One of the most agreeable articles I've read on the subject of extraterrestrials and SETI to date. See here (http://www.space.com/searchforlife/seti_culture_070503.html).

The issue regarding what we can expect to come up with after our - at least - decades long search for irrevocable evidence of our equals off the Earth.

One idea, contained in the article, which I have advocated on this website in previous posts, concerrns the human view of the world, and the universality of mathematics, the scientific method, and physics.

All the above named areas of study are, most certainly, a derivation of humanities interaction with the Universe as a whole.

What if, indeed, our whole view of the Universe (mathematics, physics, hydrogen line included) are not universal? What if everything that we "know" now, can be categorised as just an aspect of terrestrial human culture?

I guess it wouldn't greatly change the way we see our Universe, ;) but surely, it would give us a new sense of our place in it. In reality, no matter how many dissimilar forms of complexity - i.e. life - are out there we'd truly be alone in our Universe.

SETI is a waste of time, any aliens would have to be practically on our door step for any worthwhile conversation to take place, what are the chances of finding aliens able to communicate with us within 50lys?

SETI is a waste of time, any aliens would have to be practically on our door step for any worthwhile conversation to take place, what are the chances of finding aliens able to communicate with us within 50lys?Closest planet detected so far is 10.5 ly away (http://www.space.com/scienceastronomy/extrasolar_planets-9.html). Not habitable... but... read the caption in the link.

I don't think SETI is a waste of time, though. If anything, it's dangerous, but how else are we going to find out? Have to try something. Such as the way of human discovery.

SETI involves a lot more than communication, however. It certainly encompasses a lot of "Earth" science - learning about ours, and our sister species here on Earth, to see if we can try to gauge what to expect of species elsewhere. If nothing else, it improves our knowledge of what we have here.

I must say, though, that the communications aspect of SETI is pretty much a blind search, because you truly can't know what "methods of communication" another form of complexity in the Universe might use (another way of saying this is just how they would interact with the Universe).

For me, the possibilities are endless...

He is right :L SETI is a waste of time, it's also useless even if we do find anything. The truth is SETI is only meant to destroy Christianity as a religion, I hope they suceed too!

What if, indeed, our whole view of the Universe (mathematics, physics, hydrogen line included) are not universal? What if everything that we "know" now, can be categorised as just an aspect of terrestrial human culture?Observation suggest that they are universal. We have no reason to hypothesise otherwise, although please do question the status quo. You might find an interesting point.

darksidz i think religion doesnt have to do anything with science and speking about seti i dont think mats would help to communicate a lot,but what do u know :o

... SETI is a waste of time, it's also useless even if we do find anything.Can you explain why you think so?The truth is SETI is only meant to destroy Christianity as a religion, I hope they suceed too!You can drop that notion in the religions forum and we'll correct it from there. :D

Thanks for the laugh though.

Observation suggest that they are universal.Our observations, of course, which is the crux of the matter - I was wondering if I should drop this in the philsophy part of the forum, but then it'd be boring.We have no reason to hypothesise otherwise,SETI forces us to, I think. What is the reason for the so far "null" result in com SETI?although please do question the status quo. You might find an interesting point.Of course - if we don't there will be no advance in scientific thought and practice.

All the above named areas of study are, most certainly, a derivation of humanities interaction with the Universe as a whole.

What if, indeed, our whole view of the Universe (mathematics, physics, hydrogen line included) are not universal? What if everything that we "know" now, can be categorised as just an aspect of terrestrial human culture?



It would not surprise me in the least. It has been proven by some geniuses that the universe is "more than the sum of its parts". This to me appears to be a rather bold lie if at all it is.

We already have had some self-centered presumptions obliterated such as assuming life couldn't thrive or exist in certain environments. Considering this, it is possible there may be lifeforms which will not or could not communicate with us because they are too different.

We only know what our developed senses can detect. The possibilities are endless, as I bet there are many things right around us which we aren't concretely aware of yet.

I know little of astronomy so I'm presuming that since the time of the big bang, most planets and lifeforms should be about the same age as ours, wouldn't that negate the possibility of a more highly evolved or technologically advanced lifeform? That is a depressing thought from the point of view of communication, also we don't even know if it would be friend, foe or neutral. Also, it's intrigueing we are as much an accident as the dinosaurs becoming extinct, yet they definitely would still be dominant otherwise. This has some rather unsettling and yet limitless possiblities of lifeforms but I question the possiblity of a more evolved lifeform.

We already have had some self-centered presumptions obliterated such as assuming life couldn't thrive or exist in certain environments. Considering this, it is possible there may be lifeforms which will not or could not communicate with us because they are too different.

We only know what our developed senses can detect. The possibilities are endless, as I bet there are many things right around us which we aren't concretely aware of yet.Agreed to the fullest.I know little of astronomy so I'm presuming that since the time of the big bang, most planets and lifeforms should be about the same age as ours, wouldn't that negate the possibility of a more highly evolved or technologically advanced lifeform? That is a depressing thought from the point of view of communication, also we don't even know if it would be friend, foe or neutral. Also, it's intrigueing we are as much an accident as the dinosaurs becoming extinct, yet they definitely would still be dominant otherwise. This has some rather unsettling and yet limitless possiblities of lifeforms but I question the possiblity of a more evolved lifeform.Well, current estimates of the age of some macroscopic parameters of our universe:

~156,000,000,000 (156 BILLION (http://www.space.com/scienceastronomy/mystery_monday_040524.html)) light years "wide", at least - and counting... and growing - surely means that the small fraction of our Galaxy that SETI has explored so far, probably tells us nothing about complex life forms in the other ~100% of the universe that we are yet to reach.

~13.6 billion years old. Our Sun is only ~4.5 billion years old - i.e. our system has been around for roughly the last 1/3 of the Universe' age to date. The oldest planet discovered so far, is 12.7 billion yrs old (http://www.space.com/scienceastronomy/oldest_planet_030710-1.html).

Planets from that era have had 2-3 times as much time to evolve complex systems, than the Earth. What would humanity look like 5 billion years from now? What would our technology look like? Mind boggling.

There have been proposals that the more advanced sentient beings are, the more natural their technology will look. So, we might be looking at a star, and at the same time be looking at the most advanced pieces of energy generating craftsmanship in the universe.

Remember the whale episode of star trek? ;)

The posts to this thread and the linked-to comments (by a SETI scientist) have convinced me that I have been correct in ignoring SETI as a waste of time, financial, and intellectual resources.

I really wonder if the SETI people are charlatans or fools. To take a kinder view: Perhaps they are merely engaging in an interesting hobby with some hope of advancing the technology of observational astronomy, and do not expect to actually discover signals from an alien civilization.

Perhaps the primary talent of the SETI people is their ability to write grant proposals and get financing for their projects.

I wish I was a shrink with one of the SETI people as a patient. I wonder about their motives and thought processes. I find it hard to believe that they expect to communicate with aliens. Even the discovery of signals from an alien species is at best extremely unlikely.

There is no good reason to believe that there is another intelligent species in our galaxy, which puts the closest possibility a far ways off. How far away is the closest galaxy? How much would a signal be diminished over that much distance?

Did a SETI Scientist really speculate about an alien intelligence who interacts with reality via his sense of smell? I agree that such an alien would sure be different from us and it would be difficult to communicate with him.

The proposer of this notion might spend some time wondering how such an alien would learn about the structure of an atom or how he would develop the technology to build a telescope or an automobile. We would be able to communicate with him about as well as I communicate with my pet dog. Such an alien would have about as much comprehension of technology and physics as a dog.

BTW: Is a dog not a good example of a species who relies heavily on his sense of smell to interact with his environment?

I kind of agree with you. I think the question is, that, if we want to find other life in the universe, is SETI the correct way? Basically, they can only look for intelligent life, not for signs of "lower" lifeforms. So their chance of succes is very small. Some years ago, there was a proposal, that with special techniques and powerfull telescopes (dont ask me how this is achieved) we could analyze the atmosphere of planets up to some distance. The propsal was, that if an atmosphere contanis methane, then this could be a good indicator for life, since this is produced in large quantaties only through biological processes (or at least thats how it is on earth). I think this has better change of succes.

And in general, the laws of physics allow the emergence of life. So if they are universal, there must be life elsewhere.

MarcAC: Both you and the writer of the article have overlooked an important detail~13.6 billion years old. Our Sun is only ~4.5 billion years old - i.e. our system has been around for roughly the last 1/3 of the Universe' age to date. The oldest planet discovered so far, is 12.7 billion yrs old.

Planets from that era have had 2-3 times as much time to evolve complex systems, than the Earth. What would humanity look like 5 billion years from now? What would our technology look like? Mind boggling.12.7 billion years ago, the universe was composed almost entirely of hydrogen & helium, except for a few regions close to early galactic centers. Life requires elements like carbon, oxygen, nitrogen, et cetera. It also requires its solar system to be gravitationally stable and free from harsh radiation for 1-2 billion years. Such conditions were not available 12.7 billion years ago.

The planet referred to in that article was a gas giant like Jupiter. There are good reasons to believe that it was in a solar system with no planets like Earth, although the writer implied that such might exist (He was wrong).

The elements heavier than hydrogen & helium are created in the final stages of the life cycle of a star. When the star goes nova, it spews some of those heavy elements into interstellar space. Interstellar Gas clouds are collapsed by gravity to form new solar systems.

The first generation solar systems could only have gas giants for planets. Even the second & third generation planets are believed to be lacking in sufficient heavy elements for life to form.

I am only guessing: I think a typical stellar generation takes at least 2-3 billion years. Very massive stars have much shorter life cycles than Sol (10 billion years or so). Star with life cycles less than 1 billion years could result in generation shorter than 2 billion years, but 2-3 billion seems like a reasonable estimate.

Based on the above, it is possible that we are among the first intelligent species to have existed. Remember that the earliest solar systems with sufficient heavy elements for living creatures might have lacked other requirements. Also: There is no good reason to believe that intelligent life is an inevitable result of evolution. Some of the solar systems with life that are older than our system might not have an intelligent species.

BTW: How certain is it that an intelligent species will survive for millions of years? How likely is it that our knowledge and technology will continue to grow for a million years if we do survive that long? Can you be sure that there are no limits to scientific knowledge and technology?

Dinosaur, two things.

SETI is not a waste of time, really. It consumes only a small amount of resources, and takes a small amount of research time. It is one of those things that we HAVE to check for, even if it's obvious that it doesn't exist. For example, we still check the validity of Newton's Laws to very small distances---even though we know that they're right, there is a small chance that we'll see something new.

Second, your statement that "It is probable that we are the first intelligent species to evolve" is not necessarily accurate. There is no well-defined definition of ``intelligence'', and like others have said, the universe is a big place. Making probabilistic statements like this is really meaningless, without knowing the facts.

Finally, someone said SETI exists just to destroy Christianity. This is nonsense. First of all, only a small minority of Fundamentalist Christians would actually claim that the Bible says we are alone in the universe. Finding other civilizations is really the ultimate test for Christianity---if we find an alien species with a set of moral codes that are similar or equivalent to, say, the Ten Commandments, then there is good evidence that God really does exist. But these are topics for another forum:)

He is right :L SETI is a waste of time, it's also useless even if we do find anything. The truth is SETI is only meant to destroy Christianity as a religion, I hope they suceed too!

How do you know? What if we contact aliens and the first thing they say to us is the alien equivalent of "Christ is risen" because He went there too! :D

Our knowledge extends only to the limits of human perspective. There are wavelengths of light and sound beyond our capacity to observe. The biological constraints of humans such as size and structure trap us within physical boundaries such as pressure and temperature. The limits of time and distance mean that it is extremely probable that we will never be able to learn all of what our universe supports. However, we are fortunate for being capable of learning throughout our lives and passing knowledge on to others.

If you think about the vision of a bat, the properties of water, or any one of millions of differences between you and the rest of the universe, it should be humbling to think of how progressive our culture has been.

Our knowledge extends only to the limits of human perspective. There are wavelengths of light and sound beyond our capacity to observe.
What about indirect observation? We can "see" radio frequencies etc.

There is no good reason to believe that there is another intelligent species in our galaxy, which puts the closest possibility a far ways off. How far away is the closest galaxy? How much would a signal be diminished over that much distance?But there is good reason.

We are here. We orbit a star, typical of the most common stars in the Universe now observed.

We are on a planet, which is in the habitable zone of the star - all stars have habitable zones.

We have observed planets orbiting other stars, although for now we have not observed an Earth, we get closer and closer with each discovery.

Organic molecules are everywhere we look in the universe. Water is everywhere.

We are human, all the above compell us to look - it's a requisite of our intellectual responsibility.

The issue is, still, just that we may be looking for something that's not there, or we won't be able to recognise, because other complex forms in the universe may be so much different from us.

We won't know unless we search though.Did a SETI Scientist really speculate about an alien intelligence who interacts with reality via his sense of smell? I agree that such an alien would sure be different from us and it would be difficult to communicate with him.It was an analogy. Who knows what kind of senses they may have. They may be able to "see" magnetic fields, or subatomic particles - they'd effectively have a higher resolution view of the world than we do without our technology.

From what I've seen myself, a first impression gives me the idea that comm SETI scientists are dreamers. One I met almost gave the impression of a hippy stereotype - almost spritual. But dreamers take civilization forward. I respect them.

BenTheMan: Do you not read carefully or do you deliberately misquoate? I posted the following.Based on the above, it is possible that we are among the first intelligent species to have existed. You posted.Second, your statement that "It is probable that we are the first intelligent species to evolve" is not necessarily accurate. There is no well-defined definition of ``intelligence'', and like others have said, the universe is a big place. Making probabilistic statements like this is really meaningless, without knowing the facts.In the above two quotes, notice the bolded phrases. Semantically, your quote is far off from what I posted. Perhaps English is your second language and you do not comprehend the difference between possible & probable.

Mentioning that intelligence is not well defined is a weak ploy in this context.

It is not clear what you meant when you said: "without knowing the facts." There are no facts about extraterrestrial life. We have the history of life on Earth, biochemistry, and the astrophysics of stellar life cycles. Based on these areas of knowledge we can come to some reasonable conclusions about life elsewhere in the universe.

BTW: Do you have any understanding of the life cycle of stars and the creation of heavy elements at the end of those cycles? Are you aware that a galaxy has a small habitable zone, just as a solar system has a small habitable zone? Have you considered the history of life on Earth in forming your opinions? Without some analysis, statments like: The universe is a big place" are meaningless.

Originally Posted by Oli

Originally Posted by original
Our knowledge extends only to the limits of human perspective. There are wavelengths of light and sound beyond our capacity to observe.

What about indirect observation? We can "see" radio frequencies etc.

Agreed although we rely on technology for that. Technology has allowed us to expand our weltanschauung, our world view, beyond the biological limits of human capacity. This is why it can be humbling to think that even though we are not physically built to observe something, through the technology we have created these things can be observed, but only to the limits of our ability and knowledge in creating such technologies.

MarcAC: Both you and the writer of the article have overlooked an important detail12.7 billion years ago, the universe was composed almost entirely of hydrogen & helium, except for a few regions close to early galactic centers.The Universe is still composed almost entirely of H and He.Life requires elements like carbon, oxygen, nitrogen, et cetera. It also requires its solar system to be gravitationally stable and free from harsh radiation for 1-2 billion years. Such conditions were not available 12.7 billion years ago.

The planet referred to in that article was a gas giant like Jupiter. There are good reasons to believe that it was in a solar system with no planets like Earth, although the writer implied that such might exist (He was wrong).

The elements heavier than hydrogen & helium are created in the final stages of the life cycle of a star. When the star goes nova, it spews some of those heavy elements into interstellar space. Interstellar Gas clouds are collapsed by gravity to form new solar systems.

The first generation solar systems could only have gas giants for planets. Even the second & third generation planets are believed to be lacking in sufficient heavy elements for life to form.

I am only guessing: I think a typical stellar generation takes at least 2-3 billion years. Very massive stars have much shorter life cycles than Sol (10 billion years or so). Star with life cycles less than 1 billion years could result in generation shorter than 2 billion years, but 2-3 billion seems like a reasonable estimate. The first stars (Population III) existed some 0.1 Byr after the Big Bang. They exploded as suvernovae (and maybe hypernovae) and produced the first generation of gas clouds with heavy elements, in the period some 1 billion years or less after the Big Bang.

The environment was indeed harsh for the formation of rocky planets, and life, but it has been shown that heavy elements - particularly carbon - could have survived the radiation environment of the first stars (after being produced in supernovae) once they're thrown a good enough distance away.

I am not sure what you mean by first second and third gen. solar systems.

Remember we didn't just appear here 4.5 Byr ago. Homo Sapiens emerged some 150 Ky ago - a second ago on Universal timescales. Even a species emerging 1 Byr before us, along a similar evolutionary path (hypothetically speaking) would be super advanced - even 100 Kyr before us.Based on the above, it is possible that we are among the first intelligent species to have existed. Remember that the earliest solar systems with sufficient heavy elements for living creatures might have lacked other requirements. Also: There is no good reason to believe that intelligent life is an inevitable result of evolution. Some of the solar systems with life that are older than our system might not have an intelligent species.

BTW: How certain is it that an intelligent species will survive for millions of years? How likely is it that our knowledge and technology will continue to grow for a million years if we do survive that long? Can you be sure that there are no limits to scientific knowledge and technology?Indeed, intelligent life need not be the "end" product of evolution. Sharks, crocs, turtles and numerous unicellular organisms have been around for millions to billions of years.

And indeed, we cannot say how long we'll be around. But the more we know, the more we can manipulate ourselves and our environment to secure our longevity.

Are you aware that a galaxy has a small habitable zone...?From what I've seen, galactic habitable zones are defined by the stellar neighbourhood (naturally). There are many niches pockmarked all over the galaxy where life can sprout up. Of course, longevity becomes an increasing issue with greater complexity.

MarcAC: The galactic habitable zone for our galaxy is a ring approximately where our solar system is. I do not know how many light years it extends toward & away from galactic center.

A solar system too close to the center is bathed in harsh radiation and subject to gravitational instability due to fast moving stars. The closer you get to the center, the faster stars must move to avoid being sucked into the core. Note the following orbital speeds in our solar system.Mercury: About 30 miles/Second (48 km/Second)

Mars:: About 15 miles/second (24 km/second)

Saturn: About 6 miles/second (10 km/second).Similar increases in stellar speed occur as you get closer to the center of a galaxy. In addition, there is a lot of harsh radiation closer to galactic center. Closer to galactic a solar system like ours would be moving fast and would be surrounded by many fast moving stars. Our planet would not survive in a stable orbit for the billion or so years required for the formation of the first primitive life forms. Forget about any chance of stability long enough for more complex species to evolve.

A solar system too far from galactic center would be formed from gas clouds lacking in heavy elements. The carbon, oxygen, nitrogen, et cetera required by living creatures would not be available in sufficient quantities.

The above gives the basic idea of the galactic habitable zone.

The first solar systems would have planets composed of hydrogen & helium. The heavy elements are created in the final stages of the life cycles of certain types of stars which go nova and spew the heavy elements into interstellar gas clouds. Only certain types of stars enrich the interstellar media with heavy elements.

To generate the necessary abundances of heavy elements, it requires quite a few cycles of star formation via gravitational collapse of a vast cloud of interstellar gas, the stellar life cycle ending in a nova, and the enrichment of the interstellar gas clouds to be collapsed in the next cycle.

While solar systems rich in heavy elements could have been formed in the first 2-3 billion years after the Big Bang, these solar systems were probably formed too close to the galactic center to be in the habitable zone of their galaxies.

As I posted earlier in this thread, it is possible that we are the first intelligent species in our galaxy. It is even possible that we are the only intelligent species in our galaxy. While there are billions of stars in our galaxy, only a small percentage are in the habitable zone of the galaxy. Of that small percentage, there are many which do not have a planet in the habitable zone of the solar system. While the history of Earth indicates that life is likely to form when conditions are right for it, that history does not suggest that evolution leads inevitably to intelligent life.

The above analysis and the history of Earth strongly suggest that intelligent life is a rare phenomenon.

Dinosaur,
A solar system too close to the center is bathed in harsh radiation
I have seen you use these arguments many times before.Let's look at a few. Specifically, what radiation are you referring to? All stars emit radiation, our own included. The heliosphere is usually, what, a few light hours in radius? Or do you mean exploding supernovas? If one goes off within a few light years of a solar system, then yes, they are dangerious. The densly packed 'core' of our galaxy is a very dangerous place, but the galaxy is huge in comparison.
subject to gravitational instability due to fast moving stars. The closer you get to the center, the faster stars must move to avoid being sucked into the core.
The velocity of an individual star/solar system has no bearing on its stability. Another star passing within a few light hours/(days?) could disrupt planetary orbits, but not if they are a light year or so away. The stars in a galaxy do not orbit the center of the galaxy like planets orbit a star either. The stars in the center do orbit the supermassive black hole at high velocities. You do not have to go very far away, relatively speaking, from the center before galactic orbital patterns differ from solar system orbital patterns. Stars in the galaxy orbit more like a fixed disc, a record on a turntable of similar. This is the observation that prompted the postulation of 'dark matter', the rotation of the galaxies is not in accord with gravitational theory. Either our theories of gravity are wrong, or their has to be some kind of unseen mass 'sticking' the orbiting stars together. Rather than throw out Newtonian mechanics and GR, it was decided to postulate a dark matter to help explain the unpredicted motions.
The first solar systems would have planets composed of hydrogen & helium. The heavy elements are created in the final stages of the life cycles of certain types of stars which go nova and spew the heavy elements into interstellar gas clouds. Only certain types of stars enrich the interstellar media with heavy elements.

To generate the necessary abundances of heavy elements, it requires quite a few cycles of star formation via gravitational collapse of a vast cloud of interstellar gas, the stellar life cycle ending in a nova, and the enrichment of the interstellar gas clouds to be collapsed in the next cycle.
The first stars to form after inflation were thought to be very large, short-lived giants composed of light elements, correct. Heavier elements are formed in the interior when these stars go supernova. It does not take 'quite a few cycles' before heavy elements are produced, perhaps only two. But location in the galaxy has nothing to do with the lifespan of the early stars. Perhaps on the outer perimeter of the galaxy where star density is more sparse, it may take longer to 'seed' the area with sufficient heavy elements.
While solar systems rich in heavy elements could have been formed in the first 2-3 billion years after the Big Bang, these solar systems were probably formed too close to the galactic center to be in the habitable zone of their galaxies.
One to two billion years at most, and they do not need to be near the galactic center. The oft mentioned Zeta Reticuli star system from UFO lore is an example. It is a system of two stars 39 light years from us both of which are remarkably similar to our own sun, except they are thought to be around 8+ billion years old. IF there are any planets orbiting this system, or similar systems, in a habitable zone, any possible life forms could have a possible 3 1/2 billion year jump-start on us. What is known about this particular system so far, is that there are no large Jupiter-like planets orbiting close to the stars, which would make rocky planets in a habitable zone less likely. Our techonology is not sensitive enough so far to detect Earth-like planets in the habitable zone, if any, or Jupiter size planets farther away. But this is just one example of a system only 39 light years away. The galaxy is a big place.

MarcAC: The galactic habitable zone for our galaxy is a ring approximately where our solar system is. I do not know how many light years it extends toward & away from galactic center.O.k. yes, some 7-9 Kpc (21-27 Kly) according to what I've read (http://www.sciencemag.org/cgi/content/abstract/303/5654/59) - regarding the development and sustainability of complex (multicellular) life. Though it broadens towards the outter regions with time, according to studies by one who first attempted to define it (G. Gonzalez, or something like that). Additionally, the probabilities must be actively considered, in that probability decreases, but sheer number of stars and solar systems may allow some occurences far from the prime region of the habitable zone.A solar system too close to the center is bathed in harsh radiation and subject to gravitational instability due to fast moving stars. The closer you get to the center, the faster stars must move to avoid being sucked into the core. Note the following orbital speeds in our solar system.Mercury: About 30 miles/Second (48 km/Second)

Mars:: About 15 miles/second (24 km/second)

Saturn: About 6 miles/second (10 km/second).Similar increases in stellar speed occur as you get closer to the center of a galaxy.I prefer to look at it in terms of the density of stars, and simply their proximity - not so much their speed. No matter where you are in the galaxy, stars need only to pass close enough to some gravitational disturbances to perturb Oort clouds and planetary orbits. But yes, the density towards the centre of the galaxy makes perturbations more likely and the environment that more dangerous to complex organisms.In addition, there is a lot of harsh radiation closer to galactic center. Closer to galactic a solar system like ours would be moving fast and would be surrounded by many fast moving stars. Our planet would not survive in a stable orbit for the billion or so years required for the formation of the first primitive life forms. Forget about any chance of stability long enough for more complex species to evolve.Again, I do not think that looking at it in terms of "speed" of our system is the best way. The Sun orbits at some 250 km/s around the centre of our galaxy - it also orbits up an down through the galactic plane, completing one every 64 My or so - this has been linked to an extinction cycle (periodic decrease in biodiversity) observed throughout life's evolutionary history. And remember it is never simple - extinctions favour the proliferation of some species and serves to nullify others.A solar system too far from galactic center would be formed from gas clouds lacking in heavy elements. The carbon, oxygen, nitrogen, et cetera required by living creatures would not be available in sufficient quantities.

The above gives the basic idea of the galactic habitable zone.Indeed.The first solar systems would have planets composed of hydrogen & helium. The heavy elements are created in the final stages of the life cycles of certain types of stars which go nova and spew the heavy elements into interstellar gas clouds. Only certain types of stars enrich the interstellar media with heavy elements.

To generate the necessary abundances of heavy elements, it requires quite a few cycles of star formation via gravitational collapse of a vast cloud of interstellar gas, the stellar life cycle ending in a nova, and the enrichment of the interstellar gas clouds to be collapsed in the next cycle.Supernovae can produce several solar masses of metals in one single event. Considering the fact that "supernova-prone" (massive) stars exist for a few million years at most, before they explode, cycles of much less than a billion years are sufficient to provide very high metalicities.While solar systems rich in heavy elements could have been formed in the first 2-3 billion years after the Big Bang, these solar systems were probably formed too close to the galactic center to be in the habitable zone of their galaxies.

As I posted earlier in this thread, it is possible that we are the first intelligent species in our galaxy. It is even possible that we are the only intelligent species in our galaxy. While there are billions of stars in our galaxy, only a small percentage are in the habitable zone of the galaxy. Of that small percentage, there are many which do not have a planet in the habitable zone of the solar system. While the history of Earth indicates that life is likely to form when conditions are right for it, that history does not suggest that evolution leads inevitably to intelligent life.

The above analysis and the history of Earth strongly suggest that intelligent life is a rare phenomenon.The article I linked to above indicates that up to ~10% of the stars of the Milky Way are located in the habitable zone - i.e. 10 billion. 75% of those stars are older than the sun by up to 1 By. Taken together those two figures mean that there are up to 7,500,000,000 stars candidate for life, which is more advanced than ours by up to 1 By, providing they evolution took a smilar path.

I'd say all the data available advocates what the comm SETI scientists are doing as reasonable.

The problem is, still, just that way may not find what we're looking for, by the fact that whatever is evolved, may be so much different from us.