liquid, and that would seem to make it very unlikely that life could get started. Life would have a bit of a problem when all water turns to steam!

[D H]

What's the definition of "Earth like"? Has water and is in the green zone? The version of the equation on Wikipedia is rather different.

The version On Wikipedia is the same as that seti.org and replicated on many other sites:

 

[math]N=R^*\times f_p\times n_e\times f_l\times f_i\times f_c\times L[/math]

 

where (from http://www.seti.org/seti/seti-science/)

• N = The number of civilizations in The Milky Way Galaxy whose electromagnetic emissions are detectable.
  
• R* = The rate of formation of stars suitable for the development of intelligent life.
  
• f_p = The fraction of those stars with planetary systems.
  
• n_e = The number of planets, per solar system, with an environment suitable for life.
  
• f_l = The fraction of suitable planets on which life actually appears.
  
• f_i = The fraction of life bearing planets on which intelligent life emerges.
  
• f_c = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
  
• L = The length of time such civilizations release detectable signals into space.

 

Here's my take:

• R* = One per year, a fairly standard estimate. Blue giants and brown dwarfs don't count as "suitable" star, nor due most double stars.
  
• f_p = 0.9. I'll be generous. Most young stars appear to be surrounded by a dust nebula.
  
• n_e = 0.25. This might well be generous given the number of star systems we have observed with jovian planets orbiting very close to their star.
  
• f_l = 0.9. Life started so soon after the formation of the Earth that I will take this as a near-given.
  
• f_i = 1/1,000,000. I'll defend this later.
  
• f_c = 1/100. I'll defend this later as well.
  
• L = 100,000 years. I'm being optimistic and assuming we will make it past global warming, resource depletion, global thermonuclear war, plague, name your disaster.

 

Put this together: 1/year*0.9*0.25*0.9*1e-6*1e-3*1e5 years = 1/5,000. We are almost certainly alone in our galaxy, very likely alone in the local group, possibly alone in the Virgo supercluster, but we far from alone in the universe.

 

About my numbers:

 

Only one out of a million planets that do develop life of any sort develop "intelligent" life.

 

I'll define intelligent life as life capable of something akin to written communication. The Drake equation is missing a very big factor that militates against developing intelligent life. That big factor is the fraction of stars that develop complex life. Life on Earth was stuck in the form of single-celled organisms for 3.5 billion years or so, and then stuck in the form of non-intelligent life for the next 550 million years. The latter number indicates that intelligent life is a fluke. The first number indicates that complex life is an incredible fluke.

 

Most planets will succumb to runaway global warming (Venus) or runaway global cooling (Mars). Even Earth nearly succumbed to the latter. Of the few planets that do survive that, there are external threats such as meteors, massive solar flares, passing stars that render the solar system chaotic, gamma ray bursts, etc., and internal threats such as excessive tectonic activity and frozen tectonic activity. Of the few planets that do survive life-destroying disasters, simple life is most likely to remain simple. Of the very, very few planets that do develop complex life, life is like to remain unintelligent.

 

Only one out of a hundred planets that do develop "intelligent" life develop the ability to communicate electronically.

 

Developing the ability to communicate electronically requires plenty of resources. Without our abundant metals and combustibles we would still be hunter-gatherers. Planets that form too far out from the center of the galaxy will not have the abundant metals the propelled humanity to progress beyond the stone age. (Planets that form too close to the center of the galaxy are much more prone to life-destroying disasters.) Planets where life struggles rather than thrives will not develop the hydrocarbons that propelled the Industrial revolution. On a few planets life will develop the ability to communicate electronically at just about the same time they develop the ability to obliterate themselves. Planets on which the ability to communicate electronically for but a decade or so don't count for much.

[SkepticLance]

To DH

I like your final conclusion, but I would arrive at it a little differently.

 

I do not think we can consider that 0.9 of all planets within the liquid water zone will develop life. Even though life appeared quickly on Earth, to suggest that this is transferable to other planets is a bit naive. We do not know what the exact conditions were for life to form. There are lots of theories, but no real facts.

 

One thing is clear, though. A minor 'wrong' in initial conditions would be enough to prevent life's appearance. Too many impact events. Too elliptical an orbit. No tectonics. Perhaps the wrong mineralogy, or the wrong initial atmosphere. I would rate this factor well down - way below 0.9.

 

I would do the opposite for the percentage of planets with life that develop intelligent life. We have just Planet Earth to draw on for comparison, but it looks quite clear to me. Life evolves. While individual species can change in all sorts of weird ways, overall there is a trend towards greater complexity, and towards higher intelligence. Today, as well as humans, there are quite a number of species that are semi intelligent. I think that if life gets well established, the probability, given time, that it will develop into intelligence is quite high.

 

In the same way, if intelligence appears, then the main thing preventing them achieving electronic technology is either lack of hands (like dolphins) or lack of time. It took Homo sapiens about 200,000 years from the species first appearing to emitting radio signals. Since the galaxy is about 8 billion years old, we can assume the time required has already occurred. On Earth, about one third of all semi-intelligent species have some sort of hand equivalent. So I think the percentage here would be a lot higher than 1%.

[D H]

To DH

I like your final conclusion, but I would arrive at it a little differently.

 

I do not think we can consider that 0.9 of all planets within the liquid water zone will develop life. Even though life appeared quickly on Earth, to suggest that this is transferable to other planets is a bit naive.

Caveat: making conclusions from a sample size of one is a bit recklessness. I think we should all admit we are engaging in pseudoscience/speculation.

 

Given that, there are some signs that life on Earth originated 4.25 billion years ago and very strong signs that life originated more than 3.8 billion years ago. This has led some to believe that life is inevitable.

 

While extremely simple life might be inevitable, even less simple single celled life forms are not. It took quite some time to make the jump from primitive extremophiles to more modern single-celled life. About 40% of the totality of the existence of life is in the form of primitive extremophiles. Some planets are likely to be stuck in this mode, not even developing photosynthesis. Even with the aid of photosynthesis it took life even longer to clear the next hurdle: complex, multicellular life. About 85% of the totality of the existence of life is in the form of single-celled organisms.

 

One thing is clear, though. A minor 'wrong' in initial conditions would be enough to prevent life's appearance. Too many impact events. Too elliptical an orbit. No tectonics. Perhaps the wrong mineralogy, or the wrong initial atmosphere. I would rate this factor well down - way below 0.9.

The "too elliptical an orbit" is quite unlikely. All of the planets in our own solar system (Pluto isn't a planet) have nearly circular orbits. Models of planetary formation indicate that this isn't a fluke. The early solar system had a density gradient that increased sunward. Just as atmospheric drag acts to circularize the orbits of low Earth-orbit satellites, the initial protoplanetary disk would have acted to circularize the orbits of everything in that disk.

 

The "too many impact events" is also a bit unlikely, at least so far as primitive life forming is concerned. Life formed in the Hadean, when the Earth was still undergoing heavy meteor bombardment.

 

No tectonics: This is a good point. I would place this as part of what distinguishes a planet as having "an environment suitable for life"; category n_e. The same goes for mineralogy. As I mentioned in my first post, I was intentionally overgenerous with my assessment of 0.25 such planets per solar system. I suspect this number is much lower.

 

I would do the opposite for the percentage of planets with life that develop intelligent life. We have just Planet Earth to draw on for comparison, but it looks quite clear to me. Life evolves.

Life evolves slowly, particularly so primitive life. Our own planet is the only one we can draw upon. The initial formation of life was quick, but subsequent developments were not. I see the key events (note well: I am not a biologist) to be the development of photosynthesis, the development of a nucleus, the development of sex, the development of semi-intelligence, the development of near-intelligence (tool use), and the development of true intelligence (complex speech and writing). There is a long gap between each development. These developments, unlike life itself, are not inevitable.

 

On Earth, about one third of all semi-intelligent species have some sort of hand equivalent. So I think the percentage here would be a lot higher than 1%.

That a lot of semi-intelligent species has some sort of hand equivalent but only one has developed true intelligence indicates to me that the probability is far, far smaller than 1%. For most species there is no need to progress further than they have on the intelligence scale. Our complex brain is an incredible energy hog. Excess intelligence may well be a maladjustment in most species.

[Sisyphus]

Caveat: making conclusions from a sample size of one is a bit recklessness. I think we should all admit we are engaging in pseudoscience/speculation.

 

This can't be stressed enough in these sorts of discussions. The only thing we actually know is that life is possible in the form we see on Earth, and that it can arise on planets almost exactly like Earth. Is life on Earth the way it is out of necessity ("If the Earth was slightly different, life would be impossible."), or is it merely the product of the environment in which it arose ("If the Earth was different, life would be different.")? We don't know. We can speculate with a fair degree of confidence what are the limiting conditions for Earth-like life (although we've been surprised a few times in that area right here on Earth), but we don't know in what other forms "life" is possible, if any, and thus we don't know what conditions it requires. It's all just variations on interpreting the Anthropic Principle.

[SkepticLance]

I think we all agree we are engaged in strong speculation, and our conclusions are going to be highly tenuous, at best, and total garbage at worst.

 

Back to dating life. DH's reference on 4.25 billion year old carbon, while interesting, is probably a red herring. There was an article on earliest hints of life in Scientific American a year or 3 ago. They mentioned 3.8 billion year old traces of hydrocarbons in Canadian rocks, but were not prepared to draw conclusions, and really thought that, too, was a red herring. They also mentioned the 3.5 billion year old rocks in Western Australia that have been interpreted as early stromatolite (photosynthetic bacteria) fossils. SciAm thought that was pretty uncertain also.

 

In addition, there is iron oxide in ancient rocks. Apparently the Earth is rich in iron compounds, and these tend to oxidise when exposed to pure oxygen. There is a one billion year period of about 3.5 billion years ago to 2.5 billion years ago where the rocks are very rich in iron oxide. This has been interpreted by some researchers as meaning that photosynthetic organisms were releasing oxygen into the atmosphere over that time. After that time, all the available iron was oxidised and oxygen could build up in the air rather than being absorbed by iron.

 

Anyway, SciAm suggests that it is likely the first life on Earth appeared 3 to 4 billion years ago. The evidence is too poor to be more definitive. My own feeling is that the first life appeared 3.5 to 4 billion years ago, and the purported stromatolite fossils in Australia were probably real - but that is just my opinion.

 

If this is the case, life appeared 500 million to 1 billion years after the formation of the Earth. This is not, in my utterly humble opinion, strong evidence for the universality of life!

 

We are speculating. We do not know what conditions are needed for life to form. We know it happened here, but cannot assume that it happened anywhere else. Once the first life appeared, then if no nasty accidents happen, it has the chance to evolve into more robust forms. The first life, though, without strong adaptations to changes in the environment, must have been very vulnerable to extinction.

 

It is probably no accident that the first life appeared on Earth in the period after the early bombardment by meteors was pretty much over. Models indicate that the massive impact events took about 500 million years.

 

One part of the model indicates that the presense of Jupiter in its special orbit was crucial to removing the space debris causing the bombardment. Without Jupiter, or if Jupiter was elsewhere, this would not have stopped.

 

Amont the 200 odd extra-solar planets discovered, giants like Jupiter are not normally found anywhere near the special orbit Jupiter has. Indeed, many are so close to their sun that they would be inside the orbit of Mercury. These close in giant planets would be inimical to the development of life, since meteorite impacts would continue unabated within the liquid water zone.

 

I also repeat that most of these extra-solar planets have strongly elliptical orbits. The evidence strongly suggests that the nearly circular orbits in our solar system are very unusual.

 

My conclusion is that the beginnings of life outside Earth are probably rare.

[iNow]

Rare like 1 in 4, or rare like 1 in 10^30?

[SkepticLance]

To iNow

 

I think we have agreed that this is speculation. Thus, putting firm numbers on anything is exceedingly risky. I can only offer you a guess, and my guess may be way out.

 

As a guess, I would say somewhere between 1 in 1000 and 1 in a million, but as stated, I could be way wrong.

[D H]

I think we all agree we are engaged in strong speculation, and our conclusions are going to be highly tenuous, at best, and total garbage at worst.

 

Back to dating life. DH's reference on 4.25 billion year old carbon, while interesting, is probably a red herring. There was an article on earliest hints of life in Scientific American a year or 3 ago. They mentioned 3.8 billion year old traces of hydrocarbons in Canadian rocks, but were not prepared to draw conclusions, and really thought that, too, was a red herring.

While the exact date of the origin of life is uncertain, but it widely viewed as occurring very soon after the Earth become even slightly hospitable to life.

 

Lance, you completely ignored the article about the inevitability of life. Science is by no means a pseudoscience rag.

 

In addition, there is iron oxide in ancient rocks. Apparently the Earth is rich in iron compounds, and these tend to oxidise when exposed to pure oxygen. There is a one billion year period of about 3.5 billion years ago to 2.5 billion years ago where the rocks are very rich in iron oxide.

That period starts rather than ends about 2.5 billion years ago. The oldest stromatolites fossils are about 2.7 billion years old. Up until that time, life was strictly anaerobic. By the time those ancient blue-green algae had finished their job, a good chunk of that ancient life was extinct because free oxygen was highly toxic to that ancient life. Photosynthesis appeared after life had been around for some time. Marking 2.7 billion years as the start of life is, IMHO, erroneous. 2.7 billion years ago demarcates the onset of non-primitive life.

 

Amont the 200 odd extra-solar planets discovered, giants like Jupiter are not normally found anywhere near the special orbit Jupiter has. Indeed, many are so close to their sun that they would be inside the orbit of Mercury.

This is more likely a result of the fact that it is much easier to see such systems than is it indicative of the relative abundance of systems like the solar system versus systems in which the dominant gas giant has migrated from the gas giant region formation region sunward. (Note well: A gas giant planet cannot form in the vicinity of Mercury's orbit.)

 

These close in giant planets would be inimical to the development of life, since meteorite impacts would continue unabated within the liquid water zone.

These systems with close-in gas giants would be inimical to the development of inner planets, period. The type II migration that takes the gas giants sunward clears most of the remnant junk from the inner star system, including any nascent planets.

 

I also repeat that most of these extra-solar planets have strongly elliptical orbits. The evidence strongly suggests that the nearly circular orbits in our solar system are very unusual.

All extant theories of planetary formation agree on one thing: Planetary orbits around single star systems should be nearly circular. The observed deviation might well be yet another selection effect. It might also indicate that most stars systems have hidden brown dwarf companions, and it might even indicate that all of our theories on planetary formation are wrong.

 

Regardless of which is the answer, you are placing star systems with hot Jupiters and planets with highly elliptical orbit in the wrong bucket. The prevalence of brown dwarfs will reduce R*. The prevalence of hot Jupiters will reduce n_e. The fraction f_l The is fraction of suitable planets on which life actually appears. The key here is "suitable planets." Neither a hot Jupiter nor a planet with a highly elliptical orbit is suitable. Double-counting will falsely reduce the final value for N.

[SkepticLance]

To DH

 

I did not ignore your reference to the inevitability of life. I was, instead, unable to access it.

 

 

You are correct to say we cannot mark a particular time as the start of life. We simply do not know. SciAm states 3 to 4 billion years ago. That is a pretty big margin of error!

 

 

You said

 

"Regardless of which is the answer, you are placing star systems with hot Jupiters and planets with highly elliptical orbit in the wrong bucket. The prevalence of brown dwarfs will reduce R*. The prevalence of hot Jupiters will reduce ne. The fraction fl The is fraction of suitable planets on which life actually appears. The key here is "suitable planets." Neither a hot Jupiter nor a planet with a highly elliptical orbit is suitable. Double-counting will falsely reduce the final value for N."

 

I may not have made this very clear. What I am trying to do is exclude entire star systems. A system with a planet Jupiter size or larger, close to the sun, will not very likely have life bearing planets. Nor will a system with highly elliptical orbits. And based on empirical evidence to date, most systems have planets with highly elliptical orbits.

 

We still need a lot more data about extra-solar star systems, but to date the results do not look promising for lots of life bearing planets.

 

A comment on the fraction of life bearing planets on which intelligence evolves. About 10% of the galaxy is composed of star systems that are 2 billion plus years older than our own. Evolution of life in these systems, if it follows a pattern roughly equivalent to Earth, would have reached a point similar to life on Earth today around 2 billion plus years ago. Thus, 10% of life bearing worlds have had heaps of time for intelligence to evolve. This is a prime reason why I think that the fraction of life bearing worlds with intelligence has to be a lot greater than 1:1,000,000.

[D H]

I may not have made this very clear. What I am trying to do is exclude entire star systems.

Perhaps I have not made myself clear. I'll be blunt. You are missing the point. Excluding entire star systems is the job of the factors R*, f_p, and n_e. It is not the job of the factor f_l, which is the probability that life will arise given that a star, its star system, and some planet(s) in that star system are suitable for life. Think of it as a conditional probability.

 

Bottom line: If you want to reduce entire star systems, attack the leading factors.

 

A comment on the fraction of life bearing planets on which intelligence evolves. About 10% of the galaxy is composed of star systems that are 2 billion plus years older than our own. Evolution of life in these systems, if it follows a pattern roughly equivalent to Earth, would have reached a point similar to life on Earth today around 2 billion plus years ago. Thus, 10% of life bearing worlds have had heaps of time for intelligence to evolve. This is a prime reason why I think that the fraction of life bearing worlds with intelligence has to be a lot greater than 1:1,000,000.

You have not addressed my contention that life on most planets will simply die out. Think of Venus and Mars. Venus didn't have a perfect collision with a Mars size object that drew off most of the initial atmosphere. Mars is a tad too small, making its tectonic activity come to a dead stop.

 

On planets where life does survive, it will reach a pre-intelligent plateau on most. On some planets life won't even progress to the stage of simple bacterial life. On others bacterial life will form, but not multicellular life. The steps from primitive to bacterial to multicellular spanned three billion years on our planet. There are several reasons to think that the odds of developing complex life alone is very, very small.

 

The Earth's Moon is one such reason. Life would have been very different without Earth's Moon. The Moon was and remains a driver of life. The Moon stripped the Earth of most of its initial atmosphere. We would be another Venus if it hadn't been for the Moon.

 

While the time span from the development of complex life to the development of intelligent life was relatively short, there is little advantage in extreme intelligence for most species. Our brains are a huge challenge in birth. During hard times, our brains represent a huge energy drain. While life is inevitable, intelligent life is not. We are a fluke.

[SkepticLance]

DH said

 

"Perhaps I have not made myself clear. I'll be blunt. You are missing the point. Excluding entire star systems is the job of the factors R*, fp, and ne. It is not the job of the factor fl, which is the probability that life will arise given that a star, its star system, and some planet(s) in that star system are suitable for life. Think of it as a conditional probability."

 

OK. I am happy to accept that qualification. However, I still think that number will be very low. Planets suitable for life will, in my opinion, be rare.

 

Re life dying out.

Yes, that remains a strong possibility, and it is a good point for you to bring up. I guess I have tended to lump that possibility in with the odds that life did not arise at all. Your point about the moon is a good one. What is your guesstimate about the percentage of planets that once developed life, only to see it die out?

 

Re intelligence.

I disagree with you in your statement about intelligence being a fluke. There are too many other quite intelligent species. The bottlenose dolphin, for example, has a larger brain than humans, and a brain to body weight ratio that is quite similar. While we do not know how smart it actually is, it's large brain must carry at least the adaptive disadvantages of the human large brain, such as energy demand, and difficulty with birth.

 

http://practicalethics.net/blog/?s=untoothed

 

I quote :

 

"Dolphins rank higher in encephalization quotient (EQ), the ratio of the brain volume to the surface area of the body, than great apes and have been placed only second to humans. The EQ is significant because it gets higher as the subjects’ social structures get more complex. But some suggest that the EQ measurement may be underestimated in dolphins because of the additional weight of blubber in the cetacean body (see Marino, further resources below). This indicates that dolphins, therefore, may have at least the marine parallel to the human EQ."

 

Intelligence, albeit less than human, is also found in octopi, some parrots, some crows, other cetaceans, other great apes, elephants and some carnivores. Humans have been intelligent for a tiny fraction of the time life has existed, and given more time, many of those species would probably develop human or greater smarts.

[iNow]

Caveat: making conclusions from a sample size of one is a bit recklessness.

 

This can't be stressed enough in these sorts of discussions. .

 

This point seemed to need repeating.

[D H]

OK. I am happy to accept that qualification. However, I still think that number will be very low. Planets suitable for life will, in my opinion, be rare.

The only reason I didn't attack the first three numbers, R*, f_p, and n_e, is because I easily arrived at a number that indicates we are quite alone. So, attacking the first three numbers:

• R*
  This is the rate of formation of stars suitable for life. The overall star formation in the Milky Way is widely regarded to be on the order of one or so per day. Using one as the value of R* is ludicrous because
  • Multiple star systems are much less suitable for life than are single star systems in part because of the orbit stability problem. Most star systems are multiples. The observational evidence of elliptical orbits among exoplanets suggests that even most apparently single star systems are in fact multiples; the partner is a brown dwarf.
    
  • The center of the galaxy is much less suitable for life because there are too many other stars nearby. Those nearby stars can perturb the planets' orbits, or go supernovae, or emit a GRB. Most star formation occurs in regions of the galaxy that aren't suitable for life.
    
  • Massive stars are less hospitable to life for the simple that massive stars have too short a life span. In our solar system, 4.6 billion years passed between the formation of the solar system to the development of life capable of performing interstellar communication. The star has to smallish so that its lifespan is at least on the order of the timespan needed to form communicative intelligent life.

  In short, the standard value of [math]R^\ast = 1[/math] is far too high. I would venture it is an order of magnitude to high at a bare minimum, and is much more likely possibly two or three orders of magnitude too high.

   

   

  [*]f_p

  This is the fraction of suitable stars that form planetary systems. I'll leave this one alone. I suspect that most "suitable" stars form planetary systems of some form.

   

   

  [*]n_e

  Lance, this is the number that you are arguing should be drastically reduced. I simply took the low end of the widely used range of values. You are essentially arguing that this low end is far too high. Recent observations of exoplanets indicates that this number might well be much less than one because our solar system might be a fluke. (Then again, maybe we are seeing so many more hot Jupiters than anything else because the current planet detection capabilities see hot Jupiters and not much else.) Some argue the other way: That this number is greater than one. In our solar system, Venus, Earth, and Mars all qualify as planets "suitable for life" in the minds of some.

 

Re life dying out.

Yes, that remains a strong possibility, and it is a good point for you to bring up. I guess I have tended to lump that possibility in with the odds that life did not arise at all.

You shouldn't do that. With the Drake equation as it stands, that is a part of the probability of the formation of intelligent life. The Drake equation is a conditional probability equation. It is never a good idea to double count things in such equations.

 

Your point about the moon is a good one. What is your guesstimate about the percentage of planets that once developed life, only to see it die out?

I'm not even going to try.

 

Re intelligence.

I disagree with you in your statement about intelligence being a fluke. ... Intelligence, albeit less than human, is also found in octopi, some parrots, some crows, other cetaceans, other great apes, elephants and some carnivores. Humans have been intelligent for a tiny fraction of the time life has existed, and given more time, many of those species would probably develop human or greater smarts.

I disagree here. Near-intelligence has arisen many times. Communicative intelligence, once. Near-intelligence is a great evolutionary advantage. What is the evolutionary advantage to our level of intelligence?

 

We are, in a way, less intelligent than many of the species you mentioned. Non-human near-intelligent species use their brains to observe the world around them. Evolution channeled a lot of that brain power toward communicative and reasoning capabilities in our brains. Chimps, for example, have much better short-term memory than do we. The only reason this redirection was advantageous was because we had already developed an upright posture that freed our hands. In most species, such a development would be disadvantageous. Communicative intelligence might well have arisen multiple times only to disappear because in most species, true intelligence confers no advantage but does confers lots of disadvantages.

[SkepticLance]

DH

 

I definitely agree with you that communicating civilisations in our galaxy will be rare, and might well be restricted to planet Earth. Your comments about star formation are excellent.

 

I still disagree on intelligence. This thing about near intelligence, is that it has existed for less than 50 million years, and only that long in 'handicapped' species with no hand equivalent. Against a backdrop of 3 to 4 billion years with life, that is an 'eyeblink' of time. Near intelligence in terrestrial species with hands has existed for a very few millions of years. Given more time, I suspect, it would appear in many species.

 

The major advantages of intelligence are :

1. Social factors - hence breeding

2. Tool manufacture and use

3. Communication and cooperative hunting.

 

Of those three, I would say the main driver of human evolution has been the tools. And we already see chimpanzees beginning down that road. In the wild, they use stones to crack nuts, twigs to fish out termites, and even sharp sticks as spears in hunting. Given time, the chimps would very likely become our second fully intelligent species.

[Sayonara]

I still disagree on intelligence. This thing about near intelligence, is that it has existed for less than 50 million years, and only that long in 'handicapped' species with no hand equivalent.

 

We can't say "less than" here; that's speculation.

[SkepticLance]

To Sayonara, I do not quite understand your point, or why speculation is wrong. This whole thread is speculative.

 

However, my 50 million number comes from whales. They appear to be the oldest of the 'reasonably intelligent' mammals. Apes, elephants and the smarter carnivores did not have large brains till later. Whales with reasonable size brains were found as fossils in sediments dated to 50 million years ago. To the best of knowledge, all other smart mammals are younger. Similarly, there are no clear cut fossils of 'brainy birds' that far back.

[Sayonara]

Fossils aren't the best way to establish the intelligence of every animal that has ever lived, especially when we are talking about this "near intelligence" (which you introduced), or intelligences which are measured differently to our own.

 

I'm quite sure you realise I am not saying you are wrong. I am simply saying we can't know if you are right.

[SkepticLance]

To Sayonara

Sorry I did not fully understand what you were getting at before. Yes, I agree. We cannot know for sure. We do know that modern whales and dolphins are reasonably intelligent , and I made the assumption that the earliest whales with large brain cases were also quite bright. OK, we cannot be sure.

 

The real point though, is that near intelligence has occurred only in the last few million years (maximum 50) and has already exploded into numerous semi-intelligent species. Evolution can go many different ways, but over a long enough period shows a clear trend towards producing more complex organisms. On Earth, this trend led in the end to many semi-intelligent species. We cannot be sure that this happens everywhere, but I would suspect very strongly that a large fraction of worlds with life would demonstrate the same trend to increased complexity over time. Given enough time, intelligence or near intelligence would emerge in many such worlds.

 

I would be more inclined to suspect few worlds developing life itself, but a high fraction of worlds with life developing semi or real intelligence. The idea that life must appear inevitably, or almost so, on any world that has the physical requirements, is to my mind, more an aspect of a kind of religious faith than genuine science.

[Moonwatcher]

There's an interesting debate with Frank Drake and various experts on the likely emergence of intelligence at http://radio.seti.org/past-shows.php

Go to to the sound file at March 17, 2008 "Formula One: The Drake Equation".

 

While I believe simple life may be abundant throughout the universe, I expect that 'intelligent' life is very rare. By 'intelligent' I mean having the ability to build radio receivers/transmitters. It took life on Earth 3.5 billion years to manage this with only a single species emerging as capable. Although apes and dolphins have 'potential', I think they are already too closely related to us to seriously be seen as separate data points. Remember that it took around 3 billion years of life on Earth before highly complex organisms evolved. Still, there only has to be one civilization per galaxy to make 100 billion across the universe.

 

The only other animals with 'potential' are the cephalopds, particularly octopuses. Their intelligence is obvious, although difficult to measure perhaps because their type of intelligence is so 'different' to our own. They are a very rare example of intelligence evolving separately from ours, which gives me hope that life outside the Earth isn't all just slime sloshing around in some murky alien ocean.

[Sayonara]

Lance, I don't understand your thought processes at all.

 

You say you see my point that we cannot be sure, then on the very next line you state that near intelligence has only arisen in the past 50 million years, as if it is a flat fact.