Is there life in the universe, and if so has it visited Earth?

[Schrodinger's Dog]

The only solar system we know that has an earth-like planet is ours. There is one planet out of nine with life, so a more reasonable assumption is that 10% of the planets have such property.

I think 10% is a little optimistic and wouldn't it be about 11%. Assuming even there are 9 planets that one is Earth like 11% of the time is probably not quite right.

And of course the reason we only know Earth, is to do with being unable to detect planets of such insignificant mass with current methods.

 

[_Mayday_]

There are only 8 planets in our solar system, Pluto is now classified as a dwarf planet. So a 1 in 8 chance then.

 

[CEL]

I think 10% is a little optimistic and wouldn't it be about 11%. Assuming even there are 9 planets that one is Earth like 11% of the time is probably not quite right.

And of course the reason we only know Earth, is to do with being unable to detect planets of such insignificant mass with current methods.

I used 10% because it is a round number. We can only speculate about earth-like planets around other stars. Since Sol is a very ordinary star, there is no reason to think that other G2 class stars have planetary systems much different from ours.
Someday we will have the technology to detect small planets orbiting other stars and will be able to assign other values to Drake equation. Until then I think we should use as model the only system we know.

 

[Schrodinger's Dog]

I used 10% because it is a round number. We can only speculate about earth-like planets around other stars. Since Sol is a very ordinary star, there is no reason to think that other G2 class stars have planetary systems much different from ours.
Someday we will have the technology to detect small planets orbiting other stars and will be able to assign other values to Drake equation. Until then I think we should use as model the only system we know.

Indeed, I agree. Just being pedantic I suppose, and of course introducing a little doubt.

There are only 8 planets in our solar system, Pluto is now classified as a dwarf planet. So a 1 in 8 chance then.

Well that's another thread in itself hehe. Still it rounds down to 10%.

 

[mikelepore]

The ambiguity of the phrase "earth-like" needs some attention. Let's start with that range of orbital radii in which you can have liquid water. Suppose you do find another planet with the right temperature for liquid water. That is, it it had any water -- but there isn't any water there. Then you find another planet that has the right temperatures for liquid water. Unforunately its atmosphere is all ammonia gas. Now you find another planet with the right temperatures for liquid water. Unfortunately, it has no magnetic field to deflect the solar wind. You find another planet with the right temperature for liquid water, but it's more volcanic than Io, with much of the planet's surface being destroyed on a daily basis. You find another planet with the right temperatures for liquid water, but the planet has almost no carbon atoms. I believe this is more problematic than most people think. To be a place for life to develop, you have to "win the lottery" many times.

 

[cristo]

To be a place for life to develop, you have to "win the lottery" many times.

I think when a lot of people think of intelligent life elsewhere, they think of life forms very similar to our own. However, why would any intelligent life have to be even the slightest bit similar to ours? Why would it need to be carbon based and breathe oxygen? These implicit assumptions are always in the back of peoples' minds but, in my opinion, shouldn't be. I also agree with what Stephen Hawking once said (I think), namely that if we were visited by extra terrestrial beings it would not be the pleasant experience that many people fantasize about!

 

[_Mayday_]

I think when a lot of people think of intelligent life elsewhere, they think of life forms very similar to our own. However, why would any intelligent life have to be even the slightest bit similar to ours? Why would it need to be carbon based and breathe oxygen? These implicit assumptions are always in the back of peoples' minds but, in my opinion, shouldn't be. I also agree with what Stephen Hawking once said (I think), namely that if we were visited by extra terrestrial beings it would not be the pleasant experience that many people fantasize about!

That is an interesting point actually. And by life we may be talking about a tiny bacteria, not a green man with big eyes.

The other point made on Hawking is also a good one. The thought of other life forms similar to us popping in for a few days to say hi seems far fetched, then there are also problems like communication, though I can't imagine there is much to say to a single cell organsim.

 

[binzing]

Basically, just look at the odds. There has GOT TO BE life in this universe other than on Earth.

 

[CEL]

I think when a lot of people think of intelligent life elsewhere, they think of life forms very similar to our own. However, why would any intelligent life have to be even the slightest bit similar to ours? Why would it need to be carbon based and breathe oxygen? These implicit assumptions are always in the back of peoples' minds but, in my opinion, shouldn't be. I also agree with what Stephen Hawking once said (I think), namely that if we were visited by extra terrestrial beings it would not be the pleasant experience that many people fantasize about!

The only element capable of forming the long chains necessary to life is carbon. Although some scientists have speculated on the possibility of silicon based life, this element does not produce complex enough molecules.
In order to produce energy, life must use exothermic chemical reactions. We know of two of those reactions that work on Earth's lifeforms: fermentation and oxidation. Fermentation is used by some bacteria, in the absence of oxygen, but it is inefficient and works only with primitive lifeforms. Only after ocean algae produced enough oxygen in our atmosphere more complex lifeforms could develop.
Of course other oxidants, like chlorine, could be used in the reaction, but oxygen is much more abundant and life, specially complex forms, need abundance of useful elements and compounds.

 

[russ_watters]

Oxygen is helpful for complex life, but not required for life. One thing not mentioned (recently) is liquid water. It has some unique properties (such as floating when it freezes and being a good solvent) that make it somewhat unique.

 

[Schrodinger's Dog]

Very important in shallow waters where they'd freeze from bottom to top instead of top to bottom, thus hindering any chance of life in any but deep seas. In fact it's almost uncanny, that ice is less dense than water.

 

[CEL]

Oxygen is helpful for complex life, but not required for life. One thing not mentioned (recently) is liquid water. It has some unique properties (such as floating when it freezes and being a good solvent) that make it somewhat unique.

As I mentioned in my previous post, fermentation produces energy in the absence of oxygen, but is too inefficient to allow complex life. It is very possible that primitive life exists outside Earth, using fermentation, but the only way such life could visit us is attached to meteorites. By the way, this is one of the proposed mechanisms for the beginning of life in our planet.
You are right about the need of a liquid to allow interaction between the complex molecules necessary to life. This liquid must be abundant in order to life begin and develop. Since the most abundant element in the universe is hydrogen, we must search hydrogen compounds as necessary for life. Besides water, we can think of methane, ammonia and hydrogen fluoride as simple liquids. Those are really gaseous, as is water, unless the temperature is low enough or the pressure high enough. Since at very low temperatures chemical reactions are too slow to allow complex life forms to evolve, we need an atmosphere dense enough for one of those gases to exist in liquid form.
Earth has an atmospheric pressure high enough for water to exist in liquid form in a range of temperatures ideal for life.
That is why we must look for earth-like planets in our search for intelligent life.

 

[mikelepore]

that ice is less dense than water

I can't remember why that happens. Can you tell me? The polarity of the molecule?

 

[K.J.Healey]

ALL stars should have some orbital radii where liquid water could exist, correct? Its not just surface temp but gravity, other gaseous atmosphere to keep it non vaporous, etc.

 

[DaveC426913]

I can't remember why that happens. Can you tell me? The polarity of the molecule?

Yes. The polarity of the molecule causes it to crystalize as a lattice wherein the individual atoms are forced to line up farther apart as a crystal than they can get as an amorphous liquid. Same mass but greater volume = less dense.

 

[DaveC426913]

ALL stars should have some orbital radii where liquid water could exist, correct? Its not just surface temp but gravity, other gaseous atmosphere to keep it non vaporous, etc.

Yes; it is called http://en.wikipedia.org/wiki/Habitable_zone" . But there are many factors other than stellar temp that determine whether water really can exist there.

 

[mikelepore]

Bad news for the evolution of life if the goldilocks region happens to be occupied by a planet whose orbit is so eccentric that sometimes it gets so cold that the atmosphere freezes and other times it gets so hot that the rocks melt. A crowded system can only have nearly circular orbits, but, for a system with a just a few planets, there's nothing to produce a likelihood of circular orbits. A circle is merely an ellipse whose eccentricity "e" happens to have the value 1 by coincidence.

 

[DaveC426913]

Bad news for the evolution of life if the goldilocks region happens to be occupied by a planet whose orbit is so eccentric that sometimes it gets so cold that the atmosphere freezes and other times it gets so hot that the rocks melt. A crowded system can only have nearly circular orbits, but, for a system with a just a few planets, there's nothing to produce a likelihood of circular orbits. A circle is merely an ellipse whose eccentricity "e" happens to have the value 1 by coincidence.

1] I don't know how many planets have an orbit so extreme that its temp goes from liquid water to melting rocks. I'm thinking statistically near zero.

2] I think 'older planets' is already factored into 'conducive to life'. And one property of older planets is that they tend to have more stable, more circular orbits.

3] Freezing makes it tougher on life, but not impossible.

 

[CEL]

1]
...
3] Freezing makes it tougher on life, but not impossible.

True to microbial life, but complex life needs less extreme temperatures.

 

[mikelepore]

DaveC426913:

1] I don't know how many planets have an orbit so extreme that its temp goes from liquid water to melting rocks. I'm thinking statistically near zero.

Sure. I probably overstated my example. I resubmit my point without my exaggerated example.

2] I think 'older planets' is already factored into 'conducive to life'. And one property of older planets is that they tend to have more stable, more circular orbits.

Yes, I understand what you mean. All I mean to point out is that such issues are "in" the term in the Drake equation that represents the probability: given that a star has planets, what fraction of them are "earth-like". In other words, to those who wrote earlier that our own solar system indicates a value of roughly 0.1 for this term, I'm saying, whoa, there's more to consider.

3] Freezing makes it tougher on life, but not impossible

But I think this topic isn't mainly about the resilience of life to survive, give that life already exists. We're talking mainly about the event where something happens in a solution that leads to the first life.

 

[DaveC426913]

3] Freezing makes it tougher on life, but not impossible

But I think this topic isn't mainly about the resilience of life to survive, give that life already exists. We're talking mainly about the event where something happens in a solution that leads to the first life.

The point I made is that an environment that is freezing and thawing would make it harder for life to get started, but not impossible.



Actually, recent observations of glaciers suggest that ice may actually be a boon to the formation of life. It seems that there is a phenomenon in ice that dramatically concentrates nutrients into pockets, creating uncountable numbers of crucibles for chemicals to interact.

 

[Pythagorean]

is there a current accepted "probability of life on other planets" within the science community?

 

[Poop-Loops]

Yes, I read a recent paper that places the limits on the probability of life on other planets from 0 to 1.

 

[mikelepore]

Another question -- I have heard several biologists say that all life on Earth is an example of just one creation event - that life appeared only once in over 4 billion years. Apparently there is no much similarity in the chemistry of all protista, bacteria, animals, plants, and fungi, that they can tell that all of it branched out from one common ancestor. If that's true, it makes it sound so difficult for life to appear that it's incredibly good luck that it happened at all. Or --wait a minute -- could it be, rather, that life might have appeared again independently, but the second apparance and third apperance and fourth appearance of self-replicating molecules quickly got eaten and therefore became extinct?

 

[DaveC426913]

Another question -- I have heard several biologists say that all life on Earth is an example of just one creation event - that life appeared only once in over 4 billion years. Apparently there is no much similarity in the chemistry of all protista, bacteria, animals, plants, and fungi, that they can tell that all of it branched out from one common ancestor. If that's true, it makes it sound so difficult for life to appear that it's incredibly good luck that it happened at all. Or --wait a minute -- could it be, rather, that life might have appeared again independently, but the second apparance and third apperance and fourth appearance of self-replicating molecules quickly got eaten and therefore became extinct?

Two answers:

1] The initial of life does factor into the subsequent creation of life - competition for resources and niches.

2] There are some studies going on currently to look for lifeforms on Earth that are not part of the line we know and love. They'll surely be little more complex than bacteria or algae and they'll likely be clinging at the very edge of habitable zones.

 

[Ivan Seeking]

There is also the notion of panspermia.
http://www.space.com/searchforlife/aliens_all_001027-1.html

 

[K.J.Healey]

Isn't there a possibility that any given nucleation event for life must start from the same thing, and then follow sort of a cellular-automata, making any given of number of nucleation points indistinguishable as to where the following organisms started?

If you can argue that since what's alive today appears to have stemmed from one original line, would it be unreasonable to modify that statement to say that what's alive today appears to have stemmed from one type of nucleation? And those "types" ultimately lead to this current style of life.

I don't see why we have to limit what's here to ONE exact organism.

 

[Schrodinger's Dog]

Actually this was brought up in the biology section. The idea that life must have come from a very similar line. It's not beyond the realms of possibility that either this line had an advantage that isn't readily apparent, or that conditions unknown wouldn't favour, neither is it unlikely that an organism couldn't of been driven to the brink of extinction and only certain types surviving. That said though I still think it's really odd that all life has right handed DNA, and left handed amino acids, and it's a point in favour of panspermia. Although I'm still not that convinced, it's a possibility though.

Wasn't Hoyle the last to advocate panspermia?

 

[Schrodinger's Dog]

http://www.sciencedaily.com/releases/2008/02/080228174823.htm

I've just completely and utterly ripped this off from the biology thread, but since it has some real relevance here, and not everyone goes there:

ScienceDaily (Feb. 29, 2008) — An important discovery has been made with respect to the mystery of "handedness" in biomolecules. Researchers led by Sandra Pizzarello, a research professor at Arizona State University, found that some of the possible abiotic precursors to the origin of life on Earth have been shown to carry "handedness" in a larger number than previously thought.
See also:
Plants & Animals

Pizzarello, in ASU's Department of Chemistry and Biochemistry, worked with Yongsong Huang and Marcelo Alexandre, of Brown University, in studying the organic materials of a special group of meteorites that contain among a variety of compounds, amino acids that have identical counterparts in terrestrial biomolecules. These meteorites are fragments of asteroids that are about the same age as the solar system (roughly 4.5 billion years.)

Anyone like to speculate on life being seeded from space, or panspermia?

Or what about the discovery of methane in another solar system?

http://www.independent.ie/world-news/methane-gas-find-raises-hopes-of-life-beyond-earth-1323470.html"

Methane gas find raises hopes of life beyond Earth

Scientists have for the first time detected methane gas in the atmosphere of a planet orbiting a distant star – an achievement that might soon lead to the discovery of extraterrestrial life.

Methane is an organic molecule which can be produced by biological activity but the scientists believe that its presence on this particular planet cannot be a by-product of living organisms as temperatures there are 900C – hot enough to melt silver.

However, the researchers said that just being able to detect methane on a planet beyond our own solar system shows that it is possible to find the vital signs of extraterrestrial life forms on other "extrasolar" planets more suitable to life.

"Methane is an organic molecule and so even if it is not produced by biological forces in the environment of this planet, finding methane in another planetary environment could indicate that life might be there," said Giovanna Tinetti, of University College London, who took part in the study published in the journal Nature.

 

[Snazzy]

The way I see it is that life in other planetary systems, and life in the galaxy in general, is so vastly far away from each other that by the time an intelligent life form has picked up our signals, and by the time their reply has reached us here on earth, we would have became extinct. I do hold hopes that we make contact with life forms that have already sent signals, but I wouldn't bet on it.

 

[Schrodinger's Dog]

The way I see it is that life in other planetary systems, and life in the galaxy in general, is so vastly far away from each other that by the time an intelligent life form has picked up our signals, and by the time their reply has reached us here on earth, we would have became extinct. I do hold hopes that we make contact with life forms that have already sent signals, but I wouldn't bet on it.

True but isn't that given that we can never overcome the vastness of space. I personally would be wary of the claim of never or that by the time it happened we would be extinct; we do have a few billion years to play with, despite the useless politics of the 20th century; it's possible we won't wipe ourselves out quite so soon. :/

 

[Snazzy]

That's the thing, though; I don't hold the optimistic viewpoint that we can live up to the point in the far future where our planet is enveloped by fire and brimstone (not sure how that is an optimistic perspective). In order to do that, we have to protect ourselves from ourselves. I'm not trying to be misanthropic here, but in the case that humans don't blow themselves apart, or die in hoards due to some disease, or succumb to climate change within the next century or so, I do believe we can only survive for a few hundred million years, or possibly less, before Earth becomes inhospitable due to the sun's changing nature.

 

[Schrodinger's Dog]

That's the thing, though; I don't hold the optimistic viewpoint that we can live up to the point in the far future where our planet is enveloped by fire and brimstone (not sure how that is an optimistic perspective). In order to do that, we have to protect ourselves from ourselves. I'm not trying to be misanthropic here, but in the case that humans don't blow themselves apart, or die in hoards due to some disease, or succumb to climate change within the next century or so, I do believe we can only survive for a few hundred million years, or possibly less, before Earth becomes inhospitable due to the sun's changing nature.

Why is the sun going to make Earth inhospitable to life, doesn't seem to have much of a track record lately as such, if you see what I mean. Of course there have been extinction events but there's no reason to suspect we are headed towards one atm, not a total extinction event anyway? Much more likely that we will destroy ourselves given the suns history surely?

 

[Snazzy]

Well, in about 500 million years time, some NASA sources say the sun will become about 10% more luminous, which means a lot higher surface temperatures, more vapour in the atmosphere, and a runaway greenhouse effect.

 

[Schrodinger's Dog]

Well, in about 500 million years time, some NASA sources say the sun will become about 10% more luminous, which means a lot higher surface temperatures, more vapour in the atmosphere, and a runaway greenhouse effect.

Have you got a link for that. And since we've been here \simeq 200,000 years that is a long time anyway.

 

[Snazzy]

http://image.gsfc.nasa.gov/poetry/ask/a10474.html
http://www.astronomycafe.net/qadir/q79.html
http://www.space.com/scienceastronomy/solarsystem/death_of_earth_000224.html

I completely agree with you that even 100,000 years is a long time for humans (let alone a few hundred million years) considering that we transitioned from hunter-gatherer societies just 10-20 thousand years ago, however I still believe that we are at the peak of our existence right now.

 

[Schrodinger's Dog]

http://image.gsfc.nasa.gov/poetry/ask/a10474.html
http://www.astronomycafe.net/qadir/q79.html
http://www.space.com/scienceastronomy/solarsystem/death_of_earth_000224.html

I completely agree with you that even 100,000 years is a long time for humans (let alone a few hundred million years) considering that we transitioned from hunter-gatherer societies just 10-20 thousand years ago, however I still believe that we are at the peak of our existence right now.

Ok I don't doubt that 10% increase in luminosity claim although of course that is a hypothesis based on hypothetical conditions, and as this is a speculatory thread I respect your opinions, even though I'm not so pessimistic to agree with them.

I don't think we'll die out if the Earth becomes 10% more luminous, 10% more luminous doesn't mean the whole planet is doomed and all it's life, it's not a 10% rise in temperature on Earth. And if we can develop technologies to mitigate an effect we already know is going to happen, then we might just make it yet.

 

[Snazzy]

Oh yes, I agree that a 10% increase in luminosity doesn't necessarily correlate to a 10% increase in average temperature as the estimates are hypothetical, but the risk is still there. But does the American government itself have the incentive or the motivation to look for life elsewhere? SETI used to be funded by the US government, but is now primarily funded by private sources. Even the TPF was not on NASA's 2007 budget and still remains without a launch date.

If we want to find life, I believe that the best place to start is on the Galilean moons.

 

[Schrodinger's Dog]

Oh yes, I agree that a 10% increase in luminosity doesn't necessarily correlate to a 10% increase in average temperature as the estimates are hypothetical, but the risk is still there. But does the American government itself have the incentive or the motivation to look for life elsewhere? SETI used to be funded by the US government, but is now primarily funded by private sources. Even the TPF was not on NASA's 2007 budget and still remains without a launch date.

If we want to find life, I believe that the best place to start is on the Galilean moons.

Well that's not quite the whole picture we are looking for life beyond SETI, and anyway SETI is still going strong. But it only tells us that in x many years since we've been monitoring certain communication methods, aliens with in x light years would be discovered. Life may be discovered through other means such as finding planets analogous to Earth and then using http://en.wikipedia.org/wiki/Optical_interferometry#Astronomical_Optical_Interferometry" to determine whether signs of life, such as the colour of certain elements, and more importantly the colours that denote chlorophyll exist. And let's not forget that radio communication compared to today is extremely primitive, if we consider other methods of communication we have now.

 

[CEL]

http://www.sciencedaily.com/releases/2008/02/080228174823.htm

I've just completely and utterly ripped this off from the biology thread, but since it has some real relevance here, and not everyone goes there:



Anyone like to speculate on life being seeded from space, or panspermia?

Or what about the discovery of methane in another solar system?

http://www.independent.ie/world-news/methane-gas-find-raises-hopes-of-life-beyond-earth-1323470.html"

Methane is a very simple molecule. One carbon and four hydrogen atoms. It would be amazing if it was not present around other stars. It is believed that methane molecules could be at the origin of life on Earth, but it is present in other planets and moons in our solar system and no sign of life was found until now.
The importance of the discovery is that scientists were able to detect the gas in the atmosphere of an extra solar planet, not that this is a sign of possible life.

 

[Schrodinger's Dog]

Methane is a very simple molecule. One carbon and four hydrogen atoms. It would be amazing if it was not present around other stars. It is believed that methane molecules could be at the origin of life on Earth, but it is present in other planets and moons in our solar system and no sign of life was found until now.
The importance of the discovery is that scientists were able to detect the gas in the atmosphere of an extra solar planet, not that this is a sign of possible life.

Oh yeah but since extrasolar planets are limited to planets around about the size of Uranus atm at the largest, it's about as good as it gets, until more powerful interferometry arrays are started up. It's I suppose a tantalising start.

 

[Ivan Seeking]

Seeds of Life [reportedly] Found Near Saturn

http://www.livescience.com/space/scienceastronomy/080326-cassini-flyby-update.html

Note of caution: I assume that this will be published but I don't see that it has been yet.

 

[drankin]

Until we find evidence of a single alien microbe (I don't know biology), on another planet or astroid, I'm in the camp that says there is no other life out there. It seems to me that their should be, but there is absolutely no evidence. Even on planets & moons that have or had water on them.

There is life, if only single celled, on just about every extereme environment on Earth. Why isn't there the same on other planets in our solar system that have similar conditions? I suspect there is something about Earth that we have yet to discover that allows life to thrive almost uncontrollably. Now, if we can demostrate that crops can indeed grow and thrive, animals can live a normal lifespan and reproduce on another planet within a remote lab, I'll lose some of my skepticism. It seems to me that life should be teaming in the universe but as of yet, there is absolutely no evidence.

 

[Schrodinger's Dog]

Until we find evidence of a single alien microbe (I don't know biology), on another planet or astroid, I'm in the camp that says there is no other life out there. It seems to me that their should be, but there is absolutely no evidence. Even on planets & moons that have or had water on them.

There is life, if only single celled, on just about every extereme environment on Earth. Why isn't there the same on other planets in our solar system that have similar conditions? I suspect there is something about Earth that we have yet to discover that allows life to thrive almost uncontrollably. Now, if we can demostrate that crops can indeed grow and thrive, animals can live a normal lifespan and reproduce on another planet within a remote lab, I'll lose some of my skepticism. It seems to me that life should be teaming in the universe but as of yet, there is absolutely no evidence.

So no evidence equals evidence that it does not exist? That doesn't seem very scientific...

I think given the expanse of the Universe most scientists believe that although there is no evidence (because of the expanse of the Universe) actually saying there isn't is a leap too far. I mean do you actually know how many galaxies there are? And how many stars in each one there are. Now no problem with you saying you don't recognise life as having a possibility until you see proof, but isn't that equivalent to "atheism"? Isn't agnosticism more rational than "atheism" or "theism"? Than saying until they do, I am an atheist? Ie we cannot know as yet given the evidence available. Let's face it the evidence is accumulating.

And I disagree with that the probabilities are just so unquantifiable that life existing only here seems unlikely to say the least. I might be wrong, but atheism just seems all wrong. A healthy until I see proof I will believe the chances are good, or until I see proof the chances are slim or anywhere in between seems more rational, is better than no chance matey until I see proof. There should be? Is that it? Given there should be it doesn't lead to until I see evidence there is not? That's not logical unless you are religious about the Universe, in of course my humble opinion.

No planet in our system has anything like the conditions Earth had, and we cannot detect Earth like planets atm. But the fact that many Solar systems have planets, tends to suggest that many systems might also have Earth like planets given the numbers involved, and the numbers are so huge you can't actually conceive of them.

Now don't get me wrong I don't think life is all over the place, its no doubt quite sparse, but its sparsity is probably the reason we have no direct evidence, rather than the fact that we don't equals it does not exist.

 

[cristo]

I can't remember whether I've posted in this thread or not. Anyway, in my opinion the probability of life elsewhere in the universe is very close to 1, if not 1. Even if we are the only life in our own galaxy, then there are billions of other galaxies out there. I don't see how life can be so rare so as to only flourish in 1 out of many many many planetary systems.

 

[drankin]

So no evidence equals evidence that it does not exist? That doesn't seem very scientific...

I think given the expanse of the Universe most scientists believe that although there is no evidence (because of the expanse of the Universe) actually saying there isn't is a leap too far. I mean do you actually know how many galaxies there are? And how many stars in each one there are. Now no problem with you saying you don't recognise life as having a possibility until you see proof, but isn't that equivalent to "atheism"? Isn't agnosticism more rational than "atheism" or "theism"? Than saying until they do, I am an atheist? Ie we cannot know as yet given the evidence available. Let's face it the evidence is accumulating.

And I disagree with that the probabilities are just so unquantifiable that life existing only here seems unlikely to say the least. I might be wrong, but atheism just seems all wrong. A healthy until I see proof I will believe the chances are good, or until I see proof the chances are slim or anywhere in between seems more rational, is better than no chance matey until I see proof. There should be? Is that it? Given there should be it doesn't lead to until I see evidence there is not? That's not logical unless you are religious about the Universe, in of course my humble opinion.

No planet in our system has anything like the conditions Earth had, and we cannot detect Earth like planets atm. But the fact that many Solar systems have planets, tends to suggest that many systems might also have Earth like planets given the numbers involved, and the numbers are so huge you can't actually conceive of them.

Now don't get me wrong I don't think life is all over the place, its no doubt quite sparse, but its sparsity is probably the reason we have no direct evidence, rather than the fact that we don't equals it does not exist.

SD, I'm not a scientist so I might need a little more evidence to be persuaded. Life outside Earth certainly seems possible, mathmatically plausible, but at this point in human existence it isn't a fact.

Earth is the only planet in our solar system that has an oxygen rich atmosphere. Most life here requires it. Is oxygen a requirement for life to exist on other planets though? Or does life simply adapt to it's environment and utilize the elements that exist to survive? Thinking about it gets my mind off on tangents, why does life bother to exist? What drives it and to what end? If we had reasons for these questions then I could see it developing elsewhere a bit easier to swallow. I guess I need to understand why it is here before I'll accept it would be anywhere else. It would cool if there was at least a hint that it took somewhere else in the solar system, but as of yet, we have nothing to go on.

No matter, life "is" here on Earth for some reason or result.

 

[Schrodinger's Dog]

SD, I'm not a scientist so I might need a little more evidence to be persuaded. Life outside Earth certainly seems possible, mathmatically plausible, but at this point in human existence it isn't a fact.

Earth is the only planet in our solar system that has an oxygen rich atmosphere. Most life here requires it. Is oxygen a requirement for life to exist on other planets though? Or does life simply adapt to it's environment and utilize the elements that exist to survive? Thinking about it gets my mind off on tangents, why does life bother to exist? What drives it and to what end? If we had reasons for these questions then I could see it developing elsewhere a bit easier to swallow. I guess I need to understand why it is here before I'll accept it would be anywhere else. It would cool if there was at least a hint that it took somewhere else in the solar system, but as of yet, we have nothing to go on.

No matter, life "is" here on Earth for some reason or result.

Indeed there are http://www.springerlink.com/content/l0362x108x558g62/" on this planet that don't require oxygen at all, and it seems likely that they may have existed since the formation of the earliest life forms. So I'd say it's definitely needed as a chemical oxidising agent, but it isn't essential.

The good thing about life though, is that except in our case it tends to produce an environment that leads to more life or the optimal amount of life given the conditions. So given some or a little oxygen you end up with just the right amount for life to exist. And since oxygen is fairly abundant in the remnants of stars, I don't see where there is a problem with oxygen. In fact carbon, hydrogen, nitrogen and oxygen are relatively abundant in the Universe. Throw those into the mix at the right sort of temperatures with the right sort of planet, and I'd be surprised if you didn't get life.

When your talking 1,000,000,000,000,000,000,000. Stars in the universe, it actually becomes rather far fetched to assume there isn't life elsewhere.

I doubt personally life has visited Earth except in the obvious sense, and certainly not intelligent life, which doesn't seem to exist here. But I do find the idea that life and even intelligent life does not exist somewhere else to be a rather remote possibility, if not impossible then vanishingly small. Even given the most conservative values for the http://www.markelowitz.com/drakeeqn.htm" , at least one life form per galaxy is likely, and as its recently been revealed, planets are far more common than previously thought around stars, so the estimate I gave of about 100 communicative intelligent life forms per galaxy seems a good conservative guess. It would also explain why we haven't made contact with any of them yet, given the size of the galaxy that would make intelligent life quite a rarity and the likely distances between them enormous.

Why does life bother to exist? Why does it have to have a reason? I leave such speculation in the hands of those who like ontological arguments. It could just be that life exists because given the initial conditions, it's likely too, and that is that.

 

[Ivan Seeking]

Could life exist beneath Enceladus? A recent flyby of Saturn's icy moon has bolstered this fascinating idea. Two years ago, images from the robotic Cassini spacecraft orbiting Saturn led astronomers to the undeniable conclusion that Saturn's moon Enceladus was spewing fountains of gas and ice crystals through cracks in its surface dubbed tiger stripes. Last month, Cassini dove through some of these plumes and determined that they contained water vapor laced with small amounts of methane as well as simple and complex organic molecules. Surprisingly, the plumes of Enceladus appear similar in make-up to many comets. What's more, the temperature and density of the plumes indicate they might have originated from a warmer source -- possibly a liquid source -- beneath the surface. A liquid water sea containing organic molecules is a good place to look for life. Pictured above is a vertically exaggerated close-up of some long, venting tiger stripes. The computer composite was generated from images and shadows taken during the recent Cassini flyby. Nine more flybys of Enceladus by Cassini are planned. [see NASA photo]

http://apod.nasa.gov/apod/ap080331.html

 

[CEL]

Indeed there are http://www.springerlink.com/content/l0362x108x558g62/" on this planet that don't require oxygen at all, and it seems likely that they may have existed since the formation of the earliest life forms. So I'd say it's definitely needed as a chemical oxidising agent, but it isn't essential.

...

A nitpick. Life does not need oxidation. Anaerobic bacteria use fermentation instead. Since fermentation is less efficient than oxidation, it is useful only for very simple life forms. So, if we are looking for intelligent life we must certainly look for free oxygen.

 

[Schrodinger's Dog]

A nitpick. Life does not need oxidation. Anaerobic bacteria use fermentation instead. Since fermentation is less efficient than oxidation, it is useful only for very simple life forms. So, if we are looking for intelligent life we must certainly look for free oxygen.

Assumes that complex life couldn't evolve without oxygen. I tend to agree but I'm not brave enough to stake anything on it. It's a good nitpick though.