We know that life has existed on earth for billions of years. We also know that earth-like planets exist which are much older than our own. A favourite story line in scifi is the ancient empire which vanished from the galaxy billions of years ago.

I'm just curious as to how long ago life may have started in this galaxy. Ofcourse there are many people who don't think life exists beyond the atmosphere of earth. They are welcome to their opinions but I find such a thing highly improbable. It seems more likely that life appeared in this universe the moment habitable planets became available for it.

But how long ago was that? When did planets first form? Soon after the first stars? It seems to me that there should be a lot of junk floating around from all these come and gone civilisations?

I wonder if any of it is caught within orbit of our sun.

Well I am not able to run the numbers on this one but I am sure that there is someone who can. Basically we cannot say how long there has been life in the universe just as we cannot as yet prove the existence of any life other than on earth. Nor have we found any exo-planets that mirror earth in all respects as to mass water content temperature etc.

All we can do is make some massive assumptions. First of all by assuming that for there to have been some ancient civilisation then it must have evolved from a biota on a planet which was much like earth. While this civilisation may or may not have been created by 4 limbed vertabrates we could assume that they may have been air breathing land animals who started out making simple tools from natural materials and went on to control fire as a precursor to developing other more advanced technologies. None of this may be true and we can speculate about aquatic civilisations and so on but we have to invent a lot of evolutionary and technological work-arounds to take such a species from being simple animals to becoming a civilisation. The one evolutionary technological model we know that works is the one we ourselves followed.

Given the above then you have to consider when the first planets like ours came into existence. When the first generation of stars formed in the early universe there was no way that they could form planets like ours around them because at that stage in the development of the universe the heavy elements our world is made from did not yet exist. Elements that we are familiar with Iron Copper Sodium Carbon Oxygen etc were simply not there to be used. Therefore the first stars would probably have had no planets at all or at best gathers some jupiter like gas giants.

Only when some of these early stars had completed their life cycles and died liberating the heavy elements that had been made in their interiors would the universe then have the raw materials to make planets like our own. However even when these first stars had done their bit the quantities of heavy elements would still be quite small. Therefore the available material to make ancestor earths would have been fairly thinly scattered. Consequently ancestor earth like planets would have be rare. It is only with each succeeding generation of large stars forming and dying that the pool of heavy elements grows thus allowing for more rocky planets to form.

If that does not complicate matters enough then we have to consider how probable the evolution of life is and after that how often planets that produce life go on to produce intelligent life and therefore civilisations. Once again we have no valid statistics for this at the moment. However it is reasonable to assume that not all rocky planets that form around stars go on to produce life and that not all planets that produce life go on to produce intelligent species. Therefore once again if in the early universe the number of rocky planets were fewer than now and that only a proportion of them produced life and a smaller proportion produced intelligent life.

From that is possible to imagine that the emergence of other civilisations did not start until quite late in the development of the universe. The Universe is after all around 13.5 billion years old, our own Sun is about 4.75 billion years old or put it another way our own solar system has been around for nearly 1/3 the age of the universe. Therefore given what I have already said it is quite possible that even the the very first other civilisations in universe might only have had a head start on us of only 1 billion years or possibly only a few hundred million years. In fact it would not be unreasonable to assume that perhaps there have been no other civilisations before ours. We could be the first to have come this far. That does not mean there are no other planets with life, there could well be but on none of those world's has any species experienced the very special evolutionary steps needed to move from being just wildlife to becoming a civilisation.

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Where did you hear this? I'm sure all the scientists currently looking for exoplanets would be surprised to hear it.

We think it may be possible that such planets exist. We don't know for sure, and the few "Earthlike" planets we've discovered aren't very much like Earth at all.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

Actually, I read a fascinating, short news piece in an old issue of S&T (Aug. 2001), citing a study based on modeling that concluded that it's likely that most Earth-like planets are older than Earth--if they exist, of course.

ADS link (no info aside from pub. info available to the public):
http://adsabs.harvard.edu/abs/2001S&T...102b..24M

Another link citing the same article, near the bottom of the page:
http://ast.freehostia.com/Cosmology.htm

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--The State

The big problem in answering your question is: WE JUST DONT KNOW.

We've barely been off this rock. We haven't been able to do anymore than guess about any other solar systems. We haven't even directly seen any other planets in other solar systems, just calculated that they are there due to wobbling in their parent stars.

You can't even begin to answer the questions about the elephant in front of you when you're a blind man with his hands tied behind his back.

And you can pretty much bet that 99% of all speculative guesses about something that we have zero real knowledge about will be wrong....

We think there were very bright stars Class O and class B as early as 13 billion years ago. If there were lots of them in a volume of a few hundred cubic light years, then there was plently of heavy elements to make terrestrial type planets by 12.99 billion years ago as very bright stars go supernova as soon as a million years after their birth. The most primititive life could have evolved by 12.98 years ago, and advanced life was possible 12 billion years ago, if they developed faster than on Earth. Other than our example of one, there are few good reasons to think it should take 3 billion years to develop an advanced civilization of sentient beings. As others have posted, we don't know. There is a slight possibility that humans are the first. Neil

Of course, right now we don't know whether it's a "slight possibility" or a "virtual certainty" or somewhere in between.

If you want to play with a modified Drake equation to guesstimate your own number of interstellar civilizations, you can use this web page calculator: http://frombob.to/drake.html

Keep in mind how many WAGs you're entering into the parameters, of course...

WAGs?

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

Wild A. . . ah . . .Rear-ended Guesses.

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Disclaimer: Avatar is not an official NASA image and does not imply any specific interplanetary or interstellar capability.

The Leif Ericson Cruiser

Ohhhh.
Got it. Feral donkeys.


Sorry, didn't mean to put you in arrears.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

That's a large part of the problem. The term "terrestrial planet" includes Mercury, Venus, Earth, and Mars, but the other three planets, in various ways, aren't terribly earthlike. We don't know what it takes for life to start, and we don't know how long life can typically exist on worlds where life can start. There's a window of time even for Earth to have complex life, and not all the issues are related to the lifespan of the sun.

One of my questions is, even if intelligent life develops, how common are the stable conditions required for complex and (perhaps) self sustaining civilization to develop? If there are common ice ages, asteroid impacts, massive volcanoes, or other events that can cause the collapse of developing civilizations, an intelligent species might never have the chance to develop sophisticated civilization.

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I say there is an invisible elf in my backyard. How do you prove that I am wrong?

Disclaimer: Avatar is not an official NASA image and does not imply any specific interplanetary or interstellar capability.

The Leif Ericson Cruiser

We evolved in a period of rapid change. The numerous ice ages of the Quaternary (only two million years in duration) are much more disruptive events than anything that happened in the eighteen million years of the Miocene. So perhaps other planets with inteligent life will be more stable than our own.

On the other hand, perhaps events causing rapid environmental change are necessary for the evolution of higher intelligence. I don't acually think that is the case, but I might be wrong.

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Perhaps it's not necessary, but it's plausible for higher intelligence to be more likely to evolve during such periods. During periods of gradual change, the action of natural selection may be fast enough to optimize animal and plant behavior for survival. However, during periods of rapid change, the rapid adaptability of learned behavior may be worth the extra cost in brainpower.

Hmm...this suggests an interesting Fermi paradox solution. Perhaps there's nothing unusual about Earth's environment, but there's something freakishly slow about our biosphere's rate of evolutionary change. It could be that in almost all alien biospheres, something much faster and efficient than our dna/rna sexual reproduction system evolves. Thus, the competitive advantage for even low levels of intelligence is reduced.

Rapid environmental changes may be (possibly) necessary for developing the evolutionary "shortcut" that favors additional brain growth over something slower, like, say, full-body adaptations for specializations such as fast running, but those very same unstable conditions may be bad for the development of an agrarian civilization that needs time to develop and sufficient surplus resources to allow for the luxury of time to think, ponder and experiment creatively.

Whew, one long run-on sentence.

But a group of hunter-gatherers, no matter how successful, will probably be unlikely to develop a material culture that can reach the level of an industrial revolution, nor the level of scholarship that leads to development of organized scientific analysis. So it could be that there are ETIs, just not many with cares beyond their next hunt.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

how can we know

With current tech we can't unless some aliens come and tell us, or we at least receive communication form them.

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"Any Sufficiently Analyzed Magic is Indistinguishable from SCIENCE!" -Agatha Heterodyne

"Any technology, no matter how primitive, is magic to those who don't understand it." -Florence Ambrose

Or unless we go out and look for ourselves, which won't be happening anytime soon.

__________________
"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

This makes perfect sense. First systems had no planets, then slowly more and more material became available and older systems, like Tau Ceti, which has no gas giants probably has many terrestrial worlds. Systems like our's, which has half gas giants and half terrestrial, were a transition to the newer systems we have today, which our entirely composed of massive gas giants.

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Large gas giants are just the easiest to detect using current technology. There no evidence at all that more recently formed star systems are "entirely" composed of large gas giants, there's plenty of room for unditected terrestrials out there given the limits of our observations.

__________________
"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

Astronomers classify stars as either one of three populations, based on when they were born. Our sun is in Population I. On average Pop I stars have higher metal content than Pop II stars- the latter are, by definition, older than Pop I (for any newbies viewing this, "metal" in this context is not the common everyday meaning, but elements heavier than helium). So far, no Pop III stars have been found, although it's assumed they barely had any metals at all.

Metals are needed to form planets, so it's assumed that the later in time the star formed, the greater the likelihood it is to have planets, all other things equal.

I think, but can't prove, that the newest stars are called Population Zero, which should have, on average, even higher metal content than our sun.

Because this sounds so plausible, I have to say that Pop I star systems have the best chance for giving rise to advanced life (intellectually or merely functionally)

Since we're on the tail end of all those terrestrial worlds, we probably just missed the intergalactic civilization.

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You ain't nobody 'til you've been banned.

Assuming there was one. Or that if there was, that it's over now.

__________________
"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

Consider the depressing (at least to me) thought that, if each galaxy has just one intelligent civilization, there could still be millions, even billions of such civilizations with virtually no way of knowing about each other.

The region where Earth-like planets are most frequently formed has simply moved further from the galactic centre.

What one might call the Galactic Habitable Zone (GHZ) slowly moves outwards from the centre, as seen in this graphic;
http://www.centauri-dreams.org/wp-co...table_zone.jpg
the green GHZ is expanding slowly over time, from bottom to top of that image, as more elements are gradually added to the interstellar medium over time..

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New Orion's Arm Site . The Starlark . Against a Diamond Sky (OA Novella Collection) . OA Flickr set

Man in that sentence is in plural terms. The word man can be used in plural terms look it up.

What are you referring to? Its very helpful to quote the post you are replying to, particularly threads that haven't had recent activity.

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"Any Sufficiently Analyzed Magic is Indistinguishable from SCIENCE!" -Agatha Heterodyne

"Any technology, no matter how primitive, is magic to those who don't understand it." -Florence Ambrose

I think he is refering to this quote: (RalofTyr)

"One small step for (a) man. On giant leap for mankind".

If an astronaut doesn't need good grammar, niether does you. .

Suposition. If he was talking as if he were steping on the Moon as an agent of Earth, then, it would be plural. If he were refering to himself, he was gosh darn it wrong.

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You ain't nobody 'til you've been banned.

Very early in his career Sir Fred Hoyle came to the conclusion that the big bang must be wrong because life cannot come into existence in a period anywhere near as short as 14 billion years. That was one reason he always opposed the big bang and also why he looked for organic compounds in space as solutions to some unsolved problems. His work in that area was regarded as kooky some decades back, but is now quite mainstream.

In his book "Mathematics of Evolution" (ISBN 0-9669934-0-3) Fred Hoyle gives the maths of evolution and why complex life cannot happen in a short time. It is not easy reading and to the best of my knowledge no-one has ever refuted his arguments. In essence he shows that evolution can proceed rapidly when a single base pair change makes an improvement but requires astronomically long if two base pair changes are required to get an advantage. It seems pretty unlikely that you can get all the way to something like an eye with a single base pair change at a time.

If Hoyle is right then the age of the Universe is vastly older, I would suggest more than 10^23 years, and life is almost that old. Hoyle also showed that viruses are the only sized objects that can travel interstellar distances and arrive at other planets as smaller objects cannot penetrate the solar wind and larger ones burn up in the atmosphere on re-entry. In Hoyle's view, all the genetic material on Earth came from space and is incredibly ancient. We simply have recombination and a few random base pair changes to add a bit of variety happening locally.

These ideas are not popular in our time, but the evidence against life forming rapidly has never been disputed, simply ignored. Hoyle was a great man and his work deserves better than that.

Hoyle's views hold as much relevance today as do Sigmund Freud's, which is to say, some of his ideas have a plausible and verifiable basis, but a lot of them have been shown to be plain wrong, even in his own lifetime.